Adria
Topic Author
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Ailerons On A330/340 And B777

Fri Jan 09, 2004 8:49 am

Why are there such differences in aileron position between the A330/340 and B777 family? Why does the B777 have ailerons at inner and outer part of the wing? I also noticed that with extension of flaps it moves down too. Why is that? Why doesn't Airbus put it like Boeing?
 
SlamClick
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RE: Ailerons On A330/340 And B777

Fri Jan 09, 2004 9:57 am

There are many ways to solve the engineering problems in airplane design.

I am not familiar with the technical details of the triple-seven and don't have any manuals handy but I can give you some general info. It has been a long-standing Boeing practice to use inboard and outboard ailerons. The outers would have more authority at the same amount of throw (deflection) as the inners because they are farther from the CG. More leverage. In fact, at high speed they have too much authority. And so, at high speeds, Boeing designs use the inboard ailerons and roll spoilers for roll control. The outboard ailerons fair to neutral with flap retraction.

Like many other airplanes including the DeHavilland of Canada ones, the ailerons droop with flap extension. They contribute to the production of lift at low speeds but still function in roll control.

Airbus and Boeing alike use roll spoilers for primary roll control at high speed/mach number.

In some early high-speed airplane designs there was a problem with the ailerons twisting the wing at very high speeds with the resultant high dynamic air loads. Friends have told me that the pilot checkout in the B-47 (and the B-52 also, I believe) included "aeroelasticity" demonstrations where the IP would put the plane in high speed low altitude flight, then roll the yoke laterally from stop to stop. The plane would continue straight ahead and the wings would twist. (of course they did not hold it to the stops for very long)

Roll spoilers address this problem. Typically, yoke input in the high speed regime deflects roll spoilers only until a sufficient amount of roll is called for, then the ailerons begin to operate.

Happiness is not seeing another trite Ste. Maarten photo all week long.
 
liamksa
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RE: Ailerons On A330/340 And B777

Fri Jan 09, 2004 10:26 am

Also, the effectiveness of the outboard ailerons at high speeds would twist the wing around the lateral axis (as mentioned by Slamclick) reducing the AoA on the wing with the downgoing aileron. The loss of lift on this wing would cause it to drop - ie: a commanded roll to the left would cause a roll to the right - aileron reversal.

I believe these issues contributed to other methods of roll control, ie: inboard ailerons and spoilers.
 
ArmitageShanks
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RE: Ailerons On A330/340 And B777

Fri Jan 09, 2004 10:37 am

Why not have exclusive roll-control spoilers?
 
liamksa
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RE: Ailerons On A330/340 And B777

Fri Jan 09, 2004 11:05 am

Just a guess but maybe:

- Redundancy - the failure of a single system won't leave you with no roll control

- Control - a higher roll rate can be achieved with both ailerons and spoilers operating simultaneously
 
SlamClick
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RE: Ailerons On A330/340 And B777

Fri Jan 09, 2004 11:37 am

ArmitageShanks

Couple of problems with using roll spoilers ONLY:

  1. Spoilers induce roll by reducing the lift on the "inside" or "down" wing in a turn. If an airplane is already in a critical performance regime reducing lift is not very desirable. Ailerons induce roll by increasing lift on the [up]wing with DOWN aileron and simultaneously decreasing lift on the [down] wing with UP aileron. So, spoilers only = loss of overall lift.


  2. Since spoilers induce roll by reducing lift, they don't work upside down. In fact they are not terribly effective at low speeds and high angles of bank. The only way they help a turn when in this attitude is to increase the drag on the wing with spoiler deflection. There is a chilling demonstration of this in the video of the B-52 crash at SKA (Fairchild AFB) a few years ago. You can clearly see full spoiler deflection to no effect as the plane continues to roll left and the nose begins to drop.



Happiness is not seeing another trite Ste. Maarten photo all week long.
 
bio15
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RE: Ailerons On A330/340 And B777

Fri Jan 09, 2004 1:04 pm

I am a bit confused here. I thought that roll spoilers and ailerons were used on slow speed flight and then at high speed flight only the inboard ailerons did the job. Does this hold true for any aircraft or I've had it wrong all this time? As a matter of fact I think I learned that in this forum. Thanks in advance

-Alfredo
 
cdfmxtech
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RE: Ailerons On A330/340 And B777

Fri Jan 09, 2004 3:41 pm

With the exception of the 727 and 707, every Boeing narrowbody only has one set of ailerons. The B747, B767 and B777 all have inboard ailerons in addition to the outbd set. All of these aircraft have flight spoilers that aid in roll control.

On the Narrowbodies, the outbd ailerons provide the roll before the flight spoilers mix in. The only reason you don't see the flight spoilers in high speed flight is that the amount deflection necessary to perform a turn isn't that great.

On the widebodies (and the 727), with the flaps down, the outbd and inbd ailerons are fully functional. When the flaps are up, only the Inbd aileron is functional as the outbd ailerons are mechanically locked out. As I understand it on the 707, the outbd ailerons were not mechanically locked out, but were available as a function of dynamic pressure. The more ram air acting on the ailerons, the less deflection available.

The aileron/flaperon droop, as previously stated in to increase the lift.
 
buckfifty
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RE: Ailerons On A330/340 And B777

Fri Jan 09, 2004 8:03 pm

On Airbus aircraft, there is no need for an inboard aileron, as the ailerons themselves have no mechanical linkage whatsoever. The ailerons are electrically controlled and hydraulically actuated, and are governed under software control as long as a FCC is still available.

The movement from the sidestick does not translate directly into a proportional movement on the aileron, as it is more of a roll demand command. The computer will automatically adjust however much the aileron moves according to the flight conditions, so that the aircraft will roll with the same characteristics at low speed and at high speed.

In case of the failure of all five flight control computers, only the stab trim and rudder are available for control.
 
ArmitageShanks
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RE: Ailerons On A330/340 And B777

Sat Jan 10, 2004 2:01 am

Wow, that's a great explanation SlamClick! I didn't even think about the performance hit they would have. Thanks.
 
SlamClick
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RE: Ailerons On A330/340 And B777

Sat Jan 10, 2004 2:14 am

Oops! My mistake!

In reply # 1 - last paragraph I got it backwards. The correct statement is:

On Boeing aircraft rolling the yoke first activates airleron. Then in enough roll is called for, the roll spoilers deploy.

Buckfifty got it exactly right in reply #8 about the Airbus system.

Happiness is not seeing another trite Ste. Maarten photo all week long.
 
rendezvous
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RE: Ailerons On A330/340 And B777

Sat Jan 10, 2004 4:56 pm

I read a trip report somewhere, Qantas 767 (Auckland - Sydney I think). The outboard ailerons wouldn't lock out, so they had to cruise at a reduced speed to avoid damage to the wing.

What speed would they have to cruise at to be "safe"?

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