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Aviation History
1914
1914 - 0635.PDF
There are (in the present state of the art) two prominent reasons for the adoption of a propeller for aeronautical machines of finer pitch than that of greatest efficiency; firstly there is the question of suiting the pitch of the propeller to the running speed of the engine. For the power necessary in a modern aeroplane (from 50 to loo h.p.) a stroke of about 5 ins. is found to design well in machines for short-distance flying. The rotating engine also suffers from some disadvantages on the score of exhaust silencing. The multi-cylinder Vee-type, though ordinarily not so light, power for power, as the rotating engine, has many advantages, especially, for high power and where long distances have to be negotiated. It is customary in the rotating engine to employ direct air-cooling ; Fig. 25. proportioning the engine ; now it is uneconomical both from the point of view of weight-saving, and of petrol-consumption to employ too low a piston speed ; in fact, for any given dimensions of cylinder the power developed is within limits roughly proportional to the piston-speed. Taking a piston-speed of 1,000 ft. per minute and j-in. stroke, we require 1,200 revolutions per minute —20 revo lutions per second. Assuming a velocity of flight of about 80 ft. per second the effective pitch of the screw requires to be 4 ft., or approximately equal to half the diameter of the screw, instead of at least equal to the diameter, as in a good marine propeller. It is of course not difficult to gear down from the engine to the propeller) in fact this has been frequently done, but, since gearing involves a tax of approximately 5 per cent, of the horse-power, it is evidently better to drive direct and sacrifice something in the efficiency of the propeller, more especially as this course involves a far lower orque on the propeller shaft, and consequently a lower recoil torque on the framework of the machine. 7. Motive Power Installation.—We are now faced with the consideration of the motive-power installation. At the present time, the internal-combustion engine—more definitely the petrol- motor—holds the field. No other prime mover is able to compete either on the score of weight per horse-power or fuel weight economy ; there is nothing in sight likely to oust the internal-combustion motor from its supreme position. The relative importance of lightness and economy of fuel is determined by the class of service for which the motor is required. In Fig. 27 curves are given of weight/horse-power for various motors ; ordinates represent weight of motor plus fuel, abscissas the duration of the run at full load. It can be seen at a glance from this diagram that for brief periods the weight per horse-power of the engine is the all-important factor, whereas for long runs this becomes relatively less important, the weight of petrol and lubricating oil becoming the main item. On referring to Fig. 27 it will be noted, taking the extremes, that the Gnome engine starts with a very considerable advance over the motor-car engine given for comparison, but after a run of seventeen hours of full load, the motor-car engine (repre sented for the purpose of illustration by the Daimler) by its greater economy has taken the lead. This diagram was prepared by me, some three or four years ago (see Report of the Advisory Committee for 1909-10). Many of the aeronautical motors of the present day combine weight/horse-power factor of about 4, a degree of economy that compares well with the best automobile practice. Out of a great multiplicity of types of aeronautical engine now on the market there are two types—namely, the rotating engine on the one hand and the light-weight multi-cylinder Vee-type on the other—which I consider likely to survive. The rotating type of engine gives the possibility of very complete balance with simplicity of working parts, and so provides the aeronautical constructor with an engine especially serviceable where small machines are concerned, and simplicity and upkeep are of import ance. The rotating engine is, at the present day, reasonably economical in petrol, but is grossly extravagant in lubricating oil, and consequently is at a disadvantage for long-distance work ; it will, however, probably hold its own for some time to come in with 1, \ ^ \ / / / \ 1 1 1 I ..L N K ^. / / / 1 J ~f/ ^V / I —K»; »0Q Null t* t • MOU» Fig. 26. it is, in fac?, difficult to arrange such an engine with water-cooling. The horse-power absorbed in the Gnome engine incidental to air- cooling is very great ; in the original so-called 50 h.p. Gnome (which actually gives very little over 40 h.p. in flight), the power consumed in wind-resistance, even on the test stand, amounts to something nearly 6 h.p., and it may be materially greater under flying conditions. In engines of the Vee-type water-cooling is in greater favour ; the Renault special aeronautical motor is an exception, being cooled by airblast generated by a centrifugal fan. The weight of the water- cooling system when fitted amounts at the best to o'6 lb. per horse power (with water nearly 1 lb. per horse-power), and thus constitutes a serious addition to the weight of the installation. Here again the class of service becomes important. It is evident that for short- distance flying, where engine weight is of paramount importance, it may be better to employ direct air-cooling ; when, however, a long-distance service is required, it may arise that the weight of the water-cooling system is justified. According to a recent investigation by me (see Report of the Advisory Committee, 1912-1913, p. 94), the minimum horse-power * z 0 • n ( O 0 t$ m • 0 1 •. ^"*' f^-^ i0\ 41* St ^ r* _J d ^* p-*"- 1 r- • P ~ — .-. Fig. 27. expended in cooling is a function of the area and temperature difference of the surface exposed, and there is some difficulty in providing an air-cooled engine cylinder with sufficient gill surface to keep the horse-power loss as low as desirable ; when, on the other hand, water is used as a heat carrier, the rigid limitation as to available surface no longer applies, there is some disadvantage, however, as to temperature difference. In Fig. 28 a diagram if given showing the essential relations between horse-power equiva- lent of heat dissipated per square foot surface (abscissae), tangential velocity of air (ordinates), temperature difference, and power absorbed in skin friction. It will be understood the graphs represent the minimum horse-power absorbed based on the assump tion that the air is traversing the surface along a stream-line path, and that there is no additional loss of power in eddy making. (To be continued.) 635
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