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Aviation History
1937
1937 - 0390.PDF
152 FLIGHT. FEBRUARY II, 1937. Relation between energy absorption and Debye temperature. The Debye tempera ture of a material is a quantity, having the dimensions of tempor- ature, which specifies the natural frequency of vibra tion of the atoms in the lattice. load with metal bearings because seizing troubles due to inter ruption under static load of the oil film, no such difficulties are found with Aerolite. In addition, dust and dirt do not have the deleterious effect on shafts running in Aerolite bear ings that is observed when white metal or bronze is used. Many fittings in aircraft are subjected to rather similar conditions of-slow movement, very often reciprocating in char acter, and sometimes of considerable pressure, and it may therefore be advantageous to use bushings of Aerolite instead of brass in control levers and the like. Conversely the elimination of the necessity for metal bush ings in Aerolite fittings leads to a saving in weight, and in a control lever we have made (to replace one in metal where trouble had been experienced with corrosion) the saving in weight and cost was quite considerable. In c.p. Aerolite air screw blades the Aerolite can be made to bear directly on to metal and the conditions of operation (slow reciprocating motion) are just those suited to such a combination. Research has not yet reached a point where the precise field of utility of such bearings can be defined. Their low heat conductivity (about 1/500 of that of a metal) rules them out for operation in engines, for example, though Bakelite with a graphite filter may be of service under conditions of high speed operation where the bearing loads are light. Ball and roller bearing cages are quite commonly made of synthetic material. Examples are manufactured by Ellison Insulations, Ltd. Airscrews.—One of the earliest applications of reinforced Bakelite was the construction in the United States of airscrews from impregnated and pressed cotton duck material. I believe that such airscrews were in use during the war 1914-1918 on " Liberty " engines. L. G. Nilson was granted a patent for such airscrews in 1919. The development of cgntrollable pitch airscrews of Aerolite is due in the first place to F. W. Caldwell and N. S. Clay, who, in N.A.C.A. Technical Notes Nos. 198 to 201 have given us an account of what may fairly be described as a brilliant engineering achievement. The stress at any seotion of an airscrew blade is a combina tion of tension due to centrifugal pull, and bending due to the aerodynamic thrust; there is also bending in a plane at right angles due to the aerodynamic drag force on the tip portion. Wooden blades, by virtue of their lower inertia forces, are much lighter than fixed metal blades, and the thicker sec tions give a slighlv higher drag at the gain of a slightly better static thrust. With the advent of the controllable pitch pro peller, this slight aerodynamic advantage of the wooden blade disappeared; but there remained with the metal c.p. screw- very high centrifugal and inertia forces at the blade root which had to be carried under conditions of the utmost reliability in a rotating bearing. A hub so heavy was required that many were of the opinion that at least for small aircraft the extra weight largely counterbalanced the possible increase in dispos able load due to improvement in the take-off. It will be seen that Aerolite makes a very suitable material for controllable pitch airscrew blades. Its low density results in low centrifugal and inertia forces and consequently a light hub can be used. It has good energy absorption and fatigue characteristics, in particular it is free from corrosion fatigue: its notch sensitivity is such that there need be no fear of stress concentrations. For these reasons the de Havilland Aircraft Co., Ltd., in co-operation with Bakelite, Ltd., and Aero Research, Ltd., are developing airscrew blades of this material. THE DISCUSSION MR. H. E. WIMPEKIS, President of the R.Ae.S. and Director of Scientific Research at the Air Ministry, recalled that in spite of his work at Trinity College, Cambridge, Dr. de Bruyne had found time to carry out very interesting research on synthetic resins with a view to developing them for aircraft construction. Not only so, but he had actually designed a little aeroplane, which he had called "The Snark," had flown it himself, and had made a tour of Europe on it. (See Flight of December 27, 1934, and February 14 and 21, 1935.—ED.) MR. M. LANGLEY, who opened the discussion, referred to the language difficulty. Some of the terms used were confusing to the engineer, and probably some of the aeronautical expres sions puzzled the chemist. He personally admitted that he had been floored by the word " Syneresis," and would like to know what it meant. With reference to synthetic resin materials, he would like' the lecturer to state if the properties were consistent, as with metals. If ,one put certain things "into the pot" could one be certain that what came out of the pot was always what one expected. As regards the drop in strength after ten hours of heating, at one particular tem perature, was that repeated at other temperatures? Time and temperature were apparently important factors, and as one might meet in the tropics temperatures up to 50 deg. C, although one would not, of course, meet temperatures as high as 100 deg. C, would a lower temperature sustained for a longer period of time cause the same phenomenon? He thought that in connection with reinforced synthetic resins they could probably learn a lot from concrete construction, and asked if other materials had been tried for reinforcement, such as metal mesh. What most aircraft designers would like to see were large moulded components for in large mouldings, he thought, lav the way to cheapness. MB. W. O. MANNING was interested in the compressive strength parallel to the reinforcing cords. The lecturer had given the strength at right angles to the cords. He recalled that in the early days of flying he (Mr. Manning) had experi mented with Bakelite paper for covering the leading edges of wings. Plywood had an unpleasant habit of deteriorating and showing bumps, and he found that the Bakelite paper with stood weathering perfectly. The subject of adhesion was little understood, and he wopdered why it was that although glue and cellulose would stick on glass there was difficulty with Bakelite. Initial Reinforcement MR. A. R. DUNTON pointed out that Aerolite was not a new material, and that there was a B.E.S.A. specification which covered it. He wondered why cotton was used for rein forcing, and not fine phosphor bronze or steel wire. He showed a slide of a very successful impact-testing machine with two hammers, used by Metropolitan-Vickers; this might be of, use for testing aircraft materials. He suggested that a commercial article known as Premix boards might be used for aircraft construction. It could be moulded to any desired curvature by submersing it in hot oil. It might be used for wing covering. DR. F. W. LANCHESTER complimented the makers of syn thetic resins on the progress they must have made. Many years ago he tried this material for gear wheels, but found it "crumbly," like a biscuit. He sent the first lot of material back, but the batch sent to replace it was very little better. MR. I. J. GERARD, a Senior Technical Officer at the Royal Aircraft Establishment, Farnborough, referring to the lec turer's figures for strength of a flat plate, said that buckling was of little moment; what mattered was the point at which one got complete collapse. For monocoque fuselages he thought one could use the method given by Dr. de Bruyne for calculating the strength in compression of a cylinder. He thought the figures given by the lecturer in different sec tions of the paper did not agree very well, and his .own calcu lations indicated that duralumin was approximately two and a half times as good as Aerolite. MR. C. D. PHILIPPE, of Bakelite, Ltd., explained that the name Bakelite was used to describe a bonded material, while Aerolite was a reinforced material. He explained that a great deal of work was being done and progress made behind the scenes. On the subject of metal fillers, suggested by some speakers, the problem was to get adhesion between the metal and the resin. If adhesion was not sufficient, one might as well use the wire mesh by itself and omit the resin. In con clusion, he expressed the willingness of the synthetic resin manufacturers to co-operate with the aircraft manufacturers in developing suitable materials. MR CONNOLLY, of the British Aircraft Manufacturing Co., Ltd., pointed- out that the requirements outlined about a year ago were already being met, and that it seemed quit* possible now to make a wing in synthetic resin matenah Although a wing could be made without being reinforced ny steel bars, he thought there was much to be learnt from ferro concrete designers. On the subject of glues and cements,
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