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Aviation History
1935
1935 -2- 0522.PDF
476 FLIGHT. NOVEMBER 7, 1935, COMPOSITE AIRCRAFT HE reference made by Sir Eric •. -. lations, split up into two: Lift an]T Geddes last week (see page 487) to the Mayo " composite air- craft " now under construction by Short Brothers, cf Rcchester, has once more called attention to an interesting experiment which, it is hoped, will be begun some time in the spring of next year, and which is of a character so revolutionary that it deserves a somewhat closer examination. The experiment con- sists of the use of two aircraft rigidly joined together during the take-off and the climb to the opera- tional height desired. When that has been reached the two are separated, the carried aircraft setting off on its journey and the carrier aircraft returning to the base. Although (as Flight pointed out some months ago) something of the kind was tried during the War, the scheme appears fantastic at first sight. One's reaction is to dismiss it as another of those wild suggestions made from time to time by inventors who do not fully understand the fundamental principles upon which their inventions are based. Yet, as Flight remarked about a year ago, when the scheme first "leaked out," the fact that the inventor is one whose name is well known in British avia- tion circles compels one to delve below the surface and to examine the subject more thoroughly. When, more- over, a company like Imperial Airways has taken up the invention, backed by the Air Ministry, and when a firm of the standing of Short Brothers is willing to associate itself with the scheme and to undertake the construction of two aircraft in order that a thorough test may be made, it becomes doubly necessaTy to pay more than passing attention. The Fundamental Principles Data on the Mayo '' composite aircraft'' are not yet available, but it so happens that the fundamental prin- ciples involved are quite obvious, and anyone familiar with the subject of aircraft design and performance calculations can estimate, approximately at any rate, the primary factors and advantages which enter into the problem. Basically, the raison d'etre of the composite aircraft is the get- ting into the air of a very heavily loaded aircraft with the assistance of another, and lightly loaded aircraft. The heavy power load- ing of the former is reduced by the low power loading of the latter, and similarly, the wing loading of the combined aircraft is comparatively low, although that of the carried component is high. As it may not be quite obvious why a very heavily loaded air- craft should be at an advantage, a brief outline of the problems involved may suitably be given before the more technical aspects are examined. This is not in- tended to be a treatise on per- formance calculations, but for the benefit of those who are nnt familiar with work of this kind it is necessary to go back to fundamental principles. The forces on an aeroplane are usually, for purposes of performance calcu- Fresh Interest Focused on the Remarkable Short- Mayo Scheme by Sir Eric Geddes : Some Practical Aspects of the " Mother Aircraft " Idea Discussed. By C. M. POULSEN 800 700 600 500 ^400 300 100 11u 1 l\ \1 1 11 \ s ) ( \ N j ^•s, ' / y / / / ,•-•" 50 60 70 Typical drag and '• horse-power required " curvesof a biplane of average efficiency. drag or resistance. These can either be calculated from known figuris of existing aeroplanes or they can \>- obtained from wind-tunnel results on scale models. At a given air speed the lift is proportional to the wing loading, that is to say the weight per squart foot of wing area. A machine loadtd to twenty pounds per square foot oi its wing area must travel faster to remain in steady horizontal flight than one loaded to ten pounds psr •square foot. The price which has to be paid for the lift of an aeroplane wing is the airscrew thrust required to overcome the so-called drag, or air resistance to forward motion. That drag, or resistance, arises partly from th:- wings and partly from the rest of the aeroplane. The drag cf the latter is often referred to as "parasite drag,1' because it is resistance of parts which do not contribute to the lift, and therefore do no useful work. Obviously, it is possible to construct curves which show the drag, or resistance, at any forward speed of an aeroplane if the resistance of the various component parts is known, and bearing in mind that drag increases as the square of the speed. A typical set of such curves is shown in the illus- tration on this page. It will be seen that there are two curves which represent wing drag and parasite drag re- spectively, and a third curve of total drag, which repre- sents the sum of wing drag and parasite drag. Calculating " Horse-power Required" The fourth curve on the figure represents '' horse-power required." This is calculated from the total drag curve by the simple relationship which exists between force and power (power being denned as " the rate of doing woik ")• One horse-power is 550 pound feet per second, or 33,000 pound feet per minute, or 375 pound miles per hour. It is, of course, immaterial which of these units are used, so long as they are consistent. In the figure it will be seen that, for the particular aero- plane to which the curves refer, the total resistance at 80 m.p.h. is 500 lb. or 80x500 = 40,000 pound-miles psr hour. As one h.p. is 375 pound- miles per hour, 40,000 pound- miles per hour obviously is equal to 40,000 divided by 375, or 106 horse-power. Having seen how the " hors>- power-required " curve is con- structed, it becomes possible to examine the fundamental reason for the "Mayo composite air- craft. The horse-power required curve, it will be seen, reaches a minimum value of just under ico b.h.p. at a speed of about 65 m.p.h. For the particular machine in question, this would be the most economical cruisinC speed, i.e., the speed at which the smallest amount of petrol was consumed by the engine »n keeping the aeroplane in the air. and if the object was to beat the world's endurance record, the machine would be flown at a boot that speed; not"quite, because as the machine was lightened by the consumption of fuel, the roost economical speed would change- 700 600 500 400 300 100 9a 100 no i?o 150 Vlm.p.h.)
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