term paper helicopters

8/8/2019 Term Paper Helicopters

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h o w h e l i c o p t e r s w o r k

Rotary Wing Terminology

Most helicopters the engine turns a shaft that connects to an input quill on the transmission; the

main rotor mast comes straight out of the top of the transmission and the tailrotor driveshaftconnects to an output quill 90 degrees out from the mast.

Spinning the rotor which has an aerofoil section causes lift, allowing the helicopter to risevertically or hover.


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Tilting the spinning rotor will cause flight in the direction of the tilt.

There are many terms associated with rotary wing flight and it is important for a student to

become familiar with them to understand the mechanics of rotary wing flight.

Main Rotor System

Root: The inner end of the blade where the rotors connect to the blade grips.

Blade Grips: Large attaching points where the rotor blade connects to the hub.

Hub: Sits atop the mast, and connects the rotor blades to the control tubes.

Mast: Rotating shaft from the transmission, which connects the rotor blades to the

helicopter.

Control Tubes: Push \ Pull tubes that change the pitch of the rotor blades.

Pitch Change Horn: The armature that converts control tube movement to blade pitch.

Pitch: Increased or decreased angle of the rotor blades to raise, lower, or change the

direction of the rotors thrust force.

Jesus Nut: Is the singular nut that holds the hub onto the mast. (If it fails, the next person

you see will be Jesus).

this type of rotor system pivots around the trunion to allow for blade flapping


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Swash plate

The swash plate assembly has two primary roles:

Under the direction of the collective control, the swash plate assembly can change the

angle of both blades simultaneously. Doing this increases or decreases the lift that the main

rotor supplies to the vehicle, allowing the helicopter to gain or lose altitude.

Under the direction of the cyclic control, the swash plate assembly can change the angle o

the blades individually as they revolve. This allows the helicopter to move in any direction

around a 360-degree circle, including forward, backward, left and right.

The swash plate assembly consists of two plates -- the fixed and the rotating swash plates --

shown above in blue and red, respectively.

The rotating swash plate rotates with the drive shaft (green) and the rotor's blades (grey)

because of the links (purple) that connect the rotating plate to the drive shaft.

The pitch control rods (orange) allow the rotating swash plate to change the pitch of the

rotor blades.

The angle of the fixed swash plate is changed by the control rods (yellow) attached to the

fixed swash plate.

The fixed plate's control rods are affected by the pilot's input to the cyclic and collective

controls.

The fixed and rotating swash plates are connected with a set ofbearings between the two

plates. These bearings allow the rotating swash plate to spin on top of the fixed swash plate.


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Controls

Collective:The up and down control. It puts a collective control input into the rotor system,

meaning that it puts either "all up", or "all down" control inputs in at one time through the

swash plate. It is operated by the stick on the left side of the seat, called the collective pitch

control. It is operated by the pilots left hand.

The collective lets you change the angle of attack of the main rotor simultaneously on both

blades.

Cyclic: The left and right, forward and aft control. It puts in one control input into the rotor

system at a time through the swash plate. It is also known as the "Stick". It comes out of the

centre of the floor of the cockpit, and sits between the pilots legs. It is operated by the pilots

right hand.


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The cyclic changes the angle of attack of the main rotor's wings unevenly by tilting the swash

plate assembly. On one side of the helicopter, the angle of attack (and therefore the lift) is

greater.

Pedals:These are not rudder pedals, although they are in the same place as rudder pedals on anairplane. A single rotor helicopter has no real rudder. It has instead, an anti-torque rotor (Also

known as a tail rotor), which is responsible for directional control at a hover, and aircraft trim

in forward flight. The pedals are operated by the pilots feet, just like airplane rudder pedals are.

Tandem rotor helicopters also have these pedals, but they operate both main rotor systems for

directional control at a hover.

Here are some of the component parts that make up a helicopter. While this is an example of

one specific helicopter (UH-1C), not all helicopters will have all of the parts listed here. Some

of this may be a bit more of the same old stuff we have just discussed, but it will showeverything as it relates to everything else on the aircraft and the location of each component.

