Fabric control surfaces

[pigsty]

Crawling my way through an A-26 Invader, I've decided to improve the control surfaces by gently sanding a little bit of relief into the rudder, elevators and ailerons.  This is of course because all the surfaces are fabric-covered.

 

And that's making me wonder.  The rest of the airframe is stressed-skin aluminium alloy and it has 2000hp on each wing.  Even the trim tabs have metal skins.  And it's far from alone.  Does anyone know why fabric on control surfaces persisted so long after it stopped being used everywhere else?

[cngaero]

I don't really know, but thinking aloud, could it have been for weight saving considerations as many aircraft still used mechanical controls as opposed to hydraulically operated. 

As I said, just a thought. I'm sure someone far more knowledgeable than I will be along soon to give you a more definitive answer. 

[noelh]

I'm sure there's a someone here who can provide a more detailed answer but essentially it's to do with lightness and the centre of gravity of the control surface. Metal adds weight, it also changes the CofG of the control surface. That has to be compensated for or there's the possibility of flutter. If you look at the ailerons of a Cessna light aircraft you'll see lead weights bolted onto the leading edge of the metal ailerons. One of the pre flight checks is to ensure they are still there because if they're not you will probably suffer from flutter probably at a very inconvenient moment after take off. So yes they add lead weights to aircraft. Obviously adding any weight is an undesirable thing for any aircraft. Fabric covered controls avoided problems like that. That is until speeds got a lot higher which is exactly what happened to the Spitfire when higher speeds caused problems with the fabric ailerons so later models had metal ailerons.  As bombers like the A26 weren't as fast as fighters that issue didn't arise. 

 

Extra weight in control surfaces is always undesirable. Indeed when composites were first introduced one of their initial uses was in areas like control surfaces thanks to the weight saving gains to be had. 

 

Basically if something didn't have to be metal then aircraft designers didn't use metal. You would see it all over aircraft even before composites became popular.

 

 

 

Edited October 8, 2017 by noelh

[Jamie @ Sovereign Hobbies]

Noel pretty much covered it. Another characteristic of fabric controls is that the fabric can deform. Control surfaces with concave surfaces normally increase the force required to deflect it. Sometimes that is desirable - e.g. modern light aircraft that are intended to be aerobatic must require a minimum stick force per G - this is to help the pilot gauge / appreciate what he's doing to avoid pulling the wings off etc.

 

As aircraft speeds increased the control weights tended towards being a hinderance both in combat and general fatigue on the pilot.

 

On medium twins and heavies control weight is easily offset by the use of balance tabs which look like trim tabs but are mechanically rigged to move the opposite direction to the control surface, aiding the pilot in deflecting it. They can be so powerful as to remove the need for powered flying controls or indeed any direct control over the surface - in this form they're called Servo Tabs (see Bristol Brittania for example).

 

Fighters don't need the weight and complication of balance tabs though so the correct solution, as in the Spitfire, was to stiffen the control surfaces with metal skins which kept aileron control forces managable with increasing speeds.

[Graham Boak]

Surely it is more the case that balance tabs were only coming into use during WW2 anyway - before then even trim tabs were often fixed devices bent to the desired position by ground crew.  The final production versions of the Bf109 had servo tabs on the rudder.  They are sometimes called Flettner tabs in the contemporary literature.

[pigsty]

Interesting point about flutter, since so many fabric surfaces had anti-flutter weights - you have to conclude that it's an inherent problem of hinged surfaces.  And it was in the 1930s that torsion was recognised as a precursor problem to flutter, addressed through stiffening.  That must have increased weight too - all across the surface, I'd guess, since it would have to be applied to the ribs.

 

The fabric being unable to withstand the forces from increased speed sounds like a thoroughly sound reason to do away with it.

[Jamie @ Sovereign Hobbies]

Not to be a pedant (automatically a pedant) but normally the torsional stiffness of a structure comes from the skin. The skin has to sheer to allow the structure to twist. You'll see on (properly designed) wooden aircraft both old wartime and current light aircraft that plywood skins are oriented with the grains 45deg from the spar to react against torsion. For a surface to twist, the skin has to be stretched in one plane and compressed in the perpendicular plane.

 

Early biplanes could have completely open structures on the wings and used the rigging wires for stiffness. Monocoque wings pretty much always have an enclosed D section leading edge back to the main spar as a minimum for torsional stiffness if the wing is fabric covered (e.g. DHC-1 Chipmunk).

 

TL:DR it's the skins that provide all the torsional stiffness