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Aviation History
1944
1944 - 0974.PDF
5o6 FLIGHT MAY IITH, 1944 RESEARCH achievement, and we tend to believe that there is. little more to be expected. Then there comes something in the nature of a transformation. It is often ascribed to a single cause and, generally, one can say that there is an outstanding stimulus. But if we compare the final product—in this case, the aircraft itself—before and after the event, allowing a long enough time for the situation to reach a fairly stable state, we can make a fair assessment of the relative weight of all the influences whkh have contributed to the change. I believe such an examination of the advance of aircraft between, say. 1917 and 1942, is useful in providing U3 not only with a means of examining how far we have been successful in using the results of research but also a guide to the part played by sheer engineering skill and initiative. Finally, it may serve as a base from which we may survey some of the potential advances that are now opening out to us and judge what resources we shall need in order to achieve them. I shall take two typical aircraft that were in general and successful use in rgi7 and compare them with two modern aircraft of similar duties. Naturally there are striking differ ences, and we shall find no difficulty in tracing them to their sources. But perhaps equally striking are the characteristics that have apparently undergone little change. I think, how ever, that we shall see that the effort to preserve them un changed has made as nigh a demand on research and engineer ing skill as that required to produce the more obvious improvements. Basis tor Comparison During the last war the Royal Aircraft Factory (which became the Royal Aircraft Establishment in April, 1918) pro duced many designs lor aircraft which were constructed in large numbers. One of the most successful was the S.E.5, a single-seat fighter with a 180 h.p. Hispano Suiza engine. It had a creditable history as a fighter I propose to compare it with a Spitfire. Then I shall take the Handley Page O/400 twin-engined heavy bomber and compare it with a Lancaster. I shall not be giving away any information to our enemies. They are well acquainted, in more ways than one, with both Spitfires and Lancasters. Some of them may even remember the S.E.5 and the O/400. For my purpose it is quite sufficient to take examples of marks of the modern type whose perform ance has long been surpassed. Let us first look at them in general outline. Fig. 1 shows the 1917 fighter In Fig. 2 its specifically military features have disappeared and around it is the outline of the Schneider Trophy streamlined monoplane, the essential product of the period between the two world wars. Fig. 3 shows the 1942 fighter. In Figs. 4 5 and 6 is shown the transition from the 1917 bombei, through the streamlined airliner, to the 1942 bomber. The most obvious differences are the change from biplane to monoplane and the general cleaning-up due to enclosing the crew abolishing external wing bracing, and retracting the undercarriage Comparing them type by type, the overall dimensions are not very different. The Spitfire has the same wing surface a* the S.E.5, about half the drag, nearly, twice the strength, three times the speed, four times the total weight, four times the military load, and seven times the power. The Lancaster has about half the drag of the Handley Page O/400 on the same span of wings and about three-quarters the wing surface. Its total weight is nearly five times as great, the wing loading, over six times; the power, seven times; and the military load, with a 25 per cent. greater range, over eight times Let us enquire how some of these improvements have been made. Drag Reduction The change in drag coefficient CD0 is off first interest. I have not found it possible to get accurate figures for the older aircraft, but they are approximately 0.039 for the fighter and 0.046 for the bomber. The corresponding modem figures are 0.022 for the Spitfire and 0.030 for the Lancaster. Thus, per square foot of wing surface, the total drag has been reduced to about 55 and 65 per cent, of the 1917 standard. Comparing the two fighters in more detail, we find first that the wing surface is the same for both. Disregarding induced drag (or assuming it to be the same fraction of the whole in each), the top speed at the same height will be proportional to the cube root of the thrust power divided by the drag coefficient. Since the airscrew efficiency is near enough the same for both, we may use brake power. Taking ground level powers in both cases—180 h.p. for the Hispano and 1,250 for the Merlin—the ratio is about 7. Thus the contributions to increase of speed are : *— by reduction of drag/-1-^- ) = 1.21 by increase of power (7) = 1.92. The product of these figures is 2.33.' If we assume that by supercharging it is possible to keep the Merlin power constant up to, say, 25,000 ft., where the density is approximately halved, we shall get a further rise: ^^•toy supercharging (2)3 = 1.26. The total ratio of increase is therefore nearly 3. At this point I feel that the engine people are feeling very- pleased—and we have good reason to acknowledge the success of their effort. But these figures as they stand do less than justice to the aerodynamic contribution. All the cooling required by the seven-times increased power has been provided and yet the aircraft has no more than half the drag per square foot of wetted surface. How have these improvements been made? Let us look first at the drag account (Table 1). TABLE 1 Wings Wing bracing Body and cooling . . Tail surfaces Undercarriage Total CD« S.E.5 Drag at lOOit./sec. 28 15 44 7 16 no lb. 0.039 Spitfire Drag at 100ft. sec. 20 * 38.6 4.4 631b. 0.022 To the saving of 47 lb., the most obvious contributions are from the elimination of wing bracing and undercarriage—31 lb. in all. But the body and cooling drag is actually reduced by over 10 per cent, in spite of the seven-fold increase of power. For the bomber, the reduction in Cp0 is rather less than for the fighter on account of the drag of defensive armament, but otherwise the influences operating have been much the same. Towards the end of my paper I shall say something about what further improvements in drag are in sight and what problems we have to solve in order to achieve them. Yh*d Weight Analysis Let us look next at the weight picture. The Spitfire weighs four times as much as the S.E.5; the Lancaster, nearly five times as much as the O/400. What has made it possible to carry so much additional weight per square foot of wing sur face—for the fightei four times, for the bomber six times as much? In the aircraft itself, first, the development of flaps giving higher maximum lift coefficient and higher drag; secondly, power plants of much greater power per unit weight ; and, thirdly, constant-speed airscrews to make the power fully available over a wide speed range. But larger and better air fields, permitting higher take-off and landing speeds and better flying technique have contributed even more The effective maximum lift coefficient has risen by about 65 per cent. Even so, the touchdown and take-off speeds, with the higher wing loadings, are 50 to 80 per cent, higher A comparison of the weight analyses and load factors of the fighters is given in Table 2. As a matter of interest, I have given also the weight analysis for the FW.190, Structure Power Plant Fuel Load Primary Load Factor TABLE 2 S.E.5 per cent. 29.7 37-i 15.4 17.S roo.o r> Spitfire per cent. . 28.9 38.0 '16.6 16.5 roo.o 10 FW.190 per cent. 30.9 35-7 14-3 19.1 roo.o How has this remarkable similarity of weight distribution/ been maintained? From the structural point of view, it is
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