Just mouse over the grey spots to see the explanation of the parts of the helicopter below.

The Tail Rotor

The tail rotor is very important. If you spin a rotor using an engine, the rotor will rotate, but the

engine and the helicopter will try to rotate in the opposite direction. This is called TORQUE

REACTION


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The tail rotor is used like a small propeller, to pull against torque reaction and hold the

helicopter straight.

By applying more or less pitch (angle) to the tail rotor blades it can be used to make the

helicopter turn left or right, becoming a rudder. The tail rotor is connected to the main rotor

through a gearbox. When using the tail rotor trying to compensate the torque, the result is an

excess of force in the direction for which the tail rotor is meant to compensate, which will tend

to make the helicopter drift sideways. Pilots tend to compensate by applying a little cyclicpitch, but designers also help the situation by setting up the control rigging to compensate. The

result is that many helicopters tend to lean to one side in the hover and often touch down

consistently on one wheel first. On the other hand if you observe a hovering helicopter head-on

you will often note that the rotor is slightly tilted. All this is a manifestation of the drift

phenomenon.

This picture illustrates how the helicopter moves when using the appropriate controls. Up and

Down movements are controlled by the "Collective". Side to Side and Forward and Back

motions are controlled by the "Cyclic". Lateral control (Also called directional control or


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"Yaw") is achieved by using the "Foot Pedals".

Dissymmetry of lift

One cannot begin to talk about the mechanics of helicopters until the problems associated with

rotary wing aerodynamics are understood. When the first rotary wing pioneers started trying to

make a helicopter fly, they noticed a strange problem.

The helicopters rotor system would generally work just fine until one of two things happened:

Either the aircraft began to move in any given direction, or it experienced any sort of wind

introduced into the main rotor system. Upon either of these events, the rotor system would

become unstable, and the resultant crash would usually take the life of the brave soul at the

controls. The question then was; Why does this happen? The answer is what we refer to today

as "Dissymmetry of lift".

What "Dis-Symmetry of lift" means is, when the rotor system is experiencing the same

conditions all around the perimeter of the rotors arc, all things are equal, and the system is in

balance. Once the system experiences a differential in wind speed from any angle, it becomes

unbalanced, and begins to rotate. Take for instance forward flight. Imagine a two bladed rotor

system spinning at 100 MPH.

The blade moving toward the forward end of the aircraft is going 100 MPH forward, and theblade moving toward the back of the aircraft is travelling at 100 MPH in the other direction.

This is just fine when the aircraft is not moving or is in a no wind condition. It is experiencing

100 MPH of wind in all directions, so it is totally in balance. Once the aircraft moves forward,

it begins to change this balance. If we travel 10 MPH forward, then the forward moving, or

advancing rotor blade, is experiencing 110 MPH of wind speed, and the rearward, or retreating

blade, is experiencing only 90 MPH of wind speed.

When this happens, we get an unbalanced condition, and the advancing blade experiencing


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more lift wants to climb, while the retreating blade experiences less lift and wants to drop. This

is where we get the term "Dis-Symmetry of lift". The lift is not symmetrical around the entire

rotor system.

How do we compensate for this situation? We compensate by allowing the rotor to flap. By

allowing the advancing blade to flap upward, and the retreating blade to flap downward, it

changes the angle of incidence on both rotor blades which balances out the entire rotor system.

As you can see in this simple graphic, there are a few ways to allow for blade flapping.

One is to allow the blades to flap on hinges (Articulated rotor system). Another way is to have

the whole hub swing up and down around an internal bearing called a trunion (Semi-rigid rotor

system). Unfortunately, we can not compensate completely for dis-symmetry of lift by using

blade flapping. Once the aircraft gets to a certain airspeed, and the rotor had flapped as much a

it possibly can, then "Retreating blade stall" may be experienced. In retreating blade stall, the

retreating blade can no longer compensate for dis-symmetry of lift, and the outer portions of the

blade will "Stall".

