Aero 213 Flashcards
Five advantages of thrust reversers
Reduces landing distance, reduces tyre and brake wear (especially in wet/icy conditions), improved ground handling (especially in wet/icy conditions), improves rejected takeoff margins, and allows operation from shorter runways (military), thus increasing operational flexibility.
Four issues with thrust reversers
Got gases cannot impinge on the wing, hot gases cannot be re-ingested into the engine, must be impossible to deploy in flight, and can’t generate too much lift, otherwise braking performance is compromised.
Two types of thrust reverser
Hot and cold.
Hot thrust reversers
Reverse air in/from engine core. There’s two systems. First system is clamshell doors that close within the core, and direct the hot flow through vents out the side in the reverse direction. Other system is bucket doors that extend behind the core exit and direct the hot flow rearwards.
pV
mRT
h
CpT
h0
CpT0 = h + (v^2)/2
rho2/rho1
(T2/T1)^(1/(gamma-1))
(rho2/rho1)^gamma
p2/p1
What do static properties depend on?
The molecules in the gas.
What do stagnation properties depend on?
The molecules in the gas, plus any bulk kinetic energy associated with the flow of the gas.
T0/T
1 + (M^2(gamma-1)/2)
Propulsive efficiency
A measure of how well an engine produces thrust relative to the amount of kinetic energy it adds to the airflow.
Requirement for max propulsive efficiency, nuPmax
Jet velocity needs to be low as possible, but still higher than the flight speed in order to produce thrust.
Four drawbacks of increasing the BPR
Increased drag, increased weight, installation problems due to size, and requires gearbox after BPR ~ 10.
Thermal efficiency
A measure of how well an engine adds kinetic energy to the airflow relative to the total energy input from the fuel.
Two methods of increasing the thermal efficiency
Increase the overall pressure ratio, and increase the turbine entry temperature.
Overall efficiency, nu0
nuPxnuTh = Tva/m(dot)fQlcv
Thrust specific fuel consumption, tsfc
m(dot)f/T
Four issues with an unducted fan
Not feasible to install on existing aircraft, airports would have to be adapted, fly slower than normal turbofan, and noisier.
Choked flow
M = 1
Critical pressure ratio
The pressure ratio required for choked flow.
Three benefits of having a three-shaft configuration
Independent shaft speed for optimal blade speed, shorter shafts, and modular for easy maintenance.
Turbomachinery cross-sectional area, A
πrt^2 - πrh^2 = πDz, where D is mean diameter and z is blade height.
Euler’s turbomachinery equation, P
mdot(Vtheta2U2 - Vtheta1U1) = mdotw(Vtheta2r2 - Vtheta1r1)
Blade speed, U
rw
SFEE for turbomachinery, P
(mdot)deltah0
Conservation of rothalpy
h02 - U2Vtheta2 = h01 - U1Vtheta1
V
W + U
Vtheta
Wtheta + U
Wa
Va
Reaction
A measure of the extent to which the rotor contributes to the overall static enthalpy change in the stage.
Reaction = 0.5 (turbine)
Pressure drop split evenly, meaning pressure gradients are modest. Sealing is easy.
Reaction = 0 (turbine)
Pressure drop entirely across stator, so pressure drop across stator is high, and sealing is difficult.
Reaction = 0.5 (compressor)
Pressure rise is split evenly, meaning pressure gradients are modest, so boundary layer is likely to remain attached.
Reaction = 0 (compressor)
Pressure rise is entirely over the stator, meaning the pressure gradient across the stator is high, so the boundary layer is prone to separation.
Flow coefficient, phi
Va/Um, where Um is mean blade speed.
Stage loading, psi
deltah0/Um^2 = deltaVtheta/Um
p0/p
(1 + M^2(gamma-1)/2)^gamma/(gamma-1)
