I thought it is more like a generator than a engine. Does APU really produce a signifcant thrust for the craft

Answer 1

Mostly, the answers above are correct. I work on CRJ's, a longer version of the Challenger 601. The APU is mounted in the tail, with the exhaust exiting the right side (rear on the CRJ 900) and the inlet on top of the fuselage. You can walk through APU exhaust and it's not going to knock you over, keep in mind they run at 100% while in use. Compared to the thousands of pounds of thrust generated by the engines, APU exhaust is nothing. A generator is attached to provide power for all systems and the 4 electric hydraulic pumps ( there is still one engine driven pump on each engine, but only funtions with the engine running). Also there is a load control valve for bleed air to start the engines and run the ACM's ( air conditioners). That is routed into the 10th stage bleed air ducts. 14th stage runs the thrust reversers and anti-ice, but is solely provided by the engines. So to sum it up... it's a small turbine engine that runs a generator and provides air, and no, it doesn't provide enough thrust to be of consideration.

Answer 2

Some do depending upon the direction of the exhaust. I have tech Rep for the Honeywell 150 for 10 years. In the Astra 1125 it does give about 25 lbs of thrust. However many aircraft that have an APU are not certified to use them air-born. The amount of thrust is very little, but any thrust is still thrust.

The APU is there for ground power, and allows the aircraft to run the air cycle machine ( A/C or heat) and start the main engines. It can help to share the load of the main generators, providing they are paralleled correctly. Other systems can be started so the pilots can enter flight plans into the FMS's (flight management systems)

Most APU's are turbine engines, that have a starter/generator attached through a shaft. To start the engine, the Starter/Generator acts as a starter, and begins turning the turbine, while fuel and spark are added around 60%. At 95% the engine begins to run on its own, and the Starter/generator now turns into a generator, which provides up to about 300 amps to the main ships power bus.

I have worked with 100's of aircraft with the sun strand, Genesis, Honeywell 100 & 150. Any other questions in this area please ask.

Answer 3

An Auxiliary Power Unit (APU) is a small engine that is connected to an alternator. It simply provides electrical power when the main engines are not running. The APU is usually started before the main engines, because it is easier to start the much smaller APU engine, and the power that the APU generates is sufficient to start the main engines. Many large jets cannot start their main engines from battery alone and need the extra power the APU provides to start the main engines. Also if the main engines fail in flight the APU can be started to provide electrical power. Even though most APUs are turbines (simply because the jet already has that type of fuel on board, so why carry some other type of fuel) the thrust is negligible.

Another way to think of it is to compare it to an RV-the main engine provides propulsion, while the generator (the RV's APU) just supplies power and does not add anything to getting the RV down the road.

Answer 4

Depends on the aircraft. Some APUs, which exit directly to the back, will add some small amount of thrust compared to the main engines. Other APUs (ex. B777) exit out the side, which would produce negligible thrust.

Answer 5

None at all, its an auxillary power unit, it just turns on the electrical. Its not in a position to produce thrust, although it does produce a small amount of jetwash, and heat. However on a dornier, and a Challenger 601 the APU is in the left engine, so in that case, the engine would have to be running so yes that would produce thrust. Kind of a tricky subject. But the apu alone, doesnt produce thrust.

Answer 6

An APU is a small jet-engine about 3 foot long. It generates electrical power and pneumatic power (air pressure). The amount of trust it produces is in no way enough to help in the trust for the aorcraft.

Answer 7

You couldn't even push a good sized car with an APU, well, maybe once it got started. OK, if it .started downhill

Answer 8

yah you can taxi on APU power sometimes.