casa cpl gk jan 2004

Issue 4 Date: January 2004.

CPLGeneral Knowledge.

FOR THE COMMERCIAL PILOT LICENCECASA CYBER EXAMINATIONS

This book is one of the CPL Cyber Examination Series byBob Tait

Other books in the series are:

CPL AerodynamicsBob Tait

CPL PerformanceBob Tait

CPL MeteorologyBob Tait

CPL NavigationBob Tait

CPL Air LawBob Tait HPL

HUMANPERFORMANCE&LIMITATIONS

Bob

Tait

s Aviation Theory School

Phone 07 3277 8840Fax 07 3275 2178

e-mail [email protected]

Bob Taits Aviation Theory SchoolPO Box 712ArcherfieldQueensland

4108Australia

Building 221Qantas Ave

Archerfield AirportBrisbane

WHAT THIS BOOK IS NOT

This book is not a manual on how to fly an aeroplane. It does not set out to replaceyou instructor's pre-flight briefing. However, when you have completed your study,you should be well equipped with the required background to get the very best valueout of your briefings.

WHAT THIS BOOK IS

This book is a study guide designed to prepare students for the CASA CommercialPilot Licence Australia [CPLA] Examinations. It contains a full text covering all areasof the syllabus relating to General Knowledge.

As each section of the subject is dealt with, you will be presented with exercises inthe form of a set of multi-choice questions to test your comprehension of that section.The text occasionally goes beyond the requirements of the syllabus, however theexercises are designed to indicate the level of understanding required by the CASACYBER EXAMINATION. Each exercise is accompanied by fully explained an-swers.

At the end of the book, you will find final tests with explained answers.

If you have a sound understanding of the contents of this book, you will have notrouble performing to the required standard in the CASA cyber examination inGeneral Knowldege.

ABOUT BOB TAIT.Bob Tait has been associated with flying training for more years thanhe'd like to remember. [If you really must know it's 27 years.] Heentered into aviation after 8 years in the Education Department ofQueensland as a teacher.

He gave up the science lab to start his own flying school at Inghamin Queensland where he began full time theory courses to CPL. Hehas been fully occupied with theory and flying training since then andpresently runs his own theory school at Archerfield.

He holds a Grade One Instructor Rating, Command Instrument Rating, Multi Engine Trainingapproval. He has low level aerobatic test and training approval, owns his own Pitts Special andflies helicopters for fun.

ABOUT THE CPL CYBER EXAMINATION.The CASA cyber examination for General Knowledge is a 1 hour 30 minute exam with a total of40 marks. The pass mark for the exam is 70%.

It is necessary to make application to do the exam at any one of the exam centres in Australia.Applications can be made on an exam application form available from Assessment ServicesLimited at:

GPO Box 286Canberra ATC 2601Phone 02 62628820Fax 02 62628830

If you prefer you may make application for the exams directly through the ASL web site atwww.asl.com.au

Equipment requirements for the General Knowledge Cyber Examination:-

Provided by ASL to the candidate: Scribble pad.

Provided by the Candidate: None

Further information on the CASA cyber examinations can be found on the CASA website atwww.casa.gov.au

CONTENTS

ENGINES

Engine components ----------------------------------------------------- 1.1The theoretical four-stroke cycle -------------------------------------- 1.3The induction stroke ---------------------------------------------------- 1.3The compression stroke------------------------------------------------- 1.3The power stroke -------------------------------------------------------- 1.4The exhaust stroke------------------------------------------------------- 1.4

THE BASICS

Pressure in a gas --------------------------------------------------------- 1.5Force ---------------------------------------------------------------------- 1.5Torque--------------------------------------------------------------------- 1.6Ignition timing ----------------------------------------------------------- 1.7Valve timing ------------------------------------------------------------- 1.8Volumetric efficiency --------------------------------------------------- 1.9

DETONATION AND PREIGNITION

Detonation ---------------------------------------------------------------- 1.10Preignition ---------------------------------------------------------------- 1.11Compression ratio ------------------------------------------------------- 1.12Summary------------------------------------------------------------------ 1.13

ENGINE COOLING

Requirements of air-cooled engines ---------------------------------- 1.17Distribution of engine heat --------------------------------------------- 1.17Controlling engine temperature---------------------------------------- 1.17Cowl flaps ---------------------------------------------------------------- 1.18Maximum continuous power------------------------------------------- 1.18Take-off power ---------------------------------------------------------- 1.18

THE OIL SYSTEM

Functions of the oil system --------------------------------------------- 1.19Lubrication --------------------------------------------------------------- 1.19Components of the oil system ----------------------------------------- 1.20Viscosity ------------------------------------------------------------------ 1.22Oil temperature and oil pressure--------------------------------------- 1.22Ashless dispersant oil --------------------------------------------------- 1.23Straight mineral oil ------------------------------------------------------ 1.23Oil quantity --------------------------------------------------------------- 1.23Summary------------------------------------------------------------------ 1.24Exercise GK1 with answers ------------------------------------------ 1.25

PROPELLERS

Propeller blades and relative airflow--------------------------------- 2.1Angle of attack and blade angle -------------------------------------- 2.2Propeller torque and propeller thrust --------------------------------- 2.3

CONSTANT SPEED UNITS

Changing the propeller pitch ------------------------------------------ 2.5The governor ------------------------------------------------------------ 2.6Manifold pressure and RPM ------------------------------------------ 2.9Changing engine power with a CSU --------------------------------- 2.9Manifold pressure at start-up ----------------------------------------- 2.13Propeller malfunctions ------------------------------------------------- 2.14Propeller care ----------------------------------------------------------- 2.15Exercise GK 2 with answers ---------------------------------------- 2.18

FUEL AND FUEL SYSTEMS

Aviation gasoline ------------------------------------------------------- 3.1Octane rating ------------------------------------------------------------ 3.1Mixture------------------------------------------------------------------- 3.2Mixture control --------------------------------------------------------- 3.3Aircraft fuel systems --------------------------------------------------- 3.4Gravity fed systems ---------------------------------------------------- 3.4The importance of the vent -------------------------------------------- 3.5Pressure systems -------------------------------------------------------- 3.6Fuel vaporization ------------------------------------------------------- 3.6The boost pump --------------------------------------------------------- 3.7

CARBURETTORS

The float chamber ------------------------------------------------------ 3.8The venturi -------------------------------------------------------------- 3.8The mixture control ---------------------------------------------------- 3.9The air bleed ------------------------------------------------------------ 3.9Carburettor ice ---------------------------------------------------------- 3.9Carburettor heat --------------------------------------------------------- 3.12Using carburettor heat ------------------------------------------------- 3.12Testing carburettor heat ------------------------------------------------ 3.14Advantages and disadvantages of a carburettor -------------------- 3.14

FUEL INJECTION

The fuel pump----------------------------------------------------------- 3.15The fuel/air control unit ----------------------------------------------- 3.15The fuel manifold valve ----------------------------------------------- 3.15

Fuel discharge nozzles ------------------------------------------------- 3.15Fuel pressure/flow gauge ---------------------------------------------- 3.15Specific ground range -------------------------------------------------- 3.16Advantages and disadvantages of fuel injection ------------------- 3.16

MIXTURE CONTROL AND ENGINE PERFORMANCE

Fully rich ---------------------------------------------------------------- 3.17Best power --------------------------------------------------------------- 3.17Best economy ----------------------------------------------------------- 3.17Mixture condition at take-off ----------------------------------------- 3.18Exercise GK3 with answers ------------------------------------------- 3.18

SUPERCHARGING

Effect of altitude -------------------------------------------------------- 4.1Ground boosting -------------------------------------------------------- 4.1Gear-driven supercharger --------------------------------------------- 4.2Rated boost -------------------------------------------------------------- 4.3Full throttle height ------------------------------------------------------ 4.3Turbosupercharging ---------------------------------------------------- 4.3Altitude boosting ------------------------------------------------------- 4.10Pilot handling technique ----------------------------------------------- 4.10

ELECTRICAL SYSTEM

Sources of electrical power ------------------------------------------- 4.11The battery -------------------------------------------------------------- 4.11Earth return -------------------------------------------------------------- 4.11Battery capacity --------------------------------------------------------- 4.12Monitoring the system ------------------------------------------------- 4.12The centre-zero ammeter ---------------------------------------------- 4.13The left-hand zero ammeter ------------------------------------------- 4.14The alternator v The generator---------------------------------------- 4.14Overvoltage warning lights ------------------------------------------- 4.15Overload switches ------------------------------------------------------ 4.15Relays and solenoids --------------------------------------------------- 4.15The battery master switch --------------------------------------------- 4.16External power ---------------------------------------------------------- 4.16The DOs and DONTs of the electrical system --------------------- 4.17

