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Aviation History
1912
1912 - 0548.PDF
IftfGHT JUNE 15, 1912. STRUTS.—Example showing how weight and the moment oj inertia oj each strut was calculated. Mean sum of squares in rows, A and A1 = 17-4 u u B and B1 = 23,6 C and C' = 28 it 1, Dand D> = 29-6 >» >! E and E' = 20, 9 128-5x2 Total 128-5 Or the whole area of figure s 5 =2-57 sq. inches. The weight of icoit.-runof strut is its sectional area in sq. inches, ico x 12 x 30 a.... -rt"" c Z2„"Z I o_ ::: : !S--r :: :::":::: ~^n: E„ - \lo ^L-\ _ IE E!I _ Z _ n cz "*.* . :: __. B^_ -=-, ___ • * •: 1 _n: V - __ __X _ _: *\x. :::::2f± A, x J 728 -lbs. 30 lbs. being the assumed weight of 1 cubic foot of spruce), as area x 20-83 (constant). .". in this case the weight of 100 ft.-run = 2-57 x 20-83. = 53'4 lbs. weight. Again, from above, 17-4 x (o)2= 1408-5 23-6x(7)2=ii57 28-0 x (5)*= 700 29,6x(3)"= 2665 29-9 x 1 = 29-9 (N.B.—In above, the factor 9 is the distance of row A from the major axis, &c.) The total represents the moment of inertia of the figute about its major axis. But the corresponding moment of inertia of No. 25 strut in the same units is 3,300. Reducing this to 1,000 units, the corresponding moment of inertia of No. 41 becomes-^-— x 3561-9, or '3°3 x 35DI '9 = i°79 (approx.) (-303 being, there fore, the constant referred to in the notes.) Total 3569-1 Certain figures in the above calculations, likewise some of the notes in the text, differ slightly from the manuscript as originally presented, the author having revised the proofs of this article before its appearance in FLIGHT.—ED. ® ® ® ® THE AERO ENGINE. By G. H. CHALLENGER. Pump Charging.—We have seen that the piston suction cannot induce a full charge, because the velocity of the mixture through the carburettor, induction pipes and valves is proportional to the square root of the difference in pressure so that although larger valves and ripee will ensure a better result, we cannot hope to obtain a full charge at high piston speeds without some outside aid. Various experiment! have been made on automobiles with pumps and centri fugal fans to augment the charge, but the extra complications do not appear to have been justified by the results obtained, but this need not deter the designer of aero engines, because except for track- racing automobiles the desire of automobile engine designers is to increase the power developed at low piston speeds, whereas the effect of forced feed will fall off rapidly with decrease in piston speed, as will be seen by a little consideration of Table V. The aero engine designer requires high piston speed to keep down the weight for power developed, and forced feed will help him to keep down weight still more. A cylinder already considered had a oore and stroke of 100 x 160 ; with a revolution speed of 1,200 r.p.m., it required mixture at the average rate of 1-78 cubic ft. per second during one stroke in every four. *.„Rndlal En?lnes-—A rac5ial engine made up of five such cylinders equally spaced round a single throw crank-shaft may be arranged to hrc in the following order, I, 3. 5. 2, 4, I, 3 and so on. There will be a slight overlap m the cylinders' demand for mixture due to five cylinders requiring mixture during two revolutions. The rate of total supply demanded will be therefore li times 1-78 or 2-22 cubic ( IIJLT' T\ 1! • cy,inders have *e v°l»me swept by the piston, tilled at atmospheric pressure. A simple means of assuring this supply without extra moving parts is available on the aeroplane. If the aeroplane's speed is 60 miles per hour or 88 ft per sec then neglecting various losses if the air intake to the carburettor faces the direction of flight, and has an area of 0-025 sq. ft- or a diameter o sightly more than 2 ins., air will be forced in at the rate of 2 22 cubic ft. per second. o-o^f6,, f,iCtl°a,T^CiUaU^ ?isra,te of flow with an intake ^a of 0 025 sq. ft would be diminished, due to the resistance encountered n varying the velocity of the air to maintain the rate of flow through the various restricted areas of the carburettor choke inlet valves and friction in pipes and tends, but if the area of the intake is increased Ore resistance offered by the surplus air to being set in motion will be converted into static pressure, which can be utiliseTto force he necessary air through the pipes. If required, the area of intake may be made greater than 0-025 »* ft- plus the amount required to overcome resistance in which case the surplus area wi 1 bTutflized aUotlf.^ T' Cylm(l£Vl a Pressure » than that of the Stea? "1CreaSed prCSSUre ^P^ng °n the amount of thf h,Uv7Pr?f °n and ^P*05*™ ^rokes are so ultimately connected that they must be considered together. 1 ^'E:l —' ',lc fherrnal efficiency and the mean effective pressure during the explosion stroke increase with increase of com? essbn before ,grat,on for a given rate of fuel consumption. It has been 548 Continued from page 523.) found that the pressures produced with any given mixture are proportional to the pressure before ignition, i.e., doubling the pressure of the mixture and maintaining its temperature constant before explosion doubles the explosion pressure. Comparing engine tests 9 and 11 in Table III it will be seen that a compression ratio of 3-92 gave a compression pressure of 68 lbs. per sq. in. and a mean effective pressure during the explosion stroke of 83 lbs. The maximum pressure attained by the explosion was probably 250 lbs. A compression ratio of 4-71 gave a compression ratio of 86 lbs and a mean effective pressure of 85 lbs. only 2 lbs. per sq. in. more than the previous case, whilst the maximum explosion pressure probably reached 315 lbs. Pwssure and Strength.—As the strength of parts must be proportioned to stand the maximum pressures, it is doubtful if the 2i per cent, increase in power and lower fuel consumption of the higher compression engine are worth having for the 25 per cent, increase in strength necessary to withstand the higher explosion pressures. The maximum temperatures are proportional to the maximum pressures so that the desirability of limiting the compression pressure, on air-cooled motors, in order to lessen the heating of cylinder walls is apparent. In stationary engines, where weight is not of serious importance, lull advantage can be taken of the economy obtained by high com pression. Test 12, Table III, has been inserted to show the high thermal efficiency obtained with a compression pressure of 500 lbs. lests 9, 10 and 11 also show the increase of thermal efficiency with increase of compression pressure. Compression Ratios.—We have seen by Table IV that the low compression ratio engine suffers badly from attenuation of the cnarge at high piston speeds as compared with the high compression ratio engine. The forced feed system already mentioned would wipe out the loss of power in the low compression engine to a great extent, and might even result in a greater massic power, i.e., horse power per pound weight of engine. Tabk^rT1 haS k™ Calculated with the same limiting conditions as A. 3 B. 27, TABLE VI. Unit volumes supplied with forced feed at atmos pheric pressure • Volumes of mixture and residue exhaust 'in cylinder at end of stroke Percentage of residue exhaust per unit'volume' 25 p.c «lD c Power developed with forced feed, taking that 3 P ot compression ratio of 4 as unity 1 n.so Ditto without forced feed , ° • , Relative weight of motors ... '" "' .;J Ratio of power to weight with forced feed, taking 4 ratio as unity ... . Ratio of power to weight without forced feed'." 1 o"^5 To be concluded.)
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