Aircraft General Knowledge

Stress, Fatigue and Airframe Design

Structure fails at the ultimate stress and metals will fail at less after fatigue

Metals tend to fail from tension but composites “soft fail” as fibres break and can usually be detected

Civil airliners are “fail-safe” with redundancy, damage tolerant and components have safe life (hours/cycles)

S/N curve (stress/number) helps with forecasts and increasing all-up weight by 1% can increase fatigue by 5%

Minor: probability < , Major: prob. < , Hazardous: prob. < , Catastrophic: prob. <

Duralumin is 3-4% copper, ½-1% manganese, ½- 1½% magnesium and a little silicon. Soft/workable, restricted

by temperatures, poor corrosion resistance but can be covered with alclad (aluminium sheets) to reduce this

Magnesium alloys are less dense but low operating temperatures and high susceptibility to corrosion

Titanium alloys are expensive and strong and is used for parts such as engine fire-walls

Monel is an alloy of copper and nickel with small amounts of iron and manganese: exhaust systems

Honeycomb bonding used where cellular fill bonds between two metal sheets

Composites such as carbon fibre are now used, with high tensile strength to weight ratio

Corrosion from oxidation or electrolytical where metal becomes anodic (+ erodes) or cathodic (- added to)

A monocoque fuselage relies on the shape of the other skin of the aircraft for strength (ideally circular)

Semi-monocoque incorporates monocoque and some features of a frame structure (forms other than circular)

Reinforced shell is stressed skin with shape being defined by frames, bulkheads and stringers, but now

reinforced with longerons (as per truss construction). Openings are reinforced with straps called doublers

Floor panels are normally aluminium, Kevlar or a nomex honeycomb sandwich, suspended on crossbeams

Empennage (fin and tailplane) provide stabilisation, tailplane for pitch and vertical fin for yaw, which can be

designed asymmetrical so little tail rotor thrust is required in forward flight (and TR failure may not be critical)

Maintenance: Hard time: component replaced after a set number of hours/cycles/operations, On Condition: a

component is only replaced when it is deemed to be unserviceable or out of limits

Hydraulics

Systems are powered (active) and used to transmit and increase forces; simple ones without a pump: passive

Standard hydraulic high-pressure systems use 3000psi and low-pressure 2000psi

Pascal’s law states that in an enclosed container, pressure is equal throughout the fluid (perp. to walls)

Pressure = Force/Area and so force is increased by using a small input piston and a large output piston

A perfect fluid is incompressible (really, 10% reduction with 70,000psi is good enough for assumptions)

A passive hydraulic system has pressure produced by the pilot (sometimes with valve to trap pressure)

Simplest active system has a pump, a selector and an actuator (plus non-return valves)

The main pumps are usually driven from the accessory gearbox of the engine and are therefore known as EDPs

Engine Driven Pumps. Spur gear (force fluid between the gear teeth and the housing) and rotor type pumps in

small aircraft, piston in large. Electrically or air driven pumps are used as backups (or main on small craft)

Linear actuators: single acting act in one direction against spring force, double acting balanced have ram rods

of equal area on both sides so same force in each direction, double acting unbalanced has only one ram rod

and so gives different magnitude forces on retraction and extension

Rotary actuators are hydraulic motors with speed related to rate of fluid flow (can drive backup electrics)

Seals between moving surfaces are dynamic and have backing rings to prevent the seal from rolling out

External/internal leaks cause loss of pressure and temperature rise. External also loses fluid

A four-port rotary selector is used with a double acting actuator and allows two paths for fluids: flow into the

actuator, return flow to reservoir. Two-port rotary selector has one fluid path, used for single acting actuator

Spool/pilot valves: cylindrical and form a sliding but leak-tight fit, allowing flow or blocking by the valve ‘lands’

Open-centred system is low pressure for flaps/undercarriage and only one actuator can be used at a time.

When off, low pressure and fluid passes through selector centre. On: pressure up until selector auto turns off

PRV/TRV is a Pressure (Temperature) Relief Valve which opens at cracking pressure and resets at the reseating

pressure, below the cracking pressure. If it can relieve the total pump output, it is a Full Flow Relief Valve FFRV

Non-return valves only allow fluid flow in one direction, shuttle valves allow an actuator to be supplied by a

secondary system, one-way restrictors slow services that would otherwise move too quickly

Throttling valves ensure that flow rate is correct and flow control valves are used upstream of motors to

maintain constant speed (damper fitted) and hydraulic fuses upstream of components to shut off flow if there

is a leak. Priority valves PMV isolates non-essential services if pressure falls

Mineral based fluid – DTD 585 is red and can only be used with synthetic seals (most helicopters use this)

Synthetic base – Skydrol (common: purple 500A) and are used with butyl rubber, eth/propylene or Teflon seals

Overheated fluid will become dark and more viscous, and normally fluid is filtered downstream of the pump

with a Full Flow Micronic Pressure Filter FFMPF to remove debris wider than 25 microns

Fixed volume pumps are regulated (and reduced wear and tear) by ACOV Automatic Cut-Off Valve and

accumulator must be used. Cooling and lubrication is by fluid leakage

ACOV uses a poppet valve to create an idling circuit and when open, the ACOV is “kicked-out”, The kick in

pressure is lower than the kick-out pressure and a leak can cause hydraulic hammering

Accumulators: store fluid, provide limited pressure in emergencies, dampen fluctuations, allow for thermal

expansion and cater for small internal leaks. One side is fluid, the other nitrogen gas at min pressure

With variable volume/constant pressure pumps, an ACOV is not required

Volume of pistons can be altered and is driven by the engine via the ancillary gearbox (not angled)

Maximum stroke occurs with spring pressure and minimum with system pressure

Pressure transducers sense pressure and convert the signal into an electrical output

Reservoirs hold a reserve of fluid and allow it to be de-aerated and level checks should only be done with the

system at rest. The standpipe preserves a reserve supply for emergency use

Reservoirs can be vented to the atmosphere (could boil at high altitude, leading to pump cavitation or gas

trapped in the lines/actuators) and so pressurised reservoirs are now usual

If air pressure (bled from the engine) is lost, required to be able to supply 70% of normal pressure

High pressure systems allow smaller actuators and so smaller bore piping to be used

