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Aviation History
1935
1935 - 1061.PDF
MAY 9, 1935. FLIGHT. 501 FLANGED SLEEVE lution increases the pull due to centri fugal force also increases, and ultimately a speed is reached at which the pull be comes strong enough to overcome the force of the springs, and the blades go into coarse pitch. For going back into fine pitch the pilot throttles down, and waits until the engine speed has dropped sufficiently to permit the springs to over come the centrifugal pull. It is obvious that while this system may work reasonably well, it is subject / to a very nice balance between the strength of the springs and the centri fugal pull, and it is possible to imagine conditions in which high revolutions at fine pitch would be desirable. This appears to be impossible with the Ratier system. The firm of Pierre Levasseur, specialists in aircraft designed to work with the French navy, has concentrated its attention on airscrew design and manufacture for a number of years. For instance, it was this firm which, many years ago, secured the French rights to manufacture the Reed metal airscrew under licence. The subject of controllable-pitch air screws has also been closely studied by the firm, and a great deal of experimenting has been done in an effort to discover ways and means of securing wooden airscrew blades to metal hubs. So far the solution has escaped the designers, and of recent vears they have concentrated on metal controllable-pitch airscrews. Originally the scheme adopted was to have the roots of the duralumin blades secured in sockets in the steel hub, but this type of construction was discarded, because the heavier metal was used in the greater bulk and the lighter metal in the smaller bulk, so that the construction was found to be rather heavy. In the modern P. Levasseur controllable-pitch airscrew the roots of the duralumin blades form the sockets and fit oyer projections on the hub members. This arrangement gives a very light construction and is claimed to have been found to work satisfactorily. The principle upon which the Levasseur airscrews work is extremely simple. The blade roots have an internal thread machined in them, which engages with the stubs of the steel hub. The pitch and " hand " of this thread are so chosen that centrifugal force tends to twist the blades into a coarse pitch position. At low speeds this tendency is partly resisted by rubber cords anchored to the cranks on the blade roots. Limits of pitch settings are provided by two pins located On the P. Levasseur controllable-pitch hub rubber cords are used for returning the blades to fine pitch. PISTON The Ratier two-position airscrew. The inset view shows one of the two sloping slots which, when moved forward or back by the piston, causes the pitch of the blades to change. in the walls of the blade roots, and free to swivel in tapering holes in the hub bosses. To ensure that the two blades shall be synchronised as to angular movement, a flanged sleeve is fitted. On this sleeve there are two lugs in which the ends of the pitch-limiting pins engage through ball fittings. The flange of the sleeve has two holes with which two studs on the hub can engage. Spring-loaded plungers in the ends of the pins keep the flanged sleeve in engagement with the studs, but the mechanism can be unlocked by the pilot by forcing the flanged sleeve away from the hub, thus disengaging the studs from the holes. This is done by roller-ended rods bearing on the side of the flange, near the circumference. At the take-off, when the engine speed is somewhat low, the rubber cords hold the blades in the low-pitch position. When the pilot desires to change into the high-pitch position, he operates the roller-ended rods through a suitable control in the cockpit, disengaging the studs from their holes in the flange of the sleeve, and leaving the blades free to rotate into the coarse-pitch position. For changing into low pitch again the pilot releases the pressure on the sleeve, throttles down until the rubber cords can overcome centrifugal pull, and the blades return to low pitch. The simplicity of the Levasseur airscrew is attractive, but one suspects that there may be certain practical difficulties connected with the lubrication of the thread if wear on a thread cut in duralumin is to be avoided. The Smith Airscrew A controllable-pitch airscrew introduced in America some years ago was the Smith, designed and manufactured by the Smith Engineering Co., of Cleveland, Ohio. Although used by Jimmy Doolittle and Wiley Post on some of their notable flights, this airscrew has never attained as wide adoption as the Hamilton Standard, for example, although its simplicity and the fact that it is variable over a range of angles and not confined to a two-position pitch range would have led one to expect that it had certain advantages. In the Smith controllable-pitch airscrew the blades are held in sleeves and are operated via worms and worm wheels by another sleeve, a double one, in the thrust bearing cover plate of the engine. This double sleeve passes around the airscrew shaft and does not rotate.. It is so arranged that it can slide back and forth along the axis of the airscrew shaft, and by means of an angular-cut groove sliding over a fixed pin. A movement of the sleeve causes either the left or the right-hand worm to engage a horizontal gear, thus transmitting a rotating movement in one direction or other of the airscrew blades. When in the neutral position the gear trains are at rest, so that the only time the operating mechanism is subject to wear is during the few seconds it takes to change the pitch. Electrical operation of the blades of a controllable-pitch air screw would appear to offer an attractive solution of the problem, and a couple of years ago such a type was designed by Mr. W. R. Turnbull, a Canadian who came to England during the war. The American rights in this airscrew were secured by the Curtiss company. In the Turnbull design the aluminium alloy blades are car ried in a steel hub. The blade roots have shoulders against which bear the ball bearings that take the centrifugal loads, and bending loads are taken care of by roller bearings near the inner ends of the blades. Each blade has at its extreme inner end a spur wheel which forms part of the pitch-controlling gearing. The electric motor, which is quite small and housed on the front end of the airscrew hub. is geared to these spur
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