Fabric control surfaces

[Thumpalumpacus]

I've been reading and watching a bit about 1930s aircraft, wherein the aircraft are described as all-metal, but the control surfaces, ailerons, rudder, and elevators are fabric and not metal.

My question is why? Granted there's a weight difference, but was it so much that fabric was preferred? I should think that I'd want those surfaces to be the most-robust. Am I missing something here?

 

[ThomasP]

The only aerodynamic reason I ever ran across is that the fabric covering helped damp/decrease - to a greater or lesser degree - the vibration/frequency due to the air turbulence over the control surfaces. This was in a study from the late-1920s (I think) on handling and aerodynamic forces imposed on airframes during high speed dives. The same principals are part of the mechanisms involving the use of fabrics in sound deadening, and in vibration dampening in architectural structures.

In the strength engineering sense, as speeds increased through the 1930s it was found that fabric covered control surfaces were strong enough to handle speeds upto about M0.7.

 

[GTX]

I think it was the combo of lightweight, relatively small surfaces able to be worked upon if need be and also the fact that it was already a well known and proven capability vs all metal skinning.

 

[MIflyer]

The F4U was an interesting case. A big, powerful, fast airplane. but it had fabric covered outer wings and wooden ailerons. I believe that one reason is that control surfaces had to be light and strong but also well balanced.

But I find it interesting that my Ercoupe, designed in 1940 around the same time as the F4U, was a small, low powered airplane, and while it originally had metal structure fabric covered wings, all the control surfaces are all metal, no fabric covering. Of course those control surfaces do not have to be very strong, either. The ailerons have no internal ribs, just L-shaped thin pieces of aluminum to serve as stiffeners.

I've always loved the story of how the unit where J.E. Johnson was serving found out that the later Spits had metal ailerons that improved their roll rate, an important feature when dogfighting with FW-190s'. So they contacted the maker of the ailerons and arranged to have their Spits modified accordingly at the factory - without asking anyone for permission.

 

[Shortround6]

I've been reading and watching a bit about 1930s aircraft, wherein the aircraft are described as all-metal, but the control surfaces, ailerons, rudder, and elevators are fabric and not metal.

My question is why? Granted there's a weight difference, but was it so much that fabric was preferred? I should think that I'd want those surfaces to be the most-robust. Am I missing something here?

Might I suggest buying a used copy of
The Airplane and Its Engine by C. H. Chatfield as they can often be found at reasonable prices used ($10-20)
Be careful as there at least 5 editions, 1928, 1932, 1936, 1940 and 1949.
I have the 1936 and 1949 editions and the information and illustrations/photos change considerably.
The books are around 400 pages and give a very good understanding of most basic subjects.
The 1936 book starts with chapter 1. Essential parts of the Airplane and by page 72 and chapter VI it starts on engines, 3 chapters followed by one on propellers and one on the complete powerplant. Last chapter is on Aircraft instruments and accessories. So there is a lot of information.

In the mid 30s 'all-metal' pretty much meant the structure. Even fabric covered wings and fuselage were referred to as all-metal.
This might have been more common in the US were the government banned wooden construction in commercial passenger carrying planes of over a certain capacity (seat count).
It had some bearing on maintenance, inspections, and so on.

The 1930s was period of very rapid development in a lot of areas. Materials was one of them. The change from wood to Aluminum was not quite as quick as many of us think. It took a while to get the proper aluminum alloys and coatings. Britain used various combinations of steel and other things. The US didn't use much plain steel in large aircraft but did use some stainless steel.
Fabric coverings for control surfaces also worked because very thin metal, while strong, dented/wrinkled too easily. Getting the right thickness or spacing of the ribs or different alloy/heat treatment took a while. On the light aircraft using a nearly rib-less construction traded skin weight for structure (ribs).

 

[Holtzauge]

I would say the main reason for using fabric on control surfaces has to do with them having to be statically balanced around their hinge axis.

This is usually accomplished by placing lead forward of the hinge line to counteract the weight of the aileron behind it. And since the hinge line is usually so far forward, if you add weight on the aileron surface as such by replacing the fabric with metal skins, then the short moment of arm means you have to add a lot of extra lead.

So it's not the added weight of the metal skinning as such that is the problem, but rather the increased amount of balancing lead required to get it back in balance around the hinge line.

 

[z42]

On the topic of control surfaces, why don't we see 'flying tail' type designs in the WWII era? That is, instead of elevators the entire horizontal stabilizer rotates. There were apparently some WWI era aircraft with this kind of design, but then it died away, until again later during the cold war requirements of supersonic flight caused them to reappear.

At least in boats and ships, it seems well established that spade rudders (the nautical equivalent of flying tails) have lower drag than rudders mounted on a skeg.

 

[SaparotRob]

..There were apparently some WWI era aircraft with this kind of design,..

Simplicity of construction?

 

[MIflyer]

What would you use to actuate the moveable horizontal stabilizers? WWII aircraft pretty much did not have hydraulic, electric, or pneumatic power systems, the P-38L being one of the few exceptions. The later P-38's had hydraulically boosted ailerons, but the pilots only turned them on when they were going to be doing serious dogfighting where a high rate of roll was desirable - the boost also rendered the P-38's almost unflyable in terms of straight and level flight, so it had to be left turned off most of the time. There is the problem of the system needing to sense speed and altitude to know how much boost to apply; this is called "feel force" and as far as I know, no one had that in WWII, which explains the P-38's problems.

The F-86 had hydraulically power boosted controls with a flying tail starting in the F model, observable by the larger fairings in front of the horizontal stabilizer.

 

[Thumpalumpacus]

On the topic of control surfaces, why don't we see 'flying tail' type designs in the WWII era? That is, instead of elevators the entire horizontal stabilizer rotates. There were apparently some WWI era aircraft with this kind of design, but then it died away, until again later during the cold war requirements of supersonic flight caused them to reappear.