THE IGNITION SYSTEM

Dual ignition ------------------------------------------------------------ 4.19Magnetos ---------------------------------------------------------------- 4.19The impulse coupling -------------------------------------------------- 4.20Magneto switches ------------------------------------------------------ 4.21Exercise GK4 with answers ----------------------------------------- 4.22

HYDRAULICS

The basic hydraulic system ------------------------------------------------------ 5.1The pressure regulator ------------------------------------------------------------ 5.2The system relief valve ----------------------------------------------------------- 5.2System faults ----------------------------------------------------------------------- 5.3Foot-operated hydraulic brakes -------------------------------------------------- 5.3Air in the brake system ----------------------------------------------------------- 5.4A leak in the brake system ------------------------------------------------------- 5.4

UNDERCARRIAGE SYSTEMS

Tailwheel/Nosewheel ------------------------------------------------------------- 5.5Fixed undercarriages/Retractable undercarriages ----------------------------- 5.5Spring steel strut ------------------------------------------------------------------- 5.5Bungees ----------------------------------------------------------------------------- 5.5Oleo-pneumatic struts ------------------------------------------------------------- 5.5Downlocks-------------------------------------------------------------------------- 5.7Uplocks ----------------------------------------------------------------------------- 5.7Squat switches --------------------------------------------------------------------- 5.7

AIRCRAFT INSTRUMENTS -THE GYRO INSTRUMENTS

Rigidity and Precession ----------------------------------------------------------- 5.9Gyro power sources --------------------------------------------------------------- 5.10The Directional Gyro ------------------------------------------------------------- 5.11Mechanical drift ------------------------------------------------------------------- 5.12Apparent drift ---------------------------------------------------------------------- 5.12The Artificial Horizon ------------------------------------------------------------ 5.13The Turn Coordinator ------------------------------------------------------------- 5.15

THE PRESSURE INSTRUMENTS

Dynamic pressure ----------------------------------------------------------------- 5.17Static pressure ---------------------------------------------------------------------- 5.17The Air Speed Indicator ---------------------------------------------------------- 5.19The Altimeter ---------------------------------------------------------------------- 5.23The Vertical Speed Indicator ---------------------------------------------------- 5.24The Magnetic Compass ----------------------------------------------------------- 5.27Compass errors -------------------------------------------------------------------- 5.28Angle of dip ------------------------------------------------------------------------ 5.28Acceleration error ----------------------------------------------------------------- 5.29Turning error ----------------------------------------------------------------------- 5.30Exercise GK5 with answers

FIRE PROTECTION SYSTEMS

Water -------------------------------------------------------------------------------- 5.33Dry Powder ------------------------------------------------------------------------- 5.33Non combustible gas -------------------------------------------------------------- 5.33Fire Detectors ---------------------------------------------------------------------- 5.34

AUTOPILOTS

Single axis autopilot--------------------------------------------------------------- 5.37Two axis autopilot ----------------------------------------------------------------- 5.38Exercise GK5 with answers ---------------------------------------------------- 5.39

Aeroplane General Knowledge - Final Test 1Aeroplane General Knowledge - Final Test 2Aeroplane General Knowledge - Final Test 3Answers to Final Tests

CPL GENERAL KNOWLEDGE ALL RIGHTS RESERVED 2002 1.1

ENGINES

THE BASIC COMPONENTS

Although there are many types and sizes of internal combustion engines, all of them have certainbasic components in common.

As the name implies, an internal combustion engine burns fuel internally within a closed chamber.The component in which the combustion takes place is called a cylinder. The cylinder consists ofa lower cylindrical tube called the barrel and an upper section, called the cylinder head, whichcontains spark plugs, valves and passage ways [ports] to allow the flow of gas into and out of thecylinder

Because most aircraft engines are air-cooled, thebarrel and cylinder head are provided with cooling finswhich allow the heat of combustion to be conductedaway to the air flowing around the cylinder. Thecylinder head is hottest so it has larger fins, especiallyaround the exhaust port.

Fig 1.2 is a much simplified representation of thecylinder head which we will be using in all diagramsfrom now on. The flow of gas into or out of the cylinderis controlled by a valve. The valve fits into the cylinderhead as shown in Fig 1.3. The valve spring is a strongspring attached to the head of the valve so as to applya continuous upward force which keeps the valve heldtightly against the seat. In this closed position the valveprevents any flow of gas into or out of the cylinder viathe port.

The valve is opened to allow gas to flow. This is doneby applying a force to the top of the valve and pushingit down into the cylinder against the force of the spring.The force is applied by the rocker arm.

A push rod rides on the rotating camshaft [Fig 1.4]. Asthe cam lobe passes under the push rod, it is forced

upwards lifting one end of the rockerarm. The rocker arm rotates about thepivot forcing the valve down into theopen position.

As the camshaft continues to rotate, thevalve spring returns the valve, rockerarm and push rod back to the closedposition.

flange

coolingfins

Fig 1.1

inlet

exhaust

Fig 1.2

Fig 1.3

barrelcylinder head

sparkplug

Fig 1.4

valve

port

spark plug

cylinder head

valve springport

spark plug

valve held tightly against the seat

rocker arm

push

rod

camshaft cam lobe

pivot

valve pushed open

1.2 BOB TAIT'S AVIATION THEORY SCHOOL CPL GENERAL KNOWLEDGE

The piston is essentially a plunger which is free to moveup and down inside the cylinder barrel. A clearance isprovided between the piston and the barrel to allow forexpansion as temperature increases. Piston rings, madeof high-grade cast iron, take up the space between thepiston and the barrel wall [Fig 1.5]. The top two rings arecompression rings which maintain pressure above thepiston by preventing leakage of gas through the gapbetween the piston and the cylinder walls. The bottomring is the oil ring which prevents oil from passingthrough the gap and also keeps the cylinder wall coatedwith a film of oil. All rings transfer heat from the pistonto the cylinder and to the outside airflow via the fins.

We have seen so far that the cylinder head, along with itsports and valves, allows the passage of gas into thecylinder. The gas that passes into the cylinder is amixture of fuel and air. When the mixture is ignited bythe spark plugs, the heat of combustion causes a pressureto act on the piston. This pressure, acting over the areaof the top of the piston, generates a powerful force whichpushes the piston down the cylinder barrel.

The piston transfers this motion to the crankshaft throughthe connecting rod [Fig 1.6]. The small end of theconnecting rod is attached to a pin inside the piston andis free to swing from side to side as required. The big endof the connecting rod is attached to the crankshaftjournal. The straight-line downward motion of the pis-ton is converted to rotary motion in the crankshaft via theconnecting rod [Fig 1.7]. The crankshaft in turn, rotatesthe propeller.

Bearings must be provided to ensure minimum frictionand wear. The big end bearing fits between the big endof the connecting rod and the rotating crankshaft journal[Fig 1.7]. As the crankshaft rotates, it is held in place bythe main bearings [Fig 1.6 & 1.7].

A thin film of oil under high pressure ensures that thereis no metal-to-metal contact between the bearings andthe crankshaft. The crankshaft is the backbone of theengine and adequate lubrication of the big end and mainbearings is vital for its continued operation.

Fig 1.5

Fig 1.7[front view]

Ring

piston

connectingrod

journalcrankshaft mainbearing

Fig 1.6[side view]

big endbearing

main bearing


THE THEORETICAL FOUR-STROKE CYCLE [THE OTTO CYCLE]Named after August Otto, the German engineer who developed it, the Otto cycle consists of fourstrokes of the piston within the cylinder which provide the sequence of events required to keepthe engine running.

Fig 1.8

The induction stroke begins with the piston at the top of its travel [top dead centre -TDC]. Withthe inlet valve open, the piston moves down the cylinder to the bottom of its travel [bottom deadcentre - BDC]. As the piston moves down, the fuel-air mixture is induced [ie sucked] into thecylinder [Fig1.8]. The action is exactly the same as that of a syringe. At the end of the inductionstroke, the inlet valve closes.

Fig 1.9

The compression stroke: As the crankshaft continues to rotate, the piston is forced upwardswithin the cylinder. Since both valves are closed, the fuel and air mixture is compressed inpreparation for ignition [Fig 1.9].

inlet valveinlet valve

exhaust valve

fuel-air mixture

Induction stroke - side view front view

inlet valve

exhaust valve

Compression stroke - side view front view


Fig 1.10

The power stroke: As the piston nears the top of the compression stroke, an electric spark jumpsacross the points of the spark plugs, igniting the fuel-air mixture. The heat of combustion causesthe gases to expand rapidly, driving the piston down. As the piston moves down, the connectingrod rotates the crankshaft [Fig 1.10]. This is the only stroke of the four that delivers power to thecrank shaft. The other strokes actually take power away from the engine but they are necessaryto allow it to continue to operate. One stroke gives while the other three take - the propeller getsthe leftovers! This is the great inefficiency of the four stroke cycle.

