aero engine maintenance and repair

8/11/2019 Aero Engine Maintenance and Repair

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General Requirement of Aircraft EngineAn aircraft engine must be:

reliable, as losing power in an airplane is a substantiallygreater problem than an automobile engine seizing. Aircraft

engines operate at temperature, pressure, and speedextremes, and therefore need to operate reliably and safelyunder all these conditions.

Durable: It is the amount of engine life obtained whilemaintaining the desired reliability

lightweight, as a heavy engine increases the empty weightof the aircraft & reduces its payload.

powerful, to overcome the weight and drag of the aircraft.

smalland easily streamlined; large engines with substantialsurface area, when installed, create too much drag, wasting

fuel and reducing power output. repairable, to keep the cost of replacement down. Minor

repairs should be relatively inexpensive.

fuel efficientto give the aircraft the range the designrequires.

capable of operating at sufficient altitude for the aircraftwww.jdrr5.co.cc


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Powerplant Selection

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Types of reciprocating Engine

Inline Engine

This type of engine has cylinders linedup in one row. It typically has an evennumber of cylinders.

The biggest advantage of an inlineengine is that it allows the aircraft to bedesigned with a narrow frontal area forlow drag.

The disadvantages of an inline engineinclude a poor power-to-weight ratio,because the crankcase and crankshaftare long and thus heavy.

An in-line engine may be either air

cooled or liquid cooled, but liquid-coolingis more common because it is difficult toget enough air-flow to cool the rearcylinders directly

Example: Wright Flyer

Inverted inline engine

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Opposed or O-type Engine

An opposed-type engine hastwo banks of cylinders onopposite sides of a centrallylocated crankcase. Theengine is either air cooled or

liquid cooled Due to the cylinder layout,

reciprocating forces tend tocancel, resulting in a smooth

running engine free fromvibration.

Low weight to power ratio

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V-Type engine

Cylinders in this engine arearranged in two in-linebanks, tilted 30-60 degreesapart from each other

The vast majority of Vengines are water-cooled.

The V design provides ahigher power-to-weight ratiothan an inline engine, while

still providing a small frontalarea.

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Rotary Engine

Rotary engines have all thecylinders in a circle around thecrankcase like a radial engine(see below), but the differenceis that the crankshaft is boltedto the airframe, and the

propeller is bolted to theengine case.

The entire engine rotates withthe propeller, providing plentyof airflow for cooling

regardless of the aircraft'sforward speed.

Unfortunately, the severegyroscopic effects from theheavy rotating engine made

the aircraft very difficult to fly. Le Rhone 9C rotary aircraft enginewww.jdrr5.co.cc
http://upload.wikimedia.org/wikipedia/commons/3/3d/Le_Rhone_9C.jpg


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Radial Engine

This type of engine has one or more rows

of cylinders arranged in a circle around acentrally-located crankcase. Each row musthave an odd number of cylinders in order toproduce smooth operation

A radial engine has only one crank throwper row and a relatively small crankcase,resulting in a favorable power to weight

ratio. The lower cylinders, which are under the

crankcase, may collect oil when the enginehas been stopped for an extended periodcausing serious damage due to hydrostaticlock

In military aircraft designs, the large frontal

area of the engine acted as an extra layerof armor for the pilot. However, the largefrontal area also resulted in an aircraft witha blunt and aerodynamically inefficientprofile.

The power output varies from 100 to 3800hp

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Principles of Operation

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Reciprocating Engine Power and Efficiencies

Piston displacement: Thevolume displace by the piston is

known as piston displacement

Compression Ratio: In a pistonengine it is the ratio between

the volume of the cylinder andcombustion chamber when thepiston is at the bottom of itsstroke, and the volume of thecombustion chamber when the

piston is at the top of its stroke.The compression ratio isdefined as (Swept Volume +Clearance Volume) / Clearance

Volume www.jdrr5.co.cc


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Indicator Horse Power:

Indicated mean effective pressure is an averagepressure value that theoretically must be present

in a cylinder of an engine during the powerstroke to generate the maximum horsepowerpossible, given the pressures recorded withinthe cylinder during a dynamometer test.

IHP = PLANK/33,000 P - indicated mean effective pressure

L - length of stroke

A - area of piston head in square inches N - number of power strokes per minute

K - number of cylinders

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Brake Horsepower

The power developed to the propeller for usefulwork is called brake horsepower

Prony brake dynamometer

3300

2 rpmlengthforcebhp

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Friction Horsepower

FRICTION HORSEPOWER is the difference

between indicated horsepower and brake horse-power

Thrust Horsepower

Propeller efficiency refers to the percentage ofBrake Horsepower (BHP) which gets convertedinto useful Thrust Horsepower (THP) by thepropeller. The propeller is never 100% efficient.Therefore the propeller efficiency is always a

number less than one.

Neta is propeller efficiency.

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Propeller Efficiency

where J is Advance ratio, n is rpm and D is propeller diameter, V is TAS

The most efficient J depends

upon the propeller blade angle.

Course propellers (large blade

angles) will be more efficient at

larger advance ratios. Fine pitch

propellers will be more efficient at

small advance ratios.

When choosing a fixed pitch

propeller an aeronautical

engineer usually chooses

one, which is optimum for

cruise.

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Efficiencies

Thermal Efficiencies:

The ratio of useful work done by an engine tothe energy of the fuel it uses is called thermalefficiencies 25 to 30% useful power

15 to 20% lost in cooling

5 to 10% lost in overcoming friction 40 to 45% lost through exhaust

Useful power can be increased by increasing the

compression ratio

Indicated thermal efficiencies =

i.h.p 33000

wt of fuel burned/min heat value 778www.jdrr5.co.cc


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Mechanical Efficiencies:

In an engine it is the ratio of brake horsepower toindicated horsepower.

The factor greatest effect mechanical effect is the frictionwithin the engine. Mechanical efficiency is high when theengine is running at the rpm at which max b.h.p is

developed.

Note: Friction remain practically constant for an engine

phi

phbefficiencymechanical

..

....

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Volumetric Efficiency

It is a comparison of the volume of fuel/aircharge conducted into the cylinders to thetotal piston displacement of the engine

Factors decrease volumetric efficiency

Part throttle operation Long intake pipes of small diameter

Sharpe bend in intake pipe

Carburetor air temperature to high

Cylinder head temperature to high

Incomplete scavenging

Improper valve timingwww.jdrr5.co.cc


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Propulsive efficiency

It is the ratio of thrust horsepower to brakehorsepower. Ranges to 80-85%

Loss is due to friction and slippage

Controlling the blade angle is the bestmethod to maximize propulsive efficiency

During take off less blade angle

During high speed or diving more blade angle

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END

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