Technical Interview 3 Flashcards
How does a stall strip work?
A stall strip are small strips attached to the wing leading edge to promote airflow separation at high angles of attack so the wing root stalls before the wing tips. They are usually used on tapered and swept back wings, which have a tendency to stall at the wing tips first. They will not effect the efficiency of the wing in the cruise.
With stall strips airflow can’t stay attached so easily to the top of the wing, and it starts to separate from the wing before your wing reaches the critical AOA and stalls. This causes an “early” stall directly behind the strip, and prior to the entire wing stalling.
What is a high bypass ratio engine?
The fan at the front of the engine compresses a large volume of air which is then split into two streams. Most of the air goes around the engine core and is next ejected through a nozzle called a fan (or cold) nozzle. The rest goes through the engine core, where it is further compressed in a compressor and next then it mixes with the fuel and ignites. The hot expanding combustion outflow passes through the turbine section, spinning the turbine as it exits the engine. The spinning turbine turns the engine shaft, which spins the fan and compressor on the front of the engine. The fan compresses more air keeping this continuous cycle going.
What is the difference between a supercharger and a turbocharger?
The main reason behind supercharging and turbocharging is to force more air into the engine at high altitude flight to compensate for the reduced air density in the atmosphere.
A supercharger is connected directly to the engines crankshaft by a gear that drives its compressor forcing more air into the engines combustion chamber and achieving a higher power output.
A turbocharger is driven by a turbine that is connected to the engines exhaust. The turbine drives the compressor forcing more air into the engines combustion chamber and achieving a higher power output. Unlike a supercharger the turbocharger does not place a direct load on the engine which makes it more efficient to operate. The turbocharger can however suffer from lag when an instant power increase is required as the turbine can take longer to spool up to be effective.
What does a bypass ratio of 11:1 mean?
That 11 kg of air passes through the bypass duct for every 1 kg of air passing through the core.
What is the movement of the center of pressure during/approaching a stall?
A straight wing will stall from the wing root and the centre of pressure CP moves aft. A swept wing will stall at the tip first causing the CP to move forward.
What is a turbofan?
Its an engine where the majority of the thrust is derived from the fan, rather than from combustion gases expanding in a nozzle.
The fan at the front of the engine compresses a large volume of air which is then split into two streams. Most of the air goes around the engine core and is next ejected through a nozzle called a fan (or cold) nozzle. The rest goes through the engine core, where it is further compressed in a compressor and next then it mixes with the fuel and ignites. The hot expanding combustion outflow passes through the turbine section, spinning the turbine as it exits the engine. The spinning turbine turns the engine shaft, which spins the fan and compressor on the front of the engine. The fan compresses more air keeping this continuous cycle going.
Why do aircraft have leading edge devices?
To increase the wings chord (MAC) and/or camber and increasing it’s coefficient of lift which reduces the stall speed.
How much thrust comes from the N1 fan?
Depends on the engine.
In the case of the 737-800 CFM56, 27,300lbs total thrust at a ratio of 5.4:1
What is tire creep, and is it important?
Creep is the tendency of a tire to move around the wheel hub at touchdown, since it takes a little time for the tire to spin at the same speed as the airplane is moving.
Yes, it is important because if there is too much creep the tire can tear out the inflation valve and cause the tire to burst.
Does the N1 compressor produce thrust?
Yes, with high bypass turbofans the N1 compressor works as a huge propeller, it takes a big mass of air and gives it a relative small acceleration.
Around 70% of the air will bypass the core of the engine, only 30% of air will be compressed further and will eventually go through the combustion chamber.
“On most dual spool engines the front fan is driven by the LP turbine as the front fan is part of the LP compressor section”. (explicación de la pregunta nº 41376 avex)
Why does a wing stall?
Stall is defined as a sudden reduction in the lift generated by an aerofoil when the critical angle of attack is reached or exceeded.
The lift coefficient of an aerofoil increases with angle of attack up to a maximum (CLMAX). Any further increase above this stalling or critical angle of attack will make it impossible for the airflow to smoothly follow the upper wing contour, the flow will separate from the surface, causing CL to decrease and drag to increase rapidly.
This can lead to loss of heigh and loss of control, so its very important for the pilot to identify an impending stall.
What is N1, N2, EGT and EPR?
N1 refers to the rotational speed of the low speed spool which consists of the fan, the low pressure compressor and the low pressure turbine, all of which are connected by a concentric shaft.
N2 refers to the rotational speed of the high speed spool which consists of the high pressure compressor and the high pressure turbine.
EGT - Exhaust Gas Temperature, is the temperature of the turbine exhaust gases as they leave the turbine unit.
EPR - Engine Pressure Ratio. Its the ratio of the turbine outlet pressure to compressor inlet pressure. This is a measurement of thrust
Can a helicopter rotor blade stall?
Yes
How does anti-skid work?
The Anti-skid system increases the performance of the aircraft brakes.
It compares the speed of the aircraft with the rotational speed of each main wheel. If the speed of any wheel is too slow for the existing aircraft speed, the brake on that wheel (or wheels) is released momentarily to allow the wheel speed to increase and prevent the wheel from skidding.
Anti-skid systems are designed to minimise aquaplaning and the potential tyre damage which can occur when a wheel is locked or rotating at a speed which does not correspond to the speed of the aircraft.
The antiskid valves in an airplane’s braking system receive pressure from the brake valves and are controlled by an antiskid control unit. Wheel speed sensors send information to the control unit, which can detect when a wheel is skidding. When skidding is detected, the control unit tells the antiskid valve to release some of the brake pressure to prevent further skidding. This happens individually for each wheel during normal braking, but for pairs of wheels during alternate braking. Additionally, the system provides protection during touchdown by comparing wheel speed to groundspeed.
Where is thrust produced in a gas turbine engine?
Mostly in the fan, but also from the exhaust nozzle duct.
