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Aviation History
1936
1936 - 0355.PDF
FEBRUARY 6, 1936. FLIGHT. 3 59 AIRSCREW DEVELOPMENT Advantages and Disadvantages of Various Types of Airscrews and Their Material : The Latest Developments : A Summary of the General Conclusions in Dr. H. C. Watts' Lecture to the R.Ae.S. AIRSCREW development can conveniently be dealt with under four headings : aerodynamic design, materials, L adaptation to engine and aircraft, and fans for cool ing and ventilation. In his lecture to the Royal Aeronautical Society last Monday Dr. H. C. Watts omitted the fourth altogether, and dealt but briefly with the first. Of the other two, however, he had a good deal to say, and very interesting it proved. Airscrew design is a highly specialised business, and it is doubtful whether the non technical reader would be able to follow the arguments. It is, therefore, proposed to mention here only that part of the paper which deals with general principles and conclu sions, and to reserve for publication in The Aircraft: Engineer (monthly technical supplement to Flight) the more technical parts of the paper and the appendices. Dr. Watts, who is technical director of the Airscrew Co., Ltd., pointed out that wood is a natural absorber of vibration and therefore an ideal material to use with an internal combus tion engine with its periodic torque variations. However, it has two great disadvantages: it absorbs and gives up mois ture, and it is not hard enough to resist erosion by rain, sea spray and dust. Protective finishes and metal sheathing of the leading edge have overcome these deficiencies to some extent. So protected, the life of a wooden airscrew may exceed 3,001) flying hours. For seaplane work, however, the lecturer did not consider the standard method of metal sheathing wholly satisfactory. Dr. Watts then described the Schwarz process of protection, the British rights for which are held by the Airscrew Company. This consists in covering the airscrew with a film of cellulose acetate one millimetre thick. The leading edge is covered with a thin strip of cellulose material. Over this and the Thole of the blade and boss is a layer of fabric or high-tensile- steel gauze. The leading edge is covered with a strip of specially woven phosphor-bronze wire gauze, to which is sweated a nosepiece of brass. The airscrew, after being thus prepared, is covered with a sheet of cellulose material which is applied in a soft and plastic condition. Adhesion to the wood is obtained by placing the airscrew in a specially designed auto clave and submitting it at ordinary temperature to a pressure of several atmospheres, varying according to the timber used. The covering is forced right into the wood, through the fabric and wire gauze, and can only be removed by tearing away the wood with the covering. The resultant surface can be highly polished, and the covering is so hard that it is impossible to "amp it with steel di«s. Compressing the Root The finish seals the wood and prevents the rapid exchange "f moisture with the surrounding atmosphere. It is as resistant as metal to erosion and abrasion, and is, of course, non- corrosive. . One of the disadvantages of wood when applied to airscrews 15 its low shearing strength. This has hitherto prevented the * of wood for detachable blades such as are used in variable- fitch airscrews. Dr. Watts stated that he was able to say mat by an extension of the Schwarz process this defect had Been overcome. The wood at the root of the blade was impreg nated with a special preparation and compressed to about one- fflird of its original size. The compressed root was then Machined to the usual cylindical form and screwed into a steel leeve. further impregnation under high pressure served com- Pctely to close up any clearances left in fitting the sleeve. The expression of the root was permanent; daily- measurements n at constant normal temperature of a cube of the com posed material showed no perceptible variation. The specific Sravity vvas about I>4> and the u]timate tensile strength 14 to '5 tons per square inch. The main poition of the blade was (i c?mpressed, and had a specific gravity of 0.5 or less, so ecm' ,'?erT'a loads were less than one-half of those of any ?ft n "' metal blade. There was no limitation in size; a • blade for a 50ft. airscrew was practical if required. Ur>nng to metal airscrews. Dr. Watts recalled that the first SPECIAL COVERING PRESSED INTO WOOD UNDER HIGH PRESSURE FINISHING LACQUER CROSS SECTION OF PROPELLER SCKWARZ COVERED Section through the leading edge of a wooden airscrew blade finished with the Schwarz process. successful departure from wood was the Reed airscrew made of aluminium alloy. The sections are very thin and the blade sufficiently flexible to set itself closely along the direction of the resultant forces, thus reducing the bending moment. Unless the material is homogeneous the deflections are unequal and unbalanced moments are set up, causing vibration. Hence the need for special dynamic balancing. Thin sections give an advantage for maximum speed but tend to lose thrust at low speeds because of lower maximum lift and earlier stalling. Following closely on the Reed airscrew came the adjustable- pitch type with blades set in a central hub. This type lends itself to mass production; for any given diameter it is possible to stock a single type of blade and obtain two-, three-, and four-bladed airscrews of any desired pitch. Owing to the cylindrical root, the true blade section does not start until well out into the blade, and this results in poor cooling of the engine behind the airscrew. The weakness of this type of blade is catastrophic failure due to fatigue. Magnesium alloys have a specific gravity of r.8 as compared with 2.8 for aluminium alloys, and about the same weight- strength ratio. One might therefore expect a saving in weight of 30 per cent. Considerations of flutter limit minimum thick ness, and as flutter depends upon the modulus of rigidity of the material, the weight saving may be 15 rather than 30 per cent. Magnesium is now being extracted from its carbonates, of which plentiful supplies exist in this country. This fact, and the saving in weight, is forcing the development of the magnesium blade. Experience with magnesium blades has ' been somewhat conflicting. Some have failed in use and others have given satisfactory results over considerable periods. Magnesium alloys corrode easily and must be suitably pro tected. The chromate treatment is partially effective, but the film is easily damaged and cannot be maintained along the leading edge. Rubbing down with lanoline after each flight is necessary. It may, perhaps, be recalled that some years ago Dr. Walts was intimately concerned in the design and manufacture of the Leitner-Watts steel airscrew blade. In his paper Dr. Watts recalled that the blades produced by Metal Propellers, of Croydon, only just fell short of success. Accurate blades were produced in quantities, and their weight was low. They sus tained exhaustive spinning tests, engine bench tests, and development flight trials, and yet when put into service they
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