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Page 1: More Than Motors - Gibbs Guides · Gibbs Guides.com Gibbs Guide to Electric Power Systems: Part 2 – More Than Motors Preface . This guide is the second in the Electric Power Systems

Gibbs Guides.com Gibbs Guide to Electric Power Systems: Part 2 – More Than Motors

1 © Copyright Andrew Gibbs 2013

The Gibbs Guide to

Electric Power Systems Part 2

More ThanMore ThanMore ThanMotorsMotorsMotors

by

Andrew Gibbs

gibbsguides.com

Page 2: More Than Motors - Gibbs Guides · Gibbs Guides.com Gibbs Guide to Electric Power Systems: Part 2 – More Than Motors Preface . This guide is the second in the Electric Power Systems

Gibbs Guides.com Gibbs Guide to Electric Power Systems: Part 2 – More Than Motors

The Gibbs Guide to Electric Power Systems

Part 2

More Than Motors

A Gibbs Guides e-book

By Andrew Gibbs

gibbsguides.com

2 © Copyright Andrew Gibbs 2013

Page 3: More Than Motors - Gibbs Guides · Gibbs Guides.com Gibbs Guide to Electric Power Systems: Part 2 – More Than Motors Preface . This guide is the second in the Electric Power Systems

Gibbs Guides.com Gibbs Guide to Electric Power Systems: Part 2 – More Than Motors

Messages from Andrew Gibbs

Thank you for purchasing this e-book to Electric Power Systems. I sincerely hope it will be an enjoyable read, and that it will be a great deal of help to you. If you like the guide, please tell your modelling friends. If you have suggestions for improvements or alterations, please feel free to contact me.

Copyright matters Several months of full-time work plus a great deal of effort went into creating this guide. As the author of this e-book, I hold the copyright to it. I make my living from helping my fellow modellers by writing about modelling matters, so I do have to charge for some products such as this e-book. There’s lots of high quality free-to-access information on my website at www.gibbsguides.com which you are welcome to access and share. There’s also a completely free, high quality e-zine which you are most welcome to sign up for at this site. I make sure my products offer excellent value by providing high quality information at a reasonable price. In return, I ask that you respect my right to copyright and do not share this publication with others without my permission. If you did this, you would deprive me of the chance of a sale. And if that happened I might have to stop writing Gibbs Guides and get a proper job!

Andrew Gibbs

Cover photograph

The cover photograph shows a 65 inch span Hangar 9 P51D in Miss America colours. The careful pilot is busy working through his Before Take Off checklist.

3 © Copyright Andrew Gibbs 2013

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Gibbs Guides.com Gibbs Guide to Electric Power Systems: Part 2 – More Than Motors

DISCLAIMER

Electric power system components such as motors, batteries, electronic speed controllers, propellers, chargers and so on are supplied with instructions. Additional information may also be available on the supplier’s and/or manufacturer’s website. You are strongly encouraged to read and implement any such instructions, especially those instructions relating to safety. Such instructions must be accurately followed without deviation. None of the information within this guide is intended to overrule such instructions, and where any disagreement exists, always follow the instructions of the manufacturer and contact them or the supplier of your equipment for advice about the particular circumstances of your application. All possible care has been taken with this guide and the information within it is offered in good faith; nevertheless, in using this guide you do so at your own risk and absolve the author of any liability whatsoever in respect of death, personal injury, damage to property or any other kind of accident. The Gibbs Guides terms and conditions state that by purchasing this guide, you have already indicated that you accept these terms.

Copyright notice This guide is copyright Andrew Gibbs. All rights strictly reserved. Storage on a retrieval system, reproduction or translation of any part of this work by any means electronic or mechanical, including photocopying, beyond that permitted by Copyright Law, without the written permission of Andrew Gibbs is unlawful.

Printing your guide By buying this e-book, you have bought a license to print a single copy single copy for your own use. If you choose to print your copy, I suggest using good quality 90 gsm paper or heavier for the pages, and thin card for the front and rear covers.

4 © Copyright Andrew Gibbs 2013

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Gibbs Guides.com Gibbs Guide to Electric Power Systems: Part 2 – More Than Motors

Contents Preface and Acknowledgements 6 Chapter 1: Propeller Primer 7 Chapter 2: Propeller Power, Efficiency & Thrust 12 Chapter 3: Reduction Gearing 19 Chapter 4: Heat, Efficiency and Cooling 22 Chapter 5: Introduction to Electronic Speed Controllers 26 Chapter 6: Electric Motor Safety 29 Chapter 7: BEC and PCO Functions of the ESC 32 Chapter 8: Opto Isolating ESCs 34

Chapter 9: Electronic Speed Controller Limitations 35 Chapter 10: Setting up Electronic Speed Controllers 37 Chapter 11: Battery Basics 41 Chapter 12: Battery Voltage Characteristics & Charging 44 Chapter 13: Wiring and Connectors 48 Chapter 14: Wiring Diagram for Single Motor Models 53 Chapter 15: Example Power Systems in Detail 54 Index 66

5 © Copyright Andrew Gibbs 2013

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Gibbs Guides.com Gibbs Guide to Electric Power Systems: Part 2 – More Than Motors

Preface This guide is the second in the Electric Power Systems series. While part 1 examined the operation of electric motors, this part follows on and discusses the remaining components of the power system such as the battery, ESC, propeller and so on.

For modellers used to dealing with internal combustion (i.c.) engines, electric flight can seem like a mysterious black art. The aim of this guide is to bridge the gap and provide an enjoyable grounding in the principles of electric powered model aircraft. Development of model power systems. Shown here are (L to R) Webra 0.40 (circa 1980), OS35 FP (1987), brushed 600 motor with gearbox, brushless Jeti Phasor 45-3 and brushless Mega 16/15/3.

The principles governing the operation of electric power systems are completely different to those of i.c. models. For modelers used to dealing only with glow, diesel or petrol fuelled engines, this new world of Volts, Amps, Watts can be rather unsettling. The aim of this series of guides is to demystify electric flight and to allow the reader to develop a sense of familiarity and comfort with the concepts involved. As with part 1, I’ve tried to make the presentation of the material as easy as possible to get to grips with. Andrew Gibbs Southampton, UK, Mar 2013

Acknowlegements

No work of technical complexity is ever produced in isolation, and this guide is no exception. I would like to thank Toni Reynaud in particular for producing most of the many wonderful diagrams and graphs. Grateful thanks are also extended to Toni, Chris Golds, Bruce Smith and my father John Gibbs, all of whom kindly reviewed the rough drafts and offered their valuable suggestions.

6 © Copyright Andrew Gibbs 2013

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Gibbs Guides.com Gibbs Guide to Electric Power Systems: Part 2 – More Than Motors

Chapter 1 Propeller Primer

The propeller is one of the most important parts of an electric power system, but it is often given only scant attention. The choice of propeller has a major effect on the way a model flies, so propellers deserve to be carefully considered as the motor that drives it, when thinking about electric power systems. The way in which propellers operate is fundamentally straightforward, although propeller theory becomes very complex indeed when it is analysed in detail. My objective here is to present a simple, concise and easy to understand introduction to propellers which can be applied to the real-life practical needs of aeromodellers. Propeller basics The propeller converts the mechanical power supplied by the electric motor into forward thrust. The main parts of a propeller, or prop for short, are shown in the diagram below:

The various parts of a propeller. The blade cross-sections show how the blades are twisted as they extend towards the tip: the blade angle flattens as it reaches the tip.

The blades of the propeller pull the model through the air by generating a force in a forwards direction, called thrust. The thrust developed by the propeller is generated in exactly the same way that a wing develops its lift, so in essence, the propeller is really little more than a rotating wing. A variety of propeller airfoils (aerofoils) are used for model aircraft. Propellers which are designed to turn relatively slowly, such as those for

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Gibbs Guides.com Gibbs Guide to Electric Power Systems: Part 2 – More Than Motors

slow-fly models, may use undercambered blades (i.e. the blades with a concave rearmost surface), similar to the undercambered wing of an aircraft designed for slow flight. Conversely, propellers designed for higher speed use have a flatter, less cambered airfoil profile.

Left: This propeller blade has been cut in half to reveal its airfoil section. The airfoil is just like that found on a wing. Right: This undercambered slow-fly prop is a good match for this slow flying vintage-style model. Although the term ‘undercambered’ is technically meaningless, it nevertheless serves as a convenient term of reference for aeromodelers. Pitch and diameter The two propeller characteristics of most interest to us are its diameter, and its pitch. Propellers are in fact defined by their diameter and pitch measurements. For example, a 12 x 8 prop has a diameter of 12 inches and a pitch of 8 inches.

In the early days of flight, propellers were called airscrews. Although a propeller does not actually move like a mechanical screw through a solid material, this concept can help to visualise the forward helical movement of propellers with various pitches.

The diameter is the diameter of the circle which the propeller tips describe as they rotate. The pitch is the distance the propeller would move forward in one revolution, if it did not have the drag of a model attached to it. The propeller pitch is determined by the angle at

8 © Copyright Andrew Gibbs 2013

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Gibbs Guides.com Gibbs Guide to Electric Power Systems: Part 2 – More Than Motors

which the propeller blades are set; a propeller with a relatively fine pitch will move forward a short distance with one revolution, while a prop with a relatively high, or coarse pitch will move further. Thus, a high speed model will prefer a relatively course pitch prop, while a slow speed model will use a finer pitch prop.

Left: Differences in pitch are not always easy to see by eye. These two 9 inch diameter props of the same propeller ‘family’ are very different in pitch, but at first glance they look very similar. Right: When clamped together, the differences in pitch are obvious. The left hand prop has 7.5 inches of pitch, while the right hand one has 4 inches. The dimensions of a prop should always be marked on at least one blade root.

Pitch speed The pitch speed of a prop is the forward speed the propeller would achieve, if no airframe drag existed. Although this is clearly a theoretical speed, a knowledge of the propeller’s pitch speed is of practical use when working with power systems. Pitch speed is found by multiplying the propeller pitch and the rpm it is turning at. For example, if a prop has a pitch of 6 inches, and is rotating at 100 revolutions per second (= 6,000 rpm), it will try to move forward 600 inches per second, equivalent to 50 feet/sec or 34 mph. The table below shows the pitch speed for a variety of propeller pitch and rpm combinations (applies to electric and i.c):

Pitch ininches 4000 5000 6000 7000 8000 9000 10000 11000 12000 13000 14000 15000 16000

4 15 19 23 27 30 34 38 42 45 49 53 57 614.5 17 21 26 30 34 38 43 47 51 55 60 64 685 19 24 28 33 38 43 47 52 57 62 66 71 76

5.5 21 26 31 36 42 47 52 57 63 68 73 78 836 23 28 34 40 45 51 57 63 68 74 80 85 917 27 33 40 46 53 60 66 73 80 86 93 99 1068 30 38 45 53 61 68 76 83 91 98 106 114 1219 34 43 51 60 68 77 85 94 102 111 119 128 136

10 38 47 57 66 76 85 95 104 114 123 133 142 15212 45 57 68 80 91 102 114 125 136 148 159 170 182

RPM in thousands

Propeller pitch speeds (mph)

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Gibbs Guides.com Gibbs Guide to Electric Power Systems: Part 2 – More Than Motors

Model aircraft propellers are available in a wide variety of blade shape and airfoil types. From left to right: A wooden electric propeller, a GWS slow fly prop, APC electric prop, another style of GWS prop, an APC slow fly prop, two more props and finally a Günther 5 x 4.3 prop, commonly used with brushed ‘400’ size motors. All the props shown here are tractor props, i.e. designed to rotate anticlockwise when viewed from in front of a model. Usefully, a reasonable approximation of pitch speed can be made by multiplying propeller pitch in inches by rpm in thousands. For example, an 8 inch prop at 5,000 rpm has a pitch speed of about 8 x 5 = 40 mph. This is very close to the actual pitch speed of 38 mph, and quite accurate enough for most modelling purposes. Slippage The drag of the model will ensure that the actual distance the propeller moves forward will be less than its pitch speed. This difference is termed ‘slippage’. A good rule of thumb is that provided a model uses an appropriate propeller, the pitch speed will be approximately 25% greater than the flying speed.

These two views of the same propeller illustrate the twisted nature of the propeller blade. This twist helps to ensure that all parts of the propeller blade work at a similar angle of attack in flight, keeping the propeller operating as efficiently as possible.

10 © Copyright Andrew Gibbs 2013

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Gibbs Guides.com Gibbs Guide to Electric Power Systems: Part 2 – More Than Motors

Pitch to diameter ratio The pitch to diameter ratio (p/d ratio) is the ratio of a propeller’s pitch to its diameter. For example, a 12 x 6 prop has a pitch of 6 inches and a diameter of 12 inches, so it has a p/d ratio of 0.5 (because 6 ÷ 12 = 0.5). Similarly, a 12 x 9 has a p/d ratio of 0.75 (because 9 ÷ 12 = 0.75), while a 12 x 12 has a p/d ratio of 1.0 (12 ÷ 12 = 1). A propeller should be chosen with a suitable p/d ratio for the model in question. Most model aircraft props are in the range 0.4 to 1.0. Slow flying models perform best using a low p/d ratio, somewhere around 0.5, average club sport models require a p/d ratio around 0.7 – 0.8 and very fast models up to 1.0. The table below outlines a suitable choice of p/d ratio for various model types.

Model type

Typical suitable propeller p/d ratio

Example propeller & its p/d ratio

Highly aerobatic (3D) models, prop-hanging (hovering flight) etc

0.3 - 0.4 12 x 4 (0.3)

Slow models e.g. WW1 biplane 0.4 - 0.6 12 x 6 (0.5) Aerobatic models where good vertical performance (high rate of climb) is req’d.

0.5 – 0.7 11 x 7 (0.63)

Slow-medium speed model e.g. vintage types

0.5 - 0.6 11 x 6 (0.54)

Medium speed models e.g. trainer 0.5 – 0.7 8 x 6 (0.75) Moderately fast sports models 0.7 – 0.8 9 x 7 (0.77) Fast models e.g. WW2 fighter 0.8 – 0.9 10 x 8 (0.8) Very fast high-performance models e.g. high-power sport or racing models

0.9 – 1.0 4.7 x 4.7 (1.0)

7 x 7 (1.0)

Left: This large Flair Fokker Dr1 triplane by Alan Gorham has plenty of drag, and so is a slow flying model. A suitable motor and prop combination will produce a relatively low pitch speed. Right: The prop of this Balsacraft Hawker Hurricane built by Stuart Warne has a relatively high pitch speed. The blue swastika markings on this model identify it as a Hurricane of the Finnish Air Force which operated the type 1940-44. The blue swastika, the ancient symbol of the sun and good luck, was used by the Finnish Air Force between 1918 and 1945. The Finnish swastika had nothing to do with the Nazi swastika, which came into being in 1935.

