Aircraft General Flashcards
Fuselage description
Semi-monocoque
Light frame with a structural skin
Components of fuselage
Frames at cross sectional points
Longerons connect them
Wing components
Ribs (and stringers connected to them) form cross sectional shape
Spar is the beam that connects ribs together and all to the fuselage
Stress
The internal resistance of a material to deformation - an internal force which opposes deformation.
Strain
The degree of deformation experienced on a material when exposed to a force of load.
5 forms of stress
- Tension
- Compression
- Torsional
- Shear
- Bending
Combination of stresses in bending
- Compression on inside of bend
- Tension on outside of bend
- Shearing between the two
Which stresses are aircraft designed to cope with?
Tension and compression
Not bending
How is stress in airframe assessed?
FM/Operating Handbook will have G force limits. If these are exceeded must report to a ground engineer as the stress limits on airframe will have been exceeded.
Required equipment to secure parked aircraft
> = 3 tie downs (with pegs)
= 2 chocks
3 types of control
i) Mechanical
ii) Hydro-mechanical
iii) Fly-by-wire
Artificial force feedback
Required for hydro-mechanical and fly by wire controls to allow pilot the sense of how controls are behaving or responding to inputs
4 stroke engine cylinder diagram
4 stroke engine phases
i) Intake
ii) Compression
iii) Power
iv) Exhaust
Name for the 4 stroke engine process
Otto cycle
Which component controls inlet and exhaust valve timing?
Camshaft
How many rotations does camshaft do in a full OTTO cycle?
One - to allow a single inlet and outlet valve open/closing in each cycle.
Valve overlap
Exhaust valve remains open until just after inlet is opened so there is some overlap where they are both open
Compression ratio
Total cylinder volume / Clearance volume
Where total volume is volume with piston at “bottom dead centre” and clearance volume is with piston at “top dead centre”
Spark timing
Just before piston reaches top dead centre to allow flame front to move through the fuel/air mixture in time for the power phase.
Magneto functionality
Magnet rotates (driven by engine) in a conductor with two coils (primary - ignition and secondary - distributor), generating a current in them.
Breakers in the primary coil cause interruptions in its voltage, causing a spike in voltage of secondary coil, which is used to create the spark.
Magneto Distributor functionality
Distributor has a rotating finger which connects secondary magento coil to one of four connections (one for each cylinder) as it rotates.
Magneto p-wire
This is the earthing wire. If it is broken the magneto will not earth and rpm is unchanged when switched off during check.
Voltage generated by magnetos
20,000 V
Distributor rotations vs crankshaft rotations
Each cylinder needs a spark once for every two rotations of the crankshaft, therefore gearing from crankshaft to distributor is such that distributor rotates once for every two turns of the crankshaft.
Ignition harness
- issue
The leads from the distributors to the sparkplugs are bound together as an ignition harness.
Leakage from the ignition harness can cause rough running.
Starter motor wiring
Starter requires high current (thick wiring) which would be difficult to connect to ignition.
So the ignition instead powers a low current solenoid, which closes the main high power circuit for the starter motor (which also powers the starter light).
2 issues with magnetos at startup
i) Magnetos rotate at half crank shaft speed, so low rpm (e.g. 120) leads to very low (e.g. 60) magneto rpm, which doesn’t generate enough current.
ii) Spark advance (spark before top dead centre of cylinder) is timed based on normal crankshaft speed, but at low rpm can be too early and cause kickback.
Impulse coupling jobs (3)
i) Switch one magneto off at startup (only need left one)
ii) Store energy from rotating magnetos initially in a spring then release to generate a high voltage
iii) Retard ignition spark at low engine rpm so that it occurs just after top dead centre
Features of impulse coupling
Requires no power. Thus can be used to allow hand starting of prop.
Dead cut
Turning ignition to off temporarily before engine shut down to check for sudden loss of power.
If engine continues it suggests one of the magnetos may not be earthing properly with ignition off - can cause accidental hand starting of prop.
This can be bad for the engine.
Tachometer
Measures “time” on the basis of rpm, so gives an idea of useage of the engine.
Hobbs meter
Simple time elapsed instrument which advances in real time whenever master switch is closed.
2 stroke engine diagram
2 stroke engine inlet/outlet functionality
The piston closes off the inlet and outlet at the appropriate times in the cycle.
Why are 2 stroke engines noisy?
Muffling the engine noise restricts the flow of exhaust fumes. The need to allow easy escape of exhaust fumes causes the noise.
