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Aviation History
1946
1946 - 1424.PDF
82 FLIGHT JULY 25TH, 1946 Power-Folding Problems Latest Requirements for Wing Folding : Lockheed Hydraulic System Employed IF one compares the Naval Air Arm of 1938-9 with theNaval Air Arm of to-day, it is patently obvious thata major change amounting almost to a re-birth has taken place during the past seven years. This instaurationinevitably put difficulties in the way of designers of air- craft, engines, and equipment in their struggle to meet theincreasingly onerous demands made of them by the fast- moving strides of modern war. A general indication of the ways in which design re-quirements have grown steadily more difficult to satisfy is given by those relating to wing folding. The most recentrequirement calls for folding and spreading of wings in winds up to 35 knots, from any quarter in the horizontalplane, and 45 knots from ahead ; the operation to take not more than 15 sec when the wings are fitted with drop tanks,guns and /or bombs, whilst an alternative method of opera- tion with the engine off must also be provided. TheAmerican types used by the Naval Air Arm are not capable of fulfilling these stringent conditions. Like all other operational aircraft, the gross weight ofNaval types has steadily increased, but as the size of lift wells and the dimensions of flight decks have not increasedin proportion, what may be termed the " density " of the aircraft has had to go up. This is exemplified by the Black-burn Firebrand, which is about twice the weight of its ancestors the Skua and Roc, whilst being approximately ofthe same span and length. The effect of this increase in '' density " is to force additional equipment, and particu-larly armament, into or on the outer panels of the wings. The wings themselves are, of course, considerably heavieras they have to carry much increased loads and, addition- ally, are usually of greater chord as well as being fitted withlarge flaps. The Tanavoidable increase in the weight of the folding portion of the wings, when combined withoperational requirements such as exemplified, makes manual operation out of the question for most newmachines. Various Methods Wing-folding geometry takes three different basic forms :First, the vertical hinge, into which category come all the biplanes and some of the more recent examples of mono-planes such as the Fulmar and Barracuda ; a folding trail- ing edge is necessary with the monoplanes, as otherwisethe hinge would come too far abaft the rear spar. Second, the- horizontal hinge, which usually embodies two hinges,one on each spar ; the Seafire, Sea-Hornet, and a number of American types come into this category. Third, theablique hinge, the first example of which was the Black- burn Skua, followed by the Roc, and more recently, theFairey Firefly and Swordfish to quote British types, whilst the Avenger is an example of American oblique hingepractice. It is interesting to note that British aircraft with the oblique hinge all fold with leading edge upward, whilein fhe American types the reverse is the case. The many factors affecting the choice of folding geometryare in themselves enough to warrant a text-book, but it is sufficient here to say that the vertical hinge type has nowbecome virtually obsolete, owing chiefly to the difficulties of manufacture and locking the long joint between thetrailing edge and the wing. On the credit side it had the advantage that considerably less power was required forfolding, both in the side wind case and if the wing had a heavy armament load. The horizontal hinge type has the advantage of reducedarea for the wind case, since if the wing is large it usually has to be hinged in two places. The greatest disadvantageis that duplicated lifting and locking gear is entailed, and the centres of gravity of the wing and armament have to be moved through a very much greater vertical distance. This gives rise to some acute problems introduced by thet- large amount of work which has to be done in the lf action, and has to be dissipated when lowering -4 spreading. Although the oblique hinge has the advantage of rela- tively small e.g. movement, the critical case is that of the 35 knot side wind which acts on the fiat-plate area of the wing. In the horizontal or oblique types of wing folding the designer is faced with the problem of providing relatively fast movement of a large mass, and stopping it safely with- out overloading the structure and mechanism at the extremes of travel. In the former case there is the addi* tional problem of a considerable variation of load due to armament, which can give rise to large changes in velocity, particularly when lowering. Power Available There are usually three power sources available for services of th'is kind, viz., hydraulic, electric and pneu- matic. The low pressures obtainable with current pneumatic equipment, combined with the inherently unsatisfactory characteristics when jack loadings are reversed, has precluded its use for this purpose—even experimentally. Electrical operation has the disability that a great deal of power is required to. complete the operation in the specified time under the combined worst Fairey's latest naval air-craft, the Spearfish, employs dual jacks to fold and spreadits wing about an oblique • hinge axis.
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