Aircraft Flashcards
Bournoulli’s equation
Static pressure + dynamic pressure = total pressure (constant)
Newton’s laws
1) A body will continue in rest or in uniform motion in straight line, unless acted upon by a force
2) Rate of change of momentum is proportional to (and in direction) of applied force
3) Action and reaction are opposite and equal
Force equation
Mass x acceleration
Piston
- Direct vs indirect fuel injection
Direct injection delivers fuel directly to cylinder heads. Highly fuel efficient but requires complex timing.
Indirect delivers fuel on low pressure, continuous flow basis to the air intake of each cylinder.
Exhaust Gas Temperature (EGT)
- How it is used
- hottest temp
This gauge is used to adjust mixture. 15:1 creates highest temperature, richer than this and extra fuel aids cooling, leaner and there is less fuel to burn.
FADEC
Full Authority Digital Engine Control
A computer controls the various power settings (throttle, mixture, cowl flaps, ignition) through a single lever.
Diesel vs petrol engine differences
No spark plugs (compression ignition).
So need higher compression ratios (14:1 to 28:1 compared to 9:1 petrol).
This requires a strong (and thus heavy) engine.
Need direct fuel injection, no carburettor, NO MIXTURE!
Hotter, so need liquid cooling usually.
Diesel vs petrol fuel
Diesel (kerosene) is heavier (SG 0.8 to 0.9) and has more controlled detonation at high compression.
Less volatile and higher flashpoint so safer and more stable.
Poor lubricant however.
Diesel engines
- Direct fuel injection
Similar to petrol fuel injection.
Engine and electric pumps with PRV deliver pressurised fuel to a control unit.
The control unit knows throttle setting and rpm and delivers pulses of fuel to a manifold, which distributes it to individual injectors (simple mechanical injectors that deliver fuel based on pressure received).
[Older system]
Diesel engines
- Common rail injection
Supply pump provides pressurised fuel to a common rail. The common rail feeds fuel to individual solenoid injectors, which themselves control when and how much fuel is injected.
An electronic control unit controls both the pressure in the common rail and the individual solenoid injectors.
Variable pitch propellers
RPM or MAP first when:
- increasing power
- decreasing power
RPM then MAP to increase power (to prevent detonation)
MAP then RPM to decrease power
Variable pitch propeller effect of increase in TAS
Increase in TAS reduces the effective AoA of blade, which the CSU is trying to keep constant.
In practice, the TAS increase reduces the force on the blades, which results in an increased RPM. The CSU responds to the increased RPM by coarsening blades.
Gas turbine cycle
- Compression
- Combustion
- Expansion
- Exhaust
Called OTTO if steps happen separately, Brayton cycle if they happen continuously.
Single Spool Axial Flow Turbojet
- nature and where they are used
Single spool means single shaft connecting turbine blades to compressor blades (multiple of each).
Axial flow means flow straight through the jet engine.
This is slim, low air mass and high velocity. Low drag (good for fast flight) but not used by airliners as it is noisy and inefficient at cruise speed.
Twin Spool Bypass Turbojet
Two shafts, connecting LP (low pressure) compressor (at front) to LP turbine (at back), and HP compressor & turbine (in-between). LP compressor feeds some air to a bypass outside the combustion/turbine zone. This is more thermally efficient (no combustion) and better at low speeds. It insulates some of the noise of the core.
Core can be smaller so less weight.
i = Bypass air mass:core air mass [or cold air:hot air]
Produces MORE pressure than single spool.
Bypass ratio
Bypass air mass / HP compressor air mass
[Need to deduct if given total air mass]
Typical bypass ratio and share of thrust
Bypass ratio 10:1
About half of thrust in bypass air vs hot core air
Centrifugal vs axial flow compressor
Can have double sided centrifugal compressor, but difficult to have multiple stages due to energy loss of passing air to the next stage, so axial more popular.
One disadvantage of axial however is that the rotors are aerofoils and can stall, whilst centrifugal aren’t aerofoils, just directing air outwards to the diffuser.
Compression ratio around 4.5:1 for centrifugal, 35:1 for axial.
Centrifugal can be shorter though, so good for APU, although need big inlet.
Axial flow compressor
- functionality
Each stage has rotor blades and a set of stator blades.
Rotor: Increase velocity, pressure and temperature
Stator: Convert kinetic energy to pressure, so decrease velocity, increase pressure and temperature
Stators also fix airflow for next stage.
Pressure increases around 10-20% each stage.
Axial flow compressor
- shape of compressor section
Need to oppose the force of high pressure further back pushing towards low pressure at front of engine.
Use a convergent duct shape called “air annulus”. The outer case size tapers whilst the shaft size increases so rotor size squeezed from both directions.
Axial compressor
- Variable inlet guide vanes & variable stator vanes
Variable inlet guide vanes adjust flow to the first stage to ensure it hits rotors at an appropriate angle of attack - essential during startup.
Variable stator vanes can do the same thing for later stages.
Axial compressor
Inter-stage bleed
- When it is used
- Change in flow over early and late stages
They open during start up to reduce angles of attack in early stages and maintain axial flow through all stages.
They INCREASE flow over early stages and REDUCE flow over later stages.
Compressor stall symptoms
- EGT - high/low
- EPR - high/low
- TGT - high/low
- Thrust
- Fuel Flow
- RPM
- Other
- High EGT
- Low EPR
- High TGT
- Reduced thrust
- Rumbling or banging from compressor
- Fluctuating rpm & fuel flow
Compressor surge
- Description
- Deep surge
- Cause
This is when pressure in combustion chamber exceeds compressor pressure and hot gases move forwards.
In deep surge combustion gases flow all the way into compressor, which can be caused by over-fuelling, but less common now with better engine controls.
Primarily caused by EXCESS FUEL, not compressor stall.
When is compressor stall most likely? (2)
Low RPM
Acceleration/deceleration
EPR
- stands for
- calculation
Engine pressure ratio
Alternative indication to N1 for engine thrust.
= Exhaust pressure / compressor inlet pressure
Variable pitch prop positions
Specific Fuel Consumption (SFC)
Fuel per unit of power.
= Fuel per hour / thrust
Effect of bleed air on:
- Fuel consumption
- EGT
Both increase.
More fuel to supply the bleed air, then extra fuel creates more temp.
Where are N1 and N2 measured
N1: Low pressure
N2: High pressure
How are generators connected?
In parallel
Common gas used in emergency landing gear and in tyres
Nitrogen
Purpose of tyre fusible plug
To allow pressure relief in high temperature system, in advance of tyre blowing and being lost
Which aircraft systems are heated by:
- Bleed air
- Electrics
Bleed air: Wing leading edge, engine intakes
Electrics: Probes, cockpit windows
