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Aviation History
1910
1910 - 0143.PDF
FEBRUARY 26, 1910. 1/ijGHU AERIAL PROPELLERS BY A NAVAL CONSTRUCTOR. (Concluded from page 125.) WE shall now deal with twin screws, each 6 ft. diameter. This means that we must design each screw for 75 lbs. thrust and proceed as before. Diameter of propeller 6 ft. For two-bladed propeller K = -58. Thrust =75 lbs. Pitch Ratio. Slip. Efficiency, r.p.m. •6 ... 140 ... 66*5 ... 1,160 .. •8 ... -237 ... 69-5 ... 985 .. i'o ... "285 ... 70-0 ... 840 .. i-2 ... -330 ... 69-0 ... 745 •• Three-bladed propeller K = -435 :- •6 ... -085 ... 67-5 ... 1,090 .. •8 ... -177 ... 7i'o ... 910 .. i*o ... -230 ... 72-5 ... 780 .. i-2 ... "273 ... 72-5 ... 680 .. Four-bladed propeller K = "375 :— •6 ... '050 ... 67-5 ... 1,050 •8 ... M47 ... 71-5 ... 880 .. I'O ... "200 ... 74-0 ... 750 1-2 ... -247 ... 74-5 ... 665 ., Suppose we have twin screws developing a thrust of 75 lbs. each, the following table as before. For two-bladed propeller K = "325 Brake h.p. Without Gearing. . 24*6 • 23-6 • 23-4 • 23-7 24-3 23-1 22*6 22 '6 24-3 22-9 22" I With Gearing. • 25-9 • 249 • 24-7 . 25'0 25-6 24-4 23*9 239 256 24'2 23-4 23'3 Pitch Ratio. •6 •8 I'O 1*2 Slip. Efficiency, r.p.m. of 8 ft. diameter Then we calculate Brake h.p. •025 •120 •170 •220 67-5 71-5 75-o 75'5 770 640 542 475 Without Gearing. . 24-3 . 22 "9 . 2I"9 21*7 For three-bladed propeller K — 244 :— I'O 1-2 For •6 •8 I'O 1*2 ... '070 ... 70-0 ... •130 ... 76-0 ... ... -175 ... 77-25 ... four-bladed propeller K •050 •100 •150 69-5 76*0 77'5 605 5i8 455 592 500 441 23'4 2I"6 2I-2 23"6 2I"6 21'I With Gearing. .. 25-4 24*0 .. 230 .. 22'8 24"5 22'7 22'3 24-7 22 "7 22'I It will be noticed that wherever possible we should increase the diameter of the propeller. The -8 ft. diameter propellers being, in every case, more efficient than those of 6 ft. diameter. If we were using propellers of 8 ft. diameter we should choose one having a pitch ratio of 12. The four-bladed requires 22 "i b.h.p. at 441 r.p.m. The three-bladed requires 22*3 b.h.p. at 455 r.p.m. The two-bladed requires 22*8 b.h.p. at 475 r.p.m. From a practical point of view we should choose the two-bladed propellers for the reasons already given. It will be noticed in the above that the least efficiency obtained was 60 per cent., and the highest 77-5 per cent. These are very good values. No propeller yet tried has given as high an efficiency as 80 per cent. It is extremely doubtful if some of the makers of aeroplanes can get as high an efficiency as 60 per cent, with their present machines. The case worked out corresponds almost exactly to that of the Wright machine. It will be remembered that the Wrights have an engine of 25 nominal brake horse power driving two two-bladed propellers of 8 ft. diameter at 450 revolutions per minute. The approximate speed is 40 miles per hour. The Wright propellers seem to be very well designed. In several of the other makes, however, 50 per cent, would be the maximum obtainable. In one particular case the approximate pitch ratio is '35. With a well-constructed propeller we could not hope to get much more than 40 per cent, efficiency with this low value of the pitch ratio. This is very bad and should certainly be improved upon. In the curves it will be noticed that the propellers give a definite amount of thrust at zero slip. This is caused by the shape of the section of the blade which is given in the next chapter. If the section of propeller-blade was perfectly fiat on both sides, the thrust at zero slip would be nothing. When we are working out the best type of propeller, we should decide each case on its merits. No definite rule can be given. If we can get an overall efficiency (that is gearing included) of 70 per cent, we should consider this very good. Three or four attempts should be sufficient to determine the propeller best suited to our purpose. CHAPTER VII.—TO Design a Specified Propeller. We shall now proceed to design the propeller which we found most suitable in the example taken in the last chapter. This, it will be remembered, was a twin-screw, two- bladed propeller of 8 ft. diameter, having a pitch ratio of 1-2 and No. (C) blade. This blade, as stated previously, has a " disc area ratio " of 'xx. The area of the disc swept out by the tip of the TT IT propeller is j x (diam.)2 = — x (96 ins.)2 where ir = ^. Hence area of propeller blade is IT •11 x — x (Q6)'2 sq. ins. = 800 sq. inches nearly. Also the pitch of the propeller is equal to the diam. x pitch ratio = 8 x fit =• 96 ft. Now the shape of the blade is rectangular with rounded corners. RULE I.—In all cases of propeller design the inner radius of the propeller blade should be £th of the outer radius. In the present instance the outer radius is 4 ft, or 48 ins. Therefore inner radius is -*/- = 8 ins. Hence the length of the blade measured radially is equal to (48 — 8) ins. = 40 ins. But the total area is 800 sq. ins. Therefore the width of blade = W°- = 20 ins. (NOTE.—The (C) blade is the widest type used.) Accordingly the developed shape of the blade is a rounded rectangle of size 40 ins. by 20 ins. RULE 2.—The thickness of the centre of the blade at the root is -gVjth the diameter of the propeller, and the tip T1Loth the diameter, the thickness diminishing uniformly as we go from the root to the tip. In this case the thickness at root = -^ x 96 ins. = i*6 ins.; thickness at tip = "53 in. 139
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