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By: - 28th September 2014 at 15:34 Permalink - Edited 1st January 1970 at 01:00
weight reduction to simplify the balance of elevator, rudder and aileron.
yves
By: - 28th September 2014 at 16:16 Permalink - Edited 1st January 1970 at 01:00
As YP posted - any excess weight behind the control surface hinge line would entail having a heavier Mass Balance Weight(s) fitted to keep the control surface C of G forward of its hinge line !
Also fabric covering reduced the use of metal - which was in critical short supply !
By: - 28th September 2014 at 16:48 Permalink - Edited 1st January 1970 at 01:00
Quite possibly the difficulty (even impossibility) of carrying out "blind" riveting; some of the skins on the Mk.I Spitfire wings and tailplane were retained by countersunk screws, not rivets.
By: - 28th September 2014 at 18:05 Permalink - Edited 1st January 1970 at 01:00
Also fabric covering reduced the use of metal - which was in critical short supply !
Keep in mind fabric control surfaces were used past the wartime period....many 50s types still had them.
By: - 28th September 2014 at 18:10 Permalink - Edited 1st January 1970 at 01:00
They will be easier to repair and are reducing the use of an essential war material
By: - 28th September 2014 at 18:24 Permalink - Edited 1st January 1970 at 01:00
In the case of the most famous fabric ailerons - spifire of course LOL
Jeffery Quill - in his autobio says that the problem developed by stages and that many people had underestimated the potential combat manoevering speeds.
He wrote that the ailerons were ok for the original wing,but inadequate for the production wing - supermarine tried out many different 'fixes' for the aileron but in the end they cured the worst of the trouble with the thin trailing edge metal skinned ailerons !
As to post war use of fabric control surfaces alluded to by JB - they would be lighter and cheaper to build than metal ones - and if it was a lowish performance aircraft then fabric was still ok... for example the spitfire fabric ailerons were acceptable up to approx 370mph.
By: - 28th September 2014 at 20:17 Permalink - Edited 1st January 1970 at 01:00
I believe there was some advantage for stability and control reasons, though I don't know what that was. Not my speciality, I'm afraid.
I don't think that metal shortage would have been the reason, else we'd have seen similar replacement elsewhere on the airframe. Neither is weight alone: remember that the skin is carrying some of the stress, so the internal structure is lighter than that of a fabric-covered structure. The weight difference overall could be small. However, weight distribution could be relevant, and if that meant ballasting the leading edge then that would increase the weight markedly.
By: - 28th September 2014 at 21:40 Permalink - Edited 1st January 1970 at 01:00
As a hangover from more sedate days - fabric covered surfaces were the preferred route but by the time 1940 came along - the RAF fighter pilots found themselves diving at speeds well in excess of normal peacetime flying (either chasing or evading) - at these speeds of 400 mph+ the fabric started to 'balloon' and hence control problems became evident !
JQ also flew a captured Me 109E in oct 1940 and to his relief found that its aileron control was as bad as the spit with fabric ailerons - the 109 being almost immovable at 400 mph
By: - 28th September 2014 at 21:43 Permalink - Edited 1st January 1970 at 01:00
. The weight difference overall could be small. However, weight distribution could be relevant, and if that meant ballasting the leading edge then that would increase the weight markedly.
To prevent 'flutter' - the C of G of the control surface has to be well forward of the control surface hinge line .
By: - 28th September 2014 at 22:28 Permalink - Edited 1st January 1970 at 01:00
bazv: yes to both your postings, though in the design stage it could be a matter of positioning your hinge line. This is more difficult to achieve once the design is locked into production at a time of great need. However, I do think that there was more to it than a hangover from "more sedate days" - pretty well every other feature of the new designs was different from the old biplanes so if the control surfaces remained fabric covered when the wing was metal, there has to have been a reason. Metal wings, why not metal control surfaces? Weight distribution might be it but I'm not sure.
By: - 28th September 2014 at 22:47 Permalink - Edited 1st January 1970 at 01:00
As a previous poster said, the lighter a control-surface is, the easier it is to balance. Without a mass-balance, there is a risk of flutter, but stiffness is also a factor in this, as is slop in hinges and linkages.
(To digress;- With regard to the Spit' ailerons, there were actually two separate problems. The original ailerons were fine at moderate speeds, at at the top-end, the fabric ballooned-out. This was fixed by skinning them in metal - a partial fix only;- The more fundamental problem was the basic design of the aileron, which led to the ailerons becoming almost impossible to move in a high-speed dive. The late wings (MkXVIII-on?) had the ailerons piano-hinged, which totally cured the problem as far as I'm aware.)
By: - 28th September 2014 at 22:54 Permalink - Edited 1st January 1970 at 01:00
How about rudders as well I am thinking Constellation here
By: - 28th September 2014 at 22:56 Permalink - Edited 1st January 1970 at 01:00
In his book JQ wrote that when the spit was designed - it was envisaged that it would be defending against bombers - it was not thought that german single seat monoplane fighters would make it as far as blighty - therefore their airships and the a/c designers were not planning for fighter/fighter combat - hence 370mph being just acceptable for aileron control authority !
In his book they tried many 'tweeks' to make the ailerons better at higher speeds and they were not expecting metal covering to make a big difference but the combination of metal skin and thin trailing edge made a huge difference to aileron loads.
I speculate that fabric surfaces were the 'norm' in the late 30's and a large design change was not envisaged,as I am sure that you are aware - the higher the speed required more mass balancing to guard against control 'flutter' and presumably the designers fought to keep the a/c AUW as low as possible,a small increase in weight behind the hinge line would require a much larger increase in mass balance weight due to the much shorter distance/arm forward of the hinge line.
