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Classa64
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Thrust From An APU?

Sat Jan 30, 2016 4:57 pm

Was looking at a great shot posted a few days ago of a A320 climbing out. I noticed the jet wash from the two main engines and also noticed the jet wash out the tail cone from the APU. Silly question; Does the APU provide any kind of thrust that helps in anyway? I would think it does not provide any usable thrust due to its size on such a big plane but I guess that's why I am asking.

C.
 
WIederling
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RE: Thrust From An APU?

Sat Jan 30, 2016 5:26 pm

Quoting Classa64 (Thread starter):

If it would provide thrust it would be inefficient as an APU, wouldn't it?
 
mmo
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RE: Thrust From An APU?

Sat Jan 30, 2016 6:08 pm

Quoting Classa64 (Thread starter):
Does the APU provide any kind of thrust that helps in anyway? I would think it does not provide any usable thrust due to its size on such a big plane but I guess that's why I am asking.

Zero thrust provided by the APU.
 
Bambel
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RE: Thrust From An APU?

Sat Jan 30, 2016 6:50 pm

From my understanding it depends on the speed of the exhaust gases. If they are faster than the plane, they schould provide thrust. But i guess that the amount – if any – is very low.

b.
 
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Horstroad
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RE: Thrust From An APU?

Sat Jan 30, 2016 8:34 pm

It preduces nearly no thrust. I held my hand in the exhaus stream of an MD11 APU (not for long obviously and wearing gloves as it is quite hot)... it is like a very hot big hair dryer.
If the APU produced any significant thrust you would see problems on models like the B777 and especially the MD11 where the exhaust is not on the centerline of the fuselage but rather to the side.
 
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Classa64
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RE: Thrust From An APU?

Sat Jan 30, 2016 8:55 pm

No thrust thanks, question answered.

I was not aware so little pressure came out the back and I never thought of side exit exhaust.

C.

Quoting WIederling (Reply 1):
Quoting Classa64 (Thread starter):

If it would provide thrust it would be inefficient as an APU, wouldn't it?

Why ( if it did ) be inefficient if it provided thrust?, engines on the wings are like giant APU's in a way just using more of there energy to move the plane.
 
mmo
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RE: Thrust From An APU?

Sat Jan 30, 2016 9:29 pm

Quoting Classa64 (Reply 5):
Why ( if it did ) be inefficient if it provided thrust?, engines on the wings are like giant APU's in a way just using more of there energy to move the plane.

The APU exhaust is just that, an exhaust. There is, depending on the aircraft a lot of piping for the exhaust to exit the aircraft and on some aircraft, it actually vents out the side. Also, the engine is relatively small and it does not produce that much thrust. It is primarily designed to provide bleed air for the the packs to use on the ground for heating and cooling and to provide bleed air for engine start. It also provides electric power through a generator.
 
Natflyer
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RE: Thrust From An APU?

Sat Jan 30, 2016 10:54 pm

Quoting Classa64 (Thread starter):

The APU increases speed by about 2 knots...per year.
 
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Starlionblue
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RE: Thrust From An APU?

Sun Jan 31, 2016 2:19 am

While the gas generator in many APUs, turboprops and turbofans is very similar, it's a matter of where you want the energy produced by the gas generator to go: output shaft, fan or exhaust.

APUs are designed to perform maximum work in the turbine which drives the output shaft. If you take out as much energy as possible as torque on the output shaft, there's not much energy left in the exhaust since you've used up all that kinetic energy. This is similar to how a turboprop or a helicopter turboshaft works, with maximum energy going to the prop/rotor and only a little bit of thrust. On the other hand, turbofans have significant thrust through the exhaust, meaning not all the energy goes to the fan. In the case of a turbojet, only enough energy goes to the compressor stages to actually compress the air, and most of the energy is converted directly to thrust in the exhaust.

Quoting Classa64 (Reply 5):
Quoting WIederling (Reply 1):
Quoting Classa64 (Thread starter):

If it would provide thrust it would be inefficient as an APU, wouldn't it?

Why ( if it did ) be inefficient if it provided thrust?, engines on the wings are like giant APU's in a way just using more of there energy to move the plane.

It think what Wlederling meant is that if you take out energy as thrust to the exhaust, this energy is not going to the output shaft and is thus wasted with regards to APU function.
 
