APU and Thrust

[shon7]

Does the APU generate any kind of thrust that would move the airplane say if the brakes were not set?

Also, for Airlines where there are procedures where the APU has to be kept running the entire flight (in case of an MEL) -- would the APU contribute at all to the Airplanes thrust or is the thrust generated negligble.

[smudgethecat]

Originally Posted by shon7

Does the APU generate any kind of thrust that would move the airplane say if the brakes were not set?
Also, for Airlines where there are procedures where the APU has to be kept running the entire flight (in case of an MEL) -- would the APU contribute at all to the Airplanes thrust or is the thrust generated negligble.

The thrust geneated by a apu exhaust is negligible and would not contribute in any way the propulsion of the A/C.

[CaptainSandL]

I am sure that I remember hearing tales from wisened old Captains about how the Trident 3 (Ahhhhh!) used to use its APU for extra thrust.

It probably didn't help, but if it makes you feel better taking off from a short runway with the APU running then why not!!!

[Intruder]

Boeing has for a while been looking into APU configurations that would give enough residual thrust that it would be part of the takeoff thrust equation. Dunno how far they've gotten, or whether the FAA likes the idea...

[barit1]

Back in my flt ops eng job the 747-200 baseline TO performance was based on APU running, and a small (250 kg?) penalty applied if it were shut down before TO. I've been told this was Mr. Boeing's way of taking credit for APU thrust, although it might have had more to do with gear retraction times or other "legit" APU functions.

Anybody have the real story?

[stancioiu]

AN 24 uses the APU thrust. In fact the APU on this plane has its own thrust lever, it is a modified version of one of the YAKs engine i think.

[brain fade]

Stanciouo

You are correct methinks. I was on a Peru AF one and the F/E told me they always have it running for take off and landing.

[Consol]

Trident

The later version of the trident actually had a fourth engine in a small tail pod just over engine two outlet. Not sure if it was also an apu but it was there for a performance boost. Check out some pics of it and you can see the pod. The idea of thrust apu has been around a long time, heard it suggested in the last few years for a newish aeroplane. All thrust helps with take off but an engine which only works for take off and is then dead weight is inherently inefficient, its just a performance fudge.

[tallsandwich]

This is a great triva question and I would love to see some real figures on different types just for fun. Surely the contribution of running the APU at take off would be to remove the aircraft's electrical load from the main engines, as opposed to actually contribute to propulsive thrust from the gas exhaust from the APU?

If not, then if I imagine that the residual output from the APU did have a non-trivial effect on aircraft thrust, then what would I do to increase this residual thrust? I would dump more (electrical) load on the APU in order to increase the fuel burn required to keep it running and hence get myself more residual thrust o/p as a result.

Following this (il)logic then requires that in order to increase the APU thrust you switch on as much electrical equipment as possible!! (assuming this does not of course degrade the o/p of the main engines).

Anyone got any concrete figures?

[FlyVMO]

I suspect that the APUs contribution to take off perf (if any), is through its use to supply air to the ACMs (cabin air). This on some types might allow a "bleeds off" take off, providing of course anti-ice is not needed.
Taking bleed air from the engines reduces thrust through the following effects, as I understand it.
1-Thrust is determined by M(V2-V1), where M is the mass of air, V1 is the inlet velocity and V2 is the exit velocity (this is simplified for brevity)
Bleeding air out of the compressor reduces the Mass of air going through the whole engine, reducing thrust.
2*-It reduces the compressor air available to cool the turbine blades, taking the engine closer to temp limits.
So if you get that bleed air from the APU, your engines could conceivably run to a higher output.
There is no shortage of people here more qualified than I, and I would be happy to be corrected if this is wrong.

*Edit: I forgot that it is assumed the mass of fuel added cancels out the mass of the air bleed, so, I guess its more of a temp limitation.

[barit1]

Re Hi-bypass engines - If N1 is used to establish TO thrust, then with bleed off you are running lower EGT for a given N1. So with bleed off (or A/C packs off on 747 classic - see note) you can set higher N1 > more thrust > more TOGW. (It is assumed the bulk of the thrust comes from fan flow, so any change in core thrust is not significant overall)

But the APU TOGW increment I mentioned earlier applied in either "packs on or off" cases.

(Note - bleed air drives hydraulics on the 747 classic - can't speak for the 744!)

[whatdoesthisbuttondo]

I don't think the APU on the 75/76 produces any thrust.

The 777 thrust exhuast is released on the left side so I doubt that that produces any useful thrust.

[H Peacock]

Clearly any exhaust from an APU, if pointing in the correct direction, must contribute something in the way of thrust. Without wishing to detour too far from the original thread, the Shorts Tucano provides 1151 hp. 1100 hp drives the prop/gearbox, and the equivalent of 51hp comes out of the exhaust eductors as thrust. I know you also get thrust out of a helicopters jet exhaust. Don't know how this engine size compares with an APU in a big jet, but wouldn't have thought they would be much smaller

[Contract Con]

H Peacock,

Allied Signal quote 600 ESHP for their 131-9 APU fitted to the B737NG and A320.

Who knows what thrust it may provide out the exhaust though.

Cheers,

Con

[pilothouse]

Some versions of B757 performance have a PENALTY for APU running, because there is drag from the open APU inlet door!