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The fin is located under the Boeing 787 APU exhaust as shown in the picture below. Does anybody know the application of this thing or it is used for 'guiding' the APU exhaust? Thanks.

enter image description here

Pondlife
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Chaintawit Singkaew
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    APU fuel drip shield. – Pritam Dec 19 '17 at 02:05
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    At a guess it’s an aerodynamic fence of some sort, most likely added during flight test as a band aid against unwanted turbulent airflow in that region. It’s a relatively simple fix for bad aerodynamic design – Romeo_4808N Dec 19 '17 at 02:38
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    It's a bib for the APU to stay tidy. – Koyovis Dec 19 '17 at 08:28
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    @Pritam: What about writing an answer? Here is more information you can use (it prevents torching). – mins Dec 19 '17 at 09:41
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    My guess is, because of the shape of the tail, air will be flowing up and meet the horizontal flow. If there is a hole in the middle there will be a lot of turbulence so they added a deflector to let them mix slowly. For a plane of this size this is more likely to reduce noise or improve the efficiency of the APU. – user3528438 Dec 19 '17 at 12:22
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    It's primary purpose is noise abatement. NASA has been encouraging mfg to install similar shielding plates near other parts of the aircraft such as the whistling wheel brake parts. The flying wing concept has the engines above the fuselage (for ground noise blocking) for the same reason. During landing, the airframe is louder than the engines. NASA has several high profile noise abatement research projects such as QAT. https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20130013127.pdf and https://www.nasa.gov/centers/langley/pdf/70882main_FS-2002-09-73-LaRC.pdf – jwzumwalt Dec 20 '17 at 11:32
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    @jwzumwalt The article mins linked shows it has nothing to do with noise. It's to avoid a flamethrower when they start it up. – TomMcW Dec 20 '17 at 21:48
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    Gas turbines don't spit flames out the back when they start up unless something has failed, and a shield on the exhaust wouldn't stop that anyway. – Penguin Dec 21 '17 at 10:34
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    All gas turbines including APU's have a drain tube from the combustion chamber that will drain excess fuel onto the ground. If there was any excess fuel in this APU, the slope of the exhaust would pool it in the engine and that fairing would not have any effect - nobody would stand for that. – jwzumwalt Dec 21 '17 at 19:17

2 Answers2

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As mentioned in the comments, this is indeed an "aerodynamic fence." Fences of this sort serve two purposes:

  1. Direct airflow; or
  2. Energize (or trip) boundary layer airflow.

The latter are normally perpendicular to the surface on which they are mounted, allowing them to protrude directly into the boundary layer of air flowing over the surface. You can picture the effect of these types of devices by looking a how water flows over and around a boulder in a stream or river. "Tripping" the flow mixes the boundary layer with the more energetic air outside the boundary layer.

It is most likely the former, a device used to direct the airflow. By the time air gets to the tail of an airplane, to the aft most part of the empennage, it is quite unsteady flow. (In this case, imagine the water flow behind a large ship.) The turbulence characteristic of this type of flow can wreak havoc for designers who want to vent exhaust gases or airflow out of the aircraft's APU. This turbulent flow can create stagnant zones which would block the air from exiting the way intended, as described here. By directing the flow of the air around the exhaust duct, this can be minimized or avoided altogether. There is, however, probably a small amount of the boundary layer that gets energized by this device, which would further help the designers manage the flow.

The article previously cited calls them vortex generators, which are explained in more detail here and correspond to 2 above. Fuel that was pooling in the APU was a result of the stagnation of the flow.

Mark Jones Jr.
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    Can you cite a source to back up your answer? The article that @mins linked in comments to the OP indicates that the purpose is to allow fuel to exit the tailcone during flight rather than pooling in the tailcone. What you describe regarding airflow is consistent with that explanation, but you're talking about exhaust gasses; the article about fuel before the APU is turned on. Can you provide sources for your answer, and/or additional clarification, reference that article? – Ralph J Jan 02 '18 at 22:38
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    Actually, it is more likely that unburned fuel pools after the APU is shut down and would re-ignite once the APU is restarted. – Mark Jones Jr. Jan 02 '18 at 23:38
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    Albeit being a technically perfect answer, it does not answer the question and it is actually formally wrong: vortex generators and/or strakes and/or fences and/or... are used to improve the airflow on the surfaces behind them. Behind that fences there's nothing, so its purpose is anything but aerodynamics... The only aerodynamic thing that it is doing is most probably producing additional drag – sophit Mar 16 '23 at 07:53
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StackExchange is so funny sometimes!
It is to simply keep fuel from the body drain tubes out of the APU exhaust.

787Engineer
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