Propulsion Flashcards
Give advantages and disadvantages of a piston-prop
Adv: Cheap and low fuel consumption at low speeds
Disadv: Heavy, produces a lot of noise and vibrations, produce less thrust as velocity increases
Around 80% efficient
Discuss the makeup and types of turbojets
- Consists of compressor, burner and turbine
- Axial and centrifugal flow turbojets
Discuss turboprops
- Outside air accelerated by propeller
- Propeller is more efficient than a turbojet because it has a greater diameter
- Intake air follows turbojet cycle
Discuss turbofans
- Turbine drives large fan attached to the shaft which accelerates bypass air.
- Has better efficiency than a turbojet at subsonic speeds but at supersonic speeds the drag of the fan increases.
Discuss Afterburners
Fuel is injected to the un-combusted air downstream from the turbine. Very fuel inefficient.
How are propulsion systems usually selected.
Best SFC for specified cruise speed.
What is a NACA flush inlet used for?
Engines in fuselage or APUs.
Why does a pitot inlet work well?
Almost 100% pressure recovery
Why do some aircraft (usually fighter jets) have variable inlet geometry?
Need different shock angles for best performance at different Mach numbers
Why is a capture area calculation important?
If the inlet is providing more air than the engine can take, the inlet is forced to spill the excess out the front.
If the inlet is not providing what the engine needs, it attempts to suck in the extra air it does have, significantly dropping thrust.
Why is there an unavoidable trade-off between thrust and efficiency?
n = 2/((V/V_0) + 1)
which implies max efficiency at V = V_0 however then the thrust would be zero.
Why are variable pitch propellers used?
To improve thrust over a broad range of speeds
When is fine pitch and coarse pitch used for propellers?
Fine (low) pitch for takeoff and coarse (high) pitch for cruise.
Why does a propeller have an aerofoil with progressively reduced pitch?
Tangential velocities increase with distance from the hub.