L-188
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RE: Thrust From An APU?

Sun Jan 31, 2016 10:55 am

If memory serves the right nacelle on the AN-24 housed a turbo-jet engine in the rear section that served as a boost nine for takeoff a swell asan APU to provide starting air for the two engines.
 
WIederling
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RE: Thrust From An APU?

Sun Jan 31, 2016 12:47 pm

Quoting Classa64 (Reply 5):
Why ( if it did ) be inefficient if it provided thrust?, engines on the wings are like giant APU's in a way just using more of there energy to move the plane.

Thrust is energy not extracted by the turbine section. An APU is for generating shaftpower and not for thrust.
exhaust thrust thus is a sign for inefficient power generation.

Quoting Starlionblue (Reply 8):

Yes.

Quoting L-188 (Reply 9):

Urgh.

.. but seems to be true: http://www.airvectors.net/avan24_1.html
interesting solution. ( thust: 485 lbf )

Then: Trident versions with 4 engines, the fourth offset in the tail but used for thrust. See:
http://en.wikipedia.org/wiki/Hawker_Siddeley_Trident#Trident_3B
HS Trident 4th Engine? (by Boeing744 Feb 21 2006 in Civil Aviation)

[Edited 2016-01-31 04:52:47]
 
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Classa64
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RE: Thrust From An APU?

Sun Jan 31, 2016 6:30 pm

Thanks again for the answers, clarification and links from everyone. I guess I was not thinking about what the energy it produced was being used for and thrust is not one of them.

C.
 
LH707330
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RE: Thrust From An APU?

Mon Feb 01, 2016 6:44 am

OP, the jet wash you're talking about is a heat shimmer, the hotter gas refracts the light and distorts the background image. So the warm air coming out of the APU exhaust was causing a shimmer due to heat, with negligible thrust.

Technically, an APU does make some thrust in situations where the exhaust is coming out at any speed faster than the plane is moving, but as others have also mentioned, it's negligible.
 
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zeke
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RE: Thrust From An APU?

Mon Feb 01, 2016 7:41 am

Quoting Classa64 (Thread starter):
Does the APU provide any kind of thrust that helps in anyway? I would think it does not provide any usable thrust due to its size on such a big plane but I guess that's why I am asking.

The APU does not provide any noticeable thrust, however it can be used to generate the bleed air used by the packs, leaving the engines with additional thrust margin (bleed demand unloaded) that can be used to improve takeoff performance.
 
jetstar
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RE: Thrust From An APU?

Sun Feb 07, 2016 6:58 pm

In order to get any thrust out of an APU you need to have a tailpipe with a smaller diameter than the turbine fan section to force the exiting gases to speed up producing thrust, without this type of tailpipe the exiting gases just disperse into the air as what happens on an APU.

This is true for any jet engine core, remove the tailpipe and you have a gas generator. Today’s large fan engines produce most of their thrust from the fan section, but the core still produces a good percentage of thrust, remove the tailpipe and now you have a very large turboprop engine.

I remember reading somewhere years ago that Boeing looked into the idea of using the tail mounted APU as an additional thrust producer for take off since once the engines are running there is no more need for the APU, so by using a tailpipe that could be partially closed the APU exhaust gases could now be used to produce some additional thrust on takeoff.

JetStar
 
Goldenshield
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RE: Thrust From An APU?

Sun Feb 07, 2016 8:46 pm

An APU is really just a turboshaft engine, like you'd see on a helicoptor. It sucks up as much energy as it can to drive the generators and provide bleed air to the pneumatic system, which in turn leaves it with little to no thrust coming out.
 
MHG
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RE: Thrust From An APU?

Sun Feb 07, 2016 9:39 pm

APU´s certainly do not deliver any relevant thrust.
Otherwise they would be very inefficient.

As turboprops are mentioned for comparison I can give some data.
Turboprop engines´s exhausts produce "some" thrust ...
One particular engine I remember is the RR Dart (used in F-27 / HS 748 / etc.)
The Dart 7 Mk532-7 (and higher versions) produces 238 Kgs of exhaust thrust.
Early versions like the Dart 6 Mk 507 only upwards from 155 Kgs.
The PW124 and PW125B of the Fokker 50 produces a similar thrust of 238 Kgs.
 
