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Aviation History
1910
1910 - 0056.PDF
BMBMHHHRMHMi ; \fiML JANUARY 22, 1910. AERIAL PROPELLERS. AND -SOME i POINTS WHICH MAKE THEM INTERESTING Concludedfrom page 37.) The Direct Drive. DECIDING in advance to use a direct-driven propeller, therefore, may very well prejudice the chance of evolving an efficient design, for the petrol motor is essentially a axis. They revolve in opposite directions at a reduced speed by means of a differential gear. Many Blades v. Few. It will be noticed that in none of the foregoing deduc tions has any account been taken of the number of blade " Flight" Copyright. Although apparently a single four-bladed propeller, the mechanism illustrated above, which is a photograph of the Howard Wright machine, consists of a pair of two-bladed propellers which revolve in opposite directions. The blades nearest the engine are larger than the others, and do two- thirds of the work. high-speed engine, and one, moreover, which has but a limited useful range. From the constructional point of view, the direct-drive is a highly convenient method rf mounting a propeller, and the mechanical efficiency of ts transmission is, of course, at a maximum. But, according to Mr. Lanches- ter,* the propellers on the ' Flight " Copyright. The propeller on the Wellman airship is an example of how a pair of tubular rods may be used for the attachment of light metal blades. Voisin flyers, which employ the direct-drive, are about 15 per cent. less efficient, as propellers, than those on the Wright flyer, which are driven by chains, so that although the relative mechanical efficiency would tend to reduce this difference, the net result would once more be a compromise. On the Howard Wright biplane there are two propellers in tandem on the crank-shaft * See FLIGHT, January gth, page 30. "Flight" Copyright. A neat method of fixing a two-bladed propeller to the crank shaft is to use the shanks of the blades as bolts, and to fasten them by nuts to the boss, as is shown in the above illustration of the Voisin flyer. which are or should be fitted to a propeller, and indeed this question has no direct effect upon the general problem. The mass of air dealt with by the propeller is represented by a cylinder of indefinite length having a diameter equal to that of the propeller itself, and the rate at which that cylinder is projected to the rear depends upon the pitch of the revolutions, and not at all, theoreti cally, upon the number of blades. Theoretically one blade properly de signed working under suitable con ditions would be sufficient, and even then there would be no need, as some inventors have imagined, to make that blade a complete helix so that it en circled its shaft. One blade would, of course, be out of balance by itself; hence it may be assumed that two " Flight " Copyright. In the Hollands propeller the blades are shaped something like cheese-cutters, having concave faces, and being tapered from the root to the tip. blades constitute a minimum, and in marine work it is common to find at least three blades because the balance is then better still. With aerial propellers, so far as they have at present been constructed, two blades are very 52
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