Landing Gear, Wheels, Tyres and Brakes

Skids are normally made from Aluminium alloy and can be fitted with steel shoes to prevent excessive wear

In an oleo pneumatic shock absorber strut, gas supports the helicopter at rest, acting as a spring to absorb

landing/taxiing loads; oil (mineral DTD 585) damps the landing load and recoil action

Older light helicopters use a metering valve and orifice in an un-separated shock absorber. Heavier helicopters

use ones with separators to ensure gas and fluid don’t mix

A heavy landing can result in the oleo strut bottoming out when piston rod travel exceeds distance available

Torque links prevent inner piston rod rotating inside the cylinder and worn torque links can cause shimmy

Sideways retracting main gear have side stays and fore/drag stays to prevent collapse. Gears that retract fore

or aft only require fore and aft bracing. Squat switches prevent gear for being moved on the ground

Geometric lock (over centre) requires mechanical force to unlock it and hook locks engage without hydraulic

pressure but require it to release (and usually used as up-locks, with geometric for down)

Gear selector is shaped like a wheel and a horn goes off if gear is up below certain height

Fully castoring nose wheels are steered by differential braking and anti-torque pedals and some helicopters

have a dedicated hydraulically powered nose wheel steering system

Light heli: passive hydraulically operated single disc brakes (like a bike) and if a brake becomes too hot, braking

action reduces (called fade). Brakes with steel discs work best when cold; carbon fibre work better when hot

Large: multi disc brakes, powered by main hydraulic system (or use a pressure reducing valve downstream of

the brake accumulator)

Automatic break wear adjustors ensure adequate clearance between the rotors and stators with brakes off

Distance between the torque plate and pressure plate is indicated by how much the retraction pin sticks out

Well based tyres are used on motor vehicles but heli tyres are so stiff that they cannot be fitted over these

Detachable flange wheel: when tyre fitted, flange is held in place by a locking ring and secured by pressure

Split hub is two parts which are cross-bolted together with sealing ring for tubeless tyres, along with lacquer to

seal the cast alloy wheels. Knurled flanges assist in preventing the tyre slipping around the wheel

Tyre is bead, which is formed around a series of steel wires, layered with nylon/rayon and these layers build up

to form the carcass. The tyre’s strength is indicated by its ply rating. If carcass layers are laid over each other,

cross ply; if in direction of travel, radial ply (add belts of steel/Kevlar for extra strength)

Crown: tread patterned area, shoulder, sidewall, bead: strong part of tyre designed to grip the wheel rim

With a new tyre fitted, it takes up to 5 landings for the bead to bed against the knurled wheel flange properly;

during this time, friction can cause circumferential movement of the tyre compared to the wheel, called creep

Moulded into rubber sidewall: max ground speed, ply rating, recommended operating pressure, awl marks

(green/grey) where small holes were made to let out gases, red spot indicating lightest point of the tyre

Circumferential grooves: longitudinal stability, water clearance; block tread: rough/unmade strips (all weather)

Marstrand tyres reduce shimmy and can be used until middle touches ground; chined tyres reflect water/spray

For a grooved tyre, minimum tread is 2mm and for block: until tread is just discernible

High tyre pressures cause crown wear and low causes shoulder wear. Cords showing is pretty bad

Aquaplaning/hydroplaning: water can’t all be cleared and forms a wedge in front which lifts tyre

Speed of aquaplaning (KT): 9 for rotating wheel, or 7.7 on non-rotating

Flotation devices can self-arm over water and deploy with water sensors; usually contain helium gas

Pneumatic Systems

Constant supply of air usually from engine driven compressor pumps: heating, instruments, hot air anti-icing,

de-icing, air starting systems, hydraulic reservoir pressurisation

Bottled Nitrogen (or other inert gas): fire extinguishers, life jackets, flotation equipment, life rafts

Systems which use the air once and then dump it overboard are called total loss systems

Adv: Less fire risk, readily available, pressurised gas as shock absorber, compressed gas occupies minimal space

Dis: not usable in quick response systems, difficult to seal in, leaks are difficult to trace

Engine bleed air for LP/HP compressor of a gas turbine engine (low pressure and available in high volume)

Engine bleeds reduce power output so often selected off during take-off (also with supercharged engines)

Bleed air is ducted into the bleed air manifold (gallery of pipes). Some older turbine aircraft didn’t have enough

bleed air for pneumatics and so has a dedicated compressor, driven from the engine’s auxiliary gearbox

Some gas turbine engines are started by directing bleed air onto an air driven starter motor (LPAS Low

Pressure Air Starter) with air obtained from APU, a ground supply or from engine already running

Life vests/rafts are usually carbon dioxide and fire extinguishers are pressurised with nitrogen

Air Conditioning

Conditioned air is air that has been controlled in respect to temperature, pressure and purity

Ambient air used for cold and hot air is heated round the exhaust (danger of CO poisoning)

Combustion heaters use fuel: pressurise air, mix with fuel, ignite with spark plug and fan driven

Larger turbo charged piston engines can use some of the ‘deck’ pressure air (outlet pressure) for heating

Engine driven roots blowers compress air into small pipes to increase temperature but performance suffers

Bootstrap: heat exchange with ram air and making charge air do work across a compressor/turbine assembly

Primary heat (bootstrap) exchanger cools air, then the (2ndary)CAU Cold Air Unit heats and re-cools it as before

An ice screen can be fitted after the (secondary) drastic cooling to prevent ice impacting in water extractor

Vapour cooling cycle uses a refrigerant, Freon, which evaporates, drawing out latent heat, leaving it heated

before being condensed by cool ram air. Freon pumps are driven by the main gearbox

Air ducts: stainless steel for high pressure/air temp, light alloy for medium and plastic/fibreglass for low

Fuel Systems

AVGAS aviation gasoline for piston, AVTUR aviation kerosene for turbine

Volatility: ability to change into a vapour, Flash/flame (or fire) point: lowest temp at which there is sufficient

vapour to support a momentary/continuous flame, AIT is Auto Ignition Temperature

The higher the octane rating, the higher the resistance to detonation (knock) and is expressed as two numbers

where the lower is resistance to knock with weak mixture (normally referred to) and higher with rich mixture