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TWA772LR
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RE: Thrust From An APU?

Mon Feb 08, 2016 6:13 am

Slightly OT, but if you had an APU from a 737 or A320 (or of similar size and output), and built a Cessna-sized airplane, would that be a viable means of propulsion?
 
Tristarsteve
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RE: Thrust From An APU?

Mon Feb 08, 2016 8:58 am

When the B777-300ER was initialy studied, Boeing suggested an APU that provided take off assistance.


http://www.flightglobal.com/pdfarchi...67.html?search=B777%20APTU%20Study
 
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Starlionblue
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RE: Thrust From An APU?

Mon Feb 08, 2016 2:19 pm

Quoting TWA772LR (Reply 17):
Slightly OT, but if you had an APU from a 737 or A320 (or of similar size and output), and built a Cessna-sized airplane, would that be a viable means of propulsion?

Sure. For example, the engines of the BAe-146 use the Lycoming T-55 engine core, which is also used as a turboshaft engine in helicopters. It all depends on where you use the power derived from the gas generator: shaft power, free turbine prop or fan.

For scale comparison purposes, I think from memory the APU on the 747 is about as powerful as one of the engines on a Twin Otter.
 
Tristarsteve
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RE: Thrust From An APU?

Mon Feb 08, 2016 4:15 pm

Quoting Starlionblue (Reply 19):
For scale comparison purposes, I think from memory the APU on the 747 is about as powerful as one of the engines on a Twin Otter.

The Tristar APU was a PW PT6 with a load compressor instead of a prop..

Just saw an ATR72 stop on the gate. The Nbr 2 engine keeps running with the prop brake on as an APU. Now that is an APU that provides thrust. 
 
apfpilot
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RE: Thrust From An APU?

Mon Feb 08, 2016 7:20 pm

Quoting Tristarsteve (Reply 20):
Now that is an APU that provides thrust.

Or is that a thruster (I know I know incorrect usage) that provides AP?
 
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HAWK21M
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RE: Thrust From An APU?

Mon Feb 29, 2016 6:44 am

Quoting Natflyer (Reply 7):

Should read as decreased if it did  

On the topic.....The thrust in flight is insignificant to contribute to an affect to the aircraft thrust which the powerplant are contributing to already.
As on ground too, its very low.
 
KC135Hydraulics
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RE: Thrust From An APU?

Sat Mar 05, 2016 6:44 pm

Interesting topic! Food for thought, the KC-135's APUs (it has two) are actually mounted inside an enclosure inside the fuselage and they exhaust straight out of the left side of the aircraft. I am sure if APUs provided any meaningful amount of thrust they would not be permitted to exhaust in this manner as it would add some measure of undesirable yawing moment if in flight.
 
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HAWK21M
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RE: Thrust From An APU?

Sun Mar 06, 2016 8:14 am

Quoting KC135Hydraulics (Reply 23):
KC-135's APUs (it has two)

Any reason why two APUs.....why incur a weight penalty....would one be unreliable.....
 
26point2
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RE: Thrust From An APU?

Sun Mar 06, 2016 11:22 am

In at least 1 case APU running makes for a worse climb. The GLEX incurs a 2% climb penalty with APU running due to drag from the open APU inlet door.
 
kalvado
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RE: Thrust From An APU?

Sun Mar 06, 2016 5:21 pm

Quoting Goldenshield (Reply 15):
An APU is really just a turboshaft engine

Thinking about turbofan as a turboshaft engine used to drive the fan (and I believe this is not completely incorrect), why does hot section of a turbofan produces noticeable thrust? I can only think of gas mixing at exit being a major difference..
 
KC135Hydraulics
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RE: Thrust From An APU?

Sun Mar 06, 2016 5:28 pm

Quoting HAWK21M (Reply 24):
Any reason why two APUs.....why incur a weight penalty....would one be unreliable.....

It has to do with nuclear alert purposes. KC-135s are often used for refueling nuclear-capable bombers, and in the event of a real-world situation when a 135 needs to launch on a moment's notice, the 135 can start two engines at once as opposed to only one, with both APUs operable. It's called EWO. In reality, for most missions, only one APU is used. Of the two APUs, only one has an electrical generator installed at any given time, and that is the APU typically used for engine starts.

