Would a slotted flap still work if the prop is located in a slot mid wing between the wing and flap?

This would be for a distributed propulsion application for an multi prop ultralight triplane, where the each prop would need to provide distributed propulsion for multiple wings vertically.

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    $\begingroup$ Because I am not sure what your arrangement is, and if I understood well from the description, and in order to avoid confusion, I think the best way to go is to provide a sketch of the cross-section and the general configuration. $\endgroup$ – ares Dec 27 '19 at 13:26

Yes. Inserting a prop aft of the wing and forward of the flap would not reduce the flap's effectiveness. If anything, the stronger, more concentrated propwash would make the flap more effective.

The answer doesn't change for a triplane. But off topic, I wonder about how to design a triplane that's light enough to be in the ultralight category, what advantages it would have over a biplane or monoplane. Commercial interest in triplanes petered out eighty years ago.


The flap will be fine. It is the propeller I am concerned about.

A propeller directly in the wake of a wing, with the propeller axis in the wing's plane, will hit the boundary layer of that wing each time the propeller blades cut through that wake. This means a sudden decrease and increase again in dynamic pressure which results in a sudden change in bending loads on this propeller blade. This will

  • reduce the lifetime of the propeller due to material fatigue, and
  • produce a distinct noise at twice the propeller frequency for a two-bladed prop (and at six times its frequency for a three-bladed prop).

It will be better to place the propeller ahead of the wing or aft of it with some distance between the trailing edge and the propeller plane in order for the wake to dissipate and for the dynamic pressure change to become less sharp.

And using several wings stacked vertically has turned out to be a poor solution maybe 100 years ago already.

  • $\begingroup$ That much racket for multiple props might well be even noisier than a brace of piston engines. Annoying when they're all just a few feet from your ears. $\endgroup$ – Camille Goudeseune Sep 13 '20 at 5:38
  • $\begingroup$ The pressure changes at the wing's trailing edge would also accelerate the aging of the relatively fragile wing fabric of an ultralight. $\endgroup$ – Camille Goudeseune Sep 13 '20 at 5:40

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