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Aviation History
1920
1920 - 1218.PDF
If K is the normal resistance coefficient of air one has in the case of the equivalent plane F = KS'V'2 and T = FV = KS'V'* F* a8 from which ~j = KS' = i^D2, which gives S' = S' • D1 • ^r. The value of q is . Da . 4 »s 4 KIt will be seen that the "qualite " independently of diameter is a constant for all members of a family of geometrically similar airscrews. Col. Renard has indicated, without actually proving the fact, that the quality q has a maximum value of :—q = 6^*, p being the efficiency, which, since the efficiency cannot exceed unity, gives as a limit q — 6 The experiments undertaken by Monsieur Riabouchinsky •at the Aero-Dynamic Laboratory at Koutchino on this matter are most valuable and most conclusive. The results given in the following table concern the two-bladed airscrew of om 30 diameter turning to 30 revolutions per second and running in a current of air perpendicular to its axis. This airscrew had 40° blade angle, at the centre. Air Speed perpendicu- lar to the axis of rotation. o 2-5 4.2 5 6.2 Thrust Kg. 0 .036 0 .050 0 .065 0.074 0 .082 TABLE I Power Absorbed H.P. 0.32 o.33 0.30 0.29 0.28 " Qual 0.08 0 .19 0.50 0.80 1.16 Some Definitions.—If V = the speed of ascent in metres per second, N the number of turns per second of the airscrew, W = the relative speed of the air for an element of the blade— ds—situated at distance R from the centre of rotation. The pitch or advance per revolution is :— H = 2ijR tan ($ + i). TT The pitch diameter ratio is h — f:. The blade area ratio is the ratio between the projected area of the actual blade surface upon a plane perpendicular NOVEMBER 25, 1920 been brought forward several projects for the building of helicopters, capable not only of sustaining themselves but also of steering and of landing properly in gliding descent in caae to the axis of rotation and the surface of the circle swept out by the screw of diameter. The curves below give for lifting airscrews the general form of the curves of variation of the1 ' qualite'' expressed as a function of the four parameters These are the speed of the perpendicular current of air, the blade area ratio, the pitch diameter ratio and the number of blades. These curves are arrived at from the work of Col. Renard and M. Riabouchinsky (Pamphlet No. 11, Bulletin de Koutchino). (2) Construction of Helicopters If the manufacturers of toy helicopters are innumerable, the various investigators who have built full-size machines to verify their ideas are, on the contrary, very few. Before the War, in France, M. Louis Breguet, the famous builder of aero- planes, had constructed a helicopter (the Breguet-Richet helicopter) which gave quite a number of interesting results. M. Breguet was, however, only concerned with the actual lifting force of airscrews, i.e., he limited his efforts solely to solving one side only of the problem, and it has been the same with all the other investigators, in particular Mr. Cooper- Hewitt, who has made static tests on lifting airscrews driven by an electric motor in America. But since 1918 there have FIG. 1. FIG. 2. of a break-down of the engines. The whole problem of the helicopter is summarised in these three conditions, of lifting, of horizontal translation and of gliding descent. The Two Types of Helicopter It is necessary in any scheme for a helicopter to split up the lifting force between two airscrews or pairs of airscrews turn- ing in opposite directions in order to avoid the rotation of the machine itself around the axis of the airscrew. One can conceive of helicopters under the three aspects shown in Figs. 1, 2 and 3, but in reality there only exist two separate types (1) the machine with the single axis and (2) that with separate axis. Advantages of the single axis : Great mechanical simplicity and consequent lightness. Advantages of the separate axis : Better aero-dynamic efficiency of the lifting airscrews. Construction of Lifting Airscrews.—It has been seen that for lifting airscrews a very large blade and ratio was compatible with high efficiency. The optimum diameter for a lifting airscrew is 7 metres, and as far as the number of blades is concerned I have always considered that 4 was the best number. The blade of a lifting airscrew is comparable in dimensions to the wing of an aeroplane, but the loads which it has to carry are not similar. Its actual construction should be carried out with spars and ribs and particularly strong bracings, the whole covered with fabric and doped exactly like the wing of a monoplane. A blade should be designed to resist the following forces ;— (1) The static loads which depend only on the weight of the machine. For these loads a suitable factor of safety would be seven. (2) Centrifugal loads. These are considerable in a screw of large diameter. A blade weighing 30 kg. 3 metres 50 radius with its centre of gravity 2 metres 50 from the axis is subject at a speed of 200 revolutions a minute to a centrifugal force. 30Fc = mw*r = -. Tl[ 2V X JCO~J ~6o~ J 2-5 =--3.3o° Kg. It is obviously necessary that the spars should be placed radially in the blade. The weight of a lifting airscrew will, all other things being equal, obviously be greater than the wing of an aeroplane of the same surface. During the recent construction of an experimental heli- copter I have found that it was very difficult to build a blade of such a lifting screw giving a high factor of safety for a weight of less than 8 kg. per square metre. In this weight it included all the bracings, fabric, dope and varnish, in fact the whole weight of the rotating wing in working order. At the same time I believe that by reducing the diameter of the screws from seven to six metres and taking speeds of rotation of the order of 150 r.p.m., the weight per square metre can be reduced to 7 kg. Further, by using a biplane construction which allows a still further slight reduction of diameter and the replacement of bracing wires by well-streamlined struts one will easily be able to reach 6 kg. per square metre. It is this figure which will be taken in the estimates which follow. The peripheral speed of a lifting blade should not exceed 50 to 60 metres per second on account of the difficulties of construction. The peripheral speed of ordinary propulsive airscrews built of wood can easily reach 300 metres per second. All the before-mentioned considerations have to be taken into account before one can attempt the serious construction of a lifting airscrew. I give below by way of example the results of an official test upon a complete model one-seventh of full size of a lifting airscrew which I have built. The full-size wing has given nnder loading tests a factor of safety of 8. TABLE 2 Date of Test—September 5, 1918 Thrust kg. .. ..38 13 18 R.P.M. of screws ... 480 778 1,008 1,169 Power absorbed at air- screw shaft H. P. .. 0.25 1.03 2.18 3.47 Barometer at time of test (corrected to o degs. C.) 776.5 mm. Temperature, 22 degs. C. 1220
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