2 stroke ignition systems
i) Dual magneto system
ii) Capacitive Discharge Ignition (CDI) system, which is light and simple but requires battery power
Lubrication of 2 stroke systems
Used to mix oil to fuel at 25:1 however getting the ratio wrong would cause problems and the burning oil would lead to blue smoke and pollution.
Modern systems deliver oil to the cylinder walls directly from a sump.
Does 2 stroke system recover oil?
No, dry sump system is a “total loss” system and has to be refilled between flights
2 stroke induction system
A “sidedraught” device uses venturi effect with air flowing over an outlet from the fuel.
A slide and needle system regulates the amount of fuel pulled in by venturi effect from a float bowl.
The fuel spraying past the needle into the air flow creates the fuel/air mix.
2 stroke cooling
Used to be air cooled, but liquid cooling is more popular as heat transfer is more uniform.
It is however more complex and heavy.
2 stroke rpm
Optimal power rpm is about 5,000 to 7,000 rpm so gearing is required to deliver propellor rotation speeds (around 1:2 to 1:2.6 ratio).
Starting a 2 stroke engine
Usually started via hand or pull cord (can also have electric starter).
Diesel engine advantages
Wide cut (as opposed to petrol narrow cut) fuel is cheaper than AVGAS and engines are more economical.
Diesel engine ignition
Impact on engine design
Spontaneous ignition of compressed fuel so no spark plugs, magnetos, camshafts (etc.) needed.
High compression ratio of 12:1 required for this however which requires the engines to be very strong.
Diesel fuel delivery
Diesel doesn’t atomise well so fuel injection is required.
With no induction manifold air pressure is needed from some kind of fan (turbo or supercharger).
Turbo vs supercharger
Turbo is a turbine compressor driven by exhaust gases, but doesn’t work well at low rpm.
Supercharger works mechanically from the engine instead and is more efficient at low rpm.
Comparison of super-turbocharged diesel and petrol engines
The super-turbocharged engine will be big, but will be around 20% lighter and maintenance/operating costs around 30% less.
Correct fuel ratio
Rich & lean ratios
Correct: 1:12
Lean: 1:20
Rich: 1:8
Term for chemically correct fuel/air mixture
ccm or stoichiometric mixture
Diagram of throttle side of carburettor
Venturi draws fuel into carburettor, butterfly valve (throttle) restricts flow of fuel/air mix
Accelerator pump
Gives an extra spurt of fuel as throttle is opened to prevent a weak-cut due to sudden high air flow in carburettor making the mixture too lean.
Diffuser
Carburettor device that premixes fuel and air to prevent excessive fuel from main jet and helps fuel vaporisation at low speeds.
How is carb fuel tank refilled
Float and needle valve
Idling jet
Allows a small amount of fuel past butterfly valve when it is almost closed to allow engine to idle
Mixture in take off and climb, reasons
Fully rich mixture used in climb and take off, when power level is high.
Excess fuel will protect against detonation, pre-ignition and cylinder overheating.
Mixture in cruise
If cruise is at below 75% max power (usually 55-65%) leaning mixture is advisable to improve fuel economy.
Mixture at high density altitude
At higher altitude air pressure reduces and so the mixture needs to be leaned to maintain an appropriate mix. This might be the case in high altitude, high temperature take offs.
How to lean mixture
Gradually lean the mix and rpm should increase as chemically correct mixture is approached. When it starts to fall again, return to the peak and stay just on rich side of peak.
Detonation
Explosive combustion in cylinders, perhaps due to high heat. This causes cylinder damage.
Pre-ignition
Combustion before the spark plugs spark, perhaps due to heat spots.
Effect of overly rich mixture
Carbon from unburnt fuel can form on the piston heads and valves.
Cylinder heads will run cool.
Effect of overly lean mixture
High cylinder head temperatures can lead to detonation and potentially serious engine issues.
Idle cut off/out
When mixture is fully leaned a needle will cut out the fuel from from the float chamber to the venturi. Engine continues to run on fuel that is present, leaving no fuel/air mixture in the manifold.
Response to suspected detonation
Enrichen the mixture
Reduce pressure in cylinders by pulling throttle back
Increase airspeed to aid cooling
Impact ice
When it happens
When below freezing water droplets impact metal surfaces of carburettor inlet and form ice.
Can occur if outside temperature is near or below zero and cloud/rain/sleet is around, or if the aircraft is freezing and enters a warm humid area (e.g. descending from cloud).