I was talking about flutter the other day and how it occurs with the ailerons able to flap out of sync with the wing. I realized that the flaps, could in theory, move in a similar way. However, the ideal flap is rigidly attached to the plane and has little to no movement.

Does the FAA/EASA consider standard flaps-- e.g. plain, slotted, fowler-- to be balanced control surfaces?


1 Answer 1


No flaps do not need to be mass balanced. They are too rigidly attached to be able to feed energy back into the main wing structure and get a self-amplifying flutter oscillation going.

To get flutter going, the surface needs a degree of freedom of movement both up and down (like an aileron), some flexibility (springiness) in its control input connection, a center of mass that is aft of the hinge axis so that vertical motions create rotating moments about the hinge, and finally, a bit of springiness in the main structure.

A non-powered aileron (operated by cables and/or rods connected directly to the control wheel) without mass balancing tends to have this problem, so mass balancing eliminates the ability of the surface to induce flutter, in spite of its flexible interface with the main wing, by eliminating rotating moments created when the surface is vibrated vertically.

For an aileron or elevator, you don't need mass balancing if the surface is connected to the parent structure with a rigid, non-compliant attachment. You can achieve this by driving the surface hydraulically, so that the only movement of the surface that is possible is that induced by the hydraulic actuators. When you see jets that have no balance horns or offset hinge lines on ailerons or elevators, it's safe to assume it has hydraulic controls.

A flap is similar to the hydraulically operated aileron, even if its attachment is mechanical. The operating mechanism creates that same rigid connection between surface and main wing so that it can't feed back energy into the wing, and thus no balancing of the surface is required.

  • $\begingroup$ Excellent points. Do you know if the FAA and EASA generally see it the same way from a regulatory perspective? $\endgroup$ Commented Apr 12, 2023 at 11:26
  • $\begingroup$ The OEM will have to demonstrate the design meets the regulatory flutter resistance requirements, so it would produce a model and analysis that would show the wing, nacelles flaps, ailerons etc are all protected against flutter. For something like a flap, analysis would show that the rigidity of the attachment does the job, and the experimental test pilots during dev/cert testing will find out if the analysis is good. The FAA has an Advisory Circular that talks about it. EASA would be similar. faa.gov/documentLibrary/media/Advisory_Circular/… $\endgroup$
    – John K
    Commented Apr 12, 2023 at 16:37
  • $\begingroup$ I supported a suspected flutter incident of an aileron on a CRJ after an in-flight vibration snag, and they then found the aileron had a lot of backlash (slop) from bearing wear (the actual vibration was coming from somewhere else as it turned out). The flutter analysis done by the flutter engineering group showed that even with all the slop, it was still flutter resistant b/c air loads kept the surface preloaded against the top of the backlash range, effectively maintaining the rigidity for flutter resistance purposes. Same thing with a flap even if there is a lot of backlash. $\endgroup$
    – John K
    Commented Apr 12, 2023 at 18:08

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