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Aviation History
1935
1935 - 1297.PDF
May 30, 1935 Supplement to B^e* FLIGHT ENGINEERING SECTION Edited by C. M. POULSEN No. 112 (V°^U0m|X) l«th Year May 30, 1935 AUTOMATIC ENGINE CONTROLS Operation of the Various Hobson Systems By ERNEST W. KNOTT, M.I.A.E., M.S.A.E. Continued from page 25) MODERN aircraft carburetters require an altitude con trol capable of weakening the mixture strength by at least 40 per cent., and it is only too obvious that abuse of this available range of mixture strength through inexperience or inattention on the part of the pilot can either cause excessive waste of fuel or, on the other hand, destructively high engine temperatures. Many attempts on long-distance flight records have ended abruptly because the pilot, in his anxiety to conserve fuel, has damaged his engine through using too weak a mixture ratio for the conditions under which the machine was being flown. Many kinds of devices for automatically operating the i R. p ml Z bJ V P < *• FUEL CONSUMPTION FIG. 8. - 1 1 1 1 1 1 1 1 r~ 1 1 1 1 1 i^-> 1 1 OR ALTITUDE 1 1 1 1 1 1 1 1 1 1 1 1 1 1 ^ *"" 1 1 1 1 1 1 1 1 VALVE OPENING -*"*»l [\ \ \; \ \ Curves of mixture strengths and cylinder temperatures : (C) weakest mixture for maintaining power ; (B) normal power mixture ("Rich Automatic") ; (D) cruising mixture strength, giving drop in revs with safe drop in temperature and steady running ("Weak Automatic") ; (A) twelve per cent, richer than (B) for take-off boost altitude valve on the carburetter have been evolved, but they all had certain weaknesses, either of a mechanical nature or else limitation of the degree and accuracy of control. The Hobson-Penn Automatic Mixture Control was de veloped along lines completely foreign to all preconceived ideas, and was based on the following discovery. Mixture Control It is a well-known fact that as mixture strengths are weakened to the point where maximum power is main tained with minimum fuel consumption—known as W.M.M.P.—engine temperatures approach a dangerously high point, so that carburetter settings are always used giving mixture strengths sufficiently richer than the ratio giving W.M.M.P. for temperatures to be safe for continuous running. It was found, however, that if the mixture strength was reduced considerably below that for W.M.M.P and it was accompanied by a fall in revolutions, engine temperatures fell also to a safe degree, and extraordinarily low fuel consumption figures were obtainable. A typical curve is shown in Fig. 8. The Hobson-Penn device, therefore, was developed as a two-stage control in which the pilot was given only two alternative mixture strengths, the normal one for maxi mum power, acceleration and manoeuvrability, and the other weaker than W.M.M.P , which accompanied by a drop in revs gave fuel economy, and thereby a consider ably extended range. Official tests have shown that this device, when properly installed, will increase the cruising range of a machine by 20 per cent, over that obtained by the most experienced test pilots. The advantage of such a device in the hands of unskilled pilots needs no elaboration, a difference of as much as 50 per cent, being commonly recorded between similai machines fitted with an ordinary manual mixture control flying under identical conditions, but with different pilots The fact that the instrument is a two-stage type renders
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