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Aviation History
1910
1910 - 0127.PDF
FEBRUARY 19, 1910. I/QGHS AERIAL PROPELLERS. BY A NAVAL CONSTRUCTOR. Continued from page 107.) CHAPTER VI. —Method of Obtaining the Best Propeller for given Conditions. IN order to design a propeller we require:— 1. Diameter of the propeller, D. 2. Pitch ratio of the propeller, P. 3. Disc area ratio. As before stated, the thrust of a propeller of diameter D advancing with a velocity V is given by T, where T = V2 D\ K being a constant obtained by a 1000 & model experiment. The value of the constant K has been worked out for four types of propeller of pitch ratio '6, -8, ro and 1*2. We should work between these limits. The method of using these curves is as follows : — Suppose we know that our aeroplane travels at a speed of 60 ft. per second (about 41 miles per hour) and s n Wi 3 1 0 3 r4 I 8 R 8 t S E 0 1 u C ul s i "" * u«vi — SCALE. 2£Siv. ^ • • OF SLIP / HE CEff! 1 / / / . / / / / 7^~ - / / / r ~^- * Z < 0 0 1 •J 5 No. (a).—Blade pitch ratio '6. requires a thrust of 150 lbs. to drive it. This example has been worked out fully below. All this work would not be necessary in an actual design, but the effect of a change in the different variables is very clearly shown in the tables. The results deserve the most careful study, and every point should be clearly understood. x. The Diameter.—This is purely arbitrary. Some times, however, it is limited by the shape of the aeroplane in the vicinity. With our present-day practice the diameter should not exceed 10 ft. We shall work out fully the results for a single and twin screw, and for each of these cases we have taken propellers of 6 ft. and 8 ft. diameter, each being of 2, 3 and 4 blades. The following is a specimen of the method of calculating the results in the following tables. Suppose we have a single screw, with two blades, and 6 ft. diameter. In the case taken Thrust = 150 lbs., Diameter = 6 ft., Velocity forward = 60 ft. per sec. Now from formula T V3 D3, 1,000 X 150 lbs. hence K = —,. w:-.~ = no. (60)2 x o3 That is, all two-bladed propellers of 6 ft. diameter travelling forward at 60 ft. per sec. should have a value of K = i*i6. •i p 41 3 6 * % O t- z. cc C > r O ij —«c u ^'' "^ CU*v„ ! 2L2£p« 1 I y SCALE C 1 i>* * \ F SUP PER. CENT! / / / / / / ^ / / / / / / '• : t 3 0 0 UJ G in 15 20 25 No. (b).—Blade pitch ratio '8. We now turn to the curves. From these we see, in the *6 pitch ratio curve, for a value of K = i-16 we have corresponding a slip of 30 per cent, and an efficiency of 60 per cent. We repeat this for the "8, roand 1*3 pitch ratio curves, and tabulate as shown. For a three-bladed propeller the value of K would be i"i6 i*i6 —, and for a four-bladed propeller K should be —— • i'33 F K 1 55 We have made tables for the three-bladed and four- bladed propellers below. 123
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