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Aviation History
1936
1936 - 0881.PDF
374 FLIGHT. APRIL 9, 1936. THE SCHWARZ AUTOMATIC V.P. AIRSCREW Extreme Simplicity the Feature of Fully Automatic Two-pitch Design : Three- pitch Type to Follow : Use of Wooden Blades THE Schwarz variable-pitch airscrew hub has been designed to provide a simple and reliable limited solution, pending the emergence at some future date of a complete solution of the problem of an infi nitely variable-pitch airscrew to fulfil ideal requirements. Even if and when the latter becomes generally available, predict the designers, it is certain that a substantial pro portion of aircraft will for many years continue tc5 be equipped with a similar variety, such as the Schwarz auto matic hub, for convenience and economy. Two-position Simplicity The Schwarz variable-pitch airscrew hub is an automatic mechanism giving fixed positions to the blades; at present there are two positions only, corresponding to the requirements respectively for (a) take-off and climb, and (b) for normal flight. In either position the airscrew is stable, while the change-over occurs automatically and positively in both direc tions at predetermined airscrew speeds. By limiting the requirements in this way to the most important needs it has been possible to simplify the design exceedingly and to make the mechanism independent of the effect of external disturbing influences, such as friction, displacement of centre of pressure, distortion of blades, etc. The principle of the mechanism is illustrated in the accom panying diagram. Each blade is mounted rotatably in the hub; attached to the, blade is an arm carrying a weight. When the airscrew is at rest a spring, actuating the weight-arm through the medium of a push-rod and link, pushes the blade into the position of minimum incidence, with the arm against a stop on the hub. When the airscrew is in motion there are two forces acting on the blade, a moment exerted by the spring, which is independent of the speed of rotation, and a centrifugal moment which increases as the square of the speed. Take-off and climb proceed at the minimum incidence and at WEIGHT COARSE.-PITCH POSITION The Schwarz automatic v.p. airscrew hub, carrying wooden blades prepared by the makers' patented process. rotational speed gradually approaching the maximum permis sible speed of the engine. On the macnine changing over from climbing attitude to level flight, the speed of rotation increases and oversteps the point at which the two opposing moments acting on tfie blades are. exactly balanced. The centrifugal moment then overcomes the spring moment, and pushes the blade into the position shown dotted (maximum incidence) with the arm against the opposite stop. The axial distance of the weight, and consequently also the centrifugal moment, are thus increased. At the same time, however, the spring moment, in spite of the increased tension of the spring, is reduced, because the altered angle of the link reduces the effec tive leverage. Thus the "change-over is quite positive from one stable position to the other, and the engine can then be throttled back quite considerably without causing the mechanism to reverse the blade position. Thus the airscrew will be kept in the position of maximum incidence required in cruising flight. Not until the engine is throttled back to the extent required for preparing to land will the mechanism change over, when the blades will revert equally positively to the minimum incidence position, in which they will be ready for a fresh take-off or climb, e.g., in the event of the pilot reopening the throttle after an unsuccessful attempt to land. Design work is in progress with a view to the provision of a third stable position of the blades, suitable for high-altitude cruising. Qerman Experience The simplicity of the arrangement has resulted in the desired maximum reliability; it is stated that even in the unlikely event of damage to the mechanism (broken spring, seizure, defective bearing, or the like) safety is not impaired, and the worst that could happen would be to continue a flight with an unfavourable blade setting. In Germany the Schwarz variable-pitch airscrew—which, incidentally, can be adapted to the nwteur canon—has been designed and tested for several types of engines. The first tests were carried out on two-bladed airscrews for the 240 n.p. Argus AS 10 engine, and on three-bladed types on the 660 h.p. Siemens SAM 22 B engine and the 730 h.p. B.M-W. VI (geared) engines. D.L.H., too, have used this airscrew on Jumo 5 Diesel engines. In addition, the Schwarz Company have production orders in hand for equipping the rlirtn HM8U engine, B.M.W. Hornet, B.M.W. VI (direct drive), and other engines up to 900 h.p. ..h Though the Schwarz V.P. hub may, if desired, be &ttFd.^Z. metal or any approved type of blade, those so far buiIt 1na been equipped with Schwarz patented wooden detacna blades of the type described in Flight of June 6, 1935- • The low weight of these blades considerably reduces cenr- fugal forces on the bearings; moreover, all forces ^exerte ^ the blade are taken up by a single bearing in which a method of elastic preloading is employed to obviate alterna. o stresses, which are highly undesirable in ball or roller l''.'a .JVj Inci-Schwarz lightweight "detachable blades are made in EngIn^j. by the Airscrew Co., Ltd., Weybridge, under licence tandard finish lof Studied in conjunction with the text, this diagram clearly shows the operating principle of the Schwarz hub. dentally, the Schwarz process is now the s all wooden airscrews for the Royal Air Force. . rrM °f In a paper read before the R.Ae.S. recently (see ' 8&ri February 6, 1936) Dr H C. Watts described the • L ^ process of protection, and also the -method of 22?r??veT blade root to prevent play developing in the steel slee
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