Several threads recently have bumped into the question of the best flap chord size . If the chord is too large, a bigger negative pitching moment is produced and this results in poorer quality turns for the airplane.
I have, through the years, built several planes with 4 inch flaps. I remember Windy, at one Nats, had a Big Jim design that had what seemed to be like 40% chord flaps. They flew well. I think that the reason they performed as well as they did was because a conscience effort was made to limit the flap deflections.
Recently, Igor has made the case to make certain that the line of curvature flows smoothly from wing across to flap, a design element that Al Rabe also promoted in his large flapped semi scale stunters.
I propose that larger flaps are perfectly usable if the deflection is limited from what you might normally use for standard 2 inch flaps. So, is there any data to support this? Yes there is.
The attached plots are from the thesis of Gregory Williamson, a student of M, Selig at UI.
https://m-selig.ae.illinois.edu/uiuc_lsat/Williamson-2012-UIUC-MS-Thesis.pdf The section of interest is a symmetrical 15% airfoil at a Reynolds number of 400,000. The flaps are not sheet flaps like ours, but integral flaps of either 20 or 30 percent chord.
The data shows that a 20% chord flap with a deflection of 30 degrees produces almost exactly the same lift as a 30% chord flap being deflected 20 degrees. One is as good as the other for supporting the plane at the bottom of and hourglass.
OK, what about the difference in pitching moment effects between the two?
As it turns out the larger flap situation (30% vs 20%) actually has a slightly smaller negative pitching moment! And, additionally due to the lesser defection, it blends into the wing curvature more smoothly than the smaller flap deflected a greater amount.
Granted, if you are on the field fixing a poor turning plane that has large flaps at standard deflections and you can’t get to the linkages, the solution is to trim the flap chord down. However, if designing from the ground up, you should match deflection to chord size. The larger the chord the smaller the deflection.