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Gibbs Guides.com Gibbs Guide to Electric Power Systems: Part 2 – More Than Motors

Chapter 2 Propeller Power, Efficiency & Thrust

Power Requirements of Propellers The amount of power required to turn a prop depends on its diameter, its pitch and the rpm we wish to turn it at. The power required is proportional to rpm³ x diameter4 x pitch. We can summarise in a non-mathematical way what this formula tells us about the way propeller changes will affect the required power:

Relationship between pitch and required power Small increases in propeller pitch require a proportionate increase in power. For example, if the pitch is increased by 10 % (e.g. an increase in pitch from 5 inches to 5.5 inches), the power required to maintain the same rpm also rises by 10 %. Relationship between rpm and required power Small increases in rpm result in a disproportionately large increase in power. This means that if for example, the rpm is increased by 10 % (e.g. an increase in rpm from 7,000 rpm to 7,700 rpm), the power required to maintain the same propeller rpm will rise by 33 %. Relationship between diameter and required power Small increases in diameter result in a disproportionately large increase in power. For example, if the diameter is increased by 10 % (e.g. an increase in diameter from 10 inches to 11 inches), the power required to maintain the same rpm will rise by 46 %.

Each propeller blade of this Hurricane taking off is absorbing 1/3 of its engine’s 1,185 hp. Conditions of high humidity are causing the vortices present at each prop tip to condense as they are left behind, resulting in continuous trail of visible moisture. The helical pattern that each blade makes through the air can clearly be seen. The Hurricane’s designer, Sidney Camm, was an aeromodeller in his youth. Photograph copyright Randy Coniam & used with his kind permission

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Gibbs Guides.com Gibbs Guide to Electric Power Systems: Part 2 – More Than Motors

Propeller efficiency and size The propeller provides thrust for a model by accelerating a mass of air. There are two ways to provide a given quantity of thrust; either by accelerating a small volume of air to high speed, or by accelerating a larger volume of air by a smaller amount.

Small volume of air accelerated to a high velocity

Larger volume of air accelerated to a less high velocity

Relatively high efficiency

Relatively low efficiencyEqual thrust and forward velocity

EFM025

The first method uses a relatively small prop turning at high rpm, while the second uses a larger propeller turning at a lower rpm. For the same thrust, less energy is required to accelerate the larger volume of air by the smaller amount. For this reason, generally speaking, the larger the prop is the more efficient it will be. One of the advantages of electric power systems is that where appropriate, they allow us to design power systems with relatively large, efficient props compared to those used for i.c models.

This large prop of this Pilatus PC21 driven by a low Kv motor at relatively low rpm. This offers a higher level of efficiency than the alternative of a smaller prop driven at higher rpm by a higher Kv motor.

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Gibbs Guides.com Gibbs Guide to Electric Power Systems: Part 2 – More Than Motors

The two large propellers of the Wright brothers’ 1905 Flyer III were geared to rotate slowly, enabling them to achieve the maximum of thrust from the mere 20 hp at their disposal. This quite remarkable machine remains one of the lowest powered successful flying machines in the world. The talented American modeler Wayne Ulery designed and built this impressive, accurate 1/10 electric powered scale RC model of the Flyer III. The model dates from 2003, and features wing warping and a pilot whose articulated limbs appear to actually move the controls. The propellers are 12 x 6 APC E cut down to 10 inches, with tips shaped to better represent the full size propeller blade. The 48.5 inch span model weighs 2 lbs. Photograph by Wayne Ulery and used with his kind permission

Airflow considerations Since a propeller is really a rotating wing, it is subject to the same aerodynamic effects as a wing. As the propeller rotates, the blades meet the incoming air at an angle. The angle which the prop blades meet the air at (called the angle of attack) depends on the propeller pitch, the forward speed of the model and the motor rpm. Like a wing, if this angle is too low or too high, the blades will not work efficiently and may even be stalled. For a given propeller, the most efficient performance will occur with a particular combination of airspeed and rpm. Static & Dynamic thrust It can be said that there are two types of thrust produced by a propeller; static thrust, and dynamic thrust. Static thrust is the thrust produced by the propeller with the model in a zero airspeed condition, while dynamic thrust is the thrust developed by the propeller when it has some forward airspeed. Static thrust is sometimes measured by modelers with the assumption that it will be a useful indicator of flying performance. However, static thrust is only of relevance in zero airspeed situations such as a) during the first instant of take off; b) during prop hanging manoeuvres and c) when considering the rotor of a helicopter in hovering flight.

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Gibbs Guides.com Gibbs Guide to Electric Power Systems: Part 2 – More Than Motors

Left: Models in flight are propelled by dynamic thrust. This ARF DH Chipmunk is the work of Andrew Weight. Right: It is static thrust that is keeping this prop hanging model airborne. A relatively large diameter, low pitch prop is fitted to this Extra 330 Shock Flyer, the model that began a whole new flying genre. This photograph was taken in 2001, and the pilot is a young Derk Van der Vecht, who now represents his native Holland in competitions. A large diameter, low pitch prop may produce an impressive amount of static thrust, but this will not necessarily translate into good flight performance. The reason for this is that as soon as a model is moving, it is the dynamic thrust that counts. The amount of dynamic thrust developed will vary depending on the chosen propeller, the propeller rpm and the airspeed of the model. Unfortunately, without access to a wind tunnel, dynamic thrust is impossible for modelers to measure. Spinners The spinner is the streamlined cone found at the centre of the propeller. The purpose of the spinner is to reduce drag by streamlining the airflow around the propeller hub and nose area. Spinners also add greatly to the appearance of a model, and are of course an essential part of most scale models. It is important to make sure that no part of a spinner rubs on the fuselage. The location of spinners means that it is often the case that its presence prevents cooling air from reaching the cooling holes in the front of the motor. In this case, an alternative means of delivering cooling air must be found such as by means of a ‘chin’ intake and suitable ducting. Spinner size The inner 1/3 or so of the propeller blades produce very little thrust, so large spinners cause very little loss of thrust. In fact, what appears to be an excessively large spinner may actually be beneficial, since it will help to smooth the flow of air over the model. Propeller Balancing An out of balance propeller on an electric model can cause a lot of vibration, which will impose a high rate of wear on motor bearings and everything else in the model. An unbalanced propeller can also waste a surprisingly high amount of power, causing a significant loss of performance. Fortunately, propellers can quite easily be balanced using a commercial balancer. It is worth purchasing a good quality balancer, as cheaper ones may not work very well.

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Gibbs Guides.com Gibbs Guide to Electric Power Systems: Part 2 – More Than Motors

For best performance, reduced noise, and increased motor life, all propellers should be balanced before use. Spinners may also need to be balanced. The chin intake on this P51 Mustang provides cooling air for the power system components.

Folding propellers Folding propellers have their blades hinged so that when the motor is stopped, the blades can fold against the fuselage. On restarting, the blades are flung out by centrifugal force to their normal position and the prop then works as normal. The aerodynamic drag of a stationary propeller is surprisingly high, but it falls to a negligible amount if the blades are folded. A ‘windmilling’ propeller (i.e. an unpowered prop, made to turn by the model’s movement through the air) is a very high source of drag. The primary application of folding props is electric gliders, which enjoy a significantly improved gliding performance once stationary prop blades are folded. They are also appropriate for hand launched models that are landed on their bellies; in this situation conventional props are often vulnerable to damage, unlike a folding prop.

Left: My hand-launched, belly landed small P51 has broken quite a number of 7 x 6 propellers in its lifetime. A folding propeller would be a much better solution for this otherwise delightful model. Right: The propeller on this competition model is in the folded, low drag position. The drag from the externally mounted ESC will probably be of some significance, but there is no denying it is well cooled.

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Gibbs Guides.com Gibbs Guide to Electric Power Systems: Part 2 – More Than Motors

Choosing a prop for a model In broad terms, we can say that propeller diameter is chosen to give the required thrust by absorbing the required power, while propeller pitch is chosen to give a suitable pitch speed. Low speed models such as biplanes tend to use props with a low p/d ratio, sport models use a medium ratio, and fast models tend to use props with a high p/d ratio. We can think of the choice of propeller as being a bit like choosing which gear to drive a car in – a low pitch prop can be thought of as being like first gear, whereas a high pitch prop is more like a higher gear. Experimentation Choosing the optimum prop for a model is an inexact process, and some experimentation is the only way to be sure of the best choice. Note that nominally similar props from different manufacturers can vary widely in airfoil and blade shape. These variations mean that the load they present to the motor will also very significantly. For example, a 7 x 6 prop from manufacturer ‘A’ can present a significantly different load to a motor compared to a 7 x 6 prop from manufacturer ‘B’. Also, propeller pitch is not always accurately specified, so a prop that is marked with a pitch of 6 inches might actually be somewhere between 5 and 7 inches in pitch. It is always worth checking the current draw of your power system any time the prop is changed. Tractor and pusher installations A conventional installation in which the propeller is at the front of a model is known as a ‘tractor’ propeller. This is because like a farm tractor, the propeller pulls the model along behind it. Conversely, models in which the propeller is behind the motor driving it are said to be ‘pusher’ installations. Conventional propellers for model aircraft are all made to turn anti-clockwise when viewed from the front of the model. In a pusher installation a tractor propeller must still be fitted so that the blades face the same way. This requires the motor to operate in the opposite direction to a tractor installation, i.e. clockwise. Alternatively, a special ‘pusher’ propeller can be used, for which the motor direction need not be changed. In contrast, i.c. engines in pusher installations must use a pusher propeller as their direction of rotation cannot usually be changed.

Left: This prop has been correctly fitted to the motor in this pusher installation. In one sense, it could be said the prop has been fitted ‘backwards’ to the motor shaft. The motor is now required to turn clockwise when viewed from the propeller. Right: This electrically assisted glider is gliding with its prop blades folded against the fuselage. This greatly reduces the drag of the stationary blades.

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Gibbs Guides.com Gibbs Guide to Electric Power Systems: Part 2 – More Than Motors

Propeller safety All propellers are potentially dangerous, even small ones. It is a wise policy to consider the propeller to be ‘live’ any time that the battery is connected to the model. There exists a possibility of the prop turning if the battery is connected, even if the transmitter is set to motor ‘off’. Inadvertent operation of the throttle, equipment failure and interference are all possibilities.

The propeller of this Scottish Aviation Bulldog has been painted in the same way that the RAF paint full size propellers. Although both blades are striped, each blade is painted differently so that when rotating, the propeller is more visible. Photograph by kind permission of Colin Low

The consequences of increasing the voltage (cell count) using the same propeller It may be that higher performance is desired for a model, and so consideration is given to changing the battery for one with an additional cell. In the previous chapter we discussed how a mere 10 % increase in propeller rpm results in a 33 % increase in the power required to maintain the same rpm. Clearly, this means that if the system voltage is increased while the propeller remains unchanged, we can expect a very substantial increase in the current that will be demanded by the motor. The table below illustrates the approximate consequences of increasing the cell count while leaving the propeller unchanged:

Increasing the cell count with an existing propeller

Change from Change

in voltageApproximate change in current

2 Cell LiPo to 3 cell LiPo + 50 % + 80 % e.g. 10 A to 18 A 3 Cell LiPo to 4 cell LiPo + 33 % + 60 % e.g. 10 A to 16 A 4 Cell LiPo to 5 cell LiPo + 25 % + 40 % e.g. 10 A to 14 A 5 Cell LiPo to 6 cell LiPo + 20 % + 30% e.g. 10 A to 13 A

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Gibbs Guides.com Gibbs Guide to Electric Power Systems: Part 2 – More Than Motors

Chapter 3 Reduction Gearing

Gearboxes were quite commonly seen on models in the days when brushed motors were a popular choice of motor. This was because many of the brushed motors in use were ferrite types, which tend to be inefficient sources of power for a model aircraft in direct drive format, but rather better once they are geared. However, these days the relatively inexpensive brushless outrunner motor is by far the most popular choice for the majority of sport and scale models. These are generally relatively low-revving (i.e. low rpm per Volt / low Kv) and are therefore able to drive a relatively large and efficient propeller without the necessity for gearing. For this reason, gearing is no longer much used for most sport and scale models. However, for certain types of application, gearing is still useful. These applications include:

(i) Some high performance models which may use a geared inrunner (ii) Some small models powered by small inrunners or perhaps brushed motors (iii) The majority of helicopters

Why use reduction gearing? The benefit of reduction gearing is that it allows a motor to turn a larger propeller than would otherwise be the case. Larger propellers are more efficient than smaller ones, so the use of a larger prop yields gains in propeller efficiency. Propeller efficiency is difficult to measure, so it is often overlooked by modelers. Whether or not we are conscious of this fact, propeller efficiency has an important influence on a model’s performance. Reduction gearing is only worth considering for slow to medium speed models. Fast models are best fitted with a small high pitch prop turned at high rpm, a requirement which can only be supplied by a direct drive motor.

Both of these models are fine fliers. Left: This reduced size Brigadier by Alan Whipp spans 36 inches and uses a small, brushed geared motor from GWS to good effect. The model weighs around 200 g and achieves 20 min. flights from a 300 mAh nicad. Right: This 1.5 times enlarged Keil Kraft Senator was built from a scaled-up plan by Louis Louth-Davis, and is powered by a geared 400 size brushed motor.

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Methods of gearing The simplest method of reducing the speed of a propeller is by using a single pair of gears. The motor drives a small pinion gear, and this drives a larger gear attached to the propeller shaft. This arrangement results in an offset between the motor and gearbox shafts. More complex epicyclic gearboxes are sometimes used, and these produce an inline gearbox in which no offset is present. Belt drives are another way of reducing propeller speed. These used to be quite common and were inherently quiet. However, perhaps because they may incur greater losses than gears, it is now very unusual to see belt drives. Gearing losses Mechanical devices are never 100 % efficient, so the use of a gearbox (or other speed reduction system such as a belt drive) will always incur losses of energy. A typical gearbox might operate with an efficiency of perhaps 95 - 97 % so the losses are small. Since these mechanical losses cost perhaps only 3 – 5 % of the input power, they are easily more than made up for by the significant efficiency gains of using a larger prop.

