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By David Monniaux (Own work) [GFDL, CC-BY-SA-3.0 or CC BY-SA 2.0 fr], via Wikimedia Commons

Many larger airplanes have gas turbine APUs (auxiliary power units). It seems that their exhaust is oriented to the rear and might provide some thrust.

How much thrust does an APU provide and would it be enough to keep flying an airplane if the main engines went out?

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    $\begingroup$ Good news: you can maintain flight with just the APU. Bad news: you can only do this until you run out of altitude and airspeed. :) Same is true without the APU (i.e. your airplane is now a glider.) $\endgroup$
    – reirab
    May 3, 2017 at 20:26
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    $\begingroup$ As an interesting aside, Cessna calculated that it's light airplane reciprocating engine exhaust was producing up to about 5-10lbs thrust so in some later models they began to angle them back instead of straight down. Not insignificant where every little bit helps. They estimated it would take about 30lbs of thrust to maintain level flight. So, if your prop comes off, firewall the engine and extend your glide! (until the engine flies apart) $\endgroup$
    – jwzumwalt
    Jul 5, 2018 at 9:44

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An APU lacks the critical component that produces thrust, a propelling nozzle.

A propelling nozzle converts a gas turbine or gas generator into a jet engine.

and

Most modern passenger and military aircraft are powered by gas turbine engines, which are also called jet engines. There are several different types of gas turbine engines, but all turbine engines have some parts in common. All gas turbine engines have a nozzle to produce thrust, to conduct the exhaust gases back to the free stream, and to set the mass flow rate through the engine. The nozzle sits downstream of the power turbine. (NASA)

Would it be enough to keep flying an airplane if the main engines went out?

It would, in the sense that it will provide electrical and pneumatic power for the different aircraft systems, but no thrust. Or airplanes would have just been built with a few of those tiny APUs. :)

Another alternative for backup systems power when airborne is the Ram Air Turbine.

RE comment about turboprops having some exhaust thrust, they also have nozzles, which an APU still lacks. (NASA)

Before entering the nozzle (...) the exhaust velocity of the core is low and contributes little thrust because most of the energy of the core exhaust has gone into turning the drive shaft.

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    $\begingroup$ Any sense of numbers? If a turbofan could produce 185kN of thrust - would the APU produce .... 1kN? $\endgroup$
    – RoboKaren
    May 3, 2017 at 21:14
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    $\begingroup$ @RoboKaren an APU does not accelerate air out the back, it's like the exhaust of a car. Negligible thrust if any. $\endgroup$
    – user14897
    May 3, 2017 at 21:25
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    $\begingroup$ @RoboKaren, the APU is designed to have just enough power to turn the attached generator. The exhaust gasses don't have much energy left. $\endgroup$
    – Jan Hudec
    May 3, 2017 at 21:51
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    $\begingroup$ If the APU could provide thrust sufficient to keep the aircraft airborne, Sullenberger's landing would have played out quite differently... $\endgroup$
    – You'reAGitForNotUsingGit
    May 4, 2017 at 0:32
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    $\begingroup$ I don't know much about the performance numbers of APUs, but I do know that they are generally a gas turbine that has a lot in common with turboprop engines, with which I am quite familiar. A PT6 for, example, will typically have an additional 5-6% of thrust energy from the exhaust in addition to the energy that goes to driving the prop. It seems that a similar phenomenon would hold true for APUs. Is there any data available on just how much thrust? $\endgroup$
    – J W
    May 4, 2017 at 1:56
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The Antonov An-26B cargo aircraft (produced in the Soviet Union) has a Tumansky RU19A-300 combined APU and jet engine mounted on the wing behind the right main engine. This unit provides additional thrust up to 800 kg to improve takeoff and safety in case of engine failure.

This information is from the accident investigation report of Polish aircraft An-26B SP-FDO in Tallinn, Estonia on March 18. 2010. During the flight from Helsinki to Tallinn, 9.5 NM from Tallinn the left engine failed, and the thrust-providing APU could not be started. The aircraft was not configured for landing, flew over the runway and made landing on snow- and ice-covered lake Ülemiste (main supply of drinking water).

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  • $\begingroup$ This sounds nearly the same as the initial plan for the HS Trident 3B's booster engine, but the APU removal scrapped due to fuel consumption. $\endgroup$
    – user71659
    Jul 11, 2018 at 3:09
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    $\begingroup$ "The RU 19-300 was shot down after takeoff. " -- man, that's a pretty bland accident report to hide such a significant event! $\endgroup$
    – quiet flyer
    Apr 25, 2020 at 19:34
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    $\begingroup$ " With RU19A-300 operating the critical engine is left engine, with inoperative RU19A-300 the critical engine is right engine." -- fascinating-- $\endgroup$
    – quiet flyer
    Apr 25, 2020 at 19:42
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The amount of thrust available from an APU would be minuscule as it is designed drive various aircraft systems and adding on a nozzling system geared for propulsion would reduce the power available for doing it's intended work.

Even if an APU was re-purposed for propulsion, it most likely would not be enough to have any significant role. While horsepower and thrust are dissimilar terms that do not have a simple conversion, we can do some comparisons to show how under-powered an APU would be

An Airbus 350 has a pair of jet engines which combine to produce about 195,00 lb-ft of torque. This same aircraft has an APU which can produce up to 1700 equivalent shaft horsepower.

The Airbus 400 has 4 turboprops that combine to produce 45,000 HP, and has a max takeoff weight roughly 1/2 that of the A300.

While I am in no way an aeronautical engineer; and these comparisons are on the order of apples and oranges, I don't think it is a grasp to say "it ain't happening" to an APU providing propulsion.

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Not alone, but APU can help to get more thrust by taking over the duty to supply the bleed air from the main engines.

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Practically speaking, APUs are not sufficiently powerful to provide the thrust necessary to maintain level flight. It is an issue of power (as well as having that power effectively converted to thrust). The APU is designed to provide electrical, bleed air (for environmental control, and for starting other engines), and in some cases they may run other accessories.

It may be possible to have more power available to the main engine(s) if the APU is used for electrical, bleed air, etc. Normally, this is not done in most aircraft.

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Thanks to pots and Federico for their answer mentioning the Tumansky RU 19A-300 combined APU and jet engine.

Further search found a NASA paper from 1968 using a GE TF-34 "convertible" turbine that could unload the generator shaft and redirect the power output with "variable inlet guide vanes" to a turbofan.

Around 8000 lbs of thrust could be created as a turbo fan alone. Combinations of thrust and shaft power were also possible.

Generally, an engine out/landing emergency will be in a speed range where a turbo fan (or prop) will be effective (around 150 - 250 knots).

So, maybe the tri-jet isn't dead, it just needs to be tweaked a bit. There are tales of using the 3rd top mounted jet to control pitch-up as well.

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