Fig 1.11

The exhaust stroke: As the crankshaft continues to rotate, the piston is forced up the cylinderonce more. This sweeps the burnt gases out of the open exhaust valve [Fig 1.11]. Near the top ofthis stroke the exhaust valve closes and the inlet valve re-opens, commencing a new cycle.

Power stroke - side view front viewinlet valve

exhaust valve

Exhaust stroke - side view front viewinlet valve

exhaust valve


ENGINE PERFORMANCE CONSIDERATIONS

THE BASICS. Pressure in a gas: Any gas or mixture of gases exposed to a surface exerts apressure on that surface. A gas is composed of many individual molecules each of which is in astate of constant random motion. The molecules are constantly colliding with each other and withthe surfaces to which they are exposed. Because each molecule has mass, it imparts a tiny forcewith each impact upon a surface. Gas pressure is simply the result of this constant molecularbombardment.

Consider a gas or mixture of gases contained within a certain space [Fig 1.12]. The pressureexerted on the walls of the container would depend upon the number of gas molecules present andthe speed at which each is travelling. The faster the speed of the molecule, the stronger the forceof collision.

The speed with which gas molecules move depends upon the temperature of the gas. The hotterthe gas, the faster each of its molecules move. The pressure of a gas in an enclosed space dependsupon three simple variables - the volume of the container, the number of molecules [ie the massof gas present] and the temperature of the gas. If you have grasped this point, you will easilyunderstand any aspect of engine performance.

Fig 1.12 below represents a gas in an enclosed space. The original sample is comprised of a certainnumber of molecules, at a particular temperature. The impact of these molecules is producing acertain pressure within the container. If temperature remains constant and the number ofmolecules within the containing space is increased, the molecular impacts with the container wallsbecomes more frequent so pressure rises.

If the number of molecules remains constant and the temperature is increased, the speed of eachmolecule increases resulting in more frequent and harder impacts. Again the pressure rises.

Fig 1.12

more m

olecul

es

more p

ressure

higher temperature

more pressure

more frequent impacts more frequent andharder impacts

originalsample


Fig 1.13

The pressure applied to the piston during the power stroketherefore, depends upon the space between the top of thepiston and the roof of the combustion chamber, the numberof molecules enclosed within that space and the temperatureof the burning gases. This pressure applied to the area of thepiston is responsible for the force transmitted to the crank-shaft.

Torque: There is more to engine performance howeverthan the force alone. When all is said and done the object ofthe exercise is to rotate the crankshaft. The tendency for thecrankshaft to rotate is measured as torque. When you placea spanner on a nut you are attempting to rotate the nut. Thetendency for the nut to rotate depends not only on the forceyou apply, but also on the distance the force acts from thecentre of rotation. This distance is called an arm [Fig 1.13].

Just after the piston leaves the TDC position on the powerstroke, the space above it is small and the molecules of gasare crowded closely together. The heat of combustioncauses a strong pressure to act on the piston. The arm onwhich the resulting force acts is equal to the perpendiculardistance from the line of the force to the centre of rotationof the crankshaft. Because the force is strong, torque is high[See Fig 1.14].

As the piston moves further down the cylinder, the spaceabove it increases and when combustion is completed, thetemperature begins to drop. This results in decreased pres-sure acting on the piston. However, during the first portionof the stroke, as the pressure begins to drop the arm in-creases, so torque remains high [See Fig 1.15].

The torque produced depends not only on the force whichacts on the piston, the other vital element is when that forceis applied. The force must be timed to act so as to takeadvantage of the longest arm. This will be in the region fromjust after TDC to the point where the connecting rod makesa right angle with the crank [Fig 1.14 & 1.15].

As the crankshaft continues to rotate beyond the positionshown in Fig 1.15, the pressure rapidly drops and the armdecreases. Very little useful work is done during this portionof the stroke. So little in fact that the exhaust valve can beopened well before BDC to allow the burnt gases to begintheir exit.

Fig 1.14

Fig 1.15

High pressure produces a strong force -torque is high.

torque

Pressure is dropping but arm is increaseing -torque is still high

torque

force

arm

torque-producingmoment


IGNITION TIMING

Since the highest pressures within the cylinder must occur at an exact time to produce the besttorque, there is one exact moment when the spark plugs must fire the spark to ignite the fuel-airmixture [charge]. Because the charge takes a finite time to burn and produce the high temperaturerequired for best pressure, the spark plugs must fire before the point where the best pressure isrequired [Fig 1.16].

The designer chooses the point where the maximum tem-perature is to occur in terms of crankshaft rotation. Afterallowing for the time required for combustion, he fixes thepoint where the spark plugs must fire [Fig 1.17].

The actual time required for the fuel to burn depends on anumber of factors including the size and shape of thecombustion chamber. However as far as the pilot is con-cerned, the time taken to burn the charge varies with theratio of fuel to air [mixture] and the temperature beforeignition.

Under normal operating conditions, the time taken to burnthe charge is fairly constant. At high RPM the crankshaftturns through a greater angle while the burning takes place.If the burn is to be complete by the position shown in Fig1.17, it would be necessary to ignite the charge earlier interms of crankshaft rotation when engine RPM is high andlater when engine RPM is low [Fig 1.18]. When the sparkis fired earlier it is said to be advanced. When it is fired laterit is said to be retarded.

Even though car engines allow for this by changing theignition timing to suit the RPM, light aircraft engines donot, except for starting. Because an aircraft engine spendsalmost all of its life operating within a very narrow band ofRPM, the ignition timing is fixed to produce a spark at about20 to 28 of crankshaft rotation before TDC [Fig 1.16].

This means that at low RPM such as during ground opera-tions, the ignition timing is not quite what it should be. Thisis no big deal because high torque is not required in thesesituations. It would be difficult [in the case of hand starting-dangerous] to start an engine with the spark timing this faradvanced. With the engine turning so slowly, the chargewould burn before the piston reached TDC, forcing thecrankshaft to stop and rotate backwards [propeller "kickback"]. During start-up, the ignition timing is automaticallyretarded.

Fig 1.16

Fig 1.17

Fig 1.18

ignitionoccurs

ignitionoccurs

crankshaft rotation while combustion takes place

combustioncompleted

ignitionlow RPM

crankshaft rotation while combustion takes place

combustioncompleted

ignitionhigh RPM


VALVE TIMING

At first glance you might imagine that, for the induction stroke, the inlet valve should open at TDCand close at BDC. Likewise, for the exhaust stroke, the exhaust valve should open at BDC andclose at TDC. This would be the case except for the inertia of the gases and the time required forthe valves to move to the fully open position. We have already seen that towards the end of thepower stroke, there is so little work being done that the exhaust valve can be opened well beforeBDC [about 55], to allow the burnt gases to begin their exit.

By the time the piston approaches TDC onthe exhaust stroke, there is a strong flow ofgas through the exhaust port. The closing ofthe exhaust valve is delayed until a little afterTDC [about 15], while the inertia of thegases continues the outward flow. Also, sincethe out-flowing exhaust gases leave a lowpressure in the cylinder, if the inlet valve isopened just before TDC [about 15], the newcharge can begin flowing in as the spentexhaust gases are leaving.

By the time the piston reaches BDC on theinduction stroke, the new charge is flowingstrongly through the inlet port. It would besilly to close the inlet valve at this point andinterrupt that strong flow. The closing of theinlet valve is delayed until well after BDC[about 60], to allow the inertia of the incom-ing charge to continue the flow.

The early opening of the exhaust valve dur-ing the power stroke is called valve lead,while the late closing of the inlet valve dur-ing the compression stroke is called valvelag. The period either side of TDC where theinlet valve is opening and the exhaust valveis closing is called valve overlap [Fig 1.19].

Lead lag and overlap are necessary to ensurethe efficient removal of the spent exhaust gasand to maximize the mass of fresh chargeinduced for the next cycle. The degree ofsuccess the engine achieves in doing this iscalled volumetric efficiency.

exhaus

tcom

pressi

on

induction

power

BDC

overlapexhaustcloses

inletopens

inletcloses[lag]

exhaustopens[lead]

Fig 1.19

Fig1.20Note that the distancethe piston travelsdown the cylinderduring the first 90 ofrotation is greater thanthe distance travelledduring the second 90of rotation.

This curious littlequirk of geometrymeans that very littlepiston movementoccurs for a largeamount of rotationnear the bottom of thestroke. That's whylead and lag are suchlarge angles.