Left: This superb scale model of a BAM Swallow was designed and by Jeremy Collins. The original machine’s Pobjoy engine was geared 3:1 to allow it to use a larger, more efficient propeller. Right: Jeremy’s model uses a large, slow revving outrunner so he had no need to gear the motor output down. However, the model does use an unusual 1:1 ratio gearbox, built into the dummy engine to achieve the offset thrust line of the full-size Pobjoy. The 200 Kv outrunner is fed by a 9S 2P LiPo pack and consumes 2,100 Watts. For this power input, the 22 x 12 prop is turned at around 5,300 rpm. Advantages and disadvantages of gearing The main advantage of reduction gearing is that it allows the use of a larger propeller than would otherwise be possible, increasing propulsion efficiency. The main disadvantages of gearing are the cost, weight and complexity of the gearbox. Gear ratios A gearbox allows us to exchange rpm for torque. For example a gearbox with a 2:1 ratio will cause the prop shaft to rotate at half the motor’s rpm, with twice the torque. Similarly a 3:1 ratio will cause the prop shaft to rotate at one third the motor’s rpm, and with three times the torque. Ratios up to about 6:1 are commonly found in modeling applications.

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Gearing down Inrunners Inrunners are relatively high speed, low torque motors, and are therefore always potential candidates for gearing. An inrunner coupled to a high ratio gearbox can make an effective and efficient power plant for electric gliders. Gearing and Outrunners Outrunner motors are inherently low rpm, high torque motors, and therefore are only very rarely found used with gearboxes in fixed wing models.

Left: This Jeti Phasor 45-3 brushless inrunner motor is connected to a gearbox, coloured gold. The gearbox shaft is offset slightly and the ratio is 2:1. This gearing allows the motor to turn a substantially larger propeller than it could in direct drive format. The efficiency gains from a larger prop more than outweigh the slight losses from the gearbox. This motor has no apertures in the case for cooling, so careful attention must be given to supplying the case with a flow of cooling air. The metal cased gearbox actually helps in this respect, since it conducts heat away from the motor and adds considerably to the surface area available for dissipating heat. Right: This impressive electric helicopter was built by talented Dutch modeler Appie van Moorst.

Helicopters Helicopters have a large rotor which is turned at low rpm. Generally helicopter rotors turn at approximately 1,000 – 2,000 rpm. This low rotor speed requires the use of a highly geared motor. Both inrunners and outrunners are used for helicopter models. Gearbox noise All gearboxes will produce some degree of noise, although the amount of noise is usually not great and is not often considered to be a nuisance. I remember seeing a geared P51 Mustang fly some years ago which emitted quite a growl from its gearbox, which I thought actually added to the model’s character! The design of the airframe will have a significant effect on the apparent volume of noise. For example, a foam-winged model with a lot of mass will tend to deaden any noise, while a very light built-up open structure with a taut covering will tend to amplify noise. A reduction in noise can often be accomplished by ensuring the propeller is properly balanced.

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Chapter 4 Heat, Efficiency and Cooling

The cause of heat All electrically conductive materials have some electrical resistance, and when a current is passed through a resistance, heat is generated. From experience, we know that all electrical items generate heat in use. An example of this is the old-fashioned one-bar electric fire; the heating element is simply a length of special wire with a suitable resistance, wound around a former. The current flowing through this resistance causes heat to be generated. Virtually all of the energy passing through the element creates heat, and since heat is the required output, it can be said that an electric heater is almost 100% efficient. Another example of electrical resistance is the old-fashioned household incandescent light bulb. The bulb’s filament is also a length of resistive wire which is heated by the passage of current such that it glows white hot. Virtually all of the energy the bulb consumes goes to produce heat, and only a tiny proportion of the energy is emitted as light. Since light is the required output, the bulb is therefore considered to have a very low efficiency. In contrast, modern LED (light emitting diode) lights are much more efficient than this, as they do not get very hot, and emit a far higher proportion of the input energy as light. It is inescapable that the motor, battery and speed controller of a model will each generate some heat, even in components of the very highest quality. The model’s power system wires and the associated connectors will also generate a tiny amount of heat, although if the wires are adequately sized, the amount of heat generated will be insignificant. The energy to create heat in a model’s power system comes from the battery, and is therefore unavailable for driving the propeller. Heat therefore represents a loss of efficiency in an electric power system.

Small increases in current will produce disproportionately large increases in the amount of heat generated. So, if a model’s propeller is changed for a larger one which draws more current, this may have unexpected consequences in terms of the amount of heat that is generated. This electrified ‘Panic’ biplane built by Andrew Weight uses a 5S 3,200 LiPo supplying an Axi 4120/14 which draws around 50 Amps turning its 14x7 APC thin electric prop. The motor is somewhat over propped, but careful throttle management gives 10 – 15 minute flights.

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The amount of heat generated depends on the resistance of the motor’s windings, the resistance of the battery, the resistance of the ESC, and on the amount of current flowing:

● The higher the current, the greater the amount of heat generated. ● The higher the resistance, the greater the amount of heat generated.

How heat varies with current It might be supposed that the amount of heat generated is proportional to the current flowing. However, this is not the case. In fact, the amount of heat generated is proportional to the square of the current. Heat generation follows the ‘square law’ To illustrate what this means, let us consider an example - suppose a motor draws a current of 10 Amps at full power with a particular propeller. The propeller is then changed for a larger one; the higher load causes the current drawn to increase to about 14 Amps. The consequence of the higher current is that the heat generated by the battery, the motor and the ESC will each double, even though the current has only risen by about 40 %. If a still larger prop is fitted so that the current becomes 20 Amps, the heat generated will be four times what it was at 10 Amps. The graph below illustrates this principle:

Relationship of Heat to Current

0

5

10

15

20

25

30

35

40

45

50

0 2 4 6 8 10 12 14 16 18 20 22

Current in Amps

Un

its

of

hea

t g

ener

ated

(W

atts

)

40 units of heat generated at 20A

20 units of heat generated at about 14A

10 units of heat generated at 10A

From the graph, we can see that small reductions in current can result in disproportionately greater gains in efficiency. However, if we want to have a high performance model, we will need to draw a lot of power, so it may appear that there is no alterative to a system consuming a high current.

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A note for the mathematically minded The amount of power generated as heat can be found using an electrical engineering formula which says:

Dissipated power = I² R

(Confusingly, the letter ‘I’ is often used in electrical calculations to represent current, and not the letter A as we might expect. This stems from the early days of electricity, when the letter ‘I’ was used to denote intensity of current.) For the non-mathematicians, we can rewrite this formula in plain English, like this:

Amount of Heat produced = Amps x Amps x Resistance.

However, in fact, there is an alternative - there are two basic ways of getting same amount of power in a system; a high voltage and a low current may be used, or alternatively a low voltage and a high current. For example, suppose we need a 500 Watt power system. Practically, we could achieve this using either a 3-cell battery or a 4-cell battery. The table illustrates this:

Factor 3-cell

system 4-cell

system System voltage 11.1 Volts 14.8 Volts Current 45 Amps 34 Amps Total power consumption 500 Watts 500 Watts

If our system has a total electrical resistance of say 0.075 Ohms we can now work out the power dissipated as heat: 3-cell system Dissipated power = I² R = 45 x 45 x 0.075 = 151 Watts Power remaining to drive the prop = 349 Watts 4-cell system Dissipated power = I² R = 34 x 34 x 0.075 = 86 Watts Power remaining to drive the prop = 414 Watts Both power systems are consuming the same 500 Watts, yet one generates 151 Watts of power as wasted heat due to resistance, while the other only generates 86 Watts of wasted heat. Note that heat is also generated in the motor for other reasons such as the effect of the changing magnetic flux on the iron-rich components, but we can ignore this for the purposes of this discussion.

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Clearly, the higher voltage system is the more efficient option. This example illustrates an important principle of electric flight – high voltage systems are inherently more efficient than low voltage ones. Both systems will allow us to enjoy a model, however all else being equal, the 4-cell system will give the model more performance and/or flight time. The table illustrates the two alternatives in mode detail:

Factor 3-cell

system 4-cell

system Total power consumption 500 Watts 500 Watts Current 45 Amps 34 Amps Resistance of system 0.075

Ohms 0.075 Ohms

Power wasted as heat 151 Watts 86 Watts Power remaining to drive the prop 349 Watts 414 Watts Percentage of 500 W available to drive the prop

70 % 83 %

All electric power systems will generate some heat, no matter how efficient the components of the power system are. Any heat produced by the power system represents wasted energy which could have been used to turn the propeller. The overall system efficiency of this Long EZ - or any other model – may be found by multiplying the efficiency of each of the components together. For example, if the battery is 80 % efficient, and the motor is 85 % efficient, the ESC 95 % efficient and the prop 70 % efficient, the overall efficiency will be 80 x 85 x 95 x 70 = 45 % overall efficiency. This may seem low, but it is still a great deal better than an i.c engine can achieve!

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Chapter 5 Introduction to Electronic Speed Controllers

In this chapter we discuss various topics relating to Electronic Speed Controllers (ESCs) for both brushed and brushless motors. As modelers, we don’t need to be knowledgeable about ESC electronics, so this section will provide only a brief overview of the electronic side of matters. We will however discuss in detail those aspects of ESC operation which will be of practical use to aeromodellers. ESC electronics Electronic speed controllers would not be possible without the invention of two types of electronic components. These are the microprocessor and the transistor. The microprocessor is the ‘brain’ of the ESC, and it is programmed during manufacture to tell the ESC how to carry out its many functions. Microprocessors are also found in other modeling devices such as ‘intelligent’ battery chargers, as well as being commonplace in everyday life. Transistors are essentially nothing more than solid state (i.e. no moving parts) electrical switches. ESCs use a metal oxide semiconductor field-effect transistor, commonly abbreviated to MOSFET or just FET. These transistors can both switch very quickly and also handle a high power. The combination of the microprocessor and the high power FET (switch) are the foundation of all ESCs in modeling use. Brushed motor ESC operation The brushed ESC is in principle very simple; control of a brushed motor is achieved by continuously switching the battery supply voltage on or off. Although the motor is supplied with either zero Volts or the full battery Voltage, the switching happens so quickly that the effect is indistinguishable from fully proportional control.

Brushed ESC operation

EFM015

Battery ON

Battery OFFTime

1/3power

2/3power

Fullpower

Motoroff

The diagram above illustrates the basic principle of operation of the brushed motor ESC. The brushless motor ESC carries out this same function to control the speed of a motor, as well as performing many other functions to make the motor operate.

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Brushless motor ESC operation For brushless motors, the operation of the ESC is a great deal more complex. In addition to providing precisely timed voltage pulses to each of the three sets of windings, the brushless ESC must also carry out the commutation function and much more besides in order to successfully operate the motor and control its speed.

Left: A brushed ESC. The red and black wires connect to the battery positive and negative respectively, while the two yellow wires connect to the motor. The battery connections must never be reversed, as this will cause immediate and irreversible damage to the ESC. Right: A brushless ESC can always be identified by the presence of three wires to connect to the motor. The colour of the ESC to motor wires is of no significance. In this case, the wires happen to be blue. Heat generation in ESCs The FETs used to perform the switching have a resistance, and so just as with motors and batteries, the current passing through the ESC will cause it to become warm. The characteristics of FETs are such that they dissipate very little power, except when switching takes place. This means that they will create the most heat when the motor is operated at part throttle. In general, the higher the capacity of the controller, the less resistance it will offer to the flow of current, and the less heat it will generate for any given current. This means that ESCs with some spare capacity should repay their additional weight and cost by being slightly more efficient. It is worth remembering the ‘square law’ which determines how much heat is generated; to recap, small increases in current will result in disproportionately large increases in generated heat. This is the reason why it is an extremely bad idea to try and use an ESC (or any other electrical component) beyond its rated current capability. This square law is the reason that small ESCs will generate much less heat than larger ones. To illustrate this, the heat generated by a 60 Amp ESC operating at its rated current will be sixteen times more than a 15 Amp ESC operating at its rated current.

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Cooling and mounting the ESC It is absolutely essential to make sure the ESC is sufficiently cooled, particularly if the BEC function is in use – if the controller overheats, the BEC function may shut down or fail, and you will lose control of your model. To prevent overheating, always make sure the ESC is supplied with a generous supply of cooling air. The ESC cannot be over cooled. It should never be mounted in protective foam, as this will prevent air from reaching its surface. Instead, allow it to hang, or for larger controllers, the ESC can be secured by attaching to the wires at each end. Make sure the ESC cannot become accidentally disconnected in flight. Cool ESCs provide more power The resistance of a power MOSFET rises with temperature, meaning that ESCs that are kept cool will present less resistance to the flow of current, and will be more efficient and allow more power to reach the motor. ESCs that are allowed to become very hot are at risk of thermal runaway, which can lead to the destruction of the ESC.

Left: The heat shrink wrapping over ESCs does tend to reduce the effect of cooling air. Right: this specialist, high power ESC has no heat-shrink covering. It is mounted on the outside of a model, and the exposed heat sink allows the airflow to dissipate the maximum possible quantity of heat. Improving ESC Cooling The electronics of the ESC is invariably covered in a protective wrapping of heat-shrink tube. This serves very well to support the wires as they leave the circuit boards and to protect the components from damage. However, heat shrink tubing is an excellent insulator of heat, so it greatly reduces the ability of the ESC to shed its heat. For this reason, high-power ESCs tend to have an aluminium heat sink in contact with the FETs that is not covered by the heat-shrink tube. This allows the heat from the FETs to be dissipated much more easily. To increase the ability of standard, heat-shrink wrapped ESC to dissipate heat, it is possible to carefully cut away a portion of the heat shrink tube to allow the FETs to be better cooled. Great care must be taken to avoid damage to the ESC during this operation, which would of course invalidate the ESCs guarantee.

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Chapter 6 Electric Motor Safety

Electric Motor Safety and the role of the ESC In modern times, the issue of safety is perhaps emphasized rather more than is necessary or sensible in many areas of life. However, when considering electric flight, safety is an issue which most definitely needs to be taken very seriously. The lack of noise, smoke and vibration from electric motors can lead to the impression of an overall lack of significant danger from this type of power source. This is an illusion. Electric motors are no safer than i.c. engines, and in fact, in my opinion, electric power actually poses more potential danger to modelers than i.c. engines. This is for the following three primary reasons:

1. An i.c engine cannot spontaneously burst into life. In contrast, this possibility is always present any time the battery is connected to an electric model’s ESC.

2. If the propeller of an i.c engine accidentally contacts your body, the tendency will be for the engine to slow down and stop. In contrast, an electric motor will not only keep going, it will do so with even more power - as we have seen when discussing propellers; the greater the load on the motor, the more power it will develop. Thus, the injuries from a given propeller and rpm combination are likely to be worse from an electric motor than a comparable i.c. engine.

3. The noise of an operating i.c. engine provides an alert that its propeller is turning. Electric motors emit almost no such audible warning.

