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Aviation History
1915
1915 - 0380.PDF
[ftJGJJT In Fig. 22 a curve of efficiency is given ; this being a curve of maxima it is higher than anything which can be hoped for in reality; the values given by the graph represent the extreme maximum transferred from Fig. 20. Fig. 22 was never looked upon as more than an initial trial, and it is clearly deficient in the matter of information as to blade number—the method is too crude to be allowed to pass. Beyond this, the basis of portions of the work has been revised, and a more accurate expression has been adopted for the diametral relation ship ; also the blade number has been dealt with on a purely dynamic basis. 24. As a mode of representing the screw propeller and the pitch- diameter relation in graphic form, Fig. 22 leaves little to be desired ; it is true that the duplication of the curve on opposite sides of the axis is unnecessary, but the diagram so drawn does more closely represent the actual propeller, and is more easily read than a graph of less " pictorial character. In Fig. 23, which represents the result of the complete investigation, the graphs on opposite sides of the axis are made to give different data ; that on the left hand is the curve based on Newtonian theory without reference to the number of blades, whereas that on the right hand represents the full solution. We shall now proceed to consider the plotting of Fig. 23 in detail, and in the first place we will go through the actual calculations point by point. Pitchjdiameter ratio. This is determined as a function of K as due to the variation in the angle of maximum efficiency, illustrated by the various curves and the graph drawn through the maxima in Fig. 20. If for any given value of A' we wish to adopt the most advantageous proportion of pitch to diameter, we must so design that the blade includes the best portion of the efficiency curve for that particular A" value and that portions of the curve showing low efficiency are discarded ; it has already been stated that in this we shall take it as a convention that the propeller disc diameter is twice that of maximum efficiency. On this basis and referring to <he figure we have, A'. 0-07 0-15 0'2 03 o-S 10 f (max, efficiency). 43" 33° 27—30' 22°—20' 160—10' 11"—10' tan 6. 932 •649 •520 •410 •290 •197 tan 6 2 •466 •325 •260 •205 •'45 •099 p x„ = _ I "46 I-02 •82 •65 •45 •31 The pitch/diameter ratio as above (last column)is, in Fig. 23, the tangent of the angle of lines passing from the origin through the corners of the rectangle representing the " cylinder " by which the propeller is defined (see scale to right and left of figure). We have next to determine the relative diameters as related to A" in terms of the optimum propeller diameter = unity. This will be calculated on the Newtonian basis, the mass of fluid passing through the propeller disc being computed on the assumption that its velocity is the mean between the initial and final, i.e., v« « = *>i + 2" where u is the velocity in question. Now,* but Thrust = m. v..-j(>ap v« [Vt + ^) (7) or, Thrust = >„>( A' + r) <8> It is convenient to express the thrust in the form usual for pressure on surfaces and other cases following the F-sq. law, thus, Thni.tt — Cap7'\° crSv«*3 For the case of the propeller of optimum A'= o-o7, we have, C »(•.,• T) -.*. I ho factor j J is due to the deduction of the central ' tiisr diameter. diameter, blind area (9) (10) where (11) i of the MAY 28, 1915. I„ (TWO Oi-Aoe) 1 AhiOfTjtfUM ftATio) I 1 u, 1 c-s I '? rov* fftAoe 04 •% r 1 •J" Basic data:—u — t/, + v-2/2. Aspect ratio = 6. £ (augmented value) = 0'017. Thrust constant on area given by dia. D; (in abs. units) C„ = 0"068. N.B.—Scale calculated from C„ as above applies throughout both as to pitch and diameter. Thrust-Cpu," _ _ pounds. 4 x 32" 2 Fig. 23. this value of C may be taken as the key value to Fig. 23, the disc area for any given thrust being calculated from equation (9), and the diameter so obtained gives the scale by which the diagram is to be read. Taking the above as unity the diameters appropriate to other values of K are obtained as follows :— A'. 0*07 0-15 0-2 0"3 o-5 1*0 A'+ 5- 2 •0725 161 •220 •345 •625 1-500 C. •068 •151 •206 •323 •58S 1-406 •068 c roo '45 33 •21 •116 •048 Re = . Diameter / • 068 1*00 •67 •575 •466 •34 "22 The values obtained in the last column of the above table, in conjunction with the PjD ratios for corresponding A" values of the table preceding, determine the plotting of the graph on the left hand of Fig. 23 ; the next problem is that of blade numbers. (To be continued.) ® ® ® ® Double Fatality In France. ACCORDING to a telegram to the Petit Parisien from Haze- brouck, a British aeroplane took fire and fell to the ground at Old Berquin, near Hasebrouck. It is stated that both the pilot and the passenger, whose names were not given, died as the result of their injuries. Fatal Accident to Albert Moreau. WHILE testing a new machine at Melun on the evening ol the 20th inst., Albert Moreau, the inventor of the " Aerostable " mono plane, fell from a height of 1,500 ft. Moreau, who was recently awarded the Legion of Honour for his war services, died while being taken to hospital. 380
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