How do airplane designers determine flap deflection angles for flaps of various % wing chords? How do airplane designers determine flap deflection angles for flap chords of various % wing chords? E.g. flap chord of 30%, 40% or 50% of wing chord.
I understand most flaps' chords are about 25% of the wing chord, but I understand Rutan's Grizzly had a flap chord of 55% of wing chord.
I assume you could use theory to figure this out, but that doesn't mean it works like that in the real world.
Is this information readily available in graph or chart form?
I thought I saw a link in a post on StackExchange re this from Peter Kaempf, but I can't find it.
As a rule of thumb, flap effectiveness (that is lift coefficient change per flap deflection angle or c$_{L\eta}$) is proportional to the square root of the flap's percentage. A more precise formula was derived by Hermann Glauert (search for the name and you will find that elusive answer by me)
What also changes is the hinge moment needed per degree of flap deflection; this, however, changes with the square of the flap's percentage. By trading off effectivity versus force, a designer will find the best flap percentage for each control surface.
Next, wind tunnel data is used for finding the effectiveness over deflection, because this value decreases with higher deflection angles and is hard to catch by CFD because viscous effects are strongest near the trailing edge and need lots of effort to model correctly. The desired control response and trim should be achievable with maximum deflections of below ±15° (the useable range is wider for smaller percentages) and the flight test will reveal whether the calculations were correct.