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Aviation History
1925
1925 - 0145.PDF
MARCH 12, 1925 THE PISTOLESI VARIABLE-PITCH AIRSCREW ONE of the problems in aeronautical engineering that has engaged the attention of designers and inventors for some considerable time past is that relating to the variable-pitch airscrew. That the variable-pitch airscrew is a desirable component will not be denied, for its advantages over the ordinary fixed airscrew are obvious. In the first place, such an airscrew can be used, within limits, on any type of engine ; in other words, it can instantly be adapted to suit the charac- teristics of any particular engine—such as variation of crank- shaft speed, etc. Secondly, the variable-pitch airscrew can be adjusted to meet the varying conditions of flight. It is this latter feature gain of 8 kms. per hour (a m.p.h.) on the horizontal speed was obtained ; a gain of two minutes in climbing to 4,250 m. (13,900 ft.), or a gain of 15 per cent, of the total time employed, and a gain of 500 m. (1,640 ft.) on the practical ceiling. Exhaustive tests have also been carried out with this airscrew on the bench, as a result of which it would seem that the method of construction employed is quite satisfactory. During test the airscrew was operated for long periods at a rotational speed of 1.4 times the normal one, being thus subjected to a centrifugal strain equal to twice that obtaining under normal conditions. The airscrew withstood the test well, and without any indication of failure in any part. AN ITALIAN VARIABLE - PITCH AIRSCREW : The " Pistolesi " airscrew, in which the pitch of the blades is varied by means of a hand-operated crank, at rear of engine. Above, on left, is a model for engines with hollow crankshafts, and on the right is seen the model for engines fitted with solid crankshafts. that makes the variable-pitch airscrew most desirable, for by its use, in conjunction with the supercharger, we can obtain the best performance out of a machine at gTeat altitudes without sacrificing the efficiency of that machine during flight near the ground. In short, the admitted advantages of the supercharger can be utilised on a more sound, or practical, scale by means of the variable-pitch airscrew than is the case at present, where compromise figures very largely. Its utility admitted, it must be assumed that its achieve- ment is beset with problems by no means easy of solution, for, although many attempts to produce a realiable variable- pitch airscrew have been made in the past by designers in both Europe and America, it cannot be said that it has arrived at that stage when it can be put into general use. There is little doubt, however, that the problem is slowly but surely being overcome, and recently some promising results have been obtained with one or two designs. One of these designs in particular, which has already passed through some practical tests with no little success, hails from Italy, and it may be of interest to our readers if we give a few brief particulars of this airscrew—although, at the moment, we are unable to go into its technical and constructional details. The airscrew in question is the " Pistolesi," which is being developed by the Societa Idrovolanti Alta Italia (S.I.A.I.) of Sesto Calende, the constructors of the famous " Savoia " aircraft. A " Pistolesi " variable-pitch airscrew was recently fitted to a Spad XIII biplane fitted with a 220 h.p. Hispano- Suiza engine, and some interesting tests were carried out with same at the experimental aerodrome at Monteceho (Rome) by the Italian Aeronautical Research Bureau. Com- parative tests were also made with the machine fitted with the " Pistolesi " airscrew and with a fixed-pitch airscrew. The results obtained were very promising, as may be gathered from the following facts: With the " Pistolesi a 1 HI ih Briefly, the " Pistolesi " variable-pitch airscrew consists of a central metal portion carrying the two wooden blades, the roots of which are set in steel muffs. The blades can be rotated within the central housing, around their longitudinal axis, by means of hand-operated gearing. One of the most difficult problems in connection with the variable pitch airscrew has beeen to effect a satisfactory union or connection between wooden blades and their central housing, or boss— especially when it is remembered that one has to contend The "Pistolesi " Variable-Pitch Air- screw : Diagram showing the type of joint employed for securing the wooden blades in the central housing. with a strain due to centrifugal force of several tons, varying from 5 tons on small airscrews, up to 30 tons for the larger ones. It was only after much investigation and the trying out of several types of joints, that the designer of the " Pistolesi " airscrew finally settled upon the system now employed on the airscrew. As may be seen from the accompanying diagram, the root of the blade has a series of conical steps 145 D2
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