ORIGINAL: Doug Cronkhite

I'll trust aerodynamics over Webster's to be correct here.

Flaps are devices designed to increase the angle of descent without increasing airspeed. At lower deflection levels they increase lift over a 'clean' wing, but at some point, you end up in a trade-off between lift and drag where lift stops increasing, while drag rises sharply. Some (most) of our models have enough thrust to overcome this drag rise, but for a really good slow flight regime, I've always found takeoff flaps to be optimal. If you have speed brakes too, that's all the better, because they'll allow you to run higher thrust levels, which gives you instant energy increase if you need to go around or climb out simply by closing them. No spool-up time or waiting for thrust.