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Aviation History
1935
1935 - 0246.PDF
SUPPLEMENT TO FLIGHT I26rf JANUARY 31, 1935. THE AIRCRAFT ENGINEER THE SPEED and DRAG of COMMERCIAL AEROPLANES By W. O. MANNING, F.R.Ae.S. Mr. Manning, one of the earliest British Aircraft Designers, calls attention to the Possibilities of obtaining extra Speed without incurring extra Cost THE question ol the effect of the speed of aircraft on running costs has recently become prominent, and there seems to be a certain school of opinion which assumes that speed must always increase cost. This opinion seems to be based on experience with railways and motor cars, but the case of aircraft is different from other means of transport. With aircraft, all the important operating costs, except the cost of fuel, decrease with increased speed. This is due to the fact that these costs depend on the hours that the machine spends in the air and it is obvious that the larger the number of miles the aeroplane can fly in an hour, the less these costs are per mile flown. There is not, as in the case of the railway, additional expense due to the effect of speed on the upkeep of a track, or, as in the case of a motor vehicle, increased wear of expensive tyres due to the same cause. So high speed of flight will be economi cal provided any increased cost of fuel does not overbalance savings in other directions. The amount of energy required to fly from one place to another is simply the resistance or drag of the aeroplane multiplied by the distance, and, assuming what is approxi mately true, that the energy content of the fuel, the thermal efficiency of the engine, and the propulsive efficiency are roughly equal for commercial types of aeroplanes, the fuel required for a flight can be ascertained b r multiplying the drag of the aeroplane by the distance flown and multi plying the result by a constant. It will be seen that speed does not come into the matter directly. It is quite true that if one takes any particular aeroplane and increases its speed by using additional horse power, the drag and consequently the fuel consumption per mile will also be increased. But this increased fuel consumption is not due to speed as such, but to the fact that speed, in this particular instance, increases drag. The "All Wing" Ideal Suppose it were possible to build an aeroplane consisting of nothing but a wing, and that this wing could be altered in area as desired during flight. As the area could be made anything that was required it would be possible to L keep the — of the machine constant and its drag constant at almost any speed at which it was flying. Hence the fuel consumption per mile would also be constant, and, therefore independent of speed, though the horse power required would increase with the speed. Double the horse power would be required to double the speed, but, as the machine would get to its destination in half the time the fuel per mile would be constant. The drag is therefore of vital importance to the com mercial operation of aircraft, and, other things being equal, the best commercial aeroplane is the one possessing the minimum drag at the required flying speed, because it not only will be operating at the minimum fuel cost but because, as we will see later on, a low drag means a larger revenue load. Yet we see photographs of commercial aeroplanes equipped with masts and flags, a most excellent way of increasing drag and reducing payloads; an addi tional drag of only ten pounds will absorb about 3J horse power at 100 miles an hour and will use about if lb. addi tional fuel in the same time, or, say an additional gallon of petrol on a flight from London to Paris and back. These considerations affect all types of aircraft. Suppose that it was required to design an aeroplane having the maximum possible range. Provided other matters were equal, the machine could be compared by the simple ratio — where P is the amount of fuel carried and D is the drag. The ratio is independent of such considerations as size, horse power, or anything else. That this is valid may be seen easily when it is remembered that fuel consumption per mile is approximatelv drag multiplied by a constant, P P so the ratio — can be counted to — where x is the fuel con- D x sumption per mile say, by means of a constant, and the range is obviously the fuel carried divided by the fuel consumption per mile. This matter of fuel consumption per mile, or drag, also affects revenue. Let it be assumed that you are being paid for the transport of goods at the rate of so much per ton mile, then the revenue that you will earn per flight will be :— R « K(W - #L)L where R is gross revenue per flight in, say, pounds sterling K is a factor converting say, ton miles into pounds sterling L is distance flown x is weight of fuel per mile say W is weight of revenue load -f- weight of fuel. R is obviously = D before the machine has started its flight as then L = D and it is also zero when xL,, which is the weight of fuel used on the flight, is equal to W, so the curve becomes zero at each end but has obviously some value in the middle. R = KWL - K.rL2 Differentiating. aR — = KW - 2K*L BL Now R, the revenue per flight, will be at a maximum when — = D, so KW - 2KAL - D T W or L = — 2X
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