I think what Bert says is technically correct (wing load doesn't change
when you open the air brakes) - but the distribution does change a lot -
especially on a ship like the N3 & N4 where the brakes are located inboard
on the inner panels. Stated differently, when the brakes are opened the
outer panels are being asked to do more work supporting the fuselage (and
non flying portions or the inner panels) than before the dive brakes were
opened. The Nimbus 3 and 4 are placarded against carrying water ballast
in the inner panel tanks with the outer panel tanks empty for structural
reasons. You also must dump the inner tanks first. The same structural
problem occurs when the dive brakes are open and that part of the inner
panel becomes "dead weight". So - while the brakes should be used to
prevent the glider getting to extreme speeds - we need to be cautious about
suggesting that nothing bad is going to happen if you open them at extreme
speeds.

Roy B. (Nimbus 3 # 65)