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Why aren’t flaperons common? In theory, they seem like a great idea. I know some planes like the 777 have them, but they’re very small in comparison to the normal flaps.

It would make sense to integrate the flaps and ailerons together, so is there any reason it’s not done more?

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Usually a simple solution is best. Complexity adds weight, cost, maintenance, failure modes, certification challenge, etc. In aviation, we should always strive for the most simple solution possible -- only adding complexity when it is required.

Start with a simple general aviation aircraft. It might not even have flaps. The ailerons are probably mechanically directly controlled by a series of pushrods, bell-cranks, and torque tubes connected to the stick.

When you add flaps (mechanically connected to the flap lever), you add a second system and you take away some of the wing trailing edge area for the ailerons. You must balance what goes to aileron vs. high lift.

As that aircraft increases in complexity, you likely go to a flap with some sort of fowler action. For a large transport aircraft, it will be a multi-element fowler flap.

Meanwhile, the aileron will almost always remain a 'plain flap' type -- because it can actuate symmetrically up and down.

When the main high lift system consists of double or triple slotted fowler flaps, the added lift you get from a small amount of plan flap is not much.

Further, now you must actuate the flaperon asymmetrically at some times and symmetrically in othter times. If these systems are mechanically connected, this means some sort of mechanical mixing. If they are actuated electrically, hydraulically, or electro-hydraulically, then there will be some level of added complexity. You might be able to figure out how to do it in hardware, but it may mean that the dreaded software comes into play. Although it may seem harmless, software is a tremendous challenge in aviation and adds much complexity and cost.

So as with everything else -- a decision is a cost benefit trade. Is some feature or technology optional or required to meet the desired performance? Does it 'buy its way on the airplane'?

For most aircraft, flaperons don't meet this standard.

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  • $\begingroup$ Also, lowering the ailerons will reduce their effectiveness for roll control. Unwise when being tossed about by gusts on final. $\endgroup$ Commented Mar 18 at 15:47
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    $\begingroup$ A counterexample for when complicated is justified: some large sailplanes and America's Cup sailboats have so many trailing-edge control surfaces, interacting in so many ways, that traditional nomenclature is abandoned. $\endgroup$ Commented Mar 18 at 15:51
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In addition to complexity there is also a safety issue.

Ailerons and flaps both change the camber of the wing. With ailerons, the difference in camber causes the plane to roll.

A malfunctioning flap system (unfortunately) will do the same thing. This is one of the reasons flaps are tucked in close to the fuselage, and ailerons are further out on the wing. In the event of a flap malfunction, ailerons are still available for controlling roll. As a last resort, rudder could also be applied.

A large airliner such as a 787, has a roll control system that uses spoilers, flaps, and ailerons, providing a greater range of roll response and safety in redundancy.

Systems like these are not needed in GA aircraft that fly in a much smaller speed envelope with simple mechanical linkages between the pilot stick/yoke and control surfaces.

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    $\begingroup$ A bigger reason for flaps inboard ailerons outboard is simply better control. Give the ailerons a bigger lever arm, and let the flaps increase washout. $\endgroup$ Commented Mar 18 at 15:47
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As the name suggests, a flaperon is a control surface that can act both as a flap and as an aileron (actually if deployed upward past the stall angle, it can even work as a spoiler to get a so-called spoileflaperon - and if mounted on a canard surface, it could even work as an elevator and be called elevatospoileflaperon 😅).

Why aren’t flaperons common

Flaperon are not that common because they are neither fish nor fowl.

If deployed one up and one down then they work just fine like any aileron; and if deployed both downward they work like a simple plain flap. Among all the flaps, a plain flap is the easiest one to be implemented: an hinge is all it takes. Anyway by an aerodynamic its performance is quite limited: it can actually even double the $C_{L_{max}}$ but it does so at the expense of a quite high increase in drag. More complicated flap configurations, like a double or even a triple slotted flap, are more efficient giving an even bigger increase of $C_{L_{max}}$ but with a definitely lower increase in drag.

Mechanically implementing its movement is also complicated since both a coupled and an opposite movement have to be achieved, which needs a complicated mixing unit. And obviously if the flaperons are used as flaps then they cannot be used as ailerons, at least not with the same authority.

For all these reasons their use is limited to all those cases where a low stall speed isn't that important and/or some proper flaps are there to be used when the flaperons act as flaps. Typically they are found on jet fighters and on jetliners where they are used as inboard aileron and as plain flap downstream from the engine exhaust.

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  • $\begingroup$ Haha what would the name of an elevator, canard, rudder, flap, and aileron be? I think another reason they’re not used too much is because if you just need a small aileron deflection, more drag would be made than normal ailerons, as they’re further out on the wing. $\endgroup$
    – Wyatt
    Commented Mar 19 at 3:38

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