For the above reasons, perhaps the most important sentence in this entire guide is this one:

TO AVOID INJURY, YOU MUST TAKE ELECTRIC MOTOR SAFETY SERIOUSLY ESC arming Electronic Speed Controllers won't ‘arm’ until they recognize a low throttle command from the receiver. This is an important and useful safety precaution. So, if the battery is plugged in with the throttle stick set anywhere above the closed (‘stop’) position, the ESC should not arm, and in theory, the motor should not turn. However, there are a number of reasons why a motor could start up unexpectedly. These include the following:

1. Electronic devices, including ESCs, do occasionally malfunction. Cases have occurred where electric motors started up even with the throttle set to ‘stop’

2. The transmitter’s throttle stick may be accidentally moved, either by you or by someone else.

3. Interference may result in a high throttle command reaching your receiver, even with your transmitter broadcasting a throttle closed (‘stop’) command.

The only safe policy for electric motors is to consider that the motor is liable to start up at any time when the battery is connected to the model. With electric power systems, the bottom line is this: the only time a ‘live’ electric motor should be considered unlikely to be able to injure you is when it does not have a propeller attached to its shaft.

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An important safety note on testing a new power system Virtually all transmitters include a servo reverse function, which allows the sense of a transmitter command to be reversed to suit the modeler’s needs. This is a very helpful facility when setting up control surfaces. This facility is also present on the throttle channel, where it is equally handy when setting up an i.c. engine installation. However, for electric models, this reversing facility poses a very significant potential danger. The throttle channel should of course be set up so that with the stick fully back, a ‘stop’ command (throttle closed) signal is received by the ESC. However, let us consider what could happen if the throttle channel were inadvertently set up in the incorrect ‘sense’. The transmitter is switched on, and we check carefully that the throttle stick is set to ‘stop’. We then connect the battery to the model. The ESC appears not to operate. We move the throttle forward a little, but nothing happens. The ESC makes noises, but no propeller rotation occurs. Except for a confusing noise, the power system appears dead. We then move the stick fully forward, hoping for a response but still the motor does not turn. The ESC’s noise changes, but we do not understand why (In fact, the ESC has just armed the motor, since it believes it has just received a ‘stop’ command). So we decide to switch the system off and investigate. Before switching off, we decide to move the stick back to the safe ‘stop’ position, whereupon the motor bursts into life. We move the stick all the way to ‘stop’ but the motor accelerates to full power. This may be very confusing, since the throttle is set closed. Now, we have a motor spinning a sharp edged propeller which is effectively out of control. Clearly, this scenario could lead to a very serious accident involving lacerations to arms, the loss of digits or even an eye.

Left: The author, holding a brushless powered Cougar 2000 fun-fly model, back in 1999. Although heavy by modern standards, the model would prop-hang on its geared brushless inrunner motor which sucked more than 500 Watts from its 16 Nicad cells. This would be a dangerous way to hold a model, if the battery was connected to the model. Right: Even a small model such as this 300 Watt Acromaster could cause serious injury if its propeller was given an opportunity to come into contact with the body. The tips of the propeller blades are sharp, and reach a very high speed when the motor is rotating.

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Bearing the above in mind, it is very wise to ensure that ALWAYS and without exception the model is securely restrained on the ground before connecting the battery. This precaution applies most especially when testing a new installation. However, it also applies to established installations, since it is quite possible for transmitter programming to inadvertently become altered. This is known to have occurred, and at least one example was caused by a cellphone (mobile telephone) being placed near to a computerized transmitter. Testing without a prop It is also worth considering testing a new system without a propeller fitted in order to remove the danger of a rotating propeller. Clearly without a propeller fitted, the potential exists for the motor to over-speed, so care must be taken to avoid full throttle operation with no load, especially if the moor is an inrunner type. Electric power is safe – provided appropriate precautions are taken Please don’t let the above discussion scare you into feeling that electric motors are too dangerous to use! As with many things, electric motors are only as safe as the user makes them. When the proper precautions are applied, electric motors represent a very safe, clean and enjoyable form of power for model aircraft.

Left: The battery of John Ranson’s Hawker Tempest, built from the Sepp Uberlacher plan is housed under this hatch. As soon as the battery is connected to the model, John takes care to keep well behind the prop. Whatever the size of the model, whenever the battery is connected, care should be taken to restrain the model and to keep body parts out of the propeller arc. This ensures that even if the motor of a model starts unexpectedly, no injury will be caused. Right: One excellent idea to increase safety is to arrange the ESC to motor wires such that one of them can be left disconnected right up until the pilot is fully ready to fly. The motor cannot run unless this connection is made so this idea allows the battery to be installed and for the flight controls to be tested without risk that the motor will burst into life. A good way of arranging this is to route one motor wire out through the side of the cowling, such as seen here with George Worley’s impressive, weathered Hangar 9 P47 Thunderbolt.

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Chapter 7 BEC and PCO Functions of the ESC

The primary purpose of the ESC is of course to control the speed of the motor. However, all types of ESC can do a lot more for us than just this. The two most important additional ESC functions are the BEC (Battery Eliminator Circuit) which provides power for the model’s receiver and servos, and the PCO (Power Cut Off) circuit which monitors the voltage of the flight battery and guards against it becoming exhausted: The Battery Eliminator Circuit (BEC) Many ESCs, especially smaller ones, incorporate a Battery Eliminator Circuit, or BEC. The BEC provides a constant voltage output, usually of 5 Volts, to supply the receiver and servos. The BEC therefore eliminates the need for a separate 4.8 Volt Hydride or Nicad receiver battery to supply the receiver and servos, saving weight and complexity. BEC current limit BEC circuits are limited in the current they can supply. The instructions should detail this aspect of an ESCs specification, and advise as to the number and type of servos that may be used. The characteristics of the electric motors used inside servos are the same as for any other electric motors; if they are required to work hard, they will draw more current. For this reason, it is important that there is no excessive friction in the linkages operating the control surfaces such as the rudder, elevator and ailerons. If these linkages are in any way sticky or have significant friction, the current demand of the servo will increase, and the limit of the BEC may be exceeded, even if the allowable number of servos is not exceeded. The Power Cut Off function (PCO) If a model was flown until the flight battery became completely exhausted, at some point the battery voltage could easily become so low that the BEC would cease to function.

Left: A BEC circuit requires an input voltage at least a Volt or two higher than 5 Volts. This requirement is easily satisfied since the flight battery of most models is at least a 2-cell LiPo, nominally rated at 7.4 Volts. Right: It’s good practice to land a model while at least 20 % of battery power remains in the battery. LiPo battery life will be extended by this means which also avoids the danger of having the motor stop unexpectedly in flight. The subject of this photograph is John Ranson’s very fine DH Hornet.

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With no power to the RC system, control would be lost of the model. To deal with this problem, all ESCs incorporate a Power Cut-Off (PCO) function. This is sometimes known instead as the Low Voltage Cut-Off, or LVC for short. The PCO circuitry monitors the flight battery voltage, and if it falls to a particular (sometimes user set) threshold, the PCO will cut the power to the motor. This voltage at which this happens is of course set to be higher than the voltage at which the battery would be totally flat. For example, for a 3 cell LiPo, the PCO might operate at 9.0 Volts. By this means, the ESC protects the supply of power to the BEC. Power can be restored to the motor by shutting the throttle and gently reopening it again, to less than full power. This causes the battery to suffer less of an on-load voltage drop, so operating at reduced power should prevent the PCO from operating again for a while whilst the model is positioned for a landing. The PCO also prevents the battery from becoming discharged to the point that it may sustain damage. Note that although the PCO exists to preserve voltage for the BEC, the two functions are entirely separate – motor power is cut by the PCO, and not by the BEC as is often incorrectly believed. Disconnecting the ESCs BEC function For some models, it is desirable to disconnect the BEC function, and to power the RC system by an alternative means. The BEC’s output can be disconnected by cutting the power supply wire (generally coloured red) of the lead connecting the receiver to the ESC. Of course, cutting this wire does not stop the ESC’s BEC circuit from working, but it does prevent its output from being used. The two remaining wires connecting the receiver to the ESC allow it to still be controlled. Remove the battery after flight Note that it is important to make sure the battery of a BEC-equipped model is removed after flight. If the ESC is left connected to the battery, it will still cause a drain of a few milliamps even if switched off. For this reason, if the battery is left connected to a model for an extended period after flying there is a good chance that the LiPo battery would become deeply discharged, causing damage and an increased risk of fire.

Left: The positive wire of this ESC-to-receiver wire has been prised out of its plastic receptacle, disabling the BEC output. The loose wire will be insulted with heat-shrink tube or insulating tape. The wire can be replaced when if needed. Right: An alternative to altering the wire of the ESC itself is to add a short extension lead between ESC and receiver, and to alter the power supply wire in this lead instead. This leaves the ESC wiring intact. Here, a section of the red (positive) wire has been removed.

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Gibbs Guides.com Gibbs Guide to Electric Power Systems: Part 2 – More Than Motors

Chapter 8 Opto Isolating ESCs

The power system of an electric model unavoidably generates some radio frequency (RF) interference. The constant switching taking place inside an ESC means that it too creates RF interference. Some of this interference will inevitably find its way to the receiver along the lead connecting the ESC to the receiver. Clearly this situation is not ideal, although for most models it does not usually pose a problem. However, for large, fast or expensive models, this factor may present a risk that is considered unacceptable, especially as large and fast models may well be flown at some distance away from the transmitter, when the received transmitter signal will be relatively weak. In such cases, an opto isolating ESC is worth considering. These are a variation on the standard ESC, and are the same as a normal ESC except for the way they connect to the receiver; inside the ESC, the receiver to ESC connection is made optically, not electrically. Since opto ESCs are electrically isolated from the receiver, there is no direct electrical connection between the two units, which ensures that no RF (radio frequency) interference fed to the receiver by the ESC.

This is John Ranson’s atmospheric electric powered Brian Taylor Me109, showing off its brutally handsome lines. Installing a high quality receiver and an opto isolating ESC is a particularly wise precaution for a large scale model like this one, the construction of which involved a great deal of work. Opto isolating ESCs may be used for any model which can carry the weight of a separate ESC battery.

Opto ESCs have no BEC function Because there is no direct electrical connection between the receiver and the ESC, opto isolating ESCs cannot offer a BEC function. The use of a separate power source for receiver and servo power is therefore required. High current ESCs tend to be of the opto type; this reflects the fact that (a) the more current the ESC handles, the more intense any RF emissions are likely to be and also that (b) larger models are more able to carry the weight of a separate receiver battery. Since the whole point of an opto ESC is to isolate the RC system from the power system, there is little point in considering the use of a UBEC fed from the flight battery to provide RC system power.

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Gibbs Guides.com Gibbs Guide to Electric Power Systems: Part 2 – More Than Motors

Chapter 9 Electronic Speed Controller Limitations

Like all things, the ESC must be used within its current and voltage limitations. The ESC can only be relied upon to perform reliably if the cooling arrangements are good enough – if insufficient cooling air reaches the ESC, it may very well overheat and become damaged and/or shut down. Cooling is an especially important consideration if the ESC is also used to supply power to the RC equipment. It is impossible to over cool an ESC, so do not be hesitant to supply it generously with cooling air. ESC Voltage limitation In addition to a current limitation, ESCs are also designed to operate with an upper voltage limit. For smaller ESCs, usually 3 or perhaps 4 LiPo cells is the upper limit. If an ability to handle a higher voltage is needed, an appropriately rated ESC must be used. ESCs are available to handle 6, 8 or even 10 or 12 LiPo cells. Don’t ever be tempted to exceed the allowable cell count; you will run a great risk of permanently damaging the controller. ESC Current limitation All ESCs are rated to handle a particular maximum current. If an ESC is made to operate at a higher current than it is rated for, it may well overheat and become damaged and/or shut down, the same as if it were not cooled properly. Do not be tempted to use an ESC at a higher current than it is designed to handle.

Left: This 70 Amp ESC is mounted on the outside of a model. The propeller wash ensures a healthy supply of cooling air. The air scoop was made from a white plastic spoon, and supplies the battery with cooling air. Right: This small brushless ESC is rated for 17 Amps. The plastic heat shrink sleeve around this ESC protects it from physical damage, but does nothing to assist cooling. If even a small quantity of cooling air can be made to flow through the ESC, so much the better. Over specifying an ESC Over specifying an ESC (e.g. selecting a 30 Amp model when the required current capability is only 20 Amps) is a very good idea. Some spare current capacity has a

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number of benefits; it will allow for propeller changes which may increase the current draw of the motor and it will operate cooler. Also, a unit used at less than its full current capacity will be more reliable, and less likely to overheat and/or shut down. For most models, the additional weight of the larger ESC is of no consequence, and the cost increment is usually only small. There is no benefit over specifying an ESC in terms of its voltage capability. Properties of wires Wires carrying electric current have a number of properties which are not necessarily obvious to the modeler. As well as electrical resistance, the wires also have a property called inductance. The switching action of the ESC unavoidably creates voltage ‘spikes’ (momentary periods of high Voltage) in the wires linking the battery to the ESC. The longer the battery-to-ESC wires are, the larger these spikes will be. To absorb the spikes, and thus prevent the voltage spikes from reaching and damaging the ESC, one or more capacitors are soldered across the input wires to the ESC. These capacitors can easily be seen on many ESCs, and without them, the ESC would be easily damaged by the spikes. Length of wires between Battery and ESC The capacitors built in to ESCs are sufficient for a total battery to ESC lead length of approximately 20 cm. Extending these leads by just a few centimeters (perhaps 5 or 6 cm) should present no problems. However, if the battery to ESC lead length needs to be significantly increased, then one of more additional capacitors will be needed to absorb the inevitable voltage spikes and prevent ESC damage. Additional capacitors must be installed as close to the ESC as possible, soldered across the input wires. Length of wires between ESC and Motor There are no restrictions on the length of wires linking the ESC to the motor; inductance is not a factor here. However, the resistance of these wires is worth considering. If the ESC-to-motor wires need to be extended, make sure they are the same size (or larger) as those of the ESC itself. It is always worth gently twisting any power system wires together to reduce the amount of RF emitted. One turn per 2.5 cm (1 inch) is sufficient.

Left: The two round objects situated at the battery end of this 4-Max 80 Amp ESC are capacitors. These are necessary to absorb the voltage spikes caused by the inductance of the battery to ESC wire. These are rated at 35 V, 470 μF. Right: All ESCs employ capacitors which are located between the wires connecting the ESC to the battery.

