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Aviation History
1916
1916 - 0094.PDF
A "POPULAR" TYPE AEROPLANE DESIGN, By C M. POULSEN. Continued from page 77.) IN our last week's instalment I made a ridiculous slip in the calculation of the wing loading, which I obtained by dividing the total weight of the machine by the weight instead of by the area of the wings. At the same time I take the opportunity to thank those readers who have written to me pointing out the slip. It is very gratifying to know that my suggested design is being so closely followed. The mistake, however, should not have occurred, and I apologise for making it, but it was, of course, obvious to anyone that it was a slip. In correcting it, when the total weight, 793 lbs., is divided by 250 instead of by 200 we get a loading of 3-17 lbs./sq. ft. which is below the 37 lbs. lift obtained at the minimum speed, so that the landing speed of our machine should be, if anything, below 35 m.p.h. Before we can be absolutely certain of obtaining the maximum speed of 65 m.p.h. we must find the total resistance of our machine at that speed. The resistance of the wings was estimated in our last issue to be about 85 lbs. at 95 ft./sec. (65 m.p.h.). There remains to find the resistance of all the other items. The simplest way of doing this is to tabulate the various items and their resistance and vertical distance from the line of thrust, as by so doing we shall see whether there is formed a thrust-resistance couple. The different items, it will be seen, have been arranged in two tables, one for those that are within the slip stream and one for those outside it. The reason for dividing them up in this manner is that the items in the slip stream are exposed to a draught of slightly greater speed than the trans- lational speed of the machine, and that this speed is practically constant for the whole speed range of the machine. With regard to the method of obtaining the resistance of the various items a few words may be of interest. In no case should these figures be taken as Table of Resistance of Parts in Slip Stream at a Velocity 0/ 95 ft.fstc. Vertical o«; . „„., instance horn. RCn?T (lbs.). Body 9/00 Engine 26-0 12 ft. of centre struts 1' 56 10 ft. of chassis struts 1 • 30 Axle 0-52 1/3 wheels 1-50 12 ft. of bracing wire 1 'DO 10ft. of lift wire ... 1-40 Tail planes... \ Tail skid ... J 3 00 Total 45-88 Line of Thrust. 0 0 + 2-50 - a-oo -3-00 -2-20 -2-0O + 0-25 0 — Moment ( + )• — — 3-90 — — — °-35 4-25 Table of Resistance of Parts Outside Slip Stream at a »f 95 //•/«•'•• Item Resistance Uem- (lbs.). Vertical Disiance from Thrust Line. 20-ft. wing struts 2-60 62-ft. bracing wire 8-70 3-ft. centre plane struts j 0-39 2/3 wheels ... 330 Total ... H'99 + 1-75 + 2-00 + 3'6o 3 25 — Moment ( + ). 4'55 17-40 1-40 — 33'35 Moment (-)• — — — 2'6o 1-56 3-20 3-20 — — 10-56 Velocity Moment (-)• — — 10-7 I0'7 absolutely correct, since with our present knowledge it is only possible to make an approximately accurate estimate. Beginning with the table of parts in the slip stream we nave a resistance of 9 lbs. for the body at a velocity of 95 ft./sec. This figure should be approximately correct. In the curve of resistance of the model B.F.36 dirigible the following formula for resistance was given. RT = o'ooogiV1'5*. Allowing for the fact that the body of our machine is octagonal instead of circular in section, I suggest taking a slightly higher figure, say RT = 0.001V2. This is, of course, quite an arbitrary value, but should, I think, be somewhere near the mark, slightly too high if anything. At 95 ft./sec. therefore the resistance of our fuselage will be o-ooi X 95 x 95=9 lbs. The resistance of the engine is taken as 26-3 lbs. This may seem somewhat high, but in the absence of reliable figures one can only make a rough estimate, and 1 have preferred to keep the figure rather high in order to be on the safe side. Roughly speaking, the projected front area of the engine, engine plate, and propeller boss is 3 sq. ft. The resistance of an area of 3 sq. ft. moving normally through the air is found by the formula R = 00015 AV2. Substituting we get R-o 0015 x 3 x 95 x 95 = 40-5 lbs. As, however, the engine is not a flat plate, we must multiply this figure with another expressing the ratio between the resistance of a flat plate and that of a body like our engine. For this purpose I suggest taking the resistance of the engine as 65 per cent, of that of a flat plate ot the same area. We then obtain resistance of engine = 40*5 x 65 = 26-3 lbs. We now come to consider the resistance of struts, and for this we have several results of tests on struts of different section made at the N.P.L. We shall choose one of a fineness ratio of 4-1 and of a certain section found at the N.P.L. to give a resistance of 0-0196 lbs. at a speed of 30 ft./sec. As the strut under test was 18 ins. long, the resistance per foot run at 30 ft./sec. 0-0196 x 12 , , = „ = 0-013. At 95 ft./sec. the resistance will be 0-13 lbs. per foot run. For the struts of the undercarriage the same figure has been retained since correction for size of strut section would not make any appreciable difference. The same applies to the tubular wheel axle, which we shall be able to make of approxi mately the same section by streamlining. As for the resistance of the wheels this is approximately 2-4 lbs. per wheel at a speed of 70 m.p.h. We shall take this as sufficiently accurate for our purpose and use this figure direct without making allowance for size of wheel and our lower speed of 65 m.p.h. For the resist ance of bracing wires we employ the formula F= KDV2, and the value of the constant K for the diameter of wire and velocity under consideration may betaken as 0-0013. This gives a resistance per ft. run at 95 ft./sec. of about 0-14 lbs., from which figure we can obtain fairly accu rately the " drift" or resistance of all the stay wires. For the resistance of tail planes and tail skid it is difficult to get accurate figures unless we have these tested in model form, but as a rough approximation I have taken 1 lb. more than that given in the N.P.L. report for estimates of tail of B.E.2, which is 2 lbs. at 60 m.p.h. The resistance of the items outside the slip stream are obtained in the same manner, and the results shown in 94
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