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Aviation History
1926
1926 - 0530.PDF
SUPPLEMENT TO FLIGHT 68 THE AIRCRAFT ENGINEER JULY 29, 1926 The accompanying curves present the results obtained in a number of tests carried out on airscrews and aircraft of various types. The method of test was similar to that emploj^ed in R & M 704, i.e., the time taken to climb 1,000 ft. at various airspeeds with engine full out is measured with aneroid and stop watch, engine revolutions and air temperature being measured at the same time. The torque is deduced from a knowledge of the variation of horse-power with height, the power of the engine as given on the test bed and the revolutions observed in flight. The thrust is given by the formula T = kD !kL W cos 0 + (A + B R/Ro) V,-2 Where A is constant and B can be taken as constant over the small range of airscrew diameters used, -— is constant for constant W and V,-, 9 is obtained from the test, being rate of climb j 3 ,. and R/Ro is known when the geometrical forward speed properties of the airscrew are known. We can therefore the noise became very great and the aircraft could only be flown for a few minutes with the airscrews running at 1,020 ft./sec. owing to the noise causing severe head and ear pains. Endeavours were made to improve matters by making the leading edges and camber of the sections on the outer third of the blade as fine as practicable, but this modification made no measurable difference in the performance and only a small decrease in noise. A third airscrew was then tried, having a maximum tip speed of 910 ft./sec. The special feature of this airscrew was that the blade width decreased very rapidly on the outer third of the radius. The results given by this airscrew confirmed the conclusion drawn from the preceding tests. The second series of tests was carried out on a single- engined machine using a timber airscrew, of which the maximum tip speed was 960 ft./sec. On analysis it was found that the efficiency was only 93 per cent, of the calculated efficiency at the highest tip speed. The airscrew was then reduced in diameter, and the chord of section at tip reduced to increase the revolutions by 290 r.p.m. over those in the previous test. This increased the tip speed to 1.075 ft./sec. EFFICIENC Y WIT H TI P SPEE D LOS S 1-0 in 3.30 o ? > -30 UJ V k. j i i I | j o * c i SIK'C MET/ rr« LE V- — i - i •—i r , I =T) TWIN T j W.Z . EK3!ME I | -4-. 1 E* r-- f JG U w • ! Si • rrv "-^ i ^ ! ^» ; K V FUL LSCA MIND ~ F LE TE5T6 o ANN M 88 hL M- j IcST 400 300 SOO 700 800 900 1000 IIOO 1200 TIP SPEED Fr/SEC. AIRSCREW TIP SPEEDS: Curves showing decrease of efficiency with increase of tip speed compare the efficiencies of various airscrews when tested on the same machine. Apart from inaccuracies of observation, the only other sources of error are instrument errors, instrument position errors and vertical air currents. The instrument and position errors were known for all the machines tested, and the tests were carried out early in the morning when the vertical air currents are small. The first series of tests was carried out upon a twin-engined machine, using first a small diameter timber airscrew with tip speeds ranging from 750-800 ft /sec. On the assumption that the tip speed loss would be negligible, this airscrew was used to determine the constants A and B, and so formed the basis of comparison for the other airscrews. Another test was then carried out using an airscrew of larger diameter and running at tip speeds of 8f>0 to 1020 ft./sec. It was immediately evident from the poorer performance obtained •with these airscrews that the loss in efficiency due to the higher tip speed outweighed the gain due to the lower slipstream velocity. On analysis it was found that the efficiency fell very rapidly above tip speeds of 850 ft./sec. and that at 1,020 ft./sec. the attained efficiency was only 90 per cent, of the calculated efficiency. Above tip speeds of 900 ft./sec. and the extra revolutions per minute did not improve the performance, thus showing that the extra power was being absorbed, partly by the increased drag, due to increased slipstream and partly due to tip speed loss ; on analysis it was found that the attained efficiency had fallen to only 87 per cent, of the calculated efficiency. The noise emitted by this airscrew was very great and, although it caused no discomfort to the pilot, the airscrew could be heard at great distances. It should be mentioned that these tests were not so accurate as those carried out on the twin-engined machine, but nevertheless provide additional evidence that the loss due to high tip speed cannot be ignored. The third investigation was carried out on a single-engined machine, using a Duralumin airscrew with thin solid blades ; from the few tests so far carried out it appears to be possible to use tip speeds of 950 ft./sec. (the highest attainable with this airscrew) without losing any efficiency or creating any undue noise. It is interesting to note that in the Schneider Cup Races the Americans have consistently reduced the tip speeds of their airscrews and that the winning machine last year had the lowest airscrew tip speed of any. 464i
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