VOLUMETRIC EFFICIENCY

Volumetric efficiency is calculated by comparing the volume of charge actually induced into thecylinder during the induction stroke [at standard sea-level temperature and pressure], with thepiston displacement [Fig 1.21]. To put it more simply, it is a measure of the success achieved indrawing gas molecules into the cylinder.

Fig 1.21

Factors affecting volumetric efficiency: Anything that reduces the mass of gas that flows intothe cylinder will reduce volumetric efficiency. If we ignore engine design features which the pilotcan do nothing about, the factors affecting volumetric efficiency include:-

Ambient air density. Obviously the number of molecules that can be drawn into the cylinder willbe controlled to a great degree by the number of molecules available in the outside air in the firstplace, ie ambient air density. Hot days and high altitudes reduce the engine's volumetricefficiency.

Throttle position. Volumetric efficiency is at its best when the engine is operating at full throttle.This results in maximum flow into the cylinders. As the throttle is closed, the flow of gases intothe cylinders is restricted, reducing volumetric efficiency and decreasing power output.

Engine RPM. We have seen that valve and ignition timing are designed around one particularRPM setting. At high RPM the velocity of the flow through the induction system increases. Thisgives rise to increased friction with the tubes, ports and valves. Also at high RPM, the inlet andexhaust valves are open for a shorter time, giving less opportunity for gas to flow into or out ofthe cylinders.

The temperature of the incoming charge. Hot air expands and becomes less dense. If the air isheated on its way to the cylinders volumetric efficiency will be reduced. This could be due to highengine temperatures or the application of carburettor heat. You will hear more about carburettorheat later.

Supercharging. A supercharger compresses the air before it enters the cylinders. This producesa much higher mass flow, increasing volumetric efficiency. More on this later.

TDC

BDC

pistondisplacement


Fig 1.22

Fig1.23

DETONATION AND PREIGNITION

During normal combustion, the temperature and pressurewithin the combustion chamber rise rapidly but smoothlyover a given time interval to reach a peak just after TDC. Thiscan be thought of as a firm but friendly hand helping to arrestthe piston's upward travel and pushing it strongly back downon the power stroke [Fig 1.22].

Detonation is caused when the temperature and pressure ofthe fuel air mixture in the combustion chamber become highenough to cause instantaneous burning [explosion] of thecharge. This releases the energy of combustion in an instantand instead of producing useful work, the piston is subject toa sudden shock which can be likened to a severe hammerblow [ Fig 1.23].

Because almost all the heat of combustion is released withthe piston poised at or near TDC, the heat is concentratedwithin the cylinder head instead of being spread more evenlydown the cylinder walls. Since the charge detonated in thefirst place because it was too hot, the rising cylinder headtemperature encourages the next charge to detonate aggra-vating the problem.

Causes of detonation. Anything that causes the temperature and/or pressure of the charge tobecome excessive before ignition is likely to promote detonation. Common causes include:-

Operating the engine at high power with inadequate cooling airflow eg a long climb at too lowan indicated air speed.

Operating at high power with very lean mixture settings. A lean mixture ie too little fuel mixedwith the air, tends to burn slowly, subjecting the cylinder to high temperatures for a longer timethan usual. At high RPM the slow-burning charge is still burning as the piston moves up on theexhaust stroke. This tends to concentrate the heat in the cylinder head. The chemical propertiesof a lean mixture make it more prone to detonation as temperature rises.

Operating the engine at high power with carburettor heat applied.

Using high manifold pressure with low RPM. This occurs when the throttle is placed near the wideopen position while the engine is under a heavy load. If you relate it to a car, this is like climbinga steep hill with the accelerator pressed to the floor without changing to a lower gear. The wideopen throttle and slow moving valves allow too much charge to enter the cylinder. When the pistoncomes up on the compression stroke, the temperature and pressure become excessive. This iscalled overboosting and it is encountered most frequently in supercharged engines with constantspeed propellers. More on this later.

Use of the incorrect grade of fuel for the engine. The ability of a fuel to tolerate high temperature/pressure without detonating is measured as octane rating.

B A NG !!


Symptoms of detonation. Detonation will be accompanied by rising engine temperatures and apower loss. In single engine aircraft the pilot may notice vibration [knocking] in the engine. Insome circumstances a pinging sound can be heard but this is often not noticed in a modern lightaircraft, especially a twin engine aircraft. If detonation continues, structural damage may occur,even to the point of melting pistons!

Pilot actions. Since the main cause of detonation is excessive engine temperature, the most urgentaction required of the pilot is to cool the engine. The most immediate cooling effect is obtainedby placing the mixture control in the fully rich position. This sends extra fuel to reduce thecombustion temperature and begin the cooling process just where it's needed most - inside thecylinders. Reducing power and opening cowl flaps will also assist. If detonation is suspectedduring a climb, the climb indicated air speed should be increased and power reduced.

PREIGNITION

As the name implies, preignition occurs when the charge is ignited before time usually by a hotspot inside the cylinder. The hot spot can be a small carbon deposit that begins to glow red hotwithin the cylinder, or even the spark plug electrodes. As temperatures within the cylinder beginto rise with the heat of compression, the hot spot ignites the charge and the burn begins before thespark plugs have fired.

Fig 1.24

In Fig 1.24 above, a hot spot ignites the charge as early as 40 before TDC. The burn commencesearly. At the appointed time, the spark plugs fire as usual but the charge is already almost burnt.The spark plugs now burn what is left and the peak combustion pressure and temperature occurmuch too early with the piston almost at TDC. This produces a serious loss of power and a veryrapid rise in cylinder head temperature as the heat of combustion is squeezed into the small spaceabove the piston. In severe cases, the high temperatures produced can cause the charge to detonateas well as preignite!

Symptoms. Preignition will be indicated by rough running, power loss and high enginetemperature. The pilot actions required are the same as those for detonation.

hot spot ignites the charge about 40 before TDC.

spark plugs fire on que, but most of the charge is already burnt.


Fig 1.25 COMPRESSION RATIO

One design feature which determines thepower available from an engine is the de-gree of compression the charge is subjectedto prior to ignition. This is determined bythe decrease in volume suffered as the pis-ton travels from BDC to TDC during thecompression stroke.

Engineers compare the volume of the chargeat the beginning of the compression stroke[marked A in Fig 1.25], with the volume ofthe charge at the end of the compressionstroke [marked B]. This is called the com-pression ratio and is expressed numericallyas A B. For most light aircraft engines itis somewhere in the order of 7 or 8 to 1.When a charge is subject to a high degree ofcompression, ie a high compression ratio, itbecomes hot. Consider Fig 1.25. The heatcontained within the charge when it is atvolume A is still present when it is squeezedto volume B. The same amount of heat ispresent but it is compressed into a muchsmaller volume - that is why the tempera-ture rises.

The higher the compression ratio, the hotterthe charge becomes before ignition. As wehave seen, increasing the temperature ofthe charge before ignition increases the

tendency for the fuel to detonate. The most important limit on how great a compression ratio anengine can have is the onset of detonation. The ability of a fuel to withstand compression withoutdetonating is indicated by its octane rating. High compression engines require the use of highoctane fuels. More on this later.

The distance the piston travels from TDC to BDC is referred to as the stroke. The distance fromthe centre of crankshaft rotation to the centre of the journal is called the throw of the crank shaft[Fig 1.26]. If you contemplate this for a moment you should see that the stroke must always betwice the throw. The length of the stroke therefore also determines the maximum arm availableto generate torque [see Fig 1.14 & 1.15].

A longer stroke provides higher torque. However as the length of the stroke is increased, the pistonmust travel at a higher speed to cover the extra distance at any given RPM setting. This causesmore internal friction and much greater stress imposed as the piston is stopped and returned at eachend of each stroke [reciprocating loads].

Fig 1.26

AB

Compression ratio = A B

BDC

TDC

strok

e

throw


SUMMARY OF FACTORS AFFECTING ENGINE POWER OUTPUT.

The flow chart in Fig 1.27 shows the relationship of all of the variables which affect the powerproduced by an internal combustion engine. Some of them, such as the area of the piston and thelength of the arm [stroke or crankshaft throw], are fixed by the engine manufacturer. Some canbe controlled by the pilot and these are the items that must be considered when it comes to enginehandling techniques.

mass of charge

temperaturepressure

areaforce

armtorque

RPMpower

1

23

45

67

89

Fig 1.27

Let's consider these factors along with the limitations which apply to each.

1 Mass of charge. This is really the number of molecules induced into the combustion chamberduring the induction stroke. Since molecules are ultimately responsible for pressure, the greaterthe number of molecules induced the greater the pressure will be. The mass of charge induceddepends upon the pressure outside the combustion chamber in the inlet port [manifold pressure]and, to a lesser extent, on the time for which the inlet valve remains open [RPM]. However as themass of charge increases, the heat of compression increases and the risk of detonation increases[overboosting]. The limitation is the onset of detonation.

