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Aviation History
1910
1910 - 0128.PDF
To get the revolutions and horse-power, having given the diameter, pitch ratio, slip, and efficiency, we proceed -as follows:— i. Revolutions.—The actual velocity forward when the revolutions, diameter, pitch ratio and slip are given, is given by V = Revs, x P X D x (i - s). Now for the 6-ft. two-bladed propeller of pitch ratio *6 as above. V - 60 ft. per sec, P = 6, D = 6 ft. s m '$0.; 60 Four-bladed propeller K = 75 :— •6 hence revs. = 24*2 per sec. = •6 x 6 x (1 - "30) 1,450 per minute. 2. Horse'Power.—In terms of the above quantities ,. . . , _» _ Thrust x Veloc. this is given by B.H.P. = FS-1 6 » 4500 x Efficiency 150 x 60 In our case this would be — rr = 27^ B.H.P. 550 x -6o ' ° ^ ; z . UJ 0 e i EFFI C % I " I S V CORV/E ,.- ?. ' ^ / i s' s S^ / / SCALE OF SLIP :PER CErHT 3 5 3 0 3 5 i 0 5 ^ / / / / « i z 0 5 uJ a 5 40 No. (c).—Blade pitch ratioJ'O. That is, an engine running at 1,450 revs, per min., and developing 27*3 brake horse-power, will give the 150 lbs. thrust necessary to drive the aeroplane at 60 ft. per second, if the diameter of the screw is 6 ft. and pitch ratio '6, that is pitch = 3-6 ft. The following are the tabulated results :— Diameter of propeller, 6 ft. Thrust developed 150 lbs. For two-bladed propeller K = n6 :— Pitch Ratio. Slip. Efficiency. r.p.m. •6 ... -310 ... 60 ... 1,450 '8 ••• '375 ••• 62 ... 1,200 I'O ... — ... — ... — I"2 ... — ... — ... — Three-bladed propeller K = -87 :— •6 ... -240 ... 63 ... i,3I5 •8 ... -325 ... 65 ... I.IIO I'O .. -366 ... 65 ... 947 ake 27 26 26 25 25 h.p. 3 4 0 2 2 "2QO •290 •335 • •380 • 64-5 ... • 67'S ... 67-0 ... 65-0 ... 1,250 . I,o6o 902 806 . 25-4 24-3 • 24-4 25-2 It will be seen that if we stick to the pitch ratios given there are nine propellers of 6 ft. diameter which will suit our purpose. We should choose that one which is most efficient pro vided the revolutions are equal approximately to the revolutions of the engine. If not we must choose another propeller which is more suitable, or we can resort to gearing. Here we come to a most important point, namely, the question of gearing down the revolutions. The higher the speed of revolution of the shaft of the engine, the lighter the engine will be; this is an im- 20 25 No. (c.)—Blade pitch ratio 1*2. portant consideration. Also for really well-designed chains and spur-gearing, well lubricated, we may expect an efficiency of 95 per cent, for the gearing alone. The advantages of gearing are obvious. We can accommodate any engine speed to the revolution of the propeller-shaft. We can use higher speed engines and so save weight. The best propeller, however, for the above case, if we were confined to 6 ft. diameter, would be the four-bladed pitch ratio "8 running direct off the engine-shaft at 1,060 revs, per min. Now suppose we take the case of a propeller of 8 ft. diameter. 124
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