AVGAS 80 grade: red, older, low-powered engines, 100 grade: green, higher-powered engines, 100LL: blue and

low lead (tetra-ethyl lead) and used in high powered engines to reduce fouling

MOGAS is motor gas and can legally be used is some low powered piston engines. Above 25%, not above

6000ft or fuel tank temp above 20 , more carburettor icing and must be recorded in tech log

Kerosene is mixed with additives to bring freezing point down; JET A: -40 and common JET A1: -47

Wide cut fuels are 70% gasoline, 30% kerosene and not approved in EASA: JET B, JP4, AVTAG DERD 2486

Fuel specifications: ASTM D 1655 for JET A/A1/B and Defence Standard 91-91 for JET A1

Rigid, flexible and integrated (in wings) fuel tanks, sometimes with baffles to prevent sloshing about

Vapour locking can reduce/stop fuel flow so auxiliary fuel pumps are used to increase pressure

Vented so air can replace fuel used, with shut-off assembly to stop spill post-crash or during manoeuvres

Large: fuel must continue to feed the engines in the event of system failure and flame arrestors for lightning

Fuel tanks have two LP booster pumps are centrifugal pumps supplying fuel at 20-100psi

LP pumps are located in feeder boxes at the bottom of fuel tanks with flapper valves (one way)

Bypass valves allow fuel to flow if the LP pumps fail (pressure sensor just before engine HP pump)

Jet pumps can be used to move large volumes of fuel from tank to tank using the venturi principle

Crossfeed valve closed for take-off and usually in flight, and activating turns LP pump of lower fuel tank off

Jet aircraft systems usually heat the fuel at the engines just before the fuel filters and cool the oil FCOC

Capacitance systems preferred to resistance as they indicate fuel mass/not volume; fuel has twice the

capacitance of air and if there is water in the tank, full scale fuel shown; fuel gauges show 0 after failure

Drip stick have a hole at the top and pulled down until dripping flow; magnetic sticks similar

Fuel jettisoning uses LP booster pumps to spray fuel into the air, whilst keeping enough to climb to 5000ft and

cruise for 30 minutes at best range engine power

Most helicopters refuelled through filling points at the top, called gravity refuelling. Some large helicopters are

fuelled from below and max pressure for this is 50psi with maximum suction for de-fuelling as -5psi

Fuel must be sampled before the first flight of every day for clarity, sediment, water and air

Water detecting paper is left in the fuel for up to 10 seconds and turns green if water is present

A six metre radius refuelling zone is taken from the open refuelling point/connection and any vents

Helicopters are refuelled before standing overnight to prevent condensation; full: fire risk, empty: explosion

Ice and Rain Protection

EU-OPS prohibit starting a flight under known/expected icing conditions unless heli certificated and equipped

Icing conditions: TAT below 10 either with visible moisture in the air or visibility below 1500m

Vibrating rod system at 40KHz until ice forms and slows it down, when warning light activates and rod heated

Pressure operated detectors have small hold on the leading edges of aerofoils (most ice build up) and note

when a change in pressure due to holes icing up occurs

Serrated rotor uses an electric motor to turn a rotor which has grooves in it next to a fixed knife

Piston engines use warm air de-icing of the carburettor and jet helicopters for anti-icing the intakes

Engine bleed air system directs hot bleed air to the compressor to prevent ice accumulation on the front frame

Rotor Ice Protection System RIPS: main rotor uses de-icing (symmetrical shedding) and tail uses anti-icing

Electric heating mats can be used for windscreen de-icing and a fan is provided for de-misting

Pitot tubes have electrical heating and static may do (with warning lights to show when heater/power fails)

Basic Electrical Theory

Electricity flows from negative to positive but is conventionally shown the other way round

Semi-conductors in their natural state are insulators and V=IR, Power = IV or

In series, sum of resistances; in parallel, inverse sum is sum of the inverses

Ammeters detect current (series) so have low resistance as do voltmeters but with high resistance (parallel)

Coulomb is unit of electrical charge and is equal to roughly 6.21 x electrons, electric field strength is in

Newton’s per coulomb (or volts per meter) and direction of field: direction of force acting on a positive charge

Direct Current Electrics

Primary cells are not rechargeable whereas secondary cells are, at a voltage of 112% of the battery voltage

Primary cell is positive carbon plate, ammonium chloride gel electrolyte and zinc shell/negative plate and 1.5V

Secondary cells are lead acid on nickel cadmium (NiCad)

Lead acid: lead peroxide positive plate, lead negative and dilute sulphuric acid and 2.2V or 2V with a load

Fully charged is 1.25-1.3 with a hydrometer, float reading low 1.2-1.24 and discharged 1.17. They need regular

maintaining/charged. NiCad are 1.3V (1.2V load on) and hold constant voltage. Recharging too fast can boil the

electrolyte and damage the battery, called thermal runaway (can also happen to lead acid but less severe)

Battery performance decreases in cold weather (internal resistance increase and so current flow reduces)

Helicopter battery voltages are either 12V or 24V. In series, same current, voltage total and same amp hours

In parallel, same voltage, same current, but increased amp hours so parallel to increase capacity

Modern helicopters use a single pole, negative earth system (less wiring but can short)

Busbars are used to distribute power and are usually heavy copper bars and there may be more than one

Solenoids operate low torque valves and switches and doll’s eye indicators use white to attract attention

Relays are electromagnets which are used to switch other electrical circuits and can be done remotely

Fleming’s left hand rule: Point field, middle current, thumb motion. In a motor, the armature carries current

and field coils are wrapped around the iron cores to create electromagnets for control

Series wound DC motors have high starting torque, parallel/shunt wound low and compound uses shunt for 60

to 70% and series for the rest, offering a wide range of torques. Split field DC motors are reversible

Rotary actuators operate through about 350 and use reversible, series wound DC motors and limit switches

turn the motor off and change the direction of the field. Used to open fuel cocks and butterfly type valves

Linear actuators/inching controls use a DC motor turning a screw jack to give precise control over the position

Generators use Fleming’s right hand rule (motors was left hand)

Simplest generator use permanent magnets but can only be used for small, low output devices. Separately

excited generators use an external power source for the field windings. Self excited ones rely on residual

magnetism in the iron core to start and if this is lost, can be field flashed by passing a current through