2 Temperature. The other item that decides the pressure achieved is the temperature ofcombustion [see 'The Basics' in page 2.1.5]. The hotter the combustion temperature, the greaterthe pressure. However, as temperature rises, the risk of detonation rises. The limitation once againis the onset of detonation or damage to engine components, especially valves, due to overheating.

3 Pressure. The pressure acting on the piston is responsible for the force transmitted to thecrankshaft. The greater the pressure the greater the force. It would be very difficult to measure thispressure directly because it changes rapidly with piston movement and internal cylindertemperature. However if RPM is constant, the pressure acting on the piston depends on themanifold pressure. Some aircraft measure manifold pressure to give the pilot an indication of howhard the piston is being pushed.

4 Area. The other factor that decides the force produced is the area over which the pressure acts.For a given pressure in the combustion chamber, the bigger the piston, the greater the resultingforce. However bigger pistons also weigh more, so reciprocating loads are increased. Anotherinteresting consequence is that if the dimensions of the combustion chamber are doubled, the areaof the piston increases fourfold, but the volume of the combustion chamber increases eightfold.It becomes difficult to get that large amount of gas to flow into and out of the cylinder in the timeavailable. Volumetric efficiency becomes a problem.


Fig 1.27

horizontally opposed

Fig1.28

mass of charge

temperaturepressure

areaforce

armtorque

RPMpower

1

23

45

67

89

5 Force. As 3 or 4 increase, the force transmitted to the crankshaft increases and power increases.The limitation on the strength of the force is the structural strength of the engine components andthe loads imposed on the bearings.

6 Arm. The longer the arm [crankshaft throw or stroke], the greater the torque generated by anygiven force. However, as the length of the stroke increases, the reciprocating loads on the pistonand bearings increase along with friction.

7 Torque. The end product of all of the items above is the rotation of the crankshaft. The tendencyof the crankshaft to rotate is measured as the product of the force and the arm. It is called torque.

8 RPM. If all other factors remain constant, the power delivered by the engine is also governedby the speed of rotation, ie the number of power strokes per minute. The higher the RPM, thehigher the power output. However as RPM increase, the reciprocating loads and friction increase.Also as engine RPM increase, the propeller RPM increase and the propeller tip speed becomesexcessive, degrading the aerodynamic efficiency of the propeller blades. Propeller tip speed is oneof the most important limitations on engine RPM. One solution is to allow the engine to run athigh RPM and transmit its power to the propeller via a gearbox which allows the propeller to turnat lower RPM but higher torque than the engine.

Another feature determining engine power output is so obvious that it is often overlooked, ie thenumber of cylinders. The most common arrangement used in light aircraft engines is to have two

rows of cylinders horizontally opposed toeach other on either side of the crankshaft[Fig 1.28]. This produces an engine with anoverall flat shape which is relatively easy tocowl. It makes the engine shorter than itwould be if the cylinders were in line on thesame side of the crankshaft. This produces aconsiderable weight saving, reduces stresson the engine mounts and also enhances air-cooling. Because each cylinder has a 'mirrorimage' on the opposite side, this type ofengine is relatively free of vibration.

On the negative side, there is a tendency foroil to 'pool' on the lowest side of the cylinderincreasing the likelihood of fouling of thebottom spark plugs, especially after longperiods of low power operation.

top view

front view


Horizontally opposed engines are commonwith four or six cylinders and some go up toeight, ie four on each side of the crank shaft.As the number of cylinders increase, thedifficulty of cooling the rear cylinders in-creases. One neat way around this problem isthe radial engine [Fig 1.29]. The cylinders arearranged in a circle. One piston is connectedto a master connecting rod, while the othershave articulating rods which are hinged to aflange on the master rod.

Radial engines always have an odd numberof cylinders, mostly seven, nine or eleven.This is necessary to allow evenly spacedpower strokes during the cycle. Since theengine must turn through two complete revo-lutions during the four strokes of the cycle,each cylinder in a radial engine must fireonce every two engine revolutions. This isdone by firing in the order shown in Fig 1.29.It is possible only with an odd number.

The big advantage of the radial engine is itsexcellent power-to-weight ratio and ease ofair-cooling. The disadvantage is its circularfrontal area which does not lend itself tostreamlining. Nowadays, in the power rangeof the radial [above about 400 hp], the turbineengine often becomes an attractive alterna-tive. Radial engines are still fairly commonhowever, in agricultural aircraft.

Left. A radial engine in a Yak 50 Aerobaticaircraft. These Russian-built engines arepopular in advanced aerobatic aircraft be-cause of their high power-to-weight ratio.

Engine deatils

9 cylinder air-cooled radial.Rated at 360 hp.

1

2

3

4 5

6

7

radial connectingrod arrangement

radial crankshaft

propellershaft

Fig 1.29


A Russian-built V12 liquid-cooled en-gine. This engine was fitted to the YakM3 fighter. This arrangement consists ofsix cylinders on either side of the crank-shaft arranged in a 'V' formation. Theextra length and tight cowling of theengine makes it unsuitable for air cool-ing. See opposite page.

Photo courtesy ofwww.flyingfighter.com.au

The Cessna 421 features a geared propeller.In this arrangement, the engine develops itspower at high RPM and is connected to thepropeller via a gearbox. The gear box hous-ing is clearly seen as a distinct hump on topof the engine cowl.

The propeller turns at lower RPM but highertorque than the engine - this overcomes theproblem of excessive propeller tip speed.

The famous Gypsy Major engine fitted to theDH-82 Tiger Moth. This engine features a drysump, storing the engine oil in an external oiltank on the side of the fuselage. A scavengepump continually removes oil from the engineand returns it to the external tank.

In this upside-down arrangement the pistons arepositioned under the crankshaft allowing greaterground clearance for the propeller.

Photo courtesy ofwww.flyingfighter.com.au


ENGINE COOLING

Almost half of the heat of combustion within the cylinders is carried to the outside atmospherewith the exhaust gases. The remainder is conducted through the cylinder walls to the cooling fins,or carried by the engine oil to the oil cooler. Both the cylinder cooling fins and the oil cooler finallypass the heat to the air flowing through the engine cowl.

The temperature of the engine at any moment depends not only upon the rate at which heat is beinggenerated, but also on the rate at which it is being carried away. This in turn depends upon themass of air flowing through the engine cowl and on how well that air is being directed to the areaswhere it is most needed.

Baffles are provided within the engine cowls to direct the airflow, while cowl flaps are providedon larger engines to increase the mass air flow [Fig 1.30]. The pilot has some degree of controlover engine temperature by controlling the rate at which heat is being generated [power], or therate at which it is being carried away [Indicated Air Speed or cowl flap position].

Above. A twelve cylinder liquid-cooled V-12 engine fitted to a Russian built Yak M3 andrated at 1290 hp. Because of the impossibility of air cooling such a long engine - the designersresorted to liquid cooling with a radiator fitted to the underbelly of the fuselage.

This beautifully restored war-bird is based at Archerfield Queensland. Check out their website at www.flyingfighters.com.au


There is more to engine temperaturecontrol than maintaining the operatingtemperature between certain limits. Theother important consideration is the rateat which temperature changes. Becausedifferent parts of an engine must bemade of different metals, the rate atwhich these parts expand when heatedand contract when cooled varies. If tem-perature is allowed to change very rap-idly, either heating or cooling, unaccept-able stresses can be placed on engine

components. In addition, the ability of the engine oil to penetrate into small clearances is affectedby its temperature [more on this next].

Because of the absence of a liquid coolant to help moderate the rate of temperature change, air-cooled engines are especially vulnerable to rapid heating and cooling. The larger and morepowerful the engine, the more critical temperature management becomes.

MAXIMUM CONTINUOUS POWER AND TAKE-OFF POWER

Aircraft engine manufacturers rate engine power output as a percentage of the maximumcontinuous power [MCP] the engine is rated to produce. The engine is capable of running at 100%MCP for all of its rated life without damage. Of course the operator is not likely to choose to dothat because of operating efficiency considerations. Maximum continuous power means exactlywhat it says - it is the maximum power that can be used continuously.

Some larger aircraft engines feature a power setting beyond MCP which is approved for use forlimited periods of time, usually about 3 to 5 minutes. This is called take-off power and it actuallyrepresents more than 100% power. Apart from the time limit, other conditions usually apply toensure adequate engine cooling. They include the use of fully rich mixture and cowl flaps open.

