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Aviation History
1940
1940 - 2292.PDF
AUGUST 15, 1940 ROjOL ELECTRICALLY OPERATED AIRSCREW A sectioned view of the new Rotol electrically operated variable-pitch airscrew. screw ceases to rotate the engine. It will be seen, on looking at any such airscrew, that the blades have now taken up an angle such that one part of the blade area is trying to rotate the airscrew in one direction, and another part tends to rotate it in the opposite direc- tion. The combined result is an out-of-balance couple in each direction insufficient to move the engine. The drag of the airscrew is now much smaller than it would be if it were windmilling, because the resultant velocity of the air in relation to all parts of the airscrew is re- duced to a figure equal to the forward speed of the aircraft itself, which is the minimum possible in the circumstances. A further reduction of drag is due to 'the fact that all parts of the blade are at angles to the "relative wind which correspond to low drag coefficients. Owing to this reduction in drag, the feathered air- screw greatly increases the range of a multi-engined aircraft when one engine has broken down. Thereby it improves the chances of the machine getting home. The actual pitch-changing mechanism of the airscrew may be hydraulically or electrically operated, but pro- vided the above conditions are met, the method of operation is purely a matter of design convenience, the effect in the "fully feathered ' position being always the same. Generally speaking, some auxiliary source of power, such as a battery and electric motor driving an oil pump, or a motor generator type of voltage booster, is used to feather the airscrew. (/) Unfeathering the Airscrew Under certain conditions it may be desirable to un- feather the airscrew in flight, and all modern designs of feathering airscrews provide a control for this pur- pose. In principle, the operation of unfeathering is merely the reverse of the feathering operation already described. Slow unfeathering is a feature of some types of airscrew so that a cold engine can be set going again as gently as possible. Naturally an airscrew which can be locked in any position has definite advantages in this respect, since the engine can be left running slowly while the aircraft is still flying at normal cruising speed. The pitch of the blades is then readjusted at intervals, so that the engine warms up and takes its load gradually instead of suddenly, as would be the case if the airscrew were violently unfeathered. (g) Reversing the Airscrew Flying boat technique in the U.S.A. has already established the advantages of a "reversible" variable- pitch airscrew for controlling taxying on the water under adverse conditions of wind and tide. Designers of variable-pitch airscrews in this country are naturally offering this same feature in their up-to-date designs. The most promising type of pitch control, so far as this feature is concerned, is at present the electric type, in which a voltage booster similar to the one used for feathering the airscrew can also be adapted for revers- ing it when required. Qeneral It must be admitted that all these features have in- creased the complication of airscrew construction far beyond what it used to be in the days of simple fixed- pitch screws. Such complication is, however, more superficial than real when the increased range of re- quirements is taken into consideration. For the future it can only be said that, no matter what complicated requirements may be asked for, our airscrew designers are ready to meet them in good time. As time goes by it is possible that the apparatus itself may be gradually simplified and lightened without de- tracting from its ability to meet all requirements, but at the moment the airscrew is rather like the under- carriage—it started life very simply and then grew up.
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