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Aviation History
1931
1931 - 0468.PDF
FLIGHT, MAY 15. 1931 •s •8 .7 j a i 5• ' a. * j 4 •a 1 •t V Mo ^\ — \ — \ ^^ l» .1 C V Eta \ W '—^ 1 • %r\ ECO* M 110 t Fig* 11. Consumption Curves. - • • -.** • - of considerable importance, there is an ample reserve ofrudder power available for control, over and above that required to balance any change of directional trim due toengine failure. In this connection, the conditions which must be satisfied for an aeroplane to fly in a straighthorizontal path do not seem to be generally recognised. In the event of wing engine failure, these conditions are asfollows: — (1) The thrust must be equal to the drag.(2) There must be no resultant lateral force. (8) There must be no moment about the centre ofgravity of the aircraft. (1) is satisfied if flight is possible and height main-tained. To satisfy (2), a force (—F) equal and opposite to the side force on the yawed rudders must be applied eitherby the aeroplane flying one wing down and thus using a component of its weight, or by flying slightly yawed, orby a combination of both. (3) is satisfied if the rudder power is such that the yawing moment is zero. From thepilot's point of view (3) must be satisfied, and should be clearly realised and allowed for by the appropriate controlsettings. The Arrangement of Fuel Tanks As the weight of fuel required for a range of 1,000 seamiles up to the " limit " range varies from about 25 to 40 per cent, of the gross weight, it will be realised that,unless special precautions are taken with regard to change of trim as the fuel is consumed, there may be insufficienttail plane adjustment or elevator control lift available for manoeuvrability. In short duration aircraft this is ofminor importance, and the C.G. of the fuel generally is close to the C.G. of the aircraft. It will be shown, in whatfollows, that ideal positions of the centre of gravity of the fuel may be obtained, which will satisfy the conditionthat, at a constant speed, say, the cruising speed, there shall be no change of fore and aft trim as the fuel isconsumed. Let us assume that the aircraft with full tanks is intrim, fore and aft, at a constant speed V. Then, if an amount of fuel is consumed, the angle of incidence will bereduced to maintain the same level speed. This gives rise to a pitching moment about any point, and the aircraft,therefore, will be out of trim in this new attitude. If, however, the centre of gravity of the remaining fuel isarranged, either by tank position and shape, or' by some form of selective overflow, to be in such a position that itprovides a moment to balance the change in aerodynamic pitching moment, the aircraft will remain in trim, at the same speed, irrespective of fuel consumption. In otherwords, if M is the pitching moment, and M' the fuel moment, then the criterion for no change of trim isdM/da = dM'jda when a is the angle of incidence. While in practice it is not always possible wholly tosatisfy this condition, it is apparent that every effort should be made to reduce the change of trim, as the fuel is con-sumed, by some such self-trimming arrangement, in such a manner that the change in control settings, to obtainbalance, should be a minimum. Fig. 10 shows the locus of the ideal position of the fuelcentre of gravity for the " Iris III " wing tanks, and the dotted line the actual fuel C.G. travel. Even as shown,there was a marked improvement over the " Iris II " arrangement, in which particular care had not been takenin the design in this respect. F uel Economy It will be apparent, as the fuel weight required for longrange is, as we have seen, a considerable proportion of the total weight, that fuel conservation is of the utmost im-portance. If it is not used economically, then, apart from increasing the all-up weight for a given range, it nullifiesto an appreciable extent the effort made to reduce struc- ture weight, the difficulties of which increase with the sizeof the aircraft. Fig. 11 shows typical throttled consumption curves, inwhich, for different weights, the sea miles per gallon are plotted against speed. The speed, corresponding to thepeak of each curve, gives the most economical cruising speed—that is, the speed at which the sea miles per gallonare a maximum, and is found to be not far different from 1.5 times the stalling speed. As these curves are comparatively flat, it will be observedthat the most economical speed is not uniquely defined, and may be exceeded with little sacrifice in fuel economy.It may be remarked that a reduction in specific fuel con- sumption has little effect on the cruising speed, but in-creases the range for the same fuel capacity. Also, these curves show that the cruising speed increaseswith weight or, the same thing, stalling speed. This is shown more clearly in Fig. 12, in which the sea miles pergallon are plotted against weight, the full-line curve for a constant speed of 95 knots, and the dotted curve at themost economical speed. From either of these curves the range may be calculated, and it is found there is littledifference in both cases, the former having the advantage in that, for the same range, the duration of flight is re-duced. As the curves are practically straight lines, it should be noted that the sea miles per gallon are, for allpractical purposes, proportional to the weight. With the advent of a reliable flowmeter, devised by thestaff of the R.A.E., which gives a direct indication of the rate of fuel consumption, it is now possible to control themixture strength, and hence to run the engine on the leanest mixture without loss of power.Extensive tests in service have shown that, with pilots flying in formation without flowmeters or specific instruc-tions as to the use of mixture control, the variation in consumption may be as high as 45 per cent., and by theuse of mixture control this figure has been reduced to 25 per cent. By fitting flowmeters as an aid to the deter-mination of the best mixture, this figure has been further reduced to within 6 per cent, and the relative consumptionreduced from .745 to .566 lb./b.h.p. /hr. •9C 80 j-70 §60a *•*)in UJ ff-40 20 1—H 1 1 40 000 FIG. C( CONSUMPTION CURVE. 7^7- 50.000 60.000 70,000 80.000 GROSS WT OF % LBS 12 90 436
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