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Aviation History
1913
1913 - 0050.PDF
l/ycSf] A Hydroplane Challenge. Mr. F. A. Mills, of 28, Mornington Road, N.W., forwards us the following :—"I notice a statement in your issue of December 21st last that a J-h.p. 4-ft. petrol-driven hydroplane—illustrated— has attained a speed of 14 m.p.h. As the owner of the fastest officially timed petrol-driven hydroplane, I hereby challenge the owner (or runner) of the above 14 m.p.h. boat to a race against my I-metre petrol hydroplane, ' Stentor Minor,' any distance up to two miles for £$ a side. To be run anywhere within a London radius of 15 miles." Mr. W. H. Amo's Twln^Tractof R.O.G. Model. The fuselage of this machine (the writer's twenty-fifth model) is 01 channel-shaped silver spruce, the main plane of 18-gauge steel wire Mr. W. H, Amo's r.o.g. Model. •covered with Jap silk, varnished after it was put on. The camber of the two centre ribs § in., the other J in. The tail and fin are made of 20-gauge steel wire ; the former has a negative angle of 1 in 36, and the latter an area of 10 sq. ins. (approx.). The last- named is, however, too large, and will be altered in my next machine. The propellers, carved from the solid, are 8-in. diameter, and have a pitch of 14*5 ins., the width of the blades at the widest part being 1 '25 ins. The motive power is ten strands of TV-in. square section rubber to each propeller—650 turns were given and on a bench test JANUARY II, 1913. they ran down in 30 sees. Best flight, 25 sees, (approx.); duration and distance, 135 yards after rising from the ground. The weight of the machine is 5 ozs., which the designer considers rather heavy. No doubt the weight could be cut down by using three Clarke's wheels instead of the one at the tail, and two 2-in. hollow cardboard ones in front. As the machine has to rise off very rough ground large wheels are a necessity. Scientific Model Building. V. Propellers, their Design and Construction. Dynamic Thrust.—In the case of a propeller the thrust is equal to:—Weight of mass of air acted on per second x by slip velocity in f eet per second. When the machine, and therefore the propeller is advancing through the air (rotating of course at the same time) it might be thought that the thrust would be less. Sir Hiram Maxim found, however, experimentally, that the thrust of a propeller travelling through the air at a speed of 40 m.p.h. fell but little, the r.p.m. being of course the same, i.e., provided the slip speed was not greater than from 60 to 70 per cent, of the theoretical speed. The explanation is that when travelling, the propeller is constantly advancing on to undisturbed or fresh air—the slip velocity is reduced —the undisturbed air being equivalent to acting upon a larger mass of air. Theoretical Pitch. In the case of a theoretically correct propeller the pitch should at all points be the same. If we therefore take a point nearer to the centre or boss than the blade tips, the angle at that point must obviously be greater in order to obtain the same pitch with a shorter base line. At the very centre of the propeller the blade must clearly be at right angles to the plane of rotation. It is often con tended that those portions of the blades near the boss do very little or no work, there is no doubt that this is more or less true, but it is certainly of primary importance that they should be so constructed as to offer no, or at any rate the minimum assistance, in the line of travel. If we cut away the inner portions of the blade, then we are driving back a lube of air, whereas what we should be driving back is a cylinder of air which suffers loss of efficiency, i.e., which is acted upon by air friction on its outside only, and not as in the former case, both inside and outside. If the pitch be not uniform, then there must be some portions of the blade which will drag through the air instead of affording useful thrust, and others which will be doing more than they ought, i.e., put air in motion which had better be left quiet. Pitch Coefficient or Pitch Ratio. If we divide the pitch of a screw by its diameter, we obtain what is known as pitch coefficient or ratio. The mean value of eighteen pitch coefficient of full-sized machines worked out by the writer some time ago came out at o'bz. In the case of the original Wright machine it was (probably) 1. It was, of course, a slow-speed propeller, about 450 r.p.m., and its efficiency was admitted on all hands. In marine propulsion this ratio is generally I "3 for a slow- speed propeller, decreasing to 0-9 for a high-speed one. In the case of rubber-driven models, this pitch ratio is often carried much higher, even to over 3, 3f being not unfrequent with a tip angle of some 450. Within limits it would appear the higher the pitch ratio the better the efficiency. The higher the pitch the slower the r.p.m. In the case of rubber-driven models we do not want the rubber to untwist (run out) too quickly, which a fine pitch propeller tends to make it do, consequently one generally finds in models built for duration or distance that a propeller of large pitch ratio is employed. In the case of power-driven models, the engine to work efficiently must revolve quickly ; one is therefore either compelled to use a fine pitch propeller, which is not efficient when revolving under 1,000 r.p.m., or one must gear down to a slow speed propeller. Now gearing always represents loss of energy, inadmissible, at any rate, in a small power-driven model, thus one finds fine pitch propellers invariably employed in this case. Intimately connected with the pitch ratio is, however, the blade area and— The diameter.—If we increase the diameter we shall obviously decrease the pitch ratio. From experiments which the writer has made with power-driven models during the last twelve months, he is certainly an advocate of propellers of large diameter for this type of model at any rate. The best results I have obtained in this case have invariably been with narrow bladed propellers of (propor tionately) large diameter. It is certainly of primary importance to engage as much air as possible. The Wrights on their machine (their propeller diameter was 8'5 ft.) engaged from six to seven times more air per h.p. than the early Farmans. Prior to this the Wright Brothers had been able with an engine developing on 14-h.p. to fly with a passenger—the ratio re air engaged : to that engaged on the Farman machine in this case being io-S : 1. The efficiency referred to is, of course, efficiency as regards weight carried per horse-power. (To be continued.) So
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