Voltage of a shunt wound generator decreases with more load, series increases, so use compound wound

In compound, shunt is lots of thin windings, whereas series is a few thick, heavy windings

Carbon pile regulator: pile compressed means resistance decreases (in series with the shunt field coil)

Vibrating contact regulator open/closes 50-200 times/second to increase/decrease resistance, hence voltage

In twin engine, busbar distribution and equalising circuit to regulate voltage. Load shedding, isolating of the

failed generator, is required to reduce the load on the remaining generator

Generator voltage is higher than battery voltage to stop reverse current, plus a relay to disconnect generator if

voltage drops, often uses a switch to manually disconnect generator/busbar called generator breaker/cut out

Occasionally, same machine is used as an engine starter motor and then switches to a generator

Vital services connected to hot/direct busbar connected to battery, others are essential and non-essential

Fuses replaced and circuit breakers can be reset once in flight, red fuse: manual reset, yellow/white: isolators

Trip free circuit breakers cannot be held in to make a faulty circuit. Bimetallic ones heated and if too much so,

bend and break away from each other. These take time so magnetic ones act on electromagnet in breaker

Extra 10% of fuses must be carried, metallic tape stops static and discharged through static wicks

Alternating Current Electrics

The RMS voltage has the same heating effect as the equivalent DC voltage and peak = RMS x

In AC generators, the commutator is replaced by slip rings which collect the current induced in the loop

AC generators are more efficient and sometimes combined with internal rectifier to power DC (alternators)

Frequency is RPM times pole pairs divided by 60 (pole pairs being pairs of magnets)

Brushless alternators are most common and voltage output is controlled by the current in the field windings

A typical three-phase aircraft AC supply will run at 115V RMS and alternators at 14V (DC through rectifier)

AC generators have field windings on the rotor with current induced in the windings on the stators

AC generators: more flexible, lighter, better power:weight, easy to change voltage with transformers, easy to

convert from AC to DC, brushless so less maintenance, produce voltage at a lower RPM than DC generators

Alternators are star-wound (connected at one end to a wye/neutral point and the other end to an output,

through a load and back via a return line. Voltage between a live line and the return is a phase voltage and

voltage between two live lines is line voltage. Line voltage is x phase voltage, line current = phase current

A failure in one phase affects all others as current in the others will no longer be balanced

With capacitors, current leads voltage by 90 . Capacitance is proportional to area of plates/distance

Capacitance in AC leads to reactance in ohms, and is inversely proportional to frequency

The induced current is inductors opposes movement, measured in Henrys and lags behind voltage

Inductors have a reactance which is directly proportional to frequency

CIVIL: In Capacitors, the current I leads the Voltage which leads the current I in inductors L

Impedance is the vector sum of resistance and reactance

Inductors act like resistors in a circuit, whereas capacitors work the other way round (as per parallel)

Power factor is the useful work divided by the total work

Frequency wild when there is no RPM control and so can only be used in lighting and heating: 208V, 22kVA

To provide a constant frequency generator, a constant speed drive unit (CSDU/CSD) is used and consists of a

hydraulic/mechanical unit, connected to the engine by a dog clutch release mechanism: CSD disconnect

Frequency is usually (quoted 400Hz) between 380-420Hz driven from 6000RPM

For paralleling, frequency adjusts real load and reactive load is controlled by voltage (trimmed)

APUs are small gas turbine engines, fuelled from main fuel supply that can provide a power to drive a three

phase generator to produce 115/120V AC at 400Hz; constant speed APUs paralleled and started electrically

Synchronous AC motors are low torque, constant speed, not self-starting; speed related to voltage frequency

Induction motors use coils of wire instead of a magnet. Difference between rotors speed and stator field is

called the slip speed, reducing efficiency, so best with light loads: fuel/hydraulic pumps, gyro rotors and torque

motors, AC actuators. Single phase motors will not be self starting but three phase will

A faulty induction motor will slow down or stop and if a short circuit occurs, motor will overheat and slow

Delta system: voltages are equal but line current is x phase current (motors, rotary rectifiers/transformers)

Transformers step up/down AC voltage and twice the turns means twice the voltage but half the current

Auto transformers tap out at the right number of turns for a specific current (for aircraft lighting)

Rectifiers allow current flow in only one direction and 6 diodes are used for three phase full wave rectification

TRU Transformer Rectifier Units are used in an AC system to power the DC busbars and charge the batteries

Transistors are low current semiconductors, using collector, emitter, base, with base controlling the current

In a split busbar system, each engine driven generator feeds a busbar and they are dis/connected by the GCB

Generator Circuit Breaker (crew or GCU Generator Control Unit)

A BTB Bus Tie Breaker is fitted between AC busbars and when closed on the ground, allows both to be

powered externally or from APU generator. Can be closed in flight to allow one generator to power both

Paralleled generators supply tie busbars and the load is shared equally between generators

GCU monitors power quality and consists of relays sensitive to generator frequency and voltage

Generator fault will cause: open GCB, disconnecting generator from its busbar, trip the exciter relay, cutting

output, then: split: close the BTB to power both busbars from main generator, parallel: open BTB to isolate

Logic Circuits and Computers

A flip flop is a semiconductor with two equally stable states; Inhibited/negated gates have one input negated

Bit is a binary digit, 32 bits make a word and a byte is a unit of storage capacity

Integrated circuit is layers of materials (MOST Metal, Oxide, Semiconductor Technique) diffused with doping

elements and etched into the correct shapes. The base P material is called the substrate

Von Neumann: Input/output facilities, processing unit, memory, programs/data sharing memory, program

steps carried out sequentially, processor and memory joined as a pair

CPU comprises the control unit, ALU Arithmetic Logic Unit and memory (made of registers, 32 bits long, fast)

RAM is volatile. DRAM is Dynamic and slower than static RAM, as it requires refreshing every few milliseconds,

but its simplicity allows four times the density

EEPROM is Electrically Erasable Programmable Read Only Memory

Engine Basic Principles

Boyle’s Law: volume of a gas is inversely proportional to its pressure at constant temperature:

Charles’ Law: volume of a given mass of gas at constant pressure is directly proportional to its absolute

thermodynamic temperature: and so combined gas law is: (think pV = nRT)

At a constant temperature, pressure will double if volume is halved

Work is force x distance and power is rate of work, so force x distance / time

Power to weight ratio (specific power output) is measured in kilowatts per kilogram kW/kg

One horsepower is 746W. IHP Indicated Horse Power is theoretical power produced and is calculated from the

pressure developed in the cylinders but takes no account of the work that must be done within the engine

Friction Horse Power FHP is the work done within the engine to drive the mechanical components

Brake Horse Power BHP or shaft horsepower is the power delivered to, for example, the rotor shaft

BHP is IHP minus the power lost through friction and compression

Efficiency is the ratio of useful power produced to the theoretical power produced (modern piston about 80%)

Piston Engines

Atmospheric air is sucked in when mixed with fuel, creating the mixture. Products are exhaust gas

Gasoline fuel is mixed with air and introduced to a cylinder through an inlet port. The inlet port is then closed

by an inlet valve. The cylinder is sealed at the other end by a moveable, but gas-tight piston. The mixture is

ignited by an electric spark plug. As combustion takes place, the hot air attempts to expand, but is confined by

the cylinder. This causes a marked increase in pressure, which forces the piston down the cylinder. The piston

is connected to a crankshaft that converts its linear motion into rotary motion (the up and down motion of the

piston is described as reciprocal movement). The natural momentum of the piston and crankshaft cause the

piston to travel back up the cylinder. As it does so, an exhaust valve opens to allow the products of combustion

to be driven out through an exhaust port. The crankshaft is either connected directly to the rotor or indirectly

through a gearbox. Piston is connected to the crankshaft by a connecting rod, off-centre from the centre of

rotation. The fuel and air are mixed in a finely measured ratio and the inlet/exhaust valves are opened by push

rods and a camshaft and then closed by spring pressure

Four strokes (suck, squeeze, bang, blow) form the four-stroke cycle or Otto cycle: Induction to draw mixture

into the cylinder, Compression to maximise the pressure produced by combustion, Power to drive the piston

down the cylinder for mechanical power, Exhaust to expel waste gases from the cylinder before restarting

Stroke is distance the piston travels between highest TDC Top Dead Centre and lowest BDC BottomDC points

Induction: inlet valve opens at TDC, cylinder moves down, and pressure (and temp) reduce, causing mixture to

be sucked into the cylinder. The inlet valve closes when piston is at BDC. This is a normally aspirated engine

Compression: pressure and temperature of mixture increases with lower volume. Spark plug ignites fuel at TDC

Combustion takes place between compression and power strokes. In the theoretical cycle, this occurs

instantaneously whilst piston is at TDC and there is no volume change during combustion. Sometimes known

as constant volume engines. Combustion in a confined space produces large pressure/temperature increase

Power: cylinder is forced down and as volume increases, pressure/temperature reduces. The average pressure

exerted on the piston is the Mean Effective Pressure MEP and is used in calculating theoretical power

Exhaust: at BDC, exhaust valve opens, pressure and temperature increase and products forced out

Compression/expansion are adiabatic as pressure changes but no heat energy added/removed

Other two as isochoric processes as pressure changes but no change in volume

Close to TDC/BDC, little linear movement though angular rotation speed of the crankshaft remains unchanged

and this is known as the ineffective crank angle, where there is little change to cylinder volume/pressure

As the exhaust stroke comes to an end but before TDC is reached, the inlet valve opens, “valve lead”. It also

remains open beyond BDC, “valve lag” but closes before the piston passes ineffective crank angle. These two

help maximise amount of mixture entering the cylinder. Lead/lag measured in degrees of crankshaft rotation

Exhaust valve is opened before BDC before the end of the power stroke: exhaust valve lead, plus lag as it

remains open slightly after TDC to allow last bits of exhaust gas to exit under their own momentum

At the end of the exhaust stroke, when both valves are open, the exiting of exhaust gases helps to draw in the

fresh mixture, called scavenging, and this overlap period is called valve overlap

Valve timing remains unchanged regardless of engine RPM (dictated by shape of cams on camshaft)

Ignition is timed to occur just before TDC so that combustion is well under way as piston starts to move down

This is called advanced ignition and the ignition is further advanced as engine RPM increases

At low RPM, ignition is retarded which could damage the engine/force the crankshaft the wrong way, so

during start, the ignition is usually retarded to around TDC

As power is only produced in a quarter of the strokes, four-cylinder engines are often used, so firing interval is

found by taking 720 divided by number of cylinders (as cycle takes two crankshaft rotations)

Classified by: number of cylinders, arrangement, method of cooling, type of fuel and air induction system and

the rotor drive arrangement (eg. In-line, radial, V engine and horizontally opposed engines)

Cylinder is made of barrel and head bolted together. Gasket between the two ensures they are airtight. Barrel

is steel and head alloy. Air cooled engines with fins on the side and liquid cooled have galleries in the casting

Two spark plugs per cylinder for redundancy, plus quicker burn when ignited from both sides

Top of piston is the crown and is tapered slightly; lower part is called the skirt. Compression rings are fitted

around the top to prevent gas escaping, made from cast iron with self-lubricating carbon. Oil control rings to

ensure that oil is spread evenly over the cylinder’s inner surface and an oil scraper ring is fitted to the piston

skirt to draw excess oil back into the crankcase

Hinged gudgeon pin connects the rod to the piston (small end) and crank pin to the crankshaft (big end)

Crankshaft lies between the crankcase and is supported in bearing journals. Need to be strong so made from

heavy alloy steels, but sometimes hollowed to save weight and to allow cooling lubricating oil to flow through

The distance between the centre of the crankshaft journal and the centre of the crank pin is the crank throw

Crank throw times two is the stroke distance, so short stroked engines save on mass

Counterweights are used to counteract the unbalancing effect of the piston and connecting rod

Crankshaft, connecting rods and pistons are collectively known as the crank assembly

Camshaft driven through gears by crankshaft and rotates at half the engine crankshaft speed; operates valves

Crankcase is main structural element and halves are cast from a light, strong alloy, such as magnesium; it

provides mounting points for the cylinders, the engine (to the airframe) and an accessory gearbox, which

drives fuel, oil, hydraulic and ignition systems (and sometimes the engine starter motor is fitted to gearbox)