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Chapter 10 Setting up Electronic Speed Controllers

Most modellers use their ESCs on a ‘plug and play’ basis, using the manufacturer’s default settings. The default settings are usually quite suitable for the average sport model, so no set up at all is required. You may however wish to adjust the way some of the ESCs features work to suit your exact requirements. This is easily accomplished. ESC sounds Modern ESCs incorporate the ability to make sounds to inform the user of its status, and to guide the user through the set up process. One example is when connected to a battery the ESC will announce that it is ‘live’ with a few tones or even a short tune. The ESC’s sounds are actually produced by the motor windings, to which the ESC sends an appropriate signal in order to create the sound. For this reason, the motor must be attached to the ESC before setting up, unless using a programming card.

Left: Setting up an ESC is a simple matter. It can either be done using the throttle stick of the transmitter, or by using a simple programming card like this one. Right: The sounds emitted during ESC set up actually come from the windings of the motor. Programming, or setting up the ESC If required, set up is generally very easy. There are three possible methods of setting up an ESC. The most basic method, which is entirely satisfactory for most models, is to programme the ESC simply by moving the throttle stick of the transmitter to a high or low position as required, responding to the ESCs various questioning beeps. A second, easier method is to use a programming card, which is an inexpensive device temporarily connected to the receiver. Lastly, sophisticated ESCs may sometimes be set up by connecting them to a personal computer (PC) with the appropriate software. The list of user-definable ESC parameters may include any or all of the following: (a) BEC voltage selection The usual output voltage of a BEC is 5.0 V. Some ESCs allow the user to specify a different voltage, for example 5.5 V or 6.0 V.

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(b) PCO Voltage threshold Most ESCs for use with LiPo batteries will automatically detect the number of cells in uses, and automatically initiate PCO function when a suitable Voltage threshold is reached. For example, an ESC used with a 3-cell LiPo would typically cut motor power when the on-load battery Voltage reaches 3.0 Volts. This Voltage threshold may often be adjusted if required. (c) PCO function operation The way the PCO function operates may be varied. For example, when the threshold voltage is reached, the voltage supply to the motor may be cut off or alternatively just reduced. (d) Switching frequency Some ESCs allow the switching frequency to be varied. The switching frequency may need to be increased to work with high speed motors, and/or those with a high number of poles.

Left: The threshold Voltage of PCO operation is usually automatic. This large Shiebe Falke FS28 motor glider, built by Martin Tremlett from the Cliff Charlesworth plan & now owned by Bob Mahoney uses a 21-cell nicad battery and a high quality brushed motor. If it were to be converted to LiPo power, and the same ESC and motor was retained, it would be necessary to confirm that the cut-off voltage was still suitable for the LiPo, as it would be dangerous to allow the battery to become over-discharged. A much better idea would be to purchase a complete new power system. Right: Four tiny EDF units power this modestly sized Vickers Valiant, designed and built by Chris Golds. Each of the KP fans turns at an incredible 80,000 rpm. This high rotational speed requires the ESCs to carry out their duties with a very high speed of switching. Not all ESCs could cope with this demanding requirement. (e) Soft start function At the instant of start up, the motor will have no rotational speed, and so would not be generating any back EMF. If full battery voltage was applied to a stationary motor, the current would momentarily be extremely high. This would waste energy, and for brushed motors will also risk damage to the brushes and commutator. To address this problem, most ESCs offer a ‘soft start’ capability. The soft start function allows full power to be developed only gradually, perhaps over a period of one second, in response to a rapid

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full throttle command. This allows the motor to develop some rpm before full battery voltage is applied, thus avoiding the problem of full voltage being applied to a stationary motor. This prevents the very high current of an instant full power start. This feature is especially useful for gearbox equipped models, since gearbox teeth would be at risk of damage from the excessive torque of a rapidly applied application of full power. The high inertia of a large propeller also means the motor will take a relatively long time to speed up extending the high current period resulting from a zero or low back EMF. (f) Brake function If a motor is spun up, and the battery is then disconnected, the speed of the motor will decay over a period of several seconds. However, if the terminals of a brushed or brushless motor are connected together after power is removed, the motor will stop almost immediately. This is the principle by which the ‘brake’ function on an ESC works; when the brake function is enabled, the ESC electronically connects the terminals of the motor together. This prevents the motor from rotating freely, braking it. The model’s forward airspeed causes the now stationary blades to fold backwards. The braking effect from a direct connection between the motor’s terminals is very powerful, so many ESCs offer a ‘soft’ or ‘hard’ braking option, allowing the user to select the degree of motor braking that is desired.

Left: An EDF Skyhawk. Adjusting the ESC’s timing of a motor can make a big difference to the current it draws. The cause of excessive current consumption may be incorrectly set motor timing. Right: With the brake function set to off (disabled) the motor will freewheel when the throttle is set closed. A freewheeling prop generates a lot of drag; even more than a stationary prop. Increased drag can be useful on approach to land. This lovely electric Bristol Freighter is seen here about to touch down. (g) Motor timing For brushless motors, the commutation and its timing is carried out by the ESC. It is sometimes useful to be able to adjust this timing – this can significantly alter the characteristics of the motor to suit the needs of the particular set up. For example, advancing the timing will result in the motor drawing more current and turning at a higher rpm with the same prop, whereas retarding it will have the opposite effect. Thus by adjusting the timing, it is possible to choose a suitable balance of power and economy for any given propeller. For correct operation, some motor types will require that the timing is adjusted from the ESC’s default settings.

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Setting up the sense of an ESC Virtually all transmitters have a servo reverse function, which allows the sense of a transmitter command to be reversed to suit the model’s needs. This is a very helpful facility when setting up control surfaces. This facility is also present on the throttle channel, where of course it would be equally handy if setting up an i.c. engine installation. However, for electric installations, this reversing facility poses a very significant potential danger. The throttle channel should of course be set up so that with the stick fully back, a ‘stop’ command (throttle closed) signal is received by the ESC. It is a wise precaution to make a preliminary check on a new system without a propeller fitted. This creates an opportunity to check the system for correct transmitter sense and correct rotational direction with a much higher degree of safety. This precaution means that a sense of familiarity can be developed with a new system before adding the element of a rotating propeller. There is no need to operate the motor at full throttle during such a preliminary test. This type of testing is wise for all motors, and makes particular sense with outrunners, which generally use relatively large props. Since these are relatively low Kv motors, they are unlikely to over-speed with no load connected. Propeller shaft direction One possible difficulty is that with no prop fitted, it can be difficult to see which way the propeller shaft is turning. A solution to this is to make a simple disc about three of four inches in diameter, divided into four quadrants, two of which are painted black, and two white. The disc is attached to the prop shaft, making the direction in which the shaft turns easy to see. The disc should be accurately made, preferably from a light weight material, such as balsa or Depron so it is not out of balance. Measuring Kv The simple tool mentioned above may also be useful at a later date, when it can be used in conjunction with a rev counter for determining the off load speed of a motor. By comparing this rpm with the battery voltage, the motor’s Kv may easily be calculated. For example, if the motor achieves 9,760 rpm off-load with a battery supply Voltage of 9.76 Volts, the motor has a Kv of 1,000 (9,760 ÷ 9.76 = 1,000).

Left: John Ranson’s superbly realistic Bristol Beaufighter. In the excitement to test a new model, it is easy to forget the danger which a rotating propeller represents. Always make sure a model is securely restrained when testing its motor. Right: A useful tool for enabling motor Kv to be measured is a disc such as this one.

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Chapter 11 Battery Basics

This chapter is intended to give you an overview of the battery types used in RC modelling. For more detailed information on each of the battery types, a separate Gibbs Guide is available. The types of batteries used by RC modellers include the following: Lithium Polymer Lithium Polymer (LiPo) batteries offer a light weight, energy dense source of power at a reasonable price. For these reasons, they are the usual battery of choice for model aircraft. However, they are relatively fragile, and do not tolerate overcharging or excessive discharging. LiPo batteries are housed in a flexible Mylar (plastic) case, so they are much less tolerant than other types of physical abuse such as being dropped or if involved in a crash. LiPo batteries have a nominal voltage of 3.7 Volts per cell (Vpc). Common battery cell counts are 2 or 3 cells (7.4 Volts) for indoor and small park flyer models, 3 or 4 cells (11.1 V or 14.8 V) for many smaller sized models, and perhaps 6 cells (22.2 V) for medium to large models such as a 60 inch span P51 Mustang. Really large and/or high power models can use up to 12 or even 14 LiPo (51.8 V) cells.

LiPo batteries are available in a wide range of capacities and cell counts. Used with care, LiPo batteries are an excellent source of power for model aircraft

Used with care and knowledge, LiPo batteries are an excellent, reliable source of power for model aircraft. However, the safety of lithium polymer batteries is a subject that needs serious attention if accidents are to be avoided. I strongly recommend the purchase of the Gibbs Guide to Lithium Polymer Batteries if you intend to use this battery type. Care must be taken to avoid both over charging and excessive discharging. The batteries should be stored with care, and only charged in

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such a place that if fire breaks out it will not cause a disaster. Modellers who have not taken sufficient care have lost workshops, garages and cars to LiPo battery fires. The cells of LiPo batteries must also be kept in a balanced state, which requires the use of a balancer when charging.

Left: This small LiPo battery was deliberately overcharged to the point where it caught fire. If a LiPo battery breaks out into fire, the flame cannot be put out. Right: Although the risk of a LiPo battery fire is slight, the consequences can be very serious. For this reason, it is an extremely good idea to use a protective charging sack like this one as a precaution when charging LiPo batteries. LiPo battery fire image by Simon Sheldon and used with his kind permission. Lead acid The principal use of lead acid batteries for electric RC aircraft modellers is as the power source for chargers. They are also commonly used for i.c. modeller’s flight boxes. Lead acid batteries are heavy, but represent an economical method of storing a relatively large quantity of electrical energy. Lead acid batteries are not able to provide a high discharge current, so they are impractical for use in model aircraft. However, they are widely used in modelling applications. A single lead acid cell provides a nominal 2 Volts. Usually, batteries contain six such cells, providing 12 Volts. Some modellers use their car battery for supplying their charger. Provided the demands on the battery are reasonable, this can work well although it should be noted that car batteries are not designed for this type of duty. However care must be taken not to over discharge the car battery, otherwise the car will be unable to start. For a small car, with a battery capacity of perhaps 20 Ah, the battery could be flattened in as few as perhaps 6 charges of a 3-cell 2,200 mAh LiPo. In contrast, a typical 12 Volt, 60 Ah leisure battery can provide somewhere around 20 charges of a 3-cell 2,200 mAh LiPo. Lead acid batteries should always be recharged immediately after use to avoid damage. Lead acid batteries require careful handling if injury is to be avoided. Care must be taken to ventilate the area in which lead acid batteries are kept and charged. Also when transporting batteries great care must be taken, especially if they are to be transported in vehicles – if the vehicle is involved in an accident the extremely high weight of a lead acid battery may cause immense injury and damage.

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Nickel Cadmium Nickel Cadmium (NiCd, or Nicad) batteries used to be in very common use for modelling applications. They used to be more or less the only choice for flight batteries, and also for receiver batteries. Nicad batteries are no longer available for modeling purposes due to their cadmium content, which gives rise to environmental concerns. At the end of their life, nicads should be disposed of responsibly as their cadmium content is very toxic and is best kept out of landfill sites. Nicad batteries have a nominal voltage of 1.2 Volts. They will tolerate some overcharging without significant damage, and will also recover following deep discharging. Nicad batteries tend also to be very durable, and there are still plenty of examples still in use. Except for avoiding short circuits and excessive overcharging, there is little of concern in terms of safety.

Left: Hydride batteries are still in common use as a power source for transmitters and receivers. Shown here is a 4-cell receiver battery. Right: Lead acid batteries are in common use in electric flight modelling, but not for flight batteries as their weight is prohibitive. Leisure batteries are the best choice as a charger power source as they are more tolerant of repeated discharge and charge cycles. Nickel Metal Hydride Nickel Metal Hydride (NiMH, or just Hydride for short) batteries are in very common use, mostly as a power supply for transmitters. Before LiPo batteries Hydrides were also commonly used as flight batteries. In many ways, Hydride batteries are similar to nicads. Hydrides also have a nominal voltage of 1.2 Volts. However they are nowhere near as durable as nicads. They will tolerate limited overcharging, but some types are damaged by deep discharging. Traditional hydrides have a rapid self discharge rate, however the latest ‘Eneloop’ or similar hydride batteries have a low self discharge rate making them more suitable for transmitter and receiver use. As with nicads, except for avoiding short circuits and excessive overcharging, there is little of concern in terms of safety.

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Chapter 12 Battery Voltage Characteristics & Charging

All varieties of battery share certain basic characteristics. For example, all types will respond in fundamentally the same way when discharged, and when charged. The battery voltage should be considered to be a baseline or ‘nominal’ voltage. For example, an 11.1 V LiPo battery is nominally 11.1 V, however its actual voltage may be either more or less than this, depending upon the circumstances. For example we instinctively know that a battery in a low state of charge has a low voltage, while a fully charged battery has a relatively high voltage. The voltage of a battery will of course change more than that of a single cell. A battery will always be in one of three states:

● Off-load, or ‘open circuit’ i.e. disconnected from any electrical load. ● On-load, i.e. under discharge ● On-charge, i.e. while being charged

Batteries of all types share certain fundamental characteristics. Left: Various LiPo batteries. Right: Three Lead Acid batteries. Off-Load (or resting) Voltage Let us first consider a cell or battery (or single cell) in an ‘off load’ condition. Its voltage will vary according to its state of charge, and we would correctly expect a fully charged cell to have a high voltage, and a discharged cell to have a low voltage. On-Load Cell Voltage When a battery is on-load, in addition to its state of charge, a second factor will affect its voltage. This factor is the electrical resistance of the cell itself, which we call its internal resistance. When a current starts flowing the effect of internal resistance is that the cell’s voltage will be slightly reduced compared to it’s off-load voltage. The cell is then said to have suffered a voltage drop. The amount of voltage drop will depend both on the internal resistance of the cell and the amount of current flowing:

The higher the electrical current (load), the greater this voltage drop will be. The higher the cell’s internal resistance, the greater the voltage drop will be.

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Relationship between battery voltage and load

0

1

2

3

4

5

6

7

8

0 2 4 6 8

Time in Seconds

Bat

tery

Vo

ltag

eHalf Power selected

Full Power selected

Motor turned off

The voltage of a battery depends on the state of charge as well as the load placed on it. This diagram shows how the battery voltage will vary depending on the load placed on it.

