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DocLightning
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744 APU Fairing?

Sun Mar 29, 2009 7:21 pm

So I was at LAX yesterday and we went right behind a 744 (actually, two of them. One Air China and one KL) when I noticed that the fairing over the APU looks radically different from the rest of the body panels. The APU fairing is made up of myriad small panels, many of them probably not much more than a foot on a side.

Anyone know why? I don't think this is the case on the 777.
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lowrider
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RE: 744 APU Fairing?

Sun Mar 29, 2009 7:49 pm

What look like panel lines are actually lines of rivets. See the following closeups for a comparison:

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Photo © Gregg Stansbery


and

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Photo © Craig Murray



APUs need to be contained in a stronger structer than the rest of the tail cone due to risk of fire and rotor burst. Some aircraft accomplish this by enclosing the APU with a shroud or box, others simply reinforce the area around it. Perhaps the paint and construction methods on the 777 hide the rivet lines better.

[Edited 2009-03-29 12:59:33]
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zanl188
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RE: 744 APU Fairing?

Sun Mar 29, 2009 7:55 pm



Quoting DocLightning (Thread starter):
The APU fairing is made up of myriad small panels, many of them probably not much more than a foot on a side.

Anyone know why? I don't think this is the case on the 777.

APU Fairing has round head rivets that protrude above the skin, most other areas are flush rivetted and the rivets don't show as much.


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Photo © Klaas Reinder Sluijs
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Photo © Craig Murray



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Photo © John F. Zielinski

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boacvc10
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RE: 744 APU Fairing?

Tue Mar 31, 2009 4:33 pm



Quoting Lowrider (Reply 1):

APUs need to be contained in a stronger structer than the rest of the tail cone due to risk of fire and rotor burst. Some aircraft accomplish this by enclosing the APU with a shroud or box, others simply reinforce the area around it. Perhaps the paint and construction methods on the 777 hide the rivet lines better.

Is there a reason for the APU on the B747 to be in line with the vertical stablizer, fuselage, yet the B777 has it on the side ?

Does it make a bit of difference ?

Also, generally (I am not a pilot) how much generating capacity does the APU have in KW, and where is its fuel tank / supply ?

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RE: 744 APU Fairing?

Tue Mar 31, 2009 4:44 pm



Quoting Boacvc10 (Reply 3):
Is there a reason for the APU on the B747 to be in line with the vertical stablizer, fuselage, yet the B777 has it on the side ?

Both the 747 and 777 have their APU mounted on the center line of the aircraft. The 777 just uses a 'screw driver' type tail cone.
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lowrider
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RE: 744 APU Fairing?

Tue Mar 31, 2009 9:50 pm



Quoting Boacvc10 (Reply 3):
Also, generally (I am not a pilot) how much generating capacity does the APU have in KW, and where is its fuel tank / supply ?

The 747 Classic has two 90 KVA gens mounted on the APU. The fuel supply is the #2 Main tank. I believe the 400 has the same gens, but a different model APU. I would imagine the fuel supply is the same source.
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tdscanuck
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RE: 744 APU Fairing?

Wed Apr 01, 2009 5:26 am



Quoting Boacvc10 (Reply 3):
Is there a reason for the APU on the B747 to be in line with the vertical stablizer, fuselage, yet the B777 has it on the side ?

Does it make a bit of difference ?

The knife-edge tail on the 777 has a slight drag advantage over the conventional cone. However, it's harder to build and has some maintenance issues related to turning the APU exhaust. It's a trade without a really clear winner...Boeing went back to the cone for the 787 so the drag benefit couldn't have been that huge.

Quoting Boacvc10 (Reply 3):
Also, generally (I am not a pilot) how much generating capacity does the APU have in KW, and where is its fuel tank / supply ?

It varies from airplane to airplane...787 will be the largest at (I believe) 250 kW. Fuel source for all Boeing-heritage twins is the no. 1 main tank (aka the left wing).

Tom.

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