Oh yeah, when it's particularly hot or humid (IE Al Udeid, Qatar), one APU does not provide sufficient bleed air, so the second is started as well, just to get one engine turning. Sometimes, even that is insufficient, and we have to include a -95 as well (air cart). Those launches suck.
 
georgiaame
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RE: Thrust From An APU?

Mon Mar 07, 2016 3:11 am

I could be and probably am very wrong, but I seem to remember that the ORIGINAL 777, the trijet that was going to be build along with the 757 and 767, was supposed to have an APU that would indeed generate thrust on take off, then throttle back to do its regular job while the other 3 engines did theirs. Feel free to correct me on this bit of aviation trivia.
 
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Moose135
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RE: Thrust From An APU?

Mon Mar 07, 2016 3:50 am

Quoting KC135Hydraulics (Reply 27):
KC-135s are often used for refueling nuclear-capable bombers, and in the event of a real-world situation when a 135 needs to launch on a moment's notice, the 135 can start two engines at once as opposed to only one, with both APUs operable.

No more cartridge starts? Those were fun in the A-model, especially when we had carts in all 4...   When I was flying them, we couldn't use the APU, as there had been several fires, and they were disabled.
 
Bambel
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RE: Thrust From An APU?

Mon Mar 07, 2016 9:35 am

Quoting kalvado (Reply 26):
Thinking about turbofan as a turboshaft engine used to drive the fan (and I believe this is not completely incorrect), why does hot section of a turbofan produces noticeable thrust? I can only think of gas mixing at exit being a major difference..

Only a guess, but i think it has to do with turbine size. If you want to extract most of the energy out off the exhaust gases the turbine would be too large. In a stationary power plant size and weight matter much less so you can use a higher yield turbine.

b.
 
Tristarsteve
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RE: Thrust From An APU?

Mon Mar 07, 2016 4:11 pm

Quoting georgiaame (Reply 28):
I could be and probably am very wrong, but I seem to remember that the ORIGINAL 777, the trijet that was going to be build along with the 757 and 767, was supposed to have an APU that would indeed generate thrust on take off

See reply 18 for details
 
georgiaame
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RE: Thrust From An APU?

Mon Mar 07, 2016 9:22 pm

Quoting Tristarsteve (Reply 31):
See reply 18 for details

Mea culpa. But at least I wasn't hallucinating, and I got the model number correct.
 
KC135Hydraulics
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RE: Thrust From An APU?

Mon Mar 07, 2016 11:31 pm

Quoting Moose135 (Reply 29):
No more cartridge starts? Those were fun in the A-model, especially when we had carts in all 4... When I was flying them, we couldn't use the APU, as there had been several fires, and they were disabled.

Definitely not. That system was long-since disabled, probably at the same time as the water tank and its associated plumbing.
 
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Moose135
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RE: Thrust From An APU?

Tue Mar 08, 2016 12:23 am

Quoting KC135Hydraulics (Reply 33):
Definitely not. That system was long-since disabled, probably at the same time as the water tank and its associated plumbing.

I assume it went away with the old engines. It was impressive seeing a dozen A-models do a quick start with 4 carts each.
 
Apprentice
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RE: Thrust From An APU?

Tue Mar 08, 2016 12:48 am

Quoting L-188 (Reply 9):

From "Loudest thrust reverser" post:
Agree with An-26 (and late An-24 versions). Cause, APU installed on them is a TUMANSKY RU-19 A300, which is a small noisy jet engine used as TO booster / Aircraft Climb Supplemental Power / Power supply in case of Main Engine(s) Fault [to complete flight]. / Auxiliary Power Unit -electrical generator- , located on RH Power Plant- LG Nacelle, with a Exhaust to short for Noise's reduction.
For comparision: RU-16 power is stated at 35.8( 800 lbf) , while main engines AI-25 have a Maximum thrust: 16.9 kN (3,800 lbf).each. During engine Mx run or A/C TO, Mx or Flight Mechanic should use plenty of Fuel Cross Feed to keep Wings leveled.
AN-26, which has been designed as a "Small Troops/ Cargo plane, requiered a "Military Power" from onset.

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