To clarify this important concept, let us now consider some examples: Low loads A cell of low internal resistance under a very low load suffers only a very small voltage drop. A cell of higher internal resistance under the same load will show a higher voltage drop. In both cases the drop will be very small if the current is very low. Higher loads A battery of low internal resistance under a heavy load will suffer a larger voltage drop compared to being under a lighter load. A battery of higher internal resistance under a heavy load will suffer the greatest voltage drop of all. The larger a battery is in terms of its physical size and/or its capacity the lower its internal resistance will tend to be. Batteries with a relatively low internal resistance are legitimately given a high C rating. All else being equal, the lower the internal resistance of a particular battery is, the more suitable that battery is for high current applications such as electrical powered models. Thus, larger cells are generally better suited to higher currents than small ones are. Note that once we stop taking current from a battery, its voltage will begin to rise back to the voltage determined by its state of charge. An important point to remember is that a battery’s internal resistance will only affect its measured voltage when it is on load. Some practical examples of voltage drop are: (a) When a lead-acid battery is used as the power source for a LiPo charger, its voltage drop under load (caused, remember, by its own internal resistance) means the charger will have to operate on a slightly reduced battery voltage. This effect can easily be seen on chargers which display the input voltage. (b) In the case of an electric LiPo powered model, the voltage drop within its drive battery (again caused by the battery’s own internal resistance) will mean a reduced voltage is available at the motor.

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Models in the moonlight! Friends Louis Louth-Davis and Alan Whipp hold their models for the camera after a relaxing summer evening flying session. Vintage style models like this pair of electric powered Mercury Matadors require only a little power to fly well. This places a relatively low load on the battery.

Voltage recovery As soon as a load is removed from a cell it will immediately start to ‘recover’ back to the off load voltage determined by its state of charge. The graph here illustrates both the voltage drop and voltage recovery effects that we have just discussed. In this example (a 2-cell LiPo in a part discharged condition) the off load battery voltage is 7.4 Volts, and it falls to about 6.7 Volts when the motor is set to half power after 2 seconds. The voltage falls further to about 6.3 Volts when full power is selected at 4 seconds. At 6 seconds the motor is switched off and the battery voltage then starts to rise again towards the off-load voltage of 7.4 Volts.

Battery Voltage under charge The voltage of a cell under charge will appear to rise; the higher the charge current is, the higher the cell’s apparent voltage will be. This characteristic is easily observed on any charger displaying battery voltage. Cell voltage under charge is a somewhat artificial figure because what is really being measured is the output voltage of the charger necessary to drive the required charge current - the higher the charge current required, and the higher the internal resistance of the cell under charge, the greater the charger voltage needs to be. Battery capacity Battery capacity is stated in terms of the current it can provide for one hour. For example, a 1,000 mAh battery should deliver a current of 1 Amp (1,000 mAh) for a period of 1 hour. Similarly, a 2,200 mAh should provide 2.2 Amps for one hour. Alternatively it should be able to provide 10 times this current for 1/10 as long; in this case 22 Amps for 6 minutes. In practice, the battery is less efficient at high currents and only a proportion of the stated capacity will be available at high currents. The reason is of course the battery’s own internal resistance; at high currents this will cause a significant amount of heat to be generated.

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Battery chargers like this one are frequently sophisticated microprocessor-controlled devices. Recharging batteries requires the voltage of the charger to be somewhat higher than that of the battery, in order to force current through the battery in a ‘backwards’ direction.

Charging Principles When charging a battery, the charger forces current ‘backwards’ through the battery i.e. in the opposite direction to normal. This is accomplished by connecting the positive of the battery charger to the positive of the battery, and of course negative to negative. The battery’s own voltage will oppose that of the charger, so the charger’s voltage must be higher than that of the battery. Thus to recharge a 12 Volt battery requires a charger voltage of perhaps 15 Volts. Recharging causes the chemical changes that occurred as the battery gave up its charge to be reversed.

This vintage Lanzo Stik flown by Mike Burke was originally built for rubber power, but was later converted to electric power by Mike. This model has no undercarriage, so the wise choice of a folding propeller greatly reduces the chances of propeller damage on landing. A 2-cell 1,300 mAh LiPo gives the pilot at least 15 minutes of enjoyable flying. The low power requirement means the power system for such a model is relatively inexpensive.

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Chapter 13 Wiring and Connectors

Size and quality The quality and size of wiring and connectors used in RC electric models is clearly of importance to the performance of the model. Wires must be of adequate cross sectional area in order to carry the required current. Connectors must be of low resistance, otherwise they will impede the flow of current and will become warm. Wire size The size of wire used by ESCs provides an excellent guide as to the correct diameter to use - a 15 Amp ESC will use wire with a relatively small cross sectional area, while larger wires are used by ESCs that can handle more current. Another useful guide is the rating of ordinary household mains wire. In the UK this is usually rated for 13 Amps. In the USA, most household wiring uses 14 AWG (15 Amp) or 12 AWG (20 Amp) wire.

Left: Comparing the current rating of an ESC with its wires is an excellent guide as to the appropriate size of wire to use. The ESC above is rated at 40 Amps and uses wire with a relatively large cross-sectional area. Right: This ESC with its significantly thinner wires is rated for the lower current of 17 Amps. Choice of connectors Choosing a connector system is an important decision to make when starting out in electric modelling. The quantity of batteries and chargers, charge leads etc can quickly mount up, and so if you change your mind about your chosen system you are in for rather a lot of work! There is no perfect connector; all types have advantages and disadvantages. Some batteries are supplied with pre-fitted connectors. It is a good idea to standardize on one connector type for your batteries, but this does not mean you should necessarily standardise on that type. All connectors will require soldering, which is an essential skill for the electric modeller. There are a multitude of connector types, from which I suggest you choose from this short list of three: Gold, or Bullet connectors These connectors are so called because they resemble bullets. They may also be known by other names such as G2 (for 2mm gold), G4, round, bunch and Corally. Many motors

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are commonly supplied with 3.5 mm gold/bullet connectors as standard. Gold/bullet connectors have remained popular over a long period, and are often favoured by experienced modelers. Gold/bullet connectors are available in several sizes including 2, 3.5, 4 and 5 mm sizes. The 2 mm size will handle up to 20 Amps, and the 4 mm size will handle up to about 50 amps. The 2mm size is particularly small and so is ideal for small models. All sizes present no difficulty to connect or separate. They are manufactured as individual connectors rather than as a moulded pair, and so are not polarized. This means that multiple batteries can easily be connected in series. This is handy if you need, for example, to connect two 3S batteries to make a 6S battery. Other connector types will require a short 3-connector harness to permit this. Gold/bullet connectors are easy to solder and there is no risk of melting a plastic connector body as they are of all metal construction. They are probably the best connector type for reliability, however unless care is taken, there is a greater risk of mis-connection and short circuits. Short circuit protection takes the form of a length of tubing fitted to exposed male connectors, or else adding a shield made from a length of large diameter heat shrink tube. Either method works well. Different makes of gold/bullet connector are available. Compatibility issues can occur but these are rare and can be easily avoided by sticking with one make.

Left: A pair of 4 mm gold/bullet connectors fitted to a battery. Note that the plug is fitted to the positive red wire. The connectors have deliberately been spaced to minimise the risk of a short circuit. If no shield is fitted, the plug must be protected by a length of suitable insulating tubing. Right: The plugs from two separate 4 mm gold/bullet battery connectors are shown here alongside each other. The left hand example has been fitted with a safety shield made from a length of 8 mm heat shrink tubing. Note that for maximum protection the safety shield must be slightly longer than the plug body. XT60 connectors. XT60 connectors are a relatively new connector, and seem to be increasing in popularity. They are available in one size only, and are rated for 60 Amps. XT60 connectors comprise a pair of 3.5mm gold/bullet connectors encased in a moulded housing. The female part is used for the battery, and the male part is soldered to the ESC leads. XT60 connectors are therefore polarized, and are helpfully marked with + (red wire) and – (black wire) leaving no doubt as to how they should be wired up and ensuring compatibility with others using this system. XT60 connectors are a little easier

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than Deans connectors to pull apart. They are larger than gold/bullet connectors, and also slightly larger than Deans connectors. Original XT60 connectors and inferior quality clones are both available.

Left: A pair of XT60 connectors. The ridged logo provides fingers with a better grip. Right: Deans connectors. This original example does not include the ridged surface of copies, making them harder to pull apart. Deans connectors Deans connectors are commonly supplied with batteries, but seem to be in decline. They will handle up to about 40 Amps. The connector body is smooth, making them hard to pull apart, although some connector bodies have a moulded-in ridge to make this easier. As with XT 60 connectors, the female part is fitted to the battery. Some modelers have reported that once they have become well used, the spring plates of these connectors gradually flatten out, so that contact between the male/female parts of the connector pair eventually becomes intermittent. This is important because any connector failures will result in a dangerous loss of control if you are relying on the BEC of an ESC to supply RC system power. Deans connectors are moulded as a pair meaning that they are polarized. They are not difficult to solder, but are perhaps a little less easy than Gold/Bullet or XT60 types.

Comparison of connector systems Factor Gold/Bullet XT60 Deans

Availability Good Good Good Ease of assembly Easy Easy OK Current rating 2mm: 20A, 4mm: 50A 60 Amps 40 Amps Series connection of multiple batteries?

Just plug together Requires additional lead

Requires additional lead

Popularity Consistently popular Increasing Decreasing Polarised? No Yes Yes Shielded? Yes, if shield added Yes Yes Durability & reliability Excellent Excellent Less good Ease of connection Easy Easy Easy Ease of disconnection Easy Quite easy Can be hard

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Author’s choice My personal choices concerning connectors are as follows: Motors are frequently supplied with 3.5mm gold/bullet connectors for ESC to motor connections, so for convenience I do not change these. For the battery to ESC connection I use 2 mm gold/bullet connectors for applications below 20 Amps and 4 mm gold/bullet connectors for 20 Amps or more. If making my connector choices afresh, I would again select 3.5mm gold/bullet connectors for ESC to motor connections, but I might choose XT60 connectors for my batteries. Tamiya connectors Some batteries for electric power models are supplied with Tamiya connectors. These were supplied with the electric powered cars that this company pioneered in starting in the late 1970s. The connector since found its way into the electric flight arena, although it is now not much seen. The Tamiya brand is synonymous with high quality, but this connector is a rare exception - the contacts are of high resistance and very quickly become distorted in use, leading to an unreliable and even higher resistance connection. Tamiya connectors are completely unsuitable for model aircraft and should not be used.

Left: A Tamiya style connector - notice that one of the sockets has opened out and the other has become distorted. Right: This interesting variant on the Gold/Bullet system uses a plastic housing to accommodate a pair of these connectors. Wiring up of Gold connectors – make the battery plug positive Sooner or later, most modellers will need to operate their charger from their car’s battery. It is possible that the charger’s output lead could accidentally come into contact with the vehicle’s bodywork if using the Gold/Bullet connector system. The problem here is that vehicle bodywork is made to be the negative part of the car’s electrical system, so if it was the positive lead from the charger that touched the vehicle’s bodywork or engine parts, a short circuit could occur. To minimise the consequences of such an accident, I recommend you wire up your connectors so that the negative (black) wire of the charger’s output lead is fitted with a plug (the ‘male’ part), the exposed part of the connector system. This means that any short circuit between the output lead plug (wired to be negative) and your car’s bodywork (also negative) will be much less serious than if it were a positive to negative accidental short circuit. Following this convention will of

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course mean that the charger positive (red) lead will be a socket. Consequently, the battery positive lead will be wired with a plug and the battery negative will be wired with a socket; pleasingly this ‘plug positive’ policy also accords with standard electrical engineering practice. A second and very useful advantage of using this convention is that batteries and charging equipment may be easily shared with other modellers. This convention may seem contrary to the ‘common-sense’ approach of keeping a positive or ‘live’ (mains electricity) connection covered up. However, in the context of low voltage DC, this approach has no value; if the positive and negative wires touch there will be a short circuit irrespective of which one was wired with the unprotected plug. It is also a good idea to make the lead fitted with the plug about 10mm shorter than its adjacent lead. This reduces the chance of a short circuit between connectors because this difference in length puts distance between the two connector ends. Using fuel tube with Gold connectors. The unshielded plug of the commonly used Gold connector system presents a risk of short circuits occurring. To guard against this, ensure that you always fit a 25mm (1 inch) length of clean brightly coloured I.C. modeller’s fuel tube over the battery’s exposed plug as a conspicuous protective sleeve whenever the battery is not in use.

Left: If you choose Gold/Bullet connectors, and you choose not to fit a shield made of heat shrink tubing then an alternative precaution should be made to guard against accidental short circuits. This unshielded connector is protected with a 25mm (1 inch) length of clean, brightly coloured i.c. fuel tube over the battery’s exposed plug whenever it is not in use. Note that a plug is fitted to the battery positive (red) connector, and the socket to the negative one. Right: Sooner or later, many modellers will need to power their charger from a car battery which is still in-situ. Adopting the connector policy described here will minimise the consequences of an accidental short circuit between the charger’s lead and the vehicle’s metal bodywork. Additional wire For most models, the length of motor, ESC and battery wires will be adequate and no additional wire will be needed. If you do need additional wire for a model, this should be sized to match the current demand of the model, using the ESC size as a guide.

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Chapter 14 Wiring Diagram for Single Motor Models

The diagram shown below illustrates how to connect the motor, ESC and battery for a single motor model. The diagram assumes the ESC’s BEC function is used to supply power for the receiver and servos. This is the simplest possible method of providing RC system power, and is suitable for the majority of small to medium size models. Any suitable connector system can be used, but if Gold/bullet connectors are chosen, as shown here, they should be used as detailed here for reasons described in the previous chapter. The RC system (receiver, servos and their associated wires and aerial or antenna) should be kept as physically distant as is practically possible from the power system (ESC, motor and power system wiring). This minimizes the chance of the power system’s radiated RF interference affecting the RC system. This precaution is especially important with non 2.4 GHz RC systems. If the motor runs in the wrong direction (it should be anticlockwise when viewed from the front of the model) then this can be corrected by changing over any two of the three motor wires.

One of the three ESC to motor wires on this brushless model has been routed outside the cowling to allow it to serve as a safety device. Unless the wire is connected, the motor cannot run.

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Chapter 15 Example Power Systems in Detail

This chapter consists of a number of example models, all chosen for their successful power systems. My definition of ‘successful’ means in its broadest sense a power system which works well, and is well matched to the model. A power system will ideally be set up so that the motor is operating at a current which returns a reasonable efficiency. It will also use a propeller that is large enough to offer reasonable efficiency. Each page of this chapter is devoted to a single model. Each entry starts with a short description of the model and highlights any particularly noteworthy points about its power system. A photograph accompanies the description, and finally a comprehensive set of data for the model and its power system completes the entry. The data includes model type and brief constructional details, its wingspan, wing area, weight, wing loading, details of the motor used including its rpm, Kv and no-load current, details of the propeller and associated data such as prop make and dimensions, p/d ratio and pitch speed. The full throttle current and power is also provided for each model. Additional details include the ESC, the important power loading in Watts per pound and an overview of how the RC system is powered. The tables include a figure for the model’s average in-flight power consumption. This figure depends of course to a considerable extent on the pilot’s flying style; pilots who like to throttle back frequently will yield a lower figure than one who uses high power for most of the time. The average in flight power loading figure in Watts per pound is nevertheless an interesting and useful reference to have available.