Poppet valves close under spring pressure and inlet valve is usually larger than the exhaust valve; made from a

special steel alloy for use with high thermal/mechanical stress and exhaust valve may contain sodium (cooling)

Two springs in opposite directions fitted to prevent valve bounce and any rotation tendency

Tappet between camshaft and push rod for smooth movement, plus a tappet clearance to allow for thermal

expansion. Correct tappet clearance ensures valves close completely at all operating temperatures (too much

clearance will prevent fully opening valve) and incorrect clearance can cause significant performance reduction

At height, less mixture drawn in so air can be forced into the cylinder using a mechanical compressor. Engines

which use these are supercharged. Pressure of mixture in measured in the inlet manifold before introduction

and displayed on MAP gauge (inHg)

With engine stopped, MAP displays atmospheric pressure, drops when engine starts and on a normally

aspirated engine, MAP always runs lower than atmospheric pressure, but supercharged can be above

Force on piston is PLAN/E (pressure x length x area x number of cylinders / effective power strokes)

Power is a function of MAP and RPM so must both be monitored; power = torque x RPM and is measured at

the gearbox between the engine and the rotor. Torque can replace PLA in PLAN/E

Mechanical efficiency of an engine is up to 80% (actual power at shaft compared to theoretical power)

Modern engines about 33% thermally efficient (ratio: work done by engine to the mechanical equivalent of the

heat energy released by combusting the fuel) and is independent of altitude, increases with compression ratio

Clearance volume: TDC to top of cylinder, swept volume: TDC to BDC, total volume: clearance + swept

Compression ratio is total volume to clearance volume, usually between 7:1 and 9:1 but if too high,

temperature increase can cause detonation. Displacement = piston area x stroke x number of cylinders

Volumetric efficiency is the ratio of the mass of mixture induced into cylinder to the mass of air (at standard

temperature and pressure) which would otherwise fill the swept volume

Influenced by: ease of entry of mixture, density of air, ease of leaving of exhaust gases

Normally aspirated, volumetric efficiency increases with altitude (drop in exhaust back pressure) and RPM

Specific Fuel Consumption SFC is the mass of fuel for a given power in a given time: kg/kWh

Moving engine parts must be lubricated with oil to reduce friction and wear and can also be used for cooling,

cleaning, protection from corrosion and used for hydraulic operations

Higher oil number is higher viscosity, and engine oil must: maintain viscosity, have a low evaporation rate,

inhibit corrosion, not react with materials it contacts, discourage the formation of sludge

Straight oil has no additives and is used on new engines to reduce the running-in time

In a dry sump system, oil is stored in a tank mounted remotely from the engine, but wet sump stores oil in the

bottom of the crankcase

Dry sump: oil is drawn from the tank by an engine driven pump with a suction filter just before the pump. The

pump forces oil through a second high-pressure filter to various strategically placed jets and drillings servicing

the major lubrication points. Highly loaded points such as the big end bearings are fed with high pressure oil

60-70psi and lightly loaded components such as the camshaft are fed with low pressure oil via a pressure-

reducing valve. Spent oil falls to the sump where is it collected by a scavenge pump which pumps the oil back

to the oil tank, via an oil cooler (more powerful than high pressure pump to ensure oil doesn’t pool in sump)

Most oil tanks have a hot well for less viscous oil after start-up until the rest of the oil has heated up

Coarse filters are fitted before pumps to protect them. Magnetic plugs/chip detectors are fitted in the oil

return lines which catch ferrous particles to be used for engine condition monitoring

Oil pumps are engine driven and normally use a spur gear system; scavenge pump has twice the capacity of

the pressure pump and pump output increases at higher engine speeds, so regulated by pressure relief valve

Fine high pressure filter removes small particles, but has a bypass valve to ensure sufficient flow if filter blocks

Oil cooler fitted in the scavenge line, made of a matrix of galleries with fins attached and an anti-surge valve

allows oil to bypass this whilst it is cold and viscous, so as not to block the cooler with “oil coring”

Oil temperature game measures after the cooler and before the pressure pump. Oil pressure measured at the

outlet of the pressure pump and normally will start to be indicated within 30 seconds of engine start

Wet sump has no scavenge pump, scavenge line or oil tank and may need more oil to ensure adequate supply

Wet sump can over-lubricate at high RPM and the oil used for splash lubrication isn’t filtered/cooled

Dry sump oil must be checked after shut down before oil drains back into the engine

Wet sump oil should be checked 20 minutes after shut down to allow oil to drain back into the sump

Oil leaking into the engine/cylinders can cause hydraulicing (inverted/radial engines). Oil can pool between

piston crown and cylinder head. Common to remove spark plugs from lower cylinders to allow oil to drain or

manually turn the engine before starting to check for signs of this

Air cooled engine: air inlet directs air over engine with fins to increase external surface area. Cylinder baffles

ensure each cylinder equally cooled. When hovering/slow, engine driven impeller draws air through a scroll

assembly, using baffles to direct the cooling air around the engine

Highest temperatures at cylinder head so CHT sensor fitted to hottest cylinder with thermocouple

Normally, CHT affected by: amount of power being produced, aircraft’s speed (mass flow of cooling air),

temperature of cooling air and ratio of mixture

Lean/weak mixture increases CHT because it burns more slowly so engine overheat is most likely at low speed

and high power (extended climb/hover) and overcooling at high speed and lower power

Modern engines are equipped with an electric starter motor, powered by the battery. During the start, the

motor drives the engine either via a gearbox or by engaging a toothed gear on the engine flywheel

As soon as the engine starts, an overrun clutch disengages the drive from the motor and if the amber starter

warning light stays on for more than 30 seconds, starter motor may have failed to disengage

As the engine begins to rotate it drives one or more magnetos, generating high tension electricity to power the

spark plugs

The purpose of the ignition system is to provide sparks of sufficient strength and duration at the right moment

in the cycle to ensure efficient combustion. Critical components of the ignition are duplicated

Magneto is an AC generator, transformer and distribution system providing high current voltage to spark plugs

independently of the aircraft battery/electrical system. Generator section comprises a 4-pole rotating

permanent magnet, surrounded by a U-shaped stator made of soft iron. Primary winding (low tension circuit