At least in boats and ships, it seems well established that spade rudders (the nautical equivalent of flying tails) have lower drag than rudders mounted on a skeg.

I think that was a matter of speed. At slower speeds for piston-engined aircraft, a flying tail wasn't needed. Maybe the weight of such a fixture (fuselage strengthening, etc) without much added benefit? I don't know, just spitballin'.

 

[z42]

What would you use to actuate the moveable horizontal stabilizers?

Why not levers, wires and pulleys, that normal control surfaces were moved by? If anything, the forces required could be less, as by choosing the pivot point suitably, the rudders can be well balanced so even relatively small forces would be required at high speeds? (No idea here, just speculating..)

 

[z42]

I think that was a matter of speed. At slower speeds for piston-engined aircraft, a flying tail wasn't needed. Maybe the weight of such a fixture (fuselage strengthening, etc) without much added benefit? I don't know, just spitballin'.

Weight might be an issue, yes? Since the flying tail rudder connects to the fuselage with only an axle, that axle needs to be strong enough to take all loads. OTOH since the entire assembly functions as a control surface rather than the small flappy things at the tail end of a traditional horizontal stabilizer, maybe you can get away with a smaller flying stabilizer than a traditional one?

 

[Holtzauge]

I think the main reason most planes have the classical horizontal stabilizer and elevator arrangement is due to stability considerations:

Anyone who has flown aircraft with all-flying tails know they are often twitchy in pitch. I've flown two gliders with all flying stabilizers: The SF-27 and standard 15 m Cirrus and they were both very sensitive and had poor pitch stability. Funnily enough, we had two Cirrus' in our glider club: One with an all flying tail and one with a conventional stabilizer/elevator assembly. During takeoffs with the former, it was very important to let the aircraft fly itself off and avoid any temptation to try to lift it off too early because that was a surefire way to get into PIO's.

But for sure, a WW2 fighter is a different beast altogether, but I still think pretty much the same problem would occur there and simply has to do with the fact that the stabilizer is fixed and this gives you better stability, both stick fixed and stick free.

And I think the only reason the ubiquitous stabilizer/elevator arrangement was abandoned, was that aircraft stated flying so fast they got into compressibility problems and the elevator lost its "bite" while the stabilizer still had control authority, and which was why you can read German WW2 accounts where they recovered form high speed dives in BF-109's and Fw-190's using the adjustable stabilizer when the elevator became ineffective.

And I believe it was the same reason the F-86 Sabre's controls got modified: The first versions with a classical stabilizer/elevator arrangement got into compressibility issues, so they addressed that by replacing it with an all flying tail.

 

[MIflyer]

Some of the Pipers have flying tails. But for aircraft such as the F-86, it was used more like a trim.

 

[PAT303]

I've always loved the story of how the unit where J.E. Johnson was serving found out that the later Spits had metal ailerons that improved their roll rate, an important feature when dogfighting with FW-190s'. So they contacted the maker of the ailerons and arranged to have their Spits modified accordingly at the factory - without asking anyone for permission.

The spitfire's fabric covered ailerons were changed to metal pretty early on but that wasn't the fix, it just addressed the ballooning of the fabric surfaces, the fix was reprofiling of the ailerons which reduced the turbulence across their surface thus improving the airflow they controlled, they also changed to piano hinges which likewise gave much improved control. .

 

[MIflyer]

the fix was reprofiling of the ailerons which reduced the turbulence across their surface thus improving the airflow they controlled

Yes, I believe that they described it as making the trailing edge blunter. Farnborough and NAA also worked on the ailerons for the Mustang and improved the roll rate quite a bit.

 

[MikeMeech]

On the topic of control surfaces, why don't we see 'flying tail' type designs in the WWII era? That is, instead of elevators the entire horizontal stabilizer rotates. There were apparently some WWI era aircraft with this kind of design, but then it died away, until again later during the cold war requirements of supersonic flight caused them to reappear.

At least in boats and ships, it seems well established that spade rudders (the nautical equivalent of flying tails) have lower drag than rudders mounted on a skeg.

Hi
An all-moving tailplane was designed for the Miles M.52 high speed research aircraft, this tailplane (and the wings which were flown first before the tailplane was fitted)) were fitted to the Miles M.3/52 'Gillette' Falcon. The tailplane had successfully flown on this aircraft by March 1945 (data and photos from 'Miles Aircraft - The Wartime Years' by Peter Amos, Air Britain 2012). Images of flying tailplane for mock up:

And when tested on the Falcon:

So in the WW2 era.
Mike

 

[SaparotRob]

Hi
An all-moving tailplane was designed for the Miles M.52 high speed research aircraft, this tailplane (and the wings which were flown first before the tailplane was fitted)) were fitted to the Miles M.3/52 'Gillette' Falcon. The tailplane had successfully flown on this aircraft by March 1945 (data and photos from 'Miles Aircraft - The Wartime Years' by Peter Amos, Air Britain 2012). Images of flying tailplane for mock up:
View attachment 827774
View attachment 827775
And when tested on the Falcon:
View attachment 827776
So in the WW2 era.
Mike

Cool!

 

[ThomasP]

WWI technology actually, although I do not know how successfully it was implemented at the time. The Morane-Saulnier G & H (the first to use it as far as I have read) and the Halberstadt D.II both had the all-flying horizontal tail. I think there were others in the WWI era, but I do not remember which airframes.

 

[pinehilljoe]

In With Wings Like Eagles, Michael Korda wrote that Sydney Camm chose to cover much of the Hurricane with fabric to allow ground crews familiar with fabric maintenance and repair to transition to the Hurricane for maintenance.