The average in-flight power consumption depends to a large extent on the pilot’s flying style.

There is a great deal of information on each page, so this chapter should contain much to interest a modeller wanting to learn more about electric power systems. This chapter is intended to be a useful resource of examples which will help you learn more about power systems, and will I hope help to guide you in making your own power system decisions.

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Multiplex Acromaster By Andrew Gibbs

This model was the subject of a magazine review, and used the manufacturer’s recommended system, which under test consumed 30 Amps and 290 Watts. It’s always worth paying attention to how a major manufacturer such as Multiplex specifies a power system. You can be sure the power system was carefully selected, tested and developed, the result being a system which is an excellent match for the model and its intended mission profile. Another reviewer running the exact same system except for a higher battery C rating (lower internal resistance) reported significantly more power - 37 Amps and 395 Watts. However, much of this difference can be accounted for by the fact that my figures were taken with a part discharged battery, while the other reviewer used a fully charged battery to find peak power values.

The installed power system provides for a very capable model which has the ability to hover as well as achieve a good turn of speed in level and climbing flight.

Multiplex Acromaster Model type and construction Quick build kit, all foam construction Wingspan & wing area 43 inches (1,095 mm), 567 sq in / 3.93 sq ft Weight and wing loading 1,063 g (2.35 lb), 9.5 oz / sq ft Motor HiMax HC 3516-1130 brushless outrunner Motor current for max efficiency Optimum working range: 10 – 34 Amps Motor data: Max Amps, Kv & Io Max 48A/15 sec, Kv: 1.130, Io: 1.8 Amps Propeller, pitch speed & p/d ratio 11 x 5.5 APC E, p/speed 46 mph, p/d ratio 0.5 Battery 3S 2,500 mAh LiPo Propeller rpm at full throttle 8,900 rpm Full throttle current & power 30 Amps / 290 Watts ESC 45 Amp Power loading 123 Watts / lb Flight time vs charge quantity 8 minutes / 1,800 mAh Average in flight power 19 Amps / 210 Watts equivalent to 89 W / lb Performance description Good speed, good rate of climb, ability to hover RC power source & no of servos ESC integral BEC, 4 mini servos.

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Hangar 9 P51D Mustang By Andrew Gibbs

This Hangar 9 P51D was the subject of a review for RC Scale magazine. The power system consists of a 6S LiPo and a relatively low Kv motor. This is a common method of powering ‘0.60 = 0.90 size’ warbirds, and it works well. The combination of components seems to be especially efficient; on a test/photographic mission involving a mix of some aerobatics with some lower throttle camera-passes the average in-flight current was equivalent to only 34 Watts per pound. The model has a nice, scale performance. The model is no slouch, but more performance would make the model more enjoyable. The performance could be significantly enhanced if desired simply by changing the propeller for one which drew more power from the battery such as a 16 x 10. The RC system is powered by two separate LiPo batteries, each feeding a separate UBEC; a 3S 800mAh pack supplies the receiver and 4 flying control standard servos, while a 3S 350 mAh example powers the single retract servo.

Hangar 9 P51 D Mustang Model type and construction ARF semi scale, all wood Wingspan & wing area 65 inches (1,650 mm), 743 sq in / 5.15 sq ft Weight and wing loading 3,998 g (8.8 lb), 27.3 oz / sq ft Motor E Flite Power 60 Motor current for max efficiency No data available Motor data: Max Amps, Kv & Io Max 55 A continuous, Kv: 400, Io: 2.7 A @ 10 V. Propeller, pitch speed & p/d ratio 15 x 8 APC E, p/speed 57 mph, p/d ratio 0.53 Battery 6S 5,000 mAh LiPo Propeller rpm at full throttle 7,500 rpm Full throttle current & power 48 Amps / 1,050 Watts ESC 70 Amp Power loading 120 Watts / lb Average in flight power 14 Amps / 300 Watts equivalent to 34 W / lb Flight time vs charge quantity 6.5 minutes / 1,500 mAh Performance description Reasonable speed and rate of climb RC power source & no of servos 2 x LiPo & UBEC systems. 5 servos total

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Lockheed C130 Hercules By Toni Reynaud

This wonderful model epitomises Toni’s philosophy of maximum fun with minimum financial outlay. The four Speed 400 7.2 V brushed motors are fed by a 3 cell LiPo. They are somewhat overloaded at full throttle but are lasting well since full throttle is used only rarely. Toni has enjoyed many flights with this model which is still on its original motors. A single brushed ESC controls all four Speed 400 brushed motors. The model is easy to fly and lands quite slowly. A GPS was installed in the model for a few flights, which revealed that its average flight speed was around 35 mph. It is interesting to compare this with the propeller pitch speed of 49 mph, suggesting the motor/prop combination results in a pitch speed which is a good match for the airframe. More information about this model can be found at the Gibbs Guides website. Incidentally, Toni created many of the excellent diagrams in this guide.

C130 Lockheed Hercules Model type and construction Built from a plan using foam & brown paper Wingspan & wing area 71 in (1,800 mm), 558 sq in / 3.87 sq ft Weight and wing loading 1,660 g / 58.5 oz (3 lb 10.5 oz), 15 oz / sq ft Motors 4 x Speed 400 7.2V brushed can motors Motor current for max efficiency Approx 3.3 Amps Motor data: Kv and Io Kv = 2,277 Io = 0.5 A Propeller, pitch speed & p/d ratio 4 x Günther 5 x 4.3, p/speed 49 mph, p/d ratio 0.86Battery 2 x 3S 1,500 mAh LiPo in parallel Propeller rpm at full throttle 12,000 rpm (estimated) Full throttle current & power 50 Amps (12.5 A per motor) / 490 Watts ESC 50 Amp brushed Power loading 134 Watts / lb Average in flight power 12 Amps / 175 Watts equivalent to 48 W / lb Flight time vs charge quantity 6 minutes / 1,560 mAh Performance Fairly fast with good rate of climb RC power source & no of servos From ESC via integral BEC. 4 servos.

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ElectRick By Rick Morris

This charming vintage style model was designed and built by Rick Morris, a retired airline pilot. No plan was drawn; the model was built by eye! It was initially powered by a small diesel engine, and later converted to quiet clean electric power. The model received its name only after it received its new power system. Rick made sure the wing spars were beefed up considerably compared to those of a comparable free flight model, so they could accommodate the higher forces that result from being able to control the model. The power system employs a small outrunner operating on a 3S (3-cell) battery. The propeller is a 10 x 3.8 APC-E slow fly and this is about as large as is safe for use with this motor and a 3S battery. Alternatively, the model could have used a 2S LiPo which would have allowed the use of a significantly larger prop. The battery is located right up in the nose, slotted in vertically just behind the motor. The propeller develops approximately 2 lbs of static thrust, although this is of course of little relevance to the flying performance.

ElectRick Model type and construction Vintage style, balsa and ply construction Wingspan & wing area 49 in (1,246 mm), 367 sq in / 2.55 sq ft Weight and wing loading 1,105 g / 39 oz (2 lbs 7 oz), 15.3 oz / sq ft Motor Hacker A20 - 20L outrunner Motor current for max efficiency Approx 8 Amps (estimated) Motor data: Max Amps, Kv & Io Max 20 A, continuous, Kv = 1,022, Io = 0.85 A Propeller, pitch speed & p/d ratio APC 10x3.8, p/speed 27 mph (est), p/d ratio 0.38 Battery 3S 2,100 mAh LiPo rated at 25 C Propeller rpm at full throttle 7,400 rpm (estimated) Full throttle volts, current & power 19.3 Amps / 205 Watts ESC 20 Amp Power loading 85 Watts / lb Flight time vs charge quantity 18 minutes / 1,500 mAh Average in flight power 5 Amps / 56 Watts equivalent to 24 W / lb Performance description Moderate speed, good rate of climb RC power source & no of servos ESC integral BEC. 2 mini servos.

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Extreme Flight Edge 540T-E ARF By Andrew Gibbs

This model is included as it makes for an interesting comparison with the Mutliplex Acromaster. The two models are very similar in size and capability. This model has a slightly lower Kv motor, which drives a bigger prop. Both models demand a very similar level of power from the battery, and the propeller rpm is the same.

Extreme Flight Edge 540T- E ARF Model type and construction ARF, traditional balsa and ply construction Wingspan & wing area 45 inches (1,143 mm), 402 sq in / 2.79 sq ft Weight and wing loading 1,172 g (2.58 lb), 14.8 oz / sq ft Motor Torque 2818T brushless outrunner Motor current for max efficiency n/a Motor data: Max Amps, Kv & Io Max / sec, Kv: 900, Io: n/a Propeller, pitch speed & p/d ratio 12 x 6 APC E, p/speed 43 mph, p/d ratio 0.5 Battery 3S 2,500 mAh LiPo Propeller rpm at full throttle 7,500 rpm (fresh battery) Full throttle current & power 31 Amps / 315 Watts ESC Airboss 35 Amp Power loading 122 Watts / lb Flight time vs charge quantity 8 minutes / 1,800 mAh Average in flight power 19 Amps / 210 Watts equivalent to 82 W / lb Performance description Good speed, good rate of climb, 3D manoeuvres RC power source & no of servos ESC integral BEC, 4 mini servos.

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Cermark Banchee E3D ARF By Andrew Gibbs

The power system of this model comprises Cermark’s recommended low Kv motor coupled to a large diameter, low pitch prop. This makes it a suitable choice for the low speed 3D manoeuvres which this model is designed to excel at. On paper, this combination of motor, battery and propeller would be expected to turn significantly faster than the measured 8,500 rpm, and should consume considerably more power than the actual consumption of 450 Watts. The battery used had a relatively high internal resistance, and this was a significant factor limiting the current and thus the motor rpm. This power system illustrates how significant the internal resistance of the battery is.

Left: The battery is located underneath the wing. Right: The Banchee in flight. This particular model proved to be a little delicate around the nose and landing gear areas, but was very entertaining to fly.

Cermark Banchee E3D ARF Model type and construction ARF, traditional balsa and ply construction Wingspan & wing area 54 inches (1,370 mm), 710 sq in / 4.93 sq ft Weight and wing loading 1,812 g (3.94 lb), 12.8 oz / sq ft Motor (manfr’s recommended) CEM-4220-770 brushless outrunner Motor current for max efficiency n/a Motor data: Max Amps, Kv & Io Max Amps: n/a, Kv: 770, Io: n/a Propeller, pitch speed & p/d ratio 15 x 8 APC E, p/speed 64 mph, p/d ratio 0.53 Battery 4S 4,000 mAh LiPo Propeller rpm at full throttle 8,500 rpm Full throttle current & power 40 Amps / 450 Watts ESC 60 Amp Power loading 114 Watts / lb Flight time vs charge quantity 6 minutes / 2,000 mAh Average in flight power 20 Amps / 288 Watts equivalent to 73 W / lb Performance description Good speed, good rate of climb, 3D manoeuvres RC power source & no of servos UBEC supplied by flight battery, 4 mini servos.

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Zephyr ARF

This vintage style model was purchased used in 2002, for the purpose of testing power s mprise 2,400 mAh Nicad cells which m t 3.3 formance. A brushed, geared 480 motor 00 mAh Nicad cells wa instead. This transformed the m n flew beautifully. ushless system is even lighter and c ner driving a fine pitch prop. This makes for a reasonably e odel easily hts for th 00 g (incl a la g; 5 g.

By Andrew Gibbs

ystems. Its original system co d a Speed 600 and 7 x lbs) with very poor perade for a heavy model (abou

and ten 1,1odel, which the

s installedThe present br

onsists of a small low Kv outrunfficient system and the m achieves long flight times. Component weige model are: fuselage: 7 uding bomb drop module, 2 standard servos andrge 35 MHz receiver); wings: 265 battery 150 g, giving a total flying weight of 1,11

Left:

The Zephtyr is not a pretty model, but it has proven to be a useful workhorse. Right: The business end. ESC and battery live in the nose area. The box underneath houses a servo operated bomb drop mechanism!

Zephyr ARF Model type and construction ARF, traditional balsa and ply construction Wingspan & wing area 62 inches (1,570 mm), 520 sq in / 3.60 sq ft Weight and wing loading 1,115 g (2.45 lb / 39.3 oz), 10.9 oz / sq ft Motor (manfr’s recommended) MP Jet AC28 / 7-35 D outrunner. Motor current for max efficiency Up to 8 Amps Motor data: Max Amps, Kv & Io Max Amps: 9 (estimated), Kv: 950, Io: n/a Propeller, pitch speed & p/d ratio 10x4.7 GWS slow fly, p/s 54 mph, p/d ratio 0.47 Battery 3S 2,200 mAh LiPo Propeller rpm at full throttle 6,540 rpm Full throttle volts, current & power 9.8 Volts, 12 Amps, 115 Watts (half charged) ESC 30 Amp (large excess capacity; hardly gets warm)Power loading 47 Watts / lb Flight time vs charge quantity 18 minutes / 1,800 mAh Average in flight power 6 Amps / 67 Watts equivalent to 28 W / lb Performance description Moderate speed, good rate of climb. RC power source & servos ESC integral BEC, 2 standard servos.

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Ay Andrew Weight

T der Andrew Weight gave it ic p d from a previous project. The motor is propped for a full throttm ethe highest safe continuous current 14 x 7, therefore presents a load wh maximum for this motor. The p ph, which is perhaps a little high for this type of model w lane is not be that a lo 4 x 5 wou trengths of low a cs. Thi y reduce the urrent draw.

vicraft Panic B

his kit built model, a British classic model design, is intended for i.c. power, but builan electr ower system derive

le current of 50 Amps. This compares with the nt of 55 Amps for 60 seconds. This figure suganufacturer’s maximum burst curr gests would be around 45 Amps. The chosen propeller, a ich is very close to the

ropeller’s pitch speed is 64 mhich, being a strutted bip inherently a high speed model. It may wellwer pitch prop such as a 1 ld be a better match for the model’s snd medium speed aerobati s propeller choice would also usefull

c

The large prop causes a high current draw. However, the motor could hardly be better ooled, and this factor combined with good throttle management prevents overheating.

