BGT exam Flashcards
What Newton’s law is the principal of a jet engine?
What forces must be balanced for steady unaccelerated level flight?
3rd law: every action has an equal and opposite reaction.
Thrust must be equal and opposite to drag
*What is boyles and Charles law?
B: PV=PV
C: V/T1=V/T2
What action specifically produces the forward thrust on a jet engine?
Again, the following utilises N3 law.
As atmospheric air is accelerated rearwards, it has an equal and opposite reaction on the engine apparatus which produces the forward acceleration. So is an internal reaction. Is NOT due to air being blasted out back.
What equation shows what the resultant reaction of the jet engine is proportional to? What is the relative size of these variables in a turbo prop/fan aircraft?
Which is in theory more efficient? Why?
Air accelerated = mass x velocity change
In a turbo prop, mass of air accelerated is greater but velocity is slower.
In turbofans, the mass of air accelerated is less but is faster… hence why they can travel faster. However, this is thermodynamically less efficient.
However, modern turbofan engines are moving larger volumes at reduced speeds to increase the propulsive efficiency (as jet/atmospheric velocity difference is reduced).
What is Bernoulli’s theorem?
It deals with the pressure of gases, which is important in a jet engine. What ways can the pressure be changed within an engine?
P + 1/2pv^2 + pgh (static + kinetic + potential = constant)
- Removing/adding heat
- Adding/removing molecules
- Changing the volume
- Changing the air velocity
How does a change in volume in subsonic flow affect velocity and pressure?
Small to large volume: pressure increases and velocity decreases
Large to small volume: pressure decreases and velocity increases.
P+ 1/2pv^2, so if static pressure increases kinetic decreases VV.
What is the Brayton cycle?
Is the term used for the continuous thermodynamic combustion cycle for a gas turbine engine. Is also known as a constant pressure cycle (as pressure constant during combustion as volume increases so speed increases).
*Describe the principal of the Brayton cycle
X4 actions are the same (intake, compression, expansion (power), exhaust).
A: ambient air is compressed towards “b” so volume decreases and pressure increases.
B: fuel is added so heat is added as it is burnt, at a relatively constant pressure towards “c” as volume increases. Pressure slightly decreases due to drag.
C: pressure losses toward “d” as gases expand back into atmosphere and some of the pressure is turned into mechanical energy by the turbine.
*Just know this…
Jet fuels are low octane and operate constant pressure
Piston engines have high octant fuel and constant volume.
What is the most common type of inlet? Explain why
Fixed geometry (pitot type) most common on turbo jet/fan aircraft that fly at high subsonic/low supersonic speeds. Makes the most use of RAM effect of air due to a/c forward speeds and has minimal losses in RAM pressure with changes to a/c attitude.
*What is the purpose of a compressor?
Primary function is to supply air in sufficient quantity to the combustors, by increasing the pressure of the mass of air received to discharge it at the pressure/quantity required.
Secondary function is to supply bleed air for various purposes.
Describe the principal of operation of the centrifugal compressor
Airflow arrives at the centre of the impeller where flow is axial, and is accelerated outwards by centrifugal reaction to its rotational speed. Air then expands in a divergent duct called a diffuser, which causes static pressure to build as velocity decreases (Bernoulli).
What is an impeller? ASK ERIC
What type of compressor does this belong too?
Is a forged disc with integral radial disposed vanes on one or both sides forming XXXXGENT passages.
Can be swept back but for ease of construction also be straight.
Are apart of the centrifugal compressor.
What is a diffuser? What compressor does it belong too?
Attached internally to the outer part of impeller, or as a seperate casing, purpose is to create a pressure rise by slowing the air velocity through divergent ducts.
The vanes of the diffuser are tangential to the impeller and the inner edges of the vanes run with the direction of the resulting airflow from the impeller.
Centrifugal compressor
What is the principal of the axial flow compressor?
Airflow and compression remain parallel to the rotational axis of the compressor. A rotor is turned at high speeds by the turbine which will therefore continuously induce air into the compressor. Rotors further accelerate air rearward onto adjacent row of stator vanes, which are divergent so air slows and static pressure builds.
What is the purpose of inlet guide vanes?
What compressor does it belong too?
Are stationary vanes that direct airflow onto the rotor at the most desirable angle.
Axial
What is the purpose of rotor blades?
What compressor does it belong too?
Purpose is to move air rearwards through each stage. They are the first component of each stage of compression. Are of aerodynamic design with varying angle of incidence/aerodynamic twist similar to propellor (decrease angle at each stage). They rotate.
Axial
What purpose are stator blades?
What compressor does it belong too?
Receive air from rotor blades at high velocity and act as a diffuser swapping Ek for pressure. Are stationary.
Axial
Where is the impeller of the centrifugal compressor located?
Can it be used in conjunction with axial?
Located between accessories and combustor
Used in combination with axial
Where is the axial flow compressor located?
Located between inlet and combustor
Benefits of a multi spool compressor
Increase the operational flexibility as they provide high compression ratios, quick acceleration and better control of stall characteristics.
The reason for this is as altitude decreases (decreasing density) the low pressure compressor (N1) can speed up and recover some of the lost pressure through the high pressure compressor.
What is a gas generator?
Any combination of a compressor and turbine is known as a gas generator or spool.
A single spool engine is Ng, whereas multi spool engines will be denotes N1 (Low px compressor ), N2 (int/high px), N3 (high px)
What is a compressor stall?
Aka thermodynamic stall, it is the abrupt loss of efficiency in an axial flow compressor when the AoA of the blades becomes excessive/depart from the designed state.
What signs are there for a compressor stall?
There are typically no symptoms or warnings for a complete compressor stall. However, if a surge is about to occur, -vibrations may be felt -temp/fuel/thrust fluctuations -high EGT -engine sneeze/bang -loss of power -A/C surge
What are the causes of the compressor stall?
Turbulent or disrupted airflow into the engine inlet (reducing gas velocity).
Excessively rich fuel flow caused by abrupt engine accelerations as engine has not had enough time to spool up (causes reduced gas velocity by increased combustion back pressure).
Excessively lean fuel flow due to abrupt engine deceleration (increases gas velocity by reducing combustor back pressure).
Contaminated/damaged compressor blades (increasing gas velocity due reduced compression)
Damaged turbine components, causing loss of power to compressor therefore lower compression (increasing gas velocity)
Engine operated outside design envelope (RPM)
What are the 2 types of compressor stall?
Positive incidence: low pressure stage at low speeds (due bad intake ie if enter sideslip to right, left engine blocked airflow so speeds into engine reduces causing an increase in AoA)
Negative incidence: high pressure stage at high speeds (if a/c flys too fast or faulty turbine so air doesn’t slow).
Purpose of the combustion chamber
Burning fuel supplied through the fuel nozzles with the air supplied by the compressor and releasing the heat in a manner that air is expanded and accelerated to give a smooth stream of uniformly heated gas at all operating conditions.