Another term once used to describe MCP is Maximum Except Take-Off [METO] power. This isa rather clumsy way of saying the maximum power that can be used continuously.Below is an extract from the Limitations section of a Cessna 210N Flight Manual.

Fig 1.30

cooling airflow

cowl flap open

LYCOMING LYCOMING LYCOMING

POWER PLANT LIMITATIONSEngine Manufacturer: Teledyne Continental.Engine Model Number: IO - 520 - LEngine Operating Limits for Take-off and Continuous Operations:Maximum Power, 5 Minutes - Take-off 300 BHP rating.

Continuous: 285 BHP rating.Maxium Engine Speed, 5 Minutes - Take-off: 2850 RPM

Continuous: 2700 RPM


THE OIL SYSTEM

Engine oil serves a number of vital functions.

1 It lubricates by providing a boundary layer of oil between moving parts toprevent metal to metal contact. This reduces friction and energy loss andprevents excessive wear and damage to engine components.

2 It cools by carrying heat away to the oil cooler where it is dissipated to the air.This is not unlike the action of water in a car engine, which carries engine heat tothe radiator.

3 It cleans by carrying away sludge and other residue from the moving parts of theengine and depositing them in the engine oil filter.

4 It seals the spaces between the cylinder walls and the piston rings preventinggases from leaking past during the compression and power strokes.

5 It protects the metal components of the engine from oxygen, water and othercorrosive agents. It forms a cushion between surfaces under high impact loads.

Lubrication: There is much more to adequate lubrication than simply throwing lots of oil about.For example, in the big end and main bearings, the oil must actually separate the two surfaces,preventing metal to metal contact. This requires more than simply making the surfaces 'wet' withoil.

The crankshaft is not completely solid. It contains channels [or ducts], to carry pressure oilinternally. The pressure oil is fed into the rotating crankshaft through specially designed oiltransfer bearings. It is then fed through ducts to the inside of the main and big end bearings, whereit forces its way between the surfaces and emerges as a mist or spray [Fig 1.31].

Fig1.31


This spray of oil is flung out by the rotation of the crankshaft to coat the inside of the cylinderand the bottom of the piston with oil [Fig 1.32]. The oil is then picked up by the piston's oil ringand spread evenly over the cylinder walls. Finally it drains back to the sump from where it isrecirculated by the oil pump. One reason for setting the engine to run at about 1000 RPM afterstart up, is to ensure sufficient rotational speed to give adequate lubrication to the cylinders.

The engine case is nota solid block of metal,it contains passageswhich deliver thepressure oil from theoil pump to specificlocations. These pas-sages are collectivelycalled the oil gallery.From the gallery, thepressure oil is intro-duced into the hollowpush rods and trans-ported to the rockerarm where it emergesas a spray.

This spray lubricatesthe rocker arm and

valve stems, then drains back to the sump to be recirculated [Fig1. 32]. For engines fitted withconstant speed propellers, the engine oil is delivered to the propeller hub via the governor whereit acts as an agent to change the pitch of the blades [more on this later].

So you see, the manufacturer has gone to a great deal of trouble to ensure the oil system does itsjob effectively. A failure of the oil system would result in engine damage in less than one minute,followed very quickly by complete failure of the major engine components.

Components of the oil system: The very heart of the oil system is the oil pump. This is usuallyan engine-driven, gear-type pump which pumps more oil than the engine requires [Fig 1.33]. Theexcess oil is returned via a pressure regulator.

Because it is so important it is not a bolt-onitem; it is built-in as an integral part of theengine case.

The pump picks up the oil directly from thesump through a screen filter. The pick-uppoint is slightly above the bottom of thesump to prevent any heavy solid particles

Fig 1.32

Fig 1.33

Pressure oil enters the hollow push rod through a hole and emerges as a misty spray to lubricate the rocker arm and valve stem. It is then drained back to the sump to be recirculated.

splas

h/spra

y lub

ricati

on

Pressure oil emerges from the inside of the big end bearings as a spray. The rotation of the crankshaft flings it onto the cylinder walls and under the pistons. It then drains back to the sump to be recirculated.

oil inoil out


Fig 1.34

Since the oil pump is designed topump more oil than the engineneeds, provision must be made forthe excess oil to return to the suc-tion side of the pump. The pres-sure regulator allows for this.When the pressure on the engineside of the pump equals or exceedsa preset level, it lifts a plunger inthe relief valve, opening a path tocirculate back to the suction side[Fig 1.34]. The system pressurecan be set by adjusting the tensionon the spring in the valve.

The oil cooler has beenmentioned previously.Sometimes called an oiltemperature regulator, itactually does more thanjust cool the oil. Coldoil is thick and difficultto pump and it does noteasily penetrate smallclearances within theengine. The last thingwe want to do when theoil is cold is put itthrough the cooler!

The oil cooler is pro-vided with a by-passvalve which reacts tothe temperature of theoil [a thermostat]. Itshunts the oil around thecooler via the by-passwhen it is cold . When

the oil temperature increases, it is directed through the cooler [Fig 1.35]. Figures 1.34 and 1.35are schematic only.

Other features of the oil system include filters and oil temperature and pressure sensors. Oil filtersare usually provided with a by-pass to allow the passage of oil if the filter becomes blocked. It isbetter to have dirty oil than no oil!

Fig 1.35

oil inoil out

bypass valve

hot oil in

cool oil out

cooling air

oil co

oler b

ypass


VISCOSITY

Technically defined as the fluid friction [or body] of an oil, viscosity can be best regarded as theoil's resistance to flow. A good example to keep in mind is honey. When it is cold, it resists flowingor spreading - it has a high viscosity. When it is warm, it flows and spreads much more easily -it has a low viscosity. Oil behaves in exactly the same way. It would be silly to try to change theoil in an engine before start up when it is cold. Its high viscosity would keep most of it inside theengine as a thick coating over the internal surfaces [like trying to pour cold honey from a jar].Warm the engine by running it for a while and the oil pours readily when the drain plug is removed.

Oil cannot lubricate the engine properly until it has reached the correct temperature. Its viscositymust be low enough to allow it to flow easily through the small clearances in the bearings andsplash and spray over the cylinder walls, rocker arms and valves. At the same time it must beviscous enough to stick to surfaces to form an unbroken film of protection. You should alwaysensure that the oil added to an engine is the correct grade as specified in the aircraft flight manual.

The most important factor governing the warm-up period for an engine is the temperature of theoil. By far the majority of engine wear occurs in the first few minutes after start-up. It is veryimportant to allow adequate time for the engine to reach operating temperature before makinghigh power demands. The bigger the engine, the more critical the warm-up becomes.

Oil temperature and oil pressure: The pressure within a fluid depends not only upon the powerof the pump, but also upon the ability of the liquid to flow. Fig 1.36 shows that if a fluid is pumpedthrough a pipe which offers no resistance to its flow, there will be no build up of pressure.However, when a resistance is offered to the flow - in this case a narrow orifice - the pressure buildsup.

The pressure produced bythe oil pump therefore,depends upon the viscos-ity of the oil and the size ofthe clearances throughwhich it must pass. Whenoil temperature becomestoo high it flows too eas-ily, causing oil pressure todrop. When oil is cold itresists flowing and pres-sure tends to increase.

A collapsed main or big-end bearing offers a largerclearance to the oil flow,causing oil pressure todrop. Oil pressure is usu-ally measured just beforethe oil passes into the en-gine where it matters most.

Fig 1.36

no resistance-no pressure

high resistance-high pressure


Ashless Dispersant Oil

Most oils used on aircraft engines contain a dispersant that suspends contaminants such as carbon,lead compounds and dirt. The dispersant helps prevent these contaminants from gatheringtogether into clumps and forming sludge which can plug oil passages. The contaminants can thenbe filtered or drained out of the system, leaving the engine free of sludge and abrasive particles.Benefits include reduced engine wear, better compression seal around the piston rings andreduced oil consumption.

Along with the dispersant, these oils also contain additives which inhibit corrosion and reducefoaming. Unlike some additives, these leave no metallic ash when they burn, hence the name'ashless'. A high ash content in oil can cause preignition and spark plug fouling.

Straight Mineral Oil

These oils do not contain any additives except for a small amount to improve viscosity at lowtemperatures. They are used during the 'run-in' period for new engines, or after replacement ofcylinders or piston rings. The higher abrasive quality of this oil helps the piston rings and cylinderwalls to wear microscopic grooves, which eventually mate each ring to the cylinder wall. Just asa bullet fired from a particular gun carries a unique pattern of grooves that match it to that barrel,so each ring 'beds' into its particular cylinder. This bedding-in process forms a good compressionseal to prevent gases from leaking past the piston during the compression and power stroke. Afterabout 50 to 100 hours of normal engine operation, the straight oil is drained and an ashlessdispersant oil replaces it.

