# Why are the flaperons on the 787 located so far in-board?

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.

• Possible duplicate of How does the particular piece of flap behind an engine on a B777 work?
– fooot
Jul 12, 2019 at 16:52
• Disagree, not curious how they work, curious why they positioned it where they did Jul 12, 2019 at 16:54
• It's explained in the answer. The location and function are related.
– fooot
Jul 12, 2019 at 16:59
• 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? Jul 12, 2019 at 17:14
• 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! Jul 12, 2019 at 17:26

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.

• 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. Jul 12, 2019 at 17:16
• @Jason this is the duplicate question's answer.
– fooot
Jul 12, 2019 at 19:30

The 737 has quite a complicated tracked mechanism to deploy its Fowler flaps. I think it's designed to allow all elements of the flaps to move parallel to each other, as opposed to perpendicular to the trailing edge. That prevents the flaps from jamming into each other at the inside corner.

The 787, on the other hand, has quite simple dropped hinged flaps. This is part of a long standing trend since the mechanically complex 747 of simplifying flap mechanisms by using more sophisticated aerodynamic design.2 So the flaps proper move a little bit aft as they extend, but I think the flaperon hinge isn't dropped, so that it doesn't extend aft and crunch into the flaps. This video shows that the trailing edge of the flaps moves aft, but that the flaperon does not. 787 Flaps during landing

I think part of the reason for the placement of the 777 flaperon is that it moves the Fowler flaps away from the jet wash. While jet wash would make the flaps more effective, it would also require them to be much stronger and heavier. Since it doesn't extend so far down and aft, the jet wash is a convenient place to put the flaperon.

This NASA paper is long, but a very readable survey of the various flap mechanisms used up until the 777. High-Lift Systems on Commercial Subsonic Airliners - Peter Rudolph

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.

• 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. Jul 12, 2019 at 18:10
• 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).
– Bram
Jul 13, 2019 at 21:23
• @Bram MLA is different from what JohnK was talking about. MLA is to reduce maneuver loads, while this involves flutter suppression/envelope expansion.
– JZYL
Jul 13, 2019 at 22:10
• I stand corrected. Misread John K's comment and interpreted the oscillations to be maneuver-induced, but they are not specified to be.
– Bram
Jul 13, 2019 at 23:34