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Why are the flaperons positioned in between the outboard and inboard flaps (see 2nd picture)? Why not have them located as far outboard as possible to achieve maximum roll authority? I would think the order would be more efficient if the main flaps were both inboard, followed by the flaperons, followed by the ailerons. Thoughts?

In a previous post "How does the particular piece of flap behind an engine on a B777 work?", Peter Kampf explains the flaps would have a gap b/w them when extended because they extend perpendicular to the hinge line and the trailing edge is non co-linear. However, the 737 tackles this problem just fine (see 1st picture) so I don't think it fully captures why the flaperon is placed where it is on the 787 and 777.

737 flaps extended

787 control surface layout

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    $\begingroup$ Possible duplicate of How does the particular piece of flap behind an engine on a B777 work? $\endgroup$ – fooot Jul 12 at 16:52
  • $\begingroup$ Disagree, not curious how they work, curious why they positioned it where they did $\endgroup$ – Jason Jul 12 at 16:54
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    $\begingroup$ It's explained in the answer. The location and function are related. $\endgroup$ – fooot Jul 12 at 16:59
  • $\begingroup$ I disagree with the explanation, please reply to my comment on Peter Kampf's answer if you can clarify. His reasoning for position were that the flaps would otherwise have a gap due to the wing trailing edge breaking to a different angle. The 737 solved this problem and were able to design their flaps to extend flush with each other. Why not use the 737 design and then move the flaperons further outboard? $\endgroup$ – Jason Jul 12 at 17:14
  • $\begingroup$ fooot, I re-clarified my questions to differ from the duplicate, can you please remove the duplicate question tag so I can spark some more interesting conversation? Thanks! $\endgroup$ – Jason Jul 12 at 17:26
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Quoting the answer @fooot linked to:

At high speed, the outboard ailerons are locked and all roll control is achieved by deflecting the inboard ailerons (and spoilers, if needed). This helps to reduce wing torsion and twist.

Yes, roll authority will be larger once the force is applied further away from the fuselage (/ roll axis), but the cost at which this comes (wing twist) is larger than that benefit.

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    $\begingroup$ Thank you, I think this is a much better answer than the so called "duplicate" question's answer. I don't think the position has to do with having a non co-linear trailing edge as much as it has to do with the structural implications of having a flaperon so far outboard. $\endgroup$ – Jason Jul 12 at 17:16
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    $\begingroup$ @Jason this is the duplicate question's answer. $\endgroup$ – fooot Jul 12 at 19:30
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I can't speak for the designers on the true rationale, but one of the considerations would be a balance between control reversal speeds and roll rate requirements at high speed. I strongly suspect that the locking of the ailerons is because the aileron aeroelastic reversal speeds don't meet the margins required under Part 25 of 14CFR.

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    $\begingroup$ On the 747-8, the aerolastic problems resulted in a spontaneous tip oscillation/vibration in certain regimes, and they ended up using the FBW outboard aileron as an "active damper" device to stop it. The FBW computer detects the oscillation and actively runs the aileron to counteract it. An even bigger software bandaid than the MAX pitch problem fix but so far almost unknown outside the business. $\endgroup$ – John K Jul 12 at 18:10
  • $\begingroup$ The 'software bandaid' you describe is more formally known as (a particular type/instance of) Maneuver Load Alleviation (MLA). It reduces the wing load by applying the wing movables (spoilers, ailerons, flaperons) to generate a force or moment in the opposite direction. This way, the wing can be lighter, driving down OEW and -- in turn -- increasing payload mass and/or decreasing fuel consumption. I'm by no means an aircraft historian, but as far as I can tell, this is nothing fundamentally new (cf. yaw dampers). $\endgroup$ – Bram Jul 13 at 21:23
  • $\begingroup$ @Bram MLA is different from what JohnK was talking about. MLA is to reduce maneuver loads, while this involves flutter suppression/envelope expansion. $\endgroup$ – Jimmy Jul 13 at 22:10
  • $\begingroup$ I stand corrected. Misread John K's comment and interpreted the oscillations to be maneuver-induced, but they are not specified to be. $\endgroup$ – Bram Jul 13 at 23:34

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