Oil Quantity

The aircraft flight manual stipulates that a minimum quantity of oil should be in the engine beforestart-up. One reason for this is to ensure that oil temperature does not become too high duringflight. Since one of the functions of the oil is to carry engine heat away, if less oil is circulated,the temperature of that oil will rise.

The oil pump will continue to pump oil until the oil level is critically low, because the pick-up pointis near the bottom of the sump. If oil was being lost to the system, the oil pump suction screenwould eventually become uncovered causing fluctuations and eventually a total loss of oilpressure. By the time this happened, the oil temperature would have become very high if the oilloss was gradual.

However if there was a large and rapid loss of oil, such as would occur with a broken lead, theremay be no noticeable increase in temperature even though the oil level is critically low.

Whenever a large oil loss is suspected, the engine should be stopped as soon as possible, even ifit means shutting down as soon as the runway is vacated.


SUMMARY FOR ENGINES AND THE OIL SYSTEM

FACTORS AFFECTING ENGINE POWER OUTPUT

Ignoring engine design characteristics over which the pilot has no control,engine power output is controlled by:

Manifold Pressure. The pressure outside the cylinder at the inlet portgoverns the mass of charge induced during the induction stroke andtherefore the pressure acting on the piston during the power stroke.It depends upon the position of the throttle.

RPM. The number of power strokes that occur per minute determine therate at which work is being done, ie power.

LIMITATIONS ON ENGINE POWER OUTPUT

As manifold pressure increases, engine power output increases. However ifmanifold pressure becomes too high detonation occurs.

As engine RPM increase, engine power output increases. However if RPMbecome too high reciprocating loads increase with the possibility of enginedamage. At high RPM the propeller tip speed approaches and exceeds thespeed of sound. This causes a loss of propeller efficiency.

VOLUMETRIC EFFICIENCY

The degree of success the engine achieves in inducing the fresh chargeduring induction and expelling the spent one during exhaust is calledvolumetric efficiency.

It depends upon ambient air density, throttle position and RPM. Anythingthat heats the incoming charge reduces volumetric efficiency.

ENGINE COOLING

Air-cooled engines are vulnerable to stress imposed by rapid heating orcooling. Engine temperature depends upon the rate at which heat is beinggenerated and the rate at which it is being carried away.

OIL SYSTEM

The viscosity of an oil depends upon its temperature. Oil temperature mustbe within certain limits to ensure adequate lubrication while maintaining anunbroken protective film over metal surfaces.

Oil quantity must be sufficient to allow the oil to carry engine heat to the oilcooler without becoming too hot.


EXERCISE GK 1Question No 1The pressure exerted on the piston during the power stroke increases as the mass of charge induced

[a] increases and combustion temperature increases[b] decreases and combustion temperature increases[c] increases and combustion temperature decreases[d] decreases and combustion temperature decreases

Question No 2As the throttle is moved towards the fully open position

[a] manifold pressure increases and mass flow decreases[b] manifold pressure decreases and mass flow increases[c] manifold pressure increases and mass flow increases[d] manifold pressure decreases and mass flow decreases

Question No 3The volumetric efficiency of an engine depends upon

[a] throttle position, ambient temperature, ambient pressure and RPM[b] throttle position only[c] throttle position, ambient temperature, ambient pressure but not RPM[d] throttle position and mixture strength

Question No 4The best action to take at the onset of detonation in an engine is

[a] lean the mixture and reduce the power[b] lean the mixture and increase the power[c] decrease the indicated air speed and maintain the power[d] select mixture fully rich and decrease the power

Question No 5The onset of detonation in an engine is indicated by

[a] vibration, rising temperatures and reduced indicated air speed[b] vibration, falling temperatures and reduced indicated air speed[c] vibration, rising temperatures and increased indicated air speed[d] vibration, falling temperatures and increased indicated air speed

Question No 6One of the limitations applying to increased RPM for increased power in a piston engine is

[a] high fuel consumption[b] excessive propeller tip speed[c] high oil pressure[d] high cylinder head temperature

Question No 7The warm up period for an engine prior to take off provides

[a] proper oil viscosity and uniform heating of engine components[c] higher oil pressure for take off[c] a means of expelling moisture from the engine crank case[d] adequate fuel pressure for take off

Question No 8If an engine is overheating during a long climb, an appropriate pilot action would be

[a] raise the nose to reduce indicated air speed[b] lean the mixture to best economy[c] reduce power and indicated air speed[d] increase indicated air speed, richen the mixture and if necessary, reduce power


Question No 9If cylinder head temperatures are becoming too low during a long descent, the pilot should

[a] reduce indicated air speed and power[b] increase indicated air speed and reduce power[c] increases indicated air speed and power[d] reduce indicated air speed and increase power

Question No 10The octane rating of a fuel is a measure of

[a] its specific gravity[b] its resistance to detonation[c] its resistance to vaporisation[d] its anti-misting properties in the event of fire

Question No 11A horizontally opposed engine should be held at about 1000 RPM after a cold start rather than idle to avoid

[a] damage due to vibration at low RPM[b] excessive cylinder wear due to poor lubrication at low RPM[c] damage due to low oil pressure at idle[d] a large increase in the time required to raise engine temperatures

Question No 12A radial engine always has an uneven number of cylinders, commonly 5, 7 or 11. This is a necessary design featureto ensure

[a] uniformly spaced power strokes during the cycle[b] adequate engine cooling[c] correct mass balancing during high power operation[d] enough space is left between cylinders for proper air cooling during flight

Question No 13The function of oil in an engine is to

[a] clean[b] lubricate[c] cool[d] all of the above

Question No 14The viscosity of an oil is a measure of

[a] the oil's ability to flow[b] the oil's resistance to flow[c] the temperature at which it will burn[d] the oils detergent properties

Question No 15The purpose of an oil cooler bypass is

[a] to prevent the oil from becoming too hot[b] to return overheated oil to the cooler[c] to prevent oil from passing through the cooler if it is already cold[d] to allow oil to bypass the cooler if the cooler becomes blocked

Question No 16If airflow to the oil cooler is interrupted by an obstruction in the duct

[a] oil temperature and oil pressure will rise[b] oil temperature will drop and oil pressure will rise[c] oil temperature will rise and oil pressure will fall[d] oil temperature and oil pressure will both fall


Question No 17One cause of high oil temperature and low oil pressure could be

[a] very low oil level in the sump[b] the oil's viscosity being too high for the engine type[c] the oil sump being overfilled[d] the oil cooler bypass not working

Question No 18The purpose of gearing a propeller in an aircraft engine is to permit the propeller to turn at

[a] higher RPM and lower torque than the engine[b] lower RPM and torque than the engine[c] lower RPM and higher torque than the engine[d] higher RPM and lower torque than the engine

Question No 19Operating an engine with too low an oil quantity will produce

[a] rising oil temperature and pressure[b] falling oil temperature and rising oil pressure[c] falling oil pressure and falling oil temperature[d] rising oil temperature and dropping oil pressure

Question No 20During a long climb, the cylinder head temperature becomes too high. This can be rectified by

[a] closing the cowl flaps[b] reducing the climbing indicated air speed[c] leaning the mixture to best power[d] richening the mixture to full rich and increasing the climbing indicated air speed

Question No 21If maximum power is applied for take-off while the oil temperature is too low

[a] the engine components could suffer stresses due to uneven heating[b] take-off manifold pressure could be lower than normal[c] cylinder head temperature would become too high during take-off[d] take-off power would be severely reduced

Question No 22If the oil pressure gauge begins to fluctuate during flight

[a] the oil temperature is too high[b] the oil pressure gauge is unserviceable[c] the oil temperature is too low[d] the oil quantity is very low

Question No 23The cause of an abnormally high oil pressure indication could be

[a] oil quantity is too low[b] oil temperature is too low[c] oil temperature is too high[d] the oil sump is overfilled

Question No 24If the oil level in an operating engine is below the specified minimum

[a] the engine could overheat at high power settings[b] oil temperature would be lower than normal[c] engine power will be reduced[d] there will be a large power loss due to increased engine friction


Question No 25To prevent excessive cooling of an engine during a long descent at a fixed throttle setting it is necessary to

[a] decrease indicated air speed and accept a reduced rate of descent[b] increase indicated air speed and accept a reduced rate of descent[c] decrease indicated air speed and increase rate of descent[d] increase indicated air speed and increase rate of descent

Question No 26If oil temperature is rising to near the red line during a long climb a remedy would be

[a] decrease power and indicated air speed[b] increase power and indicated air speed[c] decrease power and increase indicated air speed[d] increase power and decrease indicated air speed

Question No 27A high cylinder head temperature during cruise could be due to

[a] manifold pressure too low for the selected RPM[b] mixture set too rich[c] cowl flaps left open[d] detonation or pre-ignition

Question No 28Spark plug fouling would be most likely during

[a] long periods of ground operation at low power[b] climbs at high power settings[c] cruising flight in cold weather[d] operation in conditions where carburettor ice is likely to form

Question No 29One consequence of operating an engine with excessively high oil temperature is

[a] Spark plug fouling[b] inadequate lubrication of some engine parts[c] a very high oil pressure[d] sticking exhaust valves

Question No 30Oil pressure is usually measured -

[a] immediately before the pump.[b] immediately after the pump.[c] immediately before the oil enters the engine.[d] as the oil returns to the sump.


