I've read answers that said that the climb performance is usually worse with flaps extended, and the pilots would, if they could, just retract them and get into a clean config as soon as possible. However, i believe that applies mostly to transports, which have a higher gradient.

For designs like the Predator or Reaper UAV, which take-off and land essentially...flat, wouldn't flaps be more beneficial to help their climb? Albeit, having to reduce their deflection at intervals to reduce drag?

And if so, what do you think would be the altitude at which retraction would occur? I've tried to find information, but so far have only landed with a Boeing document that claims the typical flap retraction height is 3000ft.


1 Answer 1


the climb performance is usually worse with flaps extended

That depends which parameter needs to be maximized. If you want to climb as steeply as possible, flaps will definitely help. If, however, you want to cover some distance while climbing (en-route climb), you better retract them.

Flaps will shift the optimum operating point of a wing to a higher lift coefficient. Their deployment will also lower the zero-lift angle of attack so the fuselage attitude will be more negative at the same speed. This is helpful for designs like the MQ-1 and MQ-9 since their tail strike angle is only 5.3 degrees. Taking off and landing with flaps down will help to lower take-off and approach speeds and shorten their runway requirements.

The wing airfoil of both airplanes is already highly cambered which together with the high aspect ratio indicates a one-point design for longest loiter time, flying mostly at the same dynamic pressure and attitude.

Regarding drag: Flaps help to reduce drag when used properly. A simple camber flap will not change the coefficient of drag within a deflection range of ±15° (more for short flap chord fractions, less for deeper flaps) but will shift the optimum angle of attack for best L/D to higher lift coefficients with more positive flap angles. Basically, flaps allow to pick speed independent of altitude (within limits, of course). Flaps up if you need to fly faster, flaps down if you need to slow down.


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