This situation, when not immediately recognized can cause a severe loss of aircraft

controllability. This is a major airspeed limiting factor for helicopters. For many years,

aeronautical engineers have tried to figure ways to eliminate this problem and increase the

forward airspeed for single rotor helicopters. Although many breakthroughs have been made,

the manufacturers of single rotor helicopters are usually not willing to change the entire design

on their products because of the extra costs involved for little airspeed payoff. Most have

resigned themselves to slower airspeeds for their aircraft, at a lower cost and less maintenance.


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The main rotor hub, where the rotor's drive shaft and blades connect, has to be extremely strong

as well as highly adjustable. The swash plate assembly is the component that provides the

adjustability.

Counter-Rotation Vs Contra-Rotation

One thing that people often get confused with is the difference between "Contra-Rotation" and

"Counter-Rotation". The terms are used incorrectly more than you could possibly imagine in

books, manuals, and on web sites. I wanted to take this opportunity to clear up the difference

between the two.

As you can see by the first diagram, "Counter-Rotation" is where there are two individual

shafts driving two propellers or rotors in different directions. Although we have chosen to show

this example on a CH-47 Chinook from a top view, it is exactly the same on a twin engine

airplane that has one propeller turning one way, and one turning the opposite way (Like on a P-

38 "Lightning"). Sometimes, as in the case of the CH-47, the rotors will mesh, so the


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synchronization of the systems is crucial.

On airplanes, where the propellers do not mesh it is not as critical that the systems are in synch

In an airplane, if the systems are out of synch, it can put undue stress on the airframe, and cause

harmonic vibrations throughout the airframe. You can usually hear an airplane that has the

engines out of synch, as it will make a varying strobe like sound.

Each propeller in an airplane counter rotating system has its own set of mechanical controls tovary the pitch of the blades. Often it is a hydraulic system, but in some cases (Like the P-38),

other means can be employed such as electric power. In a helicopter, both rotors are

manipulated by one set of controls for the pilot.

contra-rotation

"Contra-Rotation" is where the propellers or rotors are mounted "Co-Axially", meaning one in

front of (or on top of) the other on the same axis. Usually, the drive mechanism is a single

source, but the direction of rotation is spilt by a gearbox to drive the two systems in opposite

directions. This is usually done to reduce the "P" factor or "torque" in a turn. While we havechosen to show this example in the form of a Royal Navy Fairey Gannet, it also applies to

helicopters (Like on the Soviet "Hokum").

The main use for this on a helicopter is that it negates the need for a tailrotor (Anti-torque rotor

to maintain directional control at a hover. It also tends to relieve some of the effects of

retreating blade stall as both sides of the aircraft have advancing rotor blades.

In an airplane, one set of controls will adjust the pitch of both propellers at the same time.

Usually, it is done by varying hydraulic pressure in the propeller hubs. In a helicopter, both

rotors are manipulated by a single set of pilot controls as well, but two sets of control tubesworking off of two alternately rotating swashplates are needed to adjust the rotors at the

individual hub.

The Forces At Work

There are many forces at work when a helicopter flies, and many are specific to helicopter

flight. We will touch on some of these briefly. We all know about lift, drag, gravity, and thrust,


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so discussion of these would not really be necessary. I would rather talk about specific

conditions experienced exclusively in rotary wing flight. Here are some examples.

Translating Tendency

Translating tendency is defined by the textbooks as: The tendency for a single rotor helicopter

to drift laterally, due to tail rotor thrust. One may not think about how much thrust is produced

by the tail rotor, but we must remember that the tail rotor has a 6 to 1 rotational ratio to themain rotor system.

It actually spins 6 times faster than the main rotor, so it can compensate for the torque of the

main rotor without the need for a massive tail rotor span. The thrust it produces tends to push

the aircraft sideways at a hover. We compensate for this by adding left cyclic control inputs

(On American Helicopters, the opposite in foreign manufactured aircraft, because their rotor

systems turn the opposite way from ours). This makes the helicopter hang left skid, or wheel,

low at a hover. If you ever see an American helicopter hovering, you may notice this left side

low condition. If you ask a helicopter pilot how he is doing, and he answers, " Left skid low",

that means everything is normal.

