fabric covered control surfaces 1

[GregLocock]

A correspondent asks "Looking through my "complete aircraft of WWII" I noted that a very large percentage of planes were described as being of all-metal construction "except for the control surfaces" which are "fabric covered." Why did control surfaces continue to be "fabric covered" for years after everything else was metal skinned?"



Cheers

Greg Locock


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[3DDave]

I assumed that it was for two reasons - the first is they are often the rudder and elevator which are at the extreme end of the plane and so any weight savings is magnified; there's already an abrupt structural discontinuity but, as observed in earlier planes where the solid surface gave way to fabric over the aft fuselage, the same would be more difficult with metal.

The second is that flutter is made worse if the item is heavier for its size, so this reduces the mass component that leads to flutter and moves that speed higher. Since a lot of these planes did not have hydraulics to operate the surfaces, this would avoid having the pilots need to resist it. Also, like the first reason, counter balance against flutter and G loads would be less because the surfaces weighed less, so the total mass would be reduced.

Maybe there was a control surface stitcher's union that had a good bargaining unit?

I note that a lot of planes had only the forward half of the wing sheeted in metal and the rear portion was still in fabric; again, probably weight savings as the pressure distribution over the aft of the wing is not as extreme as the portion forward of the main spar.

 

[berkshire]

Mostly for weight ,or rather the lack of it, Fabric covered surfaces were and in some cases still are, lighter and when needed, easier to mass balance. Thereby avoiding ,or at least delaying aerodynamic flutter.
B.E.

You are judged not by what you know, but by what you can do.

 

[verymadmac]

In addition to the above, for many aircraft the performance benefit wasn't considered particularly important.

The Spitfires didn't get metal covered ailerons until an increase in roll rate was required in an attempt to counter the FW190s very high roll rate.

 

[Sparweb]

Covering of control surfaces remained a material art even into the 1970's.
IIRC Some of the Beechcraft Bonanzas designed in the 60's had magnesium aileron skins.

STF

 

[drawoh]

GregLocock,

A better question may be why did they stop using fabric covered control surfaces?

As aircraft approached high Mach numbers in the late thirties, the fabric covered control surfaces ballooned out and did weird things to the handling of the aircraft. In one of his books, J.E. Gordon claims that fabric stretched over frames is just about the lightest structure you can do.

--
JHG

 

[Tmoose]

" The Spitfires didn't get metal covered ailerons until an increase in roll rate was required in an attempt to counter the FW190s very high roll rate."

I wouldn't want to have been the Spitfire pilot that found the limit of the fabric covering.

 

[Compositepro]

With carbon fiber and fiberglass we are sort-of returning back to fabric.

Durability and damage tolerance are probably a factor in why control surfaces stayed with fabric. Light structures are fragile. Control surfaces can easily be removed for repair or replacement so durability is not as important as it is for a fuselage or wing.

 

[GregLocock]

Thank you.

Cheers

Greg Locock


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[WKTaylor]

Approaching the speed-of sound, skin-panel and flight-control 'buzz' due to air-flow, shock and high torsional loads/distortion [control reversal!] is a major factor in transitioning to stiffer all-sheet-metal or metal-honeycomb or composite-honeycomb or solid-composite structure.

Also, the use of fabric covered surfaces presents an environmental deterioration problem [recurring fabric inspection and eventual replacement] that is labor/skill intensive to repair/replace. I remember my dad having the linen coverings on his 1950-era Piper Tri-Pacer being tested EVERY annual inspection for deterioration [specialized puncture-test]. 'Passing' meant that the test-hole was patched, doped re-painted and noted in maintenance log by the mechanic. 'Failure' meant the entire fabric cover had to be replaced... new material, cut, stitched, doped, painted, log-entry, etc.. by a certified mechanic.

In WWII there where thousands of low-wage, textile-skilled, workers who could install/repair/maintain fabric coverings.

And lets get serious... these WWII aircraft were simply not designed/built to last more than a few years or a few hundred hours total service, before being worn-out or destroyed or obsoleted by new models. Watch videos of B-29s being built: clad-aluminum sheet metal was left 'bare', with extruded or otherwise machined-parts simply alodined or anodized with [maybe] zinc-chromate primer on fay surfaces. Fast-simple-cheap.

My dad flew P-40s in the CBI: the general expectation for these [surviving] airframes was no-more than 500-hours and 2-years... then the bone-yard.

