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Aviation History
1917
1917 - 0142.PDF
\QHf] FEBRUARY 8, 1917. SOME PROBLEMS IN AEROPLANE CONSTRUCTION By CAPT. V. E. CLARK, CAPT. T. F. DODD, and O. E. STRAHLMANN. Concluded from page 121.) APPENDIX.IN calculating the values used in plotting the performance curves, Figs. 4 and 5, the weight of machine was assumedas 1,150 lbs., and the engine was assumed to develop 140 b.h.p. The lifting power of a wing is given by L = KyAV-,where L is the lift, K, the lift coefficient (which varies for different altitudes of the wing to the relative wind and mustbe determined by experiment), A is the area of the wings and V the air speed. Similarly the resistance of a win? isa wing is . A A Fig. 6.—Rib used in present type of wing construction. expressed by T) — K.VAV-, Kv being "a variable coefficientthat mnst be found by experiment. The speed at which the aeroplane must fly for any assumed angle of attack canbe fonnd from the lift formula. The lift in all cases, of course, is assumed to be the weight of the aeroplane. The resistanceof the wings at these speeds can then be determined and the total resistance found by adding the parasite resistance,tliat is, the resistance of the body, landing gear, &c. From the total resistance the horse-power required can be calculated andplotted against speed. The horse-power available is obtained by multiplying the efficiency of the propeller by the brakehorse-power delivered by the engine. The ribs as ordinarily used in the present type of wingconstruction are as shown in Fig. 6. The weight of such a TO SO 90 SPET-0 MILES PER HOUR "Fi%. 7. -Performance curves for reconnaissance type of aeroplane. rib for a small pursuit machine, as assumed in the abovecalculations, would be less than J 1b. The ribs would be spaced from 12 to 15 ins. along the spars. A wing completewith cover, internal bracing, &c, weighs from 0*6 to 0-7 lb. p\*r square foot. Propellers with variable-pitch angle.—Improved performanceo; an aeroplane, especially as regards radius of action, can )> brought about by means of a propeller whose pitch anglecan br varied by the pilotJwhile in flight. The liability of failure, the complexity of the mechanism and the weightadded, must be weighed against the gain obtained in the performance. The gain in efficiency of the variable-pitch-propellcr over the fixed-blade type is considerable. This increased efficiency makes available more horse-power forclimbing, giving faster climbing, and permits throttling down to attain the economical speed, and hence increases the flightradius and the time in the air with a given quantity of fuel. These facts are more clearly brought out by the approximatecurves given in Fig. 7, which give the horse-power required and horse-power available at various speeds for a fast recon-naissance type of aeroplane of refined design. The full lines give the power available for a fixed-blade propeller ;the dotted lines for a variable-angle blade. It is assumed 140 no KU % g u! SOa o W 4 60 a: a 40 \ n « \ V- <* 5 kj s / • - /. // // /t -Z.—A.r / // // 70 «0 90 IOC SPEED MILES PER HOUR Fig. 8.—Showing economical speeds of aeroplanes with fixed-blade (full lines) and variable-blade (dotted lines)propellers. that the propeller was designed for maximum efficiency atthe high speed of the aeroplane. The most evident gain made by using the variable pitch as observed from the curvesis the increased reserve >horse-power available for climbing. This particular assumed aeroplane, with full load, climbs :—With fixed blade, 650 ft. the first minute ; with variable- pitch blade, 715 ft. the first minute. The increase in theradius of action is very great, the greatest radius of action being obtained when flying at the econo-r;v:al speed of theaeroplane. Fig. 8 shows the econ^aiyc* speed in each case. On one filling of the gasoline tanks the fixed blade wouldcarry the machine about 690 miles in 10\ hours. The variable-pitch blade would carry the same machine a distanceof about 1,050 miles in 15J hours. (Were this machine driven at full power it could go but 600 miles with eitherpropeller.) These curves, while only approximate, will at least give some indication as to the value of a variable-anglepropeller, especially where great distances are to be covered. The greater efficiency of the variable pitch would be of valuein giving increased clhnbing ability at high altitudes and the possibility of reaching greater heights with a givenmachine. Another feature possible, of secondary importance, in a variable-pitch blade is that it can be rotated to give alarge negative angle of attack, or possibly reversed, when the aeroplane is on the ground making a landing, thus serving,as a brake and cutting down the distance the machine rolls on the ground. APPENDIX. The weight of assumed aeroplane fully loaded is 2,400 lbs.The brake horse-power of engine is as given in Fig. 11. The fuel capacity is 6 hours at full power. If A denotes the anglethat the helix line makes with, the base line, Fig. 9, V the translational velocity in feet per second and N the propellerspeed in revolutions per second, then the distance advanced each revolution, neglecting slip, is (V-rN) feet, which is the I42
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