LT) is wound round the stator and comprises a few thick turns of wire. Secondary winding (high tension) is

then wound over it. Primary winding circuit continuously broken and remade by a contact breaker, creating

pulses and surges in the secondary windings so high voltage pulses fed through distributor to spark plugs

Capacitor in parallel to stop arcing and the HT current is sufficiently large to jump the gap between the spark

plugs’ electrodes just as the contact breaker points open. Engine drives contact breaker and distributor rotor

by a spindle so speed of rotor and operation of contact breaker vary directly with engine RPM. The spindle

always rotates at half the crankshaft RPM

A drop of about 300-400RPM is a sign of a malfunctioning spark plug and dead-cut should be tested during

shut down: turn off magnetos and back on and engine should start shut down then resume normally

During start, magnetos need help to get going: impulse comprises a spring-loaded clutch mounted between

the drive spindle from the engine and the magneto shaft, winding and spinning the magneto. HT booster coil

induces a large current in a secondary winding and voltage is fed to a trailing brush on the distributor arm. LT

booster coil uses battery power to increase magneto output

HT leads from the distributor supply each spark plug at 30,000V. Plug is an insulated electrode in the centre

with a smaller electrode attached to the base and the spark is generated across the accurately sized gap. Oil,

carbon or lead deposits can block the gap, causing failure to spark, known as plug fouling

Fuel: significant energy contained, must be able to form vapour but not too readily, must be able to burn

quickly but with an element of control to give a manageable rise in pressure

Calorific value is energy per unit mass and flame rate is in ft/sec (avgas optimum of about 60-80ft/sec)

When fuel detonates/knocks, energy given up as heat and shock, reducing power of engine and running rough

Detonation causes: poor fuel, weak mixture ratio, raising temp before burning, high engine temperature

Detonation is extremely rapid spontaneous combustion of part of the mixture, after burning has begun

Pre-ignition is ignition of mixture before the spark plug has fired caused by local hot spots or weak mixture

Ratio of air to fuel for complete combustion is 15:1 by mass, which is stoichiometric or “ideal mixture”

With a 10% richer mixture, detonation is less likely as excess fuel absorbs some latent heat

Typically vary from 8:1 (rich) to 20:1 (lean) and at high power settings the risk of detonation increases so

mixture is further enriched to about 20%. At low idling RPM, exhaust gas isn’t all taken out and so mixture is

weakened so require enriching. Ground running with rich mixture can cause spark plug fouling

Carburettor or fuel injection systems infer the amount of air entering the engine induction system and add

sufficient fuel to maintain the correct mixture ratio

Carburettor: with engine running, fuel drawn up through a U-tube and atomised to mix with air flowing

through. Naturally regulated as at higher RPM, more air drawn through, hence more fuel

Fuel level in the float chamber is regulated by the float and is pumped under pressure to the carburettor to

stop vapour lock. Throttle butterfly valve controls the amount of air passing through the throat

Pressure balance connects intake and float chamber to ensure correct mixture across a range of airspeeds and

flight conditions. Diffuser maintains mixture ratio across a wide range of engine speeds

At altitude, progressively richer mixture is created, so mixture control (by pilot or aneroid capsule)

Slow running jet ensures sufficiently rich mixture at low RPM and idle cut-off assists engine shut-down

Power jet or economiser provides a rich mixture at high power settings to assist cylinder cooling

Rapidly opened throttle requires an accelerator pump to inject fuel into carburettor as the system catches up

At low power settings, butterfly causes a large drop in pressure/temperature and can cause icing which may

stop the engine. Can happen at moderate/high levels of humidity and any temperature

Symptoms: loss of RPM, reduced performance and inlet manifold pressure, decreased EGT, rough running and

a jammed throttle. Heater uses unfiltered air and reduces engine power, could bring carburettor into a worse

icing range, could cause ice already accumulated to break off into the engine, use of heated air at high power

could cause detonation, hot air is less dense and so will enrich the mixture

Carb heat check before flight is a 100-200RPM drop with carb heat on at about 75% RPM

Fuel injection systems: even and reliable fuel delivery, better icing protection and quicker throttle response

Direct injection systems inject precisely measured quantities of fuel into the cylinders

Indirect deliver fuel to the inlet ports and comprise: engine driven pump which supplies fuel at pressure to the

injector assembly, electrically driven pump (same), injector assembly which adjusts fuel flow according to

throttle position and mixture setting, manifold valve which distributes fuel from the injector to the fuel

nozzles, fuel nozzles which spray finely atomised fuel into the inlets of each cylinder

Monitored by measuring fuel pressure at the outlet of the fuel manifold to provide fuel flow information

To help engine starting, particularly from cold, very rich mixture is needed but over-priming can cause a fire in

the engine intake, fouling of spark plugs or may wash the lubricating oil off the cylinder walls

Air is drawn into the carburettor/injector through an intake air filter, susceptible to impact icing. Fuel injected

have an alternate air source which bypasses the filter but will not show a drop in RPM when selected

Take-off power: maximum power that can be produced but cannot be used continuously so is time/altitude

limited. Rated power: power available under specified conditions, normally a particular MAP or torque setting

(Internal) Superchargers are fitted downstream of the carburettor and compress the mixture to increase the

power output at altitude (altitude boosted) but can also help at low level (ground boosted)

Impeller rotates at 6-12 times engine speed and draws mixture into its central eye, imparts energy to it and

accelerated it towards the circumference, increasing in velocity, pressure and temperature. As it leaves, it then

enters the diffuser which trades velocity for more temperature and pressure (1.5:1 up to 4:1 pressure

increase), depending on: impeller diameter, speed of rotation and shape of impeller vanes

Impeller is driven by engine crankshaft and uses engine oil for lubrication

Amount of boost is controlled by ABCU Automatic Boost Control Unit restricting throttle butterfly movement

Full throttle height where selected boost cannot be maintained

Superchargers are less efficient than normally aspirated engines at low altitudes and requires lots of energy

Turbochargers are driven by exhaust gas and only compresses the inlet air, before fuel is added

