REVERSE THRUST PROBLEM

[silo]

It depends on the type of engines. There are 2 main types: turbojet or turbofan.
Furthermore, there are 3 types of turbofan jet engines. Low Bypass (old and usually for supersonic engines), Medium Bypass and High Bypass.
Turbofans essentially increase the size of the first-stage compressor to the point where they act as a ducted fan (or propeller) blowing air
past the "core" of the engine. The fan causes more air to flow around (bypass) the engine which in turn reduces noise and fuel
consumption.They also produce increased thrust.
The noise of a jet engine is strongly related to the velocity of the air coming out the exhaust. A turbofan has a larger mass flow of air for a
given thrust than a turbojet, so the exhaust velocity will be slower and hence the turbofan engine will be quieter than an equivalent
turbojet. High bypass engines have more air flowing around the core of the engine than low or medium bypass. These engines actually
produce relativly little thrust in the reverse direction. They create more of an air umbrella that slows the aircraft by drag. This is why very
high reverse thrust is needed to move this type of aircraft backwards, and why so much FOD is thrown around causing lots of damage.
Also the exhaust air starts to reingest pushing EGT up though the roof - hence the idea that on rollout the T/R's should be back at idle and
stowing by 80 knots or so - in normal circumstances.
The aircrafts that can 'power back' normally are the ones using a ducted fan type engine like the older JT8's on 727, 737, DC9, and the
like. These type of engines block and turn all exhaust airflow in a 45 degreee forward direction. These engines are also fairly well
protected by FOD in reverse with either very high mounts (DC9, MD80-90 types) or very long fan ducts and exhaust extensions (737).



Post Edited ( 09-16-05 08:25 )