By: - 28th September 2014 at 23:05 Permalink - Edited 1st January 1970 at 01:00
The flutter problem increases dramatically with speed - a good analogy might be (say) if you let go of a SA Bulldog stick (on the ground) - the stick would crash forward because the elevators are not heavily mass balanced.
Conversely - on a Canberra B2 (and other early marks) if you let go of the control column then it will crash back towards your lap because they are heavily mass balanced and the weights pull the elevators nose down.
So (say) on an aileron if you can keep the rearwards 'moment' to a minimum then you can keep the mass balance weight to a minimum which also helps on stress calculations for the whole assembly !
By: - 29th September 2014 at 08:54 Permalink - Edited 1st January 1970 at 01:00
More guesses
Many historic aircraft, if not all initially, had fabric covered control surfaces where otherwise the entire airframe was of metal construction. I wonder why.Could it have been simply to reduce the weight of the surfaces or was there an aerodynamic argument?
The weight saving of using fabric covering rather than a aluminium skin seems to me to have been minimal.
Great to fill this gap in my knowledge!
Mike
I don't know but an allied query in the overflowing filing cabinet of the mind are timber flaps on the Mosquito, while the ailerons and elevators were metal. Why would you make a highly stressed component like the flaps in timber, while going to the extent of making all other control surfaces in metal ?
The postulatum given in other replies covers answers such as weight, alloy material savings etc, but I wonder if there are considered structural issues involved.
For fabric covered ailerons, one thought is that as the metal wing flexed, so the weaker fabric aileron member would flex and follow the shape of the major wing member. If it did not, the aileron hingeing might bind.
Calculating the performance of complex, new stressed skin structures in the 1930's was an emerging science. Only practical experience could really determine what was going to happen to these novel structures. A smaller stressed skin aileron structure, composed of a skin of the same thickness as a larger wing structure, would be stiffer than the wing : therefore I wonder if the aileron was purposefully designed to be weaker, using the known performance of fabric covered members. In truth these were T50 steel tube spar structures with predictable characteristics of elongation. Only the ballooning of fabric emerging from new high speed flight factors created a new issue to resolve. So being then forced to use a stressed metal skin aileron, the first thought would be to make it a thin skinned structure more longitudinally flexible than a wing. In addition the fitting of a piano hinge would then force the aileron to follow the flex of the wing.
In applying these thoughts to hydraulically activated plywood Mosquito flaps, a different set of forces is in play to wing flex, which might be the ability of plywood to flex and distort under sudden high flap loads, while the flap spar was an extremely strong 3 inch diameter tubular member that would not distort or transfer destructive forces through the flap hingeing.
In other words control surfaces are purposefully designed to be 'weaker'. When you look at the flaps and brakes of a modern jetliner landing, they certainly do shake, rattle and roll.
By: - 29th September 2014 at 11:03 Permalink - Edited 1st January 1970 at 01:00
Control system stiffness plays a large role as well. Cable-driven ailerons tend to have large counter balance weight fitted. Compare the Rhönlerche which has about 3 kilos of lead in large protruding counterbalance weights, fitted after a couple of them lost their ailerons (and in one case the wings) in dives due to flutter.
By: - 29th September 2014 at 11:07 Permalink - Edited 1st January 1970 at 01:00
And to show what flutter will do to a wing, here's a movie:
- A DG glider with the aileron ballast weights taken off so that the CoG of each aileron is too far backwards and flutter starts far below Vne: https://www.youtube.com/watch?v=kQI3AWpTWhM
By: - 29th September 2014 at 13:13 Permalink - Edited 1st January 1970 at 01:00
As said above the key element is control surface flutter.
I once flew an aircraft which, unbeknown to me had an unauthorised mod to its ailerons i.e. somebody had replaced the fabric with thin plywood. This moved their cg aft of the hinge line. When I was flying this aircraft and let go of the stick above 90 knots, the stick immediately started to thrash from side to side, and the aircraft developed an alarming side to side wobble which stopped when I grabbed the stick. This is known as stick free instability and is different to wing aero-elastic flutter as demonstrated in the glider YouTube. Control surfaces can also flutter stick fixed, i.e. against the stiffness of the control rod/cables and this normally results in them falling off.
After landing and grounding the aircraft, the original design standard i.e. fabric, was reinstated and the problem was solved.
Fabric just will not work at high speed. When aircraft started pushing speeds beyond about 0.7 Mach, the fabric started to fail due to the formation of localised shock waves and intense pressure differences are beyond the strength of the fabric, i.e. there is a picture somewhere of an early Me 163b with no fabric left on its rudder after a high speed dive.
Now you can design with either metal or plywood covering the control surface but there’s a negative. You must add mass to the surface to restore its cg back to forward of its hinge line. The extra mass increases the control surface loads, which means heavier, stronger wing structure, and you need significantly more force to move the surface. The Gloster Meteor ailerons were said to be unmovable at it limiting Mach number. Hence the introduction of power flying controls, and yet more weight.
Posts: 245
By: MikeHoulder - 28th September 2014 at 15:11
Many historic aircraft, if not all initially, had fabric covered control surfaces where otherwise the entire airframe was of metal construction. I wonder why.
Could it have been simply to reduce the weight of the surfaces or was there an aerodynamic argument?
The weight saving of using fabric covering rather than a aluminium skin seems to me to have been minimal.
Great to fill this gap in my knowledge!
Mike