PS: anybody ever examine how the compound-curved fuselage of a early F4F was put together? see attached photo for the 'simple sheet-metal tricks' employed to KISS manufacturing.

http://files.engineering.com/getfil...41-ec7df9e03e54&file=F4F~Upr_Aft_Fuselage.jpg

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[verymadmac]

If one looks to the postwar 4 engine piston engine airliners these aircraft flew a lot more hours, & as far as I can see a lot had fabric covered rudders at least (DC-6, DC-7, Super Constellation, Avro Tutor, Handley Page Hastings/Hermes). While these are typically stretched versions of war / prewar designed models one would assume that if the fabric covering was imposing extra costs, it would have metalized. Maybe the costs just paled in comparison to that of the engines (the line about the best 3 engined aircraft in the world comes to mind).

One other issue with the fabric covering of control surfaces of faster aircraft is if pinking tape on the trailing edge starts tear it can act as a tab.

https://babel.hathitrust.org/cgi/imgsrv/image?id=mdp.39015004550417;seq=147;width=680

"Aircraft design through service experience" by Civil Aeronautics Administration is always an interesting read

https://babel.hathitrust.org/cgi/pt?id=mdp.39015004550417;view=1up;seq=3;size=75

 

[WKTaylor]

verymadmac... practical elements...

WWII workers/artisans were part of the aviation work-force into the early 1960s. Skilled workers were fresh/available and labor costs low for military and the airlines and general aviation! I remember 35-to-50 cents/hour was average for low-skilled work in the early 1960s. Also most airliners have very 'regular' depot/heavy maintenance requirements. Fabric work would have already been done on 'overhauled spares', so that the old surface would be removed/repaired/refurbished and then placed in spares-storage for use on another aircraft.

Today, mechanics with 'fabric skills' and even very 'thin/lightweight sheet metal skills' are becoming more-and-more scarce... and labor costs are high due to the labor intensive precise workmanship required. Homebuilt aircraft may be a latent exception... since builders time is technically worth 'nothing' [love-of-the-task].

NOTE.
The antique aircraft I work on still retains remnants of the old fabric era: a few sheet metal lightening holes... primarily in ribs and light spars within the empennage and flight controls... are still covered-over with round fabric patches [glued-in-place] [for FOD and gross moisture exclusion].

NOTE. One of the important reasons modern nuclear-warfare capable aircraft transitioned to 'all metal' structure [early aircraft were polished aluminum] was the vulnerability of the fabric coverings to 'nuclear-burst flash/heat'... the fabric simply disintegrates [fabric skin burns-off]! Very bad to have damaged/bare control surfaces for an otherwise airworthy aircraft that still needs to fly many miles 'to the fight'. Sheet metal was able to withstand flash-heating +600F long-enough to survive to cool-back-down to ambient temperatures relatively undamaged. NOT to mention that fabric offers NO electrical resistance/protection to aircraft systems from EMP/EMI. I'm pretty sure that early low-yield atom bomb drops resulted in fabric heat/blast damage on B-29s... so hydrogen-bomb test-drops, with much higher yields than atom-bombs, were NEVER done with B-29s [B-52s].

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[Compositepro]

The plot thickens...
Let's not forget to consider lightning strike protection.

 

[berkshire]

Compositepro
The most common way of lightning strike protection on the lighter fabric aircraft was avoidance, they simply did not fly near thunderstorms.
Wooden aircraft had bonding systems where a copper cable was attached to control rods to avoid bearings being spot welded, Metal aircraft used the metal frame to conduct the strike from one end of the frame to the other. However a strike through the fabric could start a fire.
Other metal aircraft like for example the Douglas DC3 had static wicks on the Fabric covered ailerons and tail surfaces to discharge current build ups away from the flammable surfaces, that together with butyrate dope which was less flammable than the old nitrate dope helped a lot.
Nowadays Polyester fabrics together with non flammable coatings like , but not limited to the Stits Poly-Fiber, or Stewart systems , have eliminated the flammable nature of fabric coverings , so these days a lightning strike on a fabric covered aircraft using these systems is a non event. The most common damage is a small hole about the size of a burn by a cigarette end.
Advanced composite aircraft with no lightning protection in the skins are far more vulnerable these days.
B.E.

You are judged not by what you know, but by what you can do.