Advantages: no power from engine, drive system less complex and lighter system, as engine RPM increases,

velocity of exhaust gas increases, thus increasing the speed of the impeller and thus amount of boost

Because it is driven by the exhaust, it can take a while to spool up as RPM increases, so “turbo lag”

Controlled by controlling impeller speed by restricting amount of exhaust gas directed onto the turbine

Waste gate uses a butterfly valve and simple systems can use a fixed restrictor and a pressure relief valve

Critical altitude is reached when the waste gate is completely closed and MAP starts to reduce

Differential controller for waste gate at all positions except fully open where density controller limits MAP

Hot air entering the engine from the turbocharger can increase detonation risk, so intercoolers are used

Diesel engines use the heat generated through the compression stroke to ignite the charge and they always

use direct fuel injection systems. So they run at higher compression ratios, e.g. 14:1 – 28:1 instead of 9:1

Diesel fuel is heavier and less volatile than avgas, making it safer and more stable, but a poorer lubricant

The higher the cetane number of the kerosene fuel, the shorter the ignition period

Advantages: more fuel efficient, more robust so less maintenance and suited to supercharging, no ignition

system makes the simpler and more reliable, kerosene is cheaper and less prone to fire/explosion

Diesel engines introduce the fuel at the last moment in the compression stroke which immediately ignites

Pump must supply an exactly metered amount of high pressure fuel to the ejector at the right point in the

cycle, so needs to: sense engine RPM, sense air density, be able to advance/retard the fuel pulse timing,

rapidly generate a series of pulses at pressure and deliver them in sequence to each injector

Common rail system uses: high pressure pump with a pressure regulator and metering valve, common rail

acting as an accumulator for high pressure rail, electromagnetically operated injectors, ECU Engine Control

Unit to monitor engine conditions and control pressure in common rail and opening/closing of injectors

When cold, glow plugs may be used to increase temperature in combustion chamber at the start

Sheathed: electrically heated coil encased in a sheath filled with magnesium oxide powder

Ceramic: special heating element with high melting point encased in silicon nitrite (more resistant)

Diesel has a lower power to weight ratio and require more cooling (liquid) using water and antifreeze

Gas Turbine Engines

Earth

Transmission

Can consist of: Drive shafts, clutch, freewheel units, main gearbox, main rotor drive shaft, tail rotor drive

shafts, dampers, intermediate and tail rotor gearboxes, rotor brake. Will: Drive main rotor mast assembly,

provide RPM reduction, provides a means of driving the tail rotor and the transmission accessories, support

main rotor assembly. Engine torque transmitted through main drive shaft or belt assembly, to the input drive,

which drives the main transmission gear trains

Clutch required between piston engine or direct drive gas turbine and the main rotor gearbox so that the

engine can be started without turning the MGB and rotating assemblies, thus reducing engine starter load

Centrifugal clutch is automatic and shoes are moved out as speed increases, driving the transmission

Belt drives use a tightener as a clutch, increasing tension until the transmission rotates

On non-clutch assembly, a freewheel coupling unit is provided in the input drive assembly (sprag clutch type)

Gearbox castings use magnesium alloy and the MGB is a reduction gearbox using planetary or spur gear

arrangement and accessories such as hydraulics and electrics are usually driven from the MGB

Tail rotor drive shafts are made from aluminium or steel tubing and if it runs super-critical (above natural

frequency), a damper is required

Intermediate gearbox provides a change in the angle of the drive and may also change speed

Tail rotor gearbox changes drive direction by about 90 and most reduce the speed; splash lubricated with air

cooling in the sump using cooling fans

Rotors

Side-by-side, coaxial (two in different directions) with increased payload for engine power, tandem (Chinook)

with good longitudinal stability and no need for anti-torque rotors, single rotor with tail rotor

Fully articulated: multiple hinges are flap hinge, drag/lag hinge and bearings allow feathering. Changes in pitch

are effected by the pitch change rods and droop stops are only operative at low RPM

Teetering (two blade): see-saw/semi-rigid with blades connected and allowed to tilt

Hingeless: Flap/lead-lag hinges are replaced my flexible elements called elastomerics

Bearingless: No hinges or bearings Elastomerics are layers of elastomer/metal and do not need lubrication

Swashplate transmits cyclic/collective movement to pitch change horns via the pitch change rods

The rotating swashplate has self-aligning bearings so there is no load on the swashplate when angled

Old blades are made from aluminium alloy with hollow leading edge spars and a light trailing edge

New are composites with main spar having a foam core with a honeycomb tail construction

Centre of pressure, centre of gravity and aerodynamic centre are kept as close together as possible

Some tail rotors incorporate a delta 3 hinge which automatically decreases pitch as the blade flaps up

Ducted (fenestron) fan reduces power demand and has 8-18 blades which are shorter and spin faster

Vertical vibration from unequal lift in main rotor blades, reduced by adjusting the length on the pitch change

links or adjusting the trim tabs to make sure the blades are all flying in the same plane of rotation

Lateral vibration with unequal distribution of mass in the main rotor disk, reduced by dynamic balancing

(weights at the blade mounting bolts) or sweeping the blades (moving fore/aft of its usual position)

Flying Controls

The non-rotating swashplate can move vertically up and down the main rotor mast, or tilt around it

Control movements from the cyclic and collective are transmitted to the non-rotating swashplate by a

combination of push pull rods, bell cranks, cables and pulleys, operating control rods or hydraulic actuators

Non-rotating swashplate will be moved at 3 different points to achieve full articulation

Control rigging and having the blade pitch control arm forward of the leading edge of the blade help with the

phase lag; the advance angle is the angle that the pitch change arm is in front of the blade’s leading edge

A synchronised elevator can be located near the aft end of the tail boom for controllability

Mixer units send combined signals to stop cyclic inputs changing collective inputs and can contain primary

stops to prevent the controls being moved further than designated limits

The spider control system is mounted on a gimballed shaft running inside a hollow main gear shaft

Tail rotor uses a spider and push pull rods, bell cranks, cables and pulleys

In a fully powered system, the helicopter cannot be controlled without hydraulic power

Trim actuators provide anchoring, friction damping and pilot functions

Series actuators are quick controls, limited to about 10% range authority

Fire and Smoke: Detection and Suppression

As in Air Law