Why fabric covered control surfaces?

[DJ Hipply]

A B-17 flown into town this week, The control surfaces were fabric covered...why. The flaps were metal.

[Calb]

1 - lighter
2 - cheaper
3 - saved valuable aluminium that was needed elsewhere
4 - easier & quicker to make field repairs
5 - every bit as safe & effective for slow flying aircraft

just a few off the top of my head

Cal

[Ivan]

the control surfaces aren't load-bearing surfaces as the fuselage is, so they don't need the extra strength aluminium gives if mounted on a metal frame

[Felix/FFDS]

control surfaces - ailerons/elevators/rudders - were subject to more stresses, especially at high speed/high G maneouvers.  Before construction techniques improved, metal surfaces could buckle under the stress, leading to fatigue, cracks and possible disintegration in flight (not a good idea).  Fabric surfaces provided for a certain "give" to the stresses.

As materials and construction techniques improved, fabric surfaces gave way to metal surfaces all over.

[Hagar]

Also to save on valuable resources. Metal was in short suppy in most countries during WWII, especially of aircraft quality. Wooden fabric covered parts could also be manufactured in any carpentry or joinery workshop. The later Bf 109s had a wooden tailplane for this reason. It turned out to be stronger than the original metal structure.