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Aviation History
1916
1916 - 1022.PDF
IfUciif] NOVEMBER 16, 1916. A REVERSIBLE TYPE OF PROPELLER. ALTHOUGH the theory of propeller design is not yet thoroughly understood, such theories as exist being in many cases contradictory, the general trend may be said to be towards an application of the theory first advanced by Drzewiecki inji882,*aiid which was .to Two of Mr. Austin's reversible propellers the effect that each element of the screw blade may be considered separately and as behaving in the same manner as if moving through the fluid in a straight line. Applied to the air screw this means, of course, that the elements along the blades are treated as aerofoils, and that sections which have been found to give good results in large size as wings of an aeroplane will, generally speaking, be found to give good results Kempston, Bedford, has been experimenting with air screws of various shapes, all having one characteristic in common, however much they differ in details—i.e., they are reversible. In the accompanying illustra tions are shown two of these propellers, one of which has a radially expanding pitch in which the chord lines of the various elements are sensibly parallel at all radii. In the other propeller, it will be seen, a more orthodox method has been followed, in that the helix angles diminish in the manner usual in a propeller of uni form pitch, while the angles of attack do not vary greatly from root to tip. What is most interesting about these propellers, however, is the fact that they are reversible, and that therefore the sections at various radii from the'centre of the propeller boss are totally different from those generally used in air screws. The peculiarity may, perhaps, be best explained by saving) that -the blades consist of the upper Reversible propeller with radially expanding pitch. on a reduced scale when forming the elements of an air screw. The general characteristics of lift coeffi cients, drift coefficients and lift/drift ratios are applic able to the elements of the screw—modified, of course, to suit constructional considerations. For a considerable time now Mr. A. Austin, of 1014 surfaces of an ordinary propeller turned round end to end and with their flat under surfaces together. It is, of course, not to be understood that this simile refers to the construction, which is of the usual laminated type, but merely to the sections. These latter, by the way, are not to be taken as absolutely correct, as Mr. __————
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