Avicraft Panic

c

Model type and construction Traditional kit, balsa and ply construction Wingspan & wing area 48 inches (1,220 mm), 960 sq in / 6.66 sq ft Weight and wing loading 2,100 g (4.63 lb), 11.2 oz / sq ft Motor (manfr’s recommended) AXI 4120/14 Gold Line brushless outrunner Motor current for max efficiency 20 – 40 Amps (efficiency 82 % or greater) Motor data: Max Amps, Kv & Io Max Amps: 55A / 60 sec, Kv: 660, Io: 2A @ 10 V Propeller, pitch speed & p/d ratio 14x7 APC thin E, p/speed 64 mph, p/d ratio 0.50 Battery 5S 3,200 mAh LiPo Propeller rpm at full throttle 9,700 rpm (estimated) Full throttle volts, current & power 50 Amps / 790 Watts ESC 70 Amp Jeti Advance Power loading 170 Watts / lb Flight time vs charge quantity 7 minutes / 2,400 mAh Average in flight power atts equivalent to 84 W / lb 21 Amps / 390 WPerformance description Good, prop hanging possible. RC power source & servos lt Separate receiver battery; 2,000 mAh, 4.8 Vo

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Electr

I o an ancient, h Mabuchi RS540 motor coupled to a small direct drive p ed model is just about the least efficient way possible to i need very little power, this combination is workable. The performance data given below is for a 7 cell Nicad, this providing the lowest power the model can fly on. The model has also flown with other batteries including 2 and 3 cell 2,200 mAh LiPo packs. However even with a 3-cell LiPo, th rtling; althow rating we fficiency and so the performance increment is only mode (25

/ lb) and the average in flight power (21 W / lb) illustrates how marginally the model is powered on 7 cells. Compo ings 405 g, battery 370 g, total 1,420 g.

ified Gentle Lady By Andrew Gibbs

converted this glider to electric p wer around the turn of the century. The use of igh rpm brushed ferrite

rop, all installed in a very low spe power this model. However, s nce powered gliders

e performance is not sta ugh the motor consumes more than twice the power ith this battery, it is ope ll past its point of maximum e

st. The small margin between the power loadingW

nent weights: Fuselage 645 g, w

Gentle Lady Model type and construction Traditional kit, balsa and ply construction Wingspan & wing area 78.25 inches (1,987 mm), 663 sq in / 4.60 sq ft Weight and wing loading 1,420 g (3.15 lb / 50 oz), 10.9 oz / sq ft Motor RS 540 (600 size, circa 1980) brushed can motor Motor current for max efficiency Approximately 5 Amps (estimated) Motor data: Max Amps, Kv & Io Max 12 A cont, Kv: 1,500, Io: 2 A (all estimated) Propeller, pitch speed & p/d ratio 8 x 4 folder, p/speed 31 mph, p/d ratio 0.50 Battery 7 cell 2,400 mAh NiCd Propeller rpm at full throttle 8,200 rpm (measured) Full throttle volts, current & power 6.8 V, 11.7 Amps, 78 Watts ESC 30 Amp Jeti brushed Power loading 25 Watts / lb Flight time vs charge quantity 14 minutes / 2,200 mAh Average in flight power 9.4 Amps / 66 Watts equivalent to 21 W / lb Performance description V dest speed & very low rate of climbery sedate; moRC power source & servos Separate 280 mAh receiver battery, 2 x mini servos

63 © Copyright Andrew Gibbs 2013

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RB

T Im nce aircraft and trainer from 1910 until the start of World War I in August 1914. In the early days of avia fer and often had a low rank. T to be t rk! The low wing loading and high drag el suggests its flying speed in s is probably no more ph. This is a great deal lower than the estimated pitch speed of 57 mph, su nd prop combination is not well matched to the airframe. The elatively s del flies v s kv) turn l

oes serve as an excellent illustration that a less than ideal power system can still allow us to enjoy a model.

umpler Taube y Chris Golds

he Taube (Dove) was used by perial Germany as a fighter, bomber, surveilla

tion, the pilot was considhe observer was considered

ered to be ‘just’ a chaufhe one doing the real wo

of this beautiful modcale-like cruise than 20 -25 m

ggesting that the motor amodel employs a small inrunner driving a r

mall prop. While the mo ery well, a more efficient power system would belower revving motor (lower ing a larger prop with a lower p/d ratio. The mode

d

Rumpler Taube (Dove) Model type and construction Balsa & light ply. Wingspan & wing area 44 inches (1,117 mm), 324 sq in / 2.25 sq ft Flying weight & wing loading 820 g (1.80 lb / 29 oz), 12.8 oz / sq ft Motor Mega 16-15-6 Amps for max efficiency & max eff. Approx 3-12 Amps (max eff. approx. 88 %) Motor data: Max Amps, Kv & Io Max approx 18 A, Kv: 1,500, Io: 0.65 A Propeller, pitch speed & p/d ratio APC-E 7 x 5, p/speed 57 mph, p/d ratio 0.71 Battery 3-cell 1,300 mAh LiPo Propeller rpm at full throttle 12,000 rpm (estimated) Full throttle current & power 14-16 Amps, 165 Watts (estimated) ESC Jeti Master 18-3p B.E.C. Power loading 60 Watts / lb Flight time vs charge quantity 6 minutes / 800 mAh (estimated) Average in flight power Approx 8 A / 88 W equivalent to approx 49 W / lb Performance description Brisk rate of climb and forward speed.

RC power source & servos os B.E.C. 2 x mini serv

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DH Venom B

hed British s. motors

format, turning a (5x4.5 in pread use for small models. ation a models. Chris

rately pushed the motor very cells to gain the maximum ance. This resulted 15 Amps at full throttle and would have

e. How replace, and this ult in a highe . A

and a LiPo instead. This t least tw , with longer duration.

y Chris Golds This successful small hand launc model was designed and built by the prolific

The model dates from a time when brushed electric scale modeller Chris Goldwere still dominant. At the time, the Speed 400 6 V motor in direct drive Günther 125 mm x 110 mm ches) prop was in widesThis motor and prop combin lso found popularity for multi enginedelibe hard by using nine NiMH possible perform in a current of resulted in a limited motor lif ever, the motors were cheap toinformed choice did res r performance than would otherwise be possiblebrushless 400 size motor battery would usually be chosen todaycombination would supply a ice the power to the prop

DH Venom Model type and construction Balsa and foam semi scale Wingspan & wing area 36 inches (914 mm), 230 sq in / 1.60 sq ft Flying weight & wing loading 700 g (1.54 lb / 24.7 oz), 15.4 oz / sq ft Motor Speed 400 6 V brushed Motor current for max efficiency Approx 4 Amps Motor data: Max Amps, Kv & Io Max approx 12A, Kv: 3,000, Io: 0.4 A Propeller, pitch speed & p/d ratio Günther 5 x 4.5, p/speed 64 mph, p/d ratio 0.9 Battery 9 cell 1,600 mAh NiMH Propeller rpm at full throttle 15,000 rpm (estimated) Full throttle current & power Approximately 15 Amps, 150 Watts ESC Unknown, but probably 20 A or higher Power loading 93 Watts / lb Flight time vs charge quantity Approx 6.5 minutes / 1,400 mAh Average in flight power Approx 13 A / 117 W equivalent to approx 76 W / lbPerformance description Sufficient, simple aerobatics possible RC power source & servos 4.8 V receiver battery

65 © Copyright Andrew Gibbs 2013

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66 © Copyright Andrew Gibbs 2013

23.5 mm gold/bullet connectors 4 AAA A B allow B Balancing B B -

46,discharging 45

- lithium polymer 41 lead acid 42

admium

44 -

liff

51 49, 50

g 15, 16, 21, 22 30

23

tor 50 15 32 65

claimer 4 24

38 54, 59

fficiency 13, 19, 22, 25 electrical 25

Electric glider 16, 18 rs Gold Line

4220-770 0 moto

6-113

6, 26, 2, 37,

tion

r

d

Index

.4 GHz 53 - wiring up 51 - XT60

mm gold/bullet connectors 49 Coolin

cromaster

30 Current irflow 13 irframe 21 Deans connecrming (ESC) 29 DH Chipmunk

DH Hornet AM Sw 20 DH Venom alsacraft 11 Dis

15 Dissipated power anchee 60 Dynamic thrust 14, 15 attery 33

capacity charging

46 47

EDF Edge 540 -

-

- - nickel c 42 - nickel metal hydride 43

- voltage characteristics 46 EC 32 B

- current limit 32 - input voltage 32 - disconnecting BEC function 33

tion - voltage selec 37 elt drive 19 B

Brigadier 19 Bristol Freighter 39 Bristol Beaufighter 40 Brushless motor 20 Bulldog 18

d) connector 48, Bullet (gol 49 Burke, Mike 47 C130 Hercules 57

ney Camm, Sid 12 Capacitor 36 Capacity (battery) 46

Cellphone 31Charging principles 47

vehicle 51, Charing from a 52 Charlesworth, C 38 Collins, Jeremy 20 Commutator 38

andy Coniam, R 12 Connectors 48 - author’s choice 51 - Deans 50 - fuel tube 52 - gold (bullet) 48, 49 - Tamiya 51

Cougar 2000

E-

Electric moto AXI 4120/14 62 -

- Cermark CEM- 60 - EFlite Power 6 r 56 - Hacker A20 - 20L 58 - Mabuchi RS540 63 - Mega 16-15-6 64

D - MP Jet AC28 / 7-35 61 - Multiplex HiMax HC 351 0 55 - Speed 400 6 V 65

- Speed 400 7.2 V 57 - Speed 480 61 - Speed 600 61 - Torque 2818T 59

ce Electrical resistan 22 k ElectRic 58

ESC 1 3 48 - arming 29 - BEC 32 - brake func 39

ed - brush 27 - capacito 36 - cooling 28 - heat generation 27 limitations 35 -

- LVC 33 - operation 26 - PCO 32 - power 28 - programming 37

37, - set up 40 - soft start 38 switching spee 38 -

- sounds 37

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Extra 330 15 Extreme Fl 0T 59 ight Edge 54

11

r 48,

g up ris

r propeller 57, 19,

20L rricane 11,

s

pattern rs

130

21,e

eti Phasor 45-3 21

13,

d battery

voltage 42

hium polymer 0, 41, 4, 47

Z osses from gearing 20

is

ff (LV )

Bob

25,

cooling 21

safety 29

windings 23, 37 er 30,

t week’s winning lottery nuickel cadmium, nicad 43, 61

l hydride

lating ESC r

g 16, 62

ower cut off)

PC21 ed d table

(p/d) tio

54

ules

DH Venom 65

Mustan

FET 25 Finnish Air ForceFires (LiPo) 42 Fokker Dr1 11 Folding propeller 16, 17, 63 Flair 11 Flyer III 13 Fuel tube 52 Gearbox, gearing 19 - losses 20 - methods 20 - noise 21 - offset 21 Gentle Lady 63 Gold (bullet) connecto s 49 - safety shield 49 - wirin 51 Golds, Ch 38 Günthe 65 GWS 61 Hacker A20- 58 Hawker Hu 12 Hawker Tempest 31 Heat 22, 23, 24 - cause 22 - ESC heat 27 Helical 12 Helicopte 21 HiMax HC 3516-1 55 Humidity 12 Inrunner 30 Inductanc 36 i.c. engine 29 J Keil Kraft Senator 19 KP 38 Kv 20 Lanzo Stik 47 Lead aci 42 - safety 42 - LED 22 Lift 7 Light bulb 22 LiPo, lit 2 4- fires 42 - guide to LiPo batteries 41

- sack 42 - safety 41 - voltage 41 Load (propeller) 17 Load (electrical) 45 Long E 25 LLouth-Davis, Lou 19 Low, Colin 18 Low voltage cut o C 33 Mahoney, 38 Me109 34 Microprocessor 25 Mobile phone 31 Mosfet 28 Motor - - Kv 13 - measuring 40 - outrunner 20 - - timing 39 - Multiplex Acromast 55 Nex mbers 85 NNickel meta 43 Opto iso 34 Outrunne 20, 40, 55, 56, 58 P47 Thunderbolt 31 P51 Mustan 56 Panic biplane 22,PCO (P 32 - voltage threshold 38 Pilatus 13 Pitch spe 9 Pitch spee 9 Pitch to diameter ra 10, 11, 17 Power 28 Power systems -65 - Avicraft Panic 62 - Acromaster 55 - C130 Herc 57 - Cermark Banchee E3D 60 - - ElectRick 58 - Extreme Flight Edge 540T 59 - Gentle Lady 63 - Hangar 9 P51 g 56 - Taube 64 - Zephyr 61 Power system testing 29

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68 © Copyright Andrew Gibbs 2013

r 60 motor (EFlite card

7 –ng ross section 7

e

direction 17

load 17

pitch 8, 9, 12, 17 nts

31, 32, 34,iring

ellers g

ESC on

23,

st 7,

ic

818T r propeller

lades

an der Vecht, Dirk 15

15, 24,

23,

g

iring diagram 53

ector 49,copy

Powe ) 56 Programming 37 Propeller 19 - balanci 15 - blade c , 8 - blade shap 10 - diameter 8, 12, 17 - - efficiency 13, 19 - folding 16 - GWS 61 - - painting 18 - parts 7 - - power requireme 12 - safety 18, 29- 31 - size 13 Pusher propeller 17 Ranson, John 40 RC system w 53 Reduction gearing 19 Resistance (electrical) 22, 27, 44, 45, 48 Resting voltage 44 RPM 9, 12 Safety of prop 18 Scottish Aviation Bulldo 18 Senator (Keil Kraft) 19 Setting up an 37 Sheldon, Sim 42 Shiebe Falke FS28 38 Shock flyer 15 Skyhawk 39 Slippage 10 Soft start 38 Sound from ESC 37 Spinner 15 Speed 480 61 Speed 600 61 Square law 27 Static thrust 14, 15 Swallow, BAM 20 Swastika 11 Switching 25 - frequency 38 Taube 64 Tempe 31 Thrust 13 - dynam 13 - static 13 Timing 39 Torque 2 59 Tracto 17

Tremlett, Martin 38 Twist in prop b 10 Uberlacher, Sepp 31 Ulery, Wayne 13 Undercambered 8 VVan Moorst, Appie 21 Vibration 29 Voltage 25 - drop 44 - off-load 44 - on load 44 - recovery 46 - spikes 36 Vortices 12 Weight, Andrew 15, 22, 62 Whipp, Alan 19 Windings 37 Wing warpin 13 Wire, wiring 36, 48 - additional wire 52 - battery to ESC wires 36 - ESC to motor wires 36 - size 48 WWorley, George 31 Wright brothers 13 XT 60 conn 50 - clone, 50 Zephyr 61