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Inboard Versus Outboard Ailerons

Sun Jul 28, 2002 8:23 am

I have noticed that some commercial aircraft have inboard ailerons and some don't. e.g. 767 and Md80. When I have flown aircraft with inboards they are used when in flight at speed but on finals mostly the outboards are used. When flying a/c without inboards it seems that the outboards never move during cruise turns. Just to add to the complexity, The classic A300's had both, but the AB6 came out with only inboards. Can someone explain the zen of this please.
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RE: Inboard Versus Outboard Ailerons

Sun Jul 28, 2002 8:53 am

GENERALLY speaking the outboards are locked out at higher speeds as airlerons that far out can cause a few complications at high speed. That is why spoilers are used in conjuction with the inboard ailerons on modern Airbus's.
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RE: Inboard Versus Outboard Ailerons

Sun Jul 28, 2002 10:44 am

I second what Pikachu said, plus:

Add in the entire Boeing Line for featuring the device known as spoilerons, introduced some 30 years ago.  Smile/happy/getting dizzy (I'm not quite sure rather the 707s had it, but it was in the 727s for sure).

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RE: Inboard Versus Outboard Ailerons

Thu Aug 01, 2002 1:44 am

To add a little more detail to the discussion, I think someone should mention the aeroelastic reasons for having two sets of ailerons. The big problem with ailerons out at the tips of long flexible wings is aileron reversal at high speed. Let's say there is airliner flying pretty fast and the pilot wants to roll to the left using the outboard ailerons. The right aileron deflects down but what happens is that instead of "picking up the right wing" the wing twists because the aileron created a nose down pitching moment on the tip section of the wing. So now the wing is twisted at the outboard section, so that it is pointing nose down while the rest of the wing remains level. The result of this is that there is now a right rolling moment on the aircraft. A left roll was desired and a right roll results, not a good situation. Not to mention that the wing structure has experienced a little more torsion than it probably wants to. So the two solutions are either inboard ailerons or spoilers. Inboard ailerons are obviously mounted on a stiffer portion of the wing and thus won't suffer the same aeroelastic effects. Spoilers offer an advantage over ailerons in that they don't create adverse yaw, but you are dumping lift...I'll hold off on that for now...
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RE: Inboard Versus Outboard Ailerons

Thu Aug 01, 2002 1:59 am

On the 727, the outboard ailerons come into play when the flaps are extended. I've never heard of the 727 flight control system called spoilerons. On 727's, when rigging out the flight controls, the ailerons and spoilers are first rigged independently of each other; once that is completed, they are hooked together and their combined operation is checked.
I have observed the operation of the outboard ailerons on 747's and have seen them operate with the flaps are up; I am guessing, but it appears that the operation of the outboard ailerons is controlled by airspeed and not by flap position.
The DC-9 has only one set of ailerons, but this airplane has a lower cruising speed (Mach .76-.78) than does the 727 (Mach .80-.82) and that might be the reason they didn't design a dual aileron system for those airplanes.
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RE: Inboard Versus Outboard Ailerons

Thu Aug 01, 2002 6:30 am

Also the spoilers which are mounted forward of the ailerons do not induce undesireable twisting of the wing (reversal) that outboard ailerons would cause at higher airspeeds.

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