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Aviation History
1944
1944 - 1758.PDF
10 FLIGHT Endurance An Elementary Outlin^&f^How Allowwectes for QkacLfo e 1 nil Kange pcusT 24TH, 1944 >ilot Can, by Simple Means, Make Strength and Direction 'By A. SIPOWICS, Sftukdron Leader, Polish Air Force ANY pilot, particularly a military ona soone^^F later f-\ comes across the problems of aircrlfLjwJ^% and / or ^ •*• endurance. Range \the distanpHroG^m be flown ; endurance the possible time Of fllgTitwith a given amount of fuel. The endurance of an aircraft depends only on the time rate of the fuel consumption and is, therefore, determined by the minimum power required for level flight. Wind has no effect on endurance except in so far as flying becomes more' difficult if the air is bumpy. Flying on weak mix- Also a tail-wind obviously influences the range. It adds to the range in proportion to the time of flying, and a lower speed giving a greater endurance may prove advan tageous. Thus in both cases is shown the important influ ence of wind on the speed to be chosen for getting maxi mum range. The Vector Diagram Before we begin to analyse this influence in detail, let us examine the terms '' head-wind '' and '' tail-wind '' ture with the lower ratio of the supercharger, with a» dealt with later in the article. A purely head- or much reduced r.p.m. and boost as possible gives the maxi mum endurance. The corresponding speed is called " the minimum-power speed." At this speed consumption is lowest and endurance reaches its maximum. It may be mentioned that low-altitude flying is advan tageous for maximum endurance because less power is required to sustain an aircraft in denser air. The minimum-power speed as a rule is only 25 per cent.-35 per cent, above the stalling speed of an aircraft. It is easily,—• found by trial and error, keeping in level flight with the least boost and r.p.m. The pilot has, however, to choose between two possibilities: either less boost and higher r.p.m. or lower r.p.m. and greater boost. These alternatives as <t rule do not give the same power for the same speed because the efficiency of the airscrew is different in the two cases. Power developed by an engine is propor 1 $ tional to the product of the absolute pressure of the mixture—- the point D,. and r.p.m. of the engine. Zero boost corresponds to 14.7 lb./sq. in. absolute pressure ; any negative or positive boost correspondingly changes this value. Let us denote Absolute pressure by p Boost pressure by b Revolutions per min. by n Power developed by h.p. p=i4«7±b; and h.p.=a.p.n where a is a coefficient of proportionality. wind is rarely encountered. Usually a pilot must cope with more or less a side-wind, and therefore it is neces sary to know how a side-wind may be replaced in our considerations by a head- or tail-wind of the strength equivalent in results to the existing side wind. For readers who have forgotten this elementary navigational problem Fig. 1 shows how it is done. • Let us assume that the proposed flight covers the distance AB and during, the flight a side-wind of strength "w" prevails. We represent the speeds of the aircraft and of the wind by vectors 6r lines, the lengths of which are proportional to their values, and the directions conform to actual directions. Let us transfer the vector w so that its arrow-head coincides with the point of destination B. From the other end of w let us describe an arc, Dj-D2, of a radius equal to the airspeed of the air craft V, so that it cuts the line AB and Let us imagine that the aircraft takes Then off at D, and flies one hour in the direction DjC. With no wind it would reach the point C. With the wind w it would be shifted to ihe point B. The ground speed of the aircraft is then represented by the line DXB. From the figure we see that the wind w increases the grouffifr speed by EB and this represents the equivalent sti'engtj? of a tail-wind. Wind and Range We will now consider only head- or tail-winds and It is the task of the pilot in determining the minimum* ..examine in detail their influence on range. As in most power speed to find out the combination of boost and r.p.m. which gives the least value for h.p. in level flight. This combination varies with altitude, so it is recommended to find out the three best combinations: for low, medium and high-altitude flight. Range Problems The problem of range is much more complicated. As we shall see later, the speed to be chosen varies to a very great extent in different conditions of flight, and it is too great a simplification to stick to one or two recommended economical speeds. A simple example will show how a aeronautical questions, an experimental determination of range is less troublesome, quicker and more accurate than intricate theoretical considerations. Every pilot who wants to determine the range of his particular aircraft in various conditions of flight can do it without difficulty in the following way: The pilot records in level flight at a chosen altitude the values of the corresponding factors: — 1, Airspeed. 2, Rev. per min. 3, Boost pressure. 4- Altitude. The last three factors determine the power out put of the engine in these conditions and thus the rate of its fuel consumption. For determination of range, only single recommended value for economical cruising, put for--—„.the rate of fuel consumption and the airspeed are neces- ward without any stipulations, may prove incorrect. The reader may come across a recommendation to fly on a certain type of aircraft at 160 m.p.h., as being the best cruising speed. Let us assume that the aircraft flies against a wind of the same strength. It is obvious that the range of the aircraft in these conditions is zero, because its ground speed is zero. On the other hand, flying at top speed (3*0 m.p.h.)' the aircraft will have a considerable ground speed (160 m.p.h.) and a range of several hundred miles, corresponding to its endurance at full throttle. sary, but a direct measurement of that rate in flight is rather difficult in an ordinary aircraft not provided with a precision fuel-consumption rate meter. The indirect way of determining the fuel-consumption rate is to refer to the engine power graphs, or performance curves, supplied on demand by the engine manufacturers. These graphs shdjw the power developed in given conditions—r.p.m., boost and altitude, as well as the corresponding fuel-consumption rate. If we know the quantity of fuel (F) taken for the flight and the hourly consumption of it (f) at the relevant
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