Settling With Power

Settling with power can be a dangerous condition that any pilot may face, and if he or she is not

on their toes, it may cause a serious uncontrollable situation. Settling with power is basically

when the helicopter settles into the rotor wash produced by its own main rotor system. It

requires 3 key elements to occur, and these conditions should be avoided in combination with

one another.

These are: A near zero airspeed, up to 100% power applied, and a better than 300 foot per

minute rate of descent. Once you have all of these situations in occurrence, the aircraft will


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settle in its own down wash from the rotor system. The only way to recover is to gain forward

airspeed and allow the rotor system to fly into "Clean air". Once the rotor system is clear of the

rotor-wash, it will become efficient again, and the settling with power conditions will cease to

exist.

This can become a real problem at an out of ground effect hover (Above 10 feet from the

ground), and during landings.

'Settling With Power' or 'Settling in your own downwash' is a dangerous situation that any

rotary wing machine can experience. The term "Vortex Ring State" is used to describe the

actual swirling of the air within the rotor system itself that causes "Settling With

Power". Vortex Ring State can begin to occur when you have 300 Feet per minute (FPM) as a

rate of descent. Pilots need to be aware of the situation and avoid it at all costs.

Dynamic Rollover

Another dangerous condition for a helicopter pilot to experience is called dynamic rollover. It

is again, where you have a series of conditions that combine to make a dangerous situation.

Once again, 3 key elements make up this hazardous condition. They are: A pivot point, a

rolling moment, and weight equal to thrust at some time during the manoeuvre. What actually

happens is that the helicopter, which is still on the ground, will start to roll over on its side

using one skid, or wheel, as the pivot point.

Once the aircraft starts to roll, a downward collective movement is the only thing that will stop

the forces in action from flipping the aircraft on its side. By reducing the collective, the thrust

to weight ratio decreases, which allows the aircraft to settle back down in a level attitude. If thiis done on sideward sloping terrain, a collective reduction performed too quickly can cause the

aircraft to roll over on the other side, down the slope. Care must be exercised when performing

slope operations, but dynamic roll over can occur on the flattest of surfaces if the pilot becomes

complacent.


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It is normal practice to tackle a slope from the side and not from the front or back because most

helicopters have skid type landing gear with no brakes. Skid gear will most likely slide down ahill if the toes or heels of the skids are pointed up hill once the power is taken away holding the

ircraft in place.Once that force is no longer applied, the weight of the aircraft will get it started sliding and,

depending on the slope, could pick up so much speed that it crashes severely at the bottom of

the hill. The ones that have wheels and brakes could slide also depending on the degree of

slope and condition of the ground.

Other reasons not to attack a slope from the front or back is that the tail boom may strike the

hill before the skids do (Again, depending on the degree of the slope) or the rotor system mayimpact the hill before the skids do. Usually, if the standard 8 degrees of slope are used as a

maximum, then a sideward approach to the slope will have the skids touching before the rotor

system. Care should be used when passengers depart the aircraft on a slope as they may walk

into the rotor if they go up hill. Always brief the passengers to leave the aircraft on the down

slope side of the aircraft.


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A rotating body acts like a gyroscope and the forces that act upon the gyroscope require someadjustment to allow for the rotation itself. A spinning body will take inputs placed at one part o

the cycle of rotation and react later in the cycle of rotation. Now without getting too technical,

the main thing to remember here is that with the rotation comes some extra planning. If you

want a control input to take effect, you just have to be a little ahead of where you want it to

happen. In this case, 90 degrees before the spot where the action you desire is to take effect is

where you have to plan to put it into the system. Input is placed in one location and as the blade

swings 90 degrees more in the direction of rotation, the desired effect will be realized.

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