Topic Author
Posts: 25
Joined: Sun Jan 13, 2002 8:52 pm

Inboard And Outboard Flaps

Tue Feb 12, 2002 10:18 pm

I have a question,

The fact that flaps are split up in inboard and outboard flaps in my opinion leaves a hole in the flap area. It that to allow jetblast to have no interference from flap proximity? I saw this on a DC-10 and just wondered.

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Joined: Wed Jul 07, 1999 11:27 am

RE: Inboard And Outboard Flaps

Tue Feb 12, 2002 10:35 pm

Yup, that would be the case in about 90% of the time.

There may also be a structural reason. Such as the wing being "kinked" in that position. So you need a space there to make sure that the flaps don't hit each other when deployed.
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Joined: Wed Oct 06, 1999 10:48 am

RE: Inboard And Outboard Flaps

Tue Feb 12, 2002 10:51 pm

That is because of the inboard Aileron. Many aircraft have two sets of ailerons. Having the ailerons as far out on the span work best for controlling roll rate. However at higher speeds they produce twisting loads on the wing which can nullify the ailerons and even reverse the aileron direction (turning the column left could make the plane bank right). There's also a risk that it could overstress the wing. So they put an inboard aileron in at about a third the plane's total span. Usually short in total span, and placed on a thicker area of the wing that isn't going to bend. At low speeds both inboard and outboard ailerons work together. Once the flaps go up as a rule the outboard ailerons are centrally locked in the neutral position and only the inboard ailerons work. At higher speeds the inboard ailerons can produce the same roll rate as the outboard aileron could at low speeds. The inboard ailerons usually have a larger range of travel as well. Where the inboard aileron is, is where the gap in the flaps are.

There are exceptions to this rule. The DC-8 has the inboard and outboard ailerons paired together, so does the A-330 and -340, and just one huge flap. There is a thrust gate though to allow the thrust to go through right behind the engine (DC-8 only).

Some aircraft feature just outboard ailerons. Such as the DC-9, 757, A-319/-320/-321, and 737. They don't have very large wingspans and their wings naturally don't flex as much at the tips. Additionally the wings are deliberately strengthened so that the ailerons do not impose twisting loads on the wings.

If you look at the 757 also, it does not have a gap. It has double slotted flaps inboard and outboard, and flap-gap is filled with a single slotted flap. They are attached and extend as one.

Additionally, some aircraft don't have outboard ailerons. The Convair 880 and 990 are examples. They feature a large inboard aileron, and I think they use spoilers to augment roll-rate at low speeds. Maybe I'm getting confused with the B-52 though. The B-52 BTW has only an inboard aileron with spoilers to assist roll-rate.


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Joined: Wed Jul 05, 2000 11:37 am

RE: Inboard And Outboard Flaps

Wed Feb 13, 2002 2:51 am

The Classic B737s have Exhaust gates...but they did away with them on the Next gen - which have continous span flaps. So the thrust must not have been that much of an issue.
Posts: 43
Joined: Mon Jul 16, 2001 2:14 pm

RE: Inboard And Outboard Flaps

Fri Feb 15, 2002 9:10 am

Hi guys

Another line of thought is that its also a redundancy issue. Inboard and outboard flaps are NO DOUBT powered via different hydraulic systems. If you lose a system/or flap group, you dont necessarily lose ALL your flaps.

Just a little thought about the safety side of things, thats all.

Posts: 518
Joined: Sun Feb 04, 2001 10:34 am

RE: Inboard And Outboard Flaps

Fri Feb 15, 2002 10:12 am


Your statement is not quite true, at least for some Boeing aircraft. The 767 flaps are all driven from the one gearbox. Same with the 737. The 767 has a hydraulic motor and an electrical motor as it's redundancy source.


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