Whats the theorical speed limit (Vne) of a fabric aircraft?

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ragflyer

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Joined
Apr 17, 2007
Messages
376
Fabric covered control surfaces are lighter than metal covered ones... so if flutter is the limiting factor, the plane with the fabric covered surfaces may well have a higher Vne than the all metal equivalent.
Yes that is exactly why you see a number of control surfaces in WW2 era airplanes covered in fabric.
 

rollerball

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Joined
Aug 3, 2018
Messages
52
Location
Aquitaine, France
The BMAA (British Microlight Aircraft Association) likes to play silly games from time to time to try and make the point that it decides the rules not you and on one such occasion I had to get my AX3 Weedhopper repermitted after repairing it after a minor accident. The BMAA had a bee in its bonnet and said that it had no record of the AX3 ever having been certified to fly with its doors off despite that fact that pilots had been doing so for years and insisted that I had to use my aircraft in the hands of a certified test pilot (who luckily I knew and provided his services FOC) to do so as part of the repermitting. As well as exploring the general envelope we also had to come up with a recommended Vne and this was in the middle of winter mind. So off we went with him flying and me in the RH seat taking down the numbers, which of course were not significantly different to the numbers for the aircraft with its doors on. Then we got to Vne and my pal asked what I wanted to take it up to. Normal Vne for the AX3 is from memory 94 mph so he looked at me, I looked at him and said 'take it the whole way'. So down went the nose and up went the airspeed. As we hurtled downwards with the airstream screaming by outside I was able to hide behind the edge of the windshield but it was still bloody cold. Unfortunately my test pilot friend wasn't so lucky as he was a bit taller than me. All the checks were finished after we attained Vne and we landed and taxied back in. When we got out of the aircraft I said how exciting the whole experience was but although my pal tried to reply he couldn't. It turned out that he couldn't move his jaw because it had become frozen up with the cold. Luckily for him it wasn't permanent though :)
 

Paul Saccani

Member
Joined
Jun 5, 2019
Messages
11
Mach speeds would be the limit due to heating but otherwise it is a matter of structural effciency. Fabric covered surfaces are very efficient at lower wing loading/speeds. As speeds increase wing torsion increases and is most efficiently reacted by a structural skin. However no reason, if you are willing to accept the small structural inefficiency, that a transonic fabric surface could not be made as long as the underlying structure is beefy enough to take the torsion loads.

Control surfaces are an exception. They where preferred in a number of high speed fighters of WW2 as they are very light (particularly aft of the hinge) and this helps prevent/minimize risk of flutter which is a concern at high speeds.
The DH 108 Swallow (Absolute World Air Speed record holder in 1948) went supersonic with a fabric covered fuselage. The crossover point where monocoque structure starts to be more efficient than rag and tube is usually taken to be about 140 knots, with a great many exceptions on either side. Fabric covered control surfaces were replaced with aluminium covered ones in WWII fighters due to the deficiencies with fabric covered ones. Fabric covered control surfaces were generally *not* preferred at high speeds, once they were actually used there. The problem with fabric covered control surfaces is not flutter, but ballooning at high speeds. Many WWII fighters had aluminium covered control surfaces retrofitted to improve high speed control. The original theory behind their adoption (at then considered high speeds) was not on the basis of flutter, but because their known deficiencies at higher speeds would increase control forces, preventing the pilot from over-stressing the aircraft. Up to a point this had some validity, that point was where the ballooning effectively locked the control surface aerodynamically. This was extremely noticeable on Spitfire mk II ailerons, for instance, resulting in a retrofit program of aluminium covering of the ailerons to the mk IIa standard. The aeroelastic (more to flutter than mass balancing) properties of fabric covered surfaces could result in “buzz” at lower speeds than with aluminium covered surfaces. Essentially, localised transonic flows and fabric did not go well together.
 
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