ANSWERS TO EXERCISE GK 1

Comment

Pressure in a gas depends upon the number of molecules present ie mass of charge,and the speed at which each molecule moves ie temperature of the charge.Manifold pressure is the pressure outside the cylinder at the inlet port. The position ofthe throttle decides how much gas flows through the inlet manifold to the port. Thehigher the manifold pressure, the greater the mass flow of gas into the cylinder whenthe inlet valve opens.See page 1.1.9.Anything that decreases the temperature of the charge will help minimise the risk ofdetonation. The most immediate effect will always be achieved by placing the mixturecontrol into fully rich. This sends extra cooling fuel to where it is most needed - theinside of the cylinder.The explosion of the charge sends shocks through the engine which are felt as vibra-tions. The sudden release of the heat of combustion while the piston is at or near TDCconcentrates the heat into the cylinder head, causing the temperature to rise and powerto drop.As propeller RPM increase, the propeller tip speed approaches and may exceed thespeed of sound. This degrades the propeller's aerodynamic efficiency.As the engine heats up, the viscosity of the oil is brought into the range required foreffective lubrication.Engine temperature depends upon the rate at which heat is being generated [power],and the rate at which it is being carried away [IAS]. The extra fuel in a rich mixturehelps reduce the temperature of combustion.This is really the opposite to question 8 above. Increase the rate at which heat is beinggenerated and decrease the rate at which it is being carried away.The higher the octane rating of a fuel, the greater its ability to withstand compressionand heat without detonating.A horizontally opposed engine relies on oil being flung from the rotating crankshaft toadequately lubricate the cylinder walls.The only way to fire every cylinder during two rotations of the crankshaft is to fireevery second cylinder in the direction of rotation. See page 2.1.15.See page 2.1.17.The higher the viscosity, the 'thicker' the oil becomes. It resists flowing and spreading.Oil coolers are more correctly called oil temperature regulators. It is important towarm the oil when it is cold, just as it is important to cool it when it is hot.Airflow through the cooler is required to carry the heat away. If the airflow is inter-rupted, the oil temperature will rise. Hot oil flows too easily and eventually thepressure will become lower.The lower the quantity of oil in the sump, the more frequently it must circulate to carryengine heat away. The oil that is present will be come hotter.Higher RPM produce more engine power. However, high RPM reduces the propeller'sefficiency. A gear box allows the engine's power to be transmitted to the propeller inthe form of greater torque but lower RPM.See question No 17See question No 8Engine warm-up is essential prior to demanding take-off power. This not only ensuresthat the oil is the correct viscosity to properly lubricate the various components, butalso lessens the 'thermal shock' of large and sudden temperature increase.If oil level is very low, as the oil moves about in flight eg in turbulence, the oil pumppick-up screen becomes uncovered causing the pump to suck air at intervals. Thiscauses fluctuations in oil pressure. To cause this to happen, the oil level would have tobe critically low. Oil temperature would be very high.Oil has a high viscosity at low temperature. This offers a high resistance to the oilpump causing oil pressure to rise.

No Answer

1 [a]

2 [c]

3 [a]4 [d]

5 [a]

6 [b]

7 [a]

8 [d]

9 [d]

10 [b]

11 [b]

12 [a]

13 [d]14 [b]15 [c]

16 [c]

17 [a]

18 [c]

19 [d]20 [d]21 [a]

22 [d]

23 [b]


Comment

The lower the quantity of oil, the hotter that oil becomes. At high power settings, thesmaller quantity of oil must carry the extra heat away from the engine.[Assuming a cruise power-on descent at high IAS.] To reduce the rate at which heat isbeing carried away from the engine, the IAS would have to be reduced. Power wouldbe kept constant, so the rate of descent would decrease.To help cool the engine, the rate at which heat is being generated [power], must bereduced. The rate at which heat is being carried away [IAS], should be increased. Theaircraft would suffer a decrease in the rate of climb.Both detonation and preignition are accompanied by a marked increase in enginetemperature.Especially in horizontally opposed engines, the oil tends to 'pool' on the bottom of thecylinder causing plug fouling at low temperatures. The bottom plugs are usually theculprits.Very high oil temperature reduces the viscosity of the oil to the point where it nolonger maintains an unbroken film over the surfaces. This increases the possibility ofmetal to metal contact and rapid engine wear.Oil pressure is usually measured just before the oil goes to do its vital work i.e. justbefore it enters the engine oil gallery.

No Answer

24 [a]

25 [a]

26 [c]

27 [d]

28 [a]

29 [b]

30 [c]


Fig 2.1

Fig 2.2 A

BCdistance travelled forward

distan

ce tr

avell

ed by

the b

lade i

n the

plan

e of r

otatio

n

A

BC

The propeller blade's motion through the air however, is a little more complex than that of a wingbecause as it advances along the flight path, it also rotates about its own shaft.

The behaviour of the relative airflow about the blade is best understood by considering each ofthese motions separately. Consider a short period of time, say 1/100 sec and imagine a rotating

propeller advancing along the flight path.

Now ignore the forward motion and consider only themotion about the propeller shaft. In Fig 2.2 at left, the lineAB represents the distance a particular point on the bladetravels about the shaft in 1/100 sec. This line wouldrepresent the motion of that point if the aeroplane werestanding still.

The line BC represents the distance the aeroplane wouldmove forward in that time. So the point we are consider-ing would move the distance AB in the plane of rotationof the propeller shaft and the distance BC in the directionof flight.

Since it does both simultaneously, the resultant path isalong the line AC. The relative airflow approaches theblade from a direction opposite to its motion ie along theline CA. The resulting angle of attack is shaded in Fig 2.2.

PROPELLERS

A propeller blade is essentially a rotating wing. Like a wing, it is a cambered aerofoil whichadvances into the relative airflow at an angle of attack. In the case of the propeller blade, the totalreaction is resolved into two component forces called propeller thrust and propeller torque [Fig2.1].

relati

ve ai

rflow

torqu

e

total reaction

thrust

relative airflow

lift

drag

total

reacti

onWING

PROPELLER BLADE


2000 RPM

2200 RPM

2400 RPM

2600 RPM

2800 RPM

80 kt

100 k

t12

0 kt

140 k

t16

0 kt

angle of attack at2400 RPM and120 kt

Fig 2.3 shows a simple device which helps inexploring the relationship between propel-ler RPM, aeroplane forward speed [TAS]and the angle of attack on the propellerblade.

The angle at which the propeller blade is setto the plane of rotation is called the bladeangle. In small training aeroplanes, this an-gle is fixed by the manufacturer and cannotbe altered by the pilot.

The angle of attack however, is the result ofthe RPM and TAS and it does change inflight as each of those factors change.

Consider Fig 2.3. The vertical scale repre-sents the distance travelled by a particularpoint in the plane of rotation at variousRPM. The higher the RPM, the further ittravels. The horizontal scale represents thedistance the aircraft travels forward at vari-ous TAS values. The actual motion of theblades and the resulting angle of attack canbe found by pulling a piece of string acrossthe diagram as shown inFig 2.4.

If you would care to do some investigatingyou will find that for any given value ofRPM, an increase in TAS will cause a de-crease in angle of attack while a decrease inTAS will produce an increase in angle ofattack.

Also, for any given TAS, an increase inRPM will cause an increase in angle ofattack while a decrease in RPM will cause adecrease in angle of attack. Note that all thewhile the blade angle remains constant.

Note also that any given angle of attack canbe achieved at only one TAS for any RPMvalue. All of these relationships will becomeimportant when we investigate the operationof the constant speed propeller shortly.

Fig 2.4

Fig 2.3

2000 RPM

2200 RPM

2400 RPM

2600 RPM

2800 RPM

160 k

t14

0 kt

120 k

t10

0 kt

80 kt

Blade angle


Fig 2.5 As the angle of attack of the propellerblade changes, the total reaction changesand so the value of propeller thrust andpropeller torque change. Most importantly,the ratio of propeller thrust to propell

casa cpl gk jan 2004

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