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Aviation History
1943
1943 - 1446.PDF
FLIGHT JUNE 3RD, 1943 POST-WAR TRANSPORT AIRCRAFT / / r / / 7/ 7— / / 1 / — ns.3 1900 1905 Designers sometimes reproach us with an over-conservative and an unduly restrictive attitude in that respect. The chief engineers of several American companies have been good enough to respond to an inquiry con- cerning the wing loading that they would think it desirable to use in transport aircraft at a the. present time if no regu- I latory restrictions had to be S considered. Although there s are wide variations in indi- * vidual opinion, the choice seems typically to fall around 30 1b. per sq. it. for an air- craft of 25,000 lb. gross weight; around 35 lb. per sq. ft. for a 60,000 lb. machine; and around 45 lb. per sq. ft. for one of 150,000 lb. These average .figures, with the exception of the last, are well within the range of practic- ability under existing Ameri- can airworthiness require- ments, but some of the designers proposed values of which that would not have been true. It is generally agreed that wing loading should increase with gross weight, but some views favour an increase as rapid as the cube root of the weight, while others prefer only about half that rate of variation, or even less. The majority favour a higher initial wing loading in an aircraft designed especially for long-range operation' than in one of similar size intended for short flights ; but the differences generally recommended are so small that the mean wing loading during the flight, allowing for half of the fuel load having been consumed, would be lower in the long-range machines. Minimum Wing Drag With a relation established between wing thickness, aspect ratio, and thickness ratio, the aspect ratio which will produce minimum wing drag at any particular value of the lift coefficient can be determined for any given Series of airfoil sections. Plotting minimum drag at optimum aspect ratio and thickness ratio against lift coefficient, in turn, the variation of wing drag with wing loading is derived. The curves presented in Fig. 4 happen to be based on an early NACA series. The aspect ratio for mini- mum drag has a value that increases only a little less rapidly than in direct proportion to the lift coefficient. The optimum span is therefore almost independent of the oper- ating value of the lift coefficient, or, for a given cruising speed, of the wing loading. In the particular case plotted, the minimum drag corre- sponded to a lift coefficient of 0.54, an aspect ratio of 11.3, and a root thickness ratio of 0.23. Improvement in the drag characteristics of airfoil sec- tions operates to reduce the optimum value of the lift coefficient as determined by an analysis of this type, since reduction of drag makes it profitable to accept the profile drag that comes from increase of area rather than the induced drag that accompanies increase of lift coefficient. It is impossible to speak freely of the characteristics of modern low-drag sections; but, on the assumption that all profile drag coefficients were reduced 50 per cent., as com- pared with those used in plotting Fig. 4, the optimum lift coefficient for minimum cruising drag would become 0.40 and the optimum aspect ratio 12.0. It may be noted as a rough-and-ready rule that the wing loading for minimum wing drag at cruising speed with 14 3 io *-& o e 4 ' 1 , _ 1 — -A // , . ,. £?- i/ / ( / 1 ! 7 - 1 .1 FIG.4 1920 1925 1930 1936 1940 present-day airfoils should be approximately 9(V/ioo)2, where V is the cruising speed in m.p.h. at a height of io,oooft., and that the span loading should be approxi- mately o.8(V/ioo)2. Drag and Wing Loading The curve of drag against aspect ratio is very flat in the neighbourhood of its minimum value, and the curve of drag against lift coefficient almost equally flat. I believe the beneficial effect of very high wing loading on drag is often overestimated. There are, on the other hand, obvious structural advantages in reducing the aspect ratio and increasing the wing loading, both in direct saving of structural weight and through reduction of the required gust load factors. The reduction of wing loading by one- third, as compared with the optimum value, should in- crease the wing drag by barely five per cent., but it will typically increase the weight of the wing by nearly 20 per cent., with some increase in the weight of other struc- tural elements as well, and it is there that the price has to be paid. Where take off quality aqd rate of climb are of paramount importance, improvements in those charac- teristics may offset the incre'ase in structural weight that comes with increase of aspect ratio. No generally appli- cable balance can be struck between the aerodynamic and the structural factors; but it seems unlikely that there will be any appreciable gain from increasing the wing load- ing to above IO(V/IOO)2, or span loading to above I.O(V/IOO)2, where V is again the cruising speed at 10,000ft. For twin-engine aircraft, for which climb with one engine inoperative is a critical condition, the span loading is better kept down to 0.9CV/100)2. With airfoil sections of the future, these optimum values may be reduced by some 10 to 20 per cent. ; and they will, of course, be reduced by any increase in cruising altitude. Wing loading and power loading remain as variables. In the next series of charts I have examined the variations of speed and costs in terms of those factors. The basic air- craft for this series of studies is a four-engine machine of 60,000 lb. gross weight, taken as approximating the size that is likely to be most used in overland operations between major centres after the war. In addition to the reduction of parasite drag that would naturally accompany the increase in size from 25,000 lb.' of the DC-3 to more than double that weight, the drag coefficient has been
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