SLIPSTREAMS

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48

ir OW that the light aeroplane has attained a degree of safety and ease of control which makes it a reasonably reliable means of transport when in charge of anyone of

normal intelligence, manufacturers are devoting more and more attention to the question of performance. A higher cruising and all-out speed seems to be the aim of all designers, and as a result engines of increased power are being adopted, and the reduction of drag by very careful streamlining and other means is the order of the day.

Head resistance is being attacked very thoroughly by American designers, and “spats,” or streamline fairing over the landing wheels and retractable undercarriages are found on quite a number of types. In this country nothing much has been done in this direction so far, although I believe that the retractable undercarriage is being experimented with by one or two firms. This feature certainly appears to be a very desirable one where more m.p.h. is being sought after, and when the various problems attached to it have been satisfactorily solved, it is not unlikely that it will ultimately replace the orthodox type of landing gear. Meanwhile aero-engine designers are hard at work trying to get more power from motors which weigh less than their prototypes, for with a high power-weight ratio a really low landing speed, a greater power reserve and a quicker cruising speed are made possible, and it is in these directions that the ‘plane for the private owner must develop. It does not seem so very long ago when we thought that anything below three lbs. per b.h.p. was a great achievement ; now it has been brought down to very nearly 1 lb. per b.h.p. with some engines—as, for example, the latest Pobjoy. This little power unit, I think, deserves to be better known. It was first built in. 1926, and although until quite recently it was never made on a production basis, several examples of the early experimental models are in use and giving satisfactory service. The latest Pobjoy, known as the ” R

“type, came through its type test very well indeed ; the petrol consumption works out at .57 pints per b.h.p. hour, and the oil, 0.9 pints per hour, but what is really remarkable is the fact that it weighs complete with exhaust pipes, hand starter, and propeller hub only 130 lbs. Its normal output is 75 h.p., and thus the powerweight ratio is 1.7 b.h.p. The makers (Pobjoy Airmotors, Ltd., of Hooton, Cheshire), in writing to me about their engine, point out that in order to avoid installation difficulties they supply the unit complete with cowlings, oil tank and cooler, all ready for mounting in the air frame. The Pobjoy ” R ” runs at a fairly high speed for an aircraft engine, with 3,000 r.p.m. as its normal speed and a geared prop. The reduction gear consists of a pair of double helical wheels, and to assist in steadying the drive, a small hollow flywheel is incorporated. One of

its salient qualities is an exceptionally smooth running at all speeds secured by virtue of its revs and seven small cylinders.

Autogiro Demonstrations.

Speed, Weight and Power.

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