The whole concept of flap speeds is really a misnomer, because the
optimum flap setting depends on angle of attack, so the flap speed
varies with weight. This is not as much of an issue if you always fly
solo and dry, but I fly a 2-place often with water, and my max gross is
almost double the empty weight.

The laminar flow airfoils used on most flapped gliders have a "drag
bucket" where the drag is much lower over a narrow range of AoA. By
adjusting the flap setting, you can move the drag bucket to a higher or
lower range. The notches in the available flap settings should be
designed to give some overlap in the drag buckets.

I have heard that the optimum flap setting is the one that produces no
force in the flap control, and I often let the flap handle float
between the 0 and -4 degree positions when in low speed cruise, but I
have no proof that this is correct. Does anyone have a reference
supporting this?

rustynuts wrote:
Can't believe that nobody knows about flap speeds?