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Aviation History
1912
1912 - 0618.PDF
f/ysHf JULY 6, 1912. Conducted by V. E, Hydro- Aeroplanes. Maximum Flotation and Working Load. REFERRING to the experiments described in last week's issue, we have been asked if we consider that the working load ought to be about one-quarter the maximum flotation. This is undoubtedly excessive, and we give this week a sectional drawing and photograph of a float in which the volume and, therefore, the fiotational capacity is considerably diminished, but the base area remains the same. Since the float should only be immersed to a certain extent, there is in'(reality very little practical loss of flotation, and considerable JOHNSON, M.A. sideways and slightly upwards off the surface. The water at the last behaving as if it had an elastic skin which was capable of sticking to the bottom of the float, and also of being stretched to a certain extent before breaking. Not only needles, but pieces of aluminium and other metallic foils of considerable weight, if of sufficient surface area, can be floated on the surface of water if they are carefully placed thereon so that this skin—which water really does possess— be not broken. If we dip into water narrow glass tubes, the water at once rushes up and stands about an inch above the general level. The tube inside is wet. The elastic skin of the water is, therefore, O I— IE in ch es Fig. 1.—Vertical section improved V.E.J, type of combined float hydroplane and aerofoil surfa«. CD = line of maximum permissible immersion. AB = line of immersion for quick rising, a = angle of incidence (approx.) at which float plane should be set. saving in useless material and head resistance, chiefly when the machine is in free flight. The transparency of the silk even when well varnished is well shown in the photograph. This transparency is a decided advantage, since any leakage can at once be detected. Base Area and Cylindrical Floats. •j The question of base area is an important one. The greater this area the less easily can it be moved vertically up and down in the water ; a cylindrical-shaped float offers but little resistance to such a Fig. 2.—Photo of float shown in vertical section in Fig. 1. movement; and this is the chief reason why such shaped floats must be placed so far ahead of the centre of gravity. Pterygoid and Apteroid Aspect. For the same reason, and also because the travel of the centre of (water) pressure on the hydroplane should be as small as possible, I believe that the float should have a pterygoid (broadside-on) and not an apteroid (end-on) aspect. The float shown in the photograph is six inches each way—it is the tail float—but the main float is 18 ins. by 6 ins., i.e., it has a pterygoid aspect of 3 to I. So long, of course, as you are driving anything through the water one would naturally place it in apteroid aspect, but a hydroplane is not driven through the water, but on its surface, as explained last week. Water Skin Effect. If we place such a float as that shown in the illustration on the surface of still water, and try to pick it straight up from the surface, a considerable force is required, but very little effort if it be slid attached to the tube, and goes on pulling up the water until the weight of the water raised above the general level is equal to the force exerted by the skin. Professor Boys has measured the force actually exerted by this elastic water skin, and found it to be three and a-quarter grains to the inch. [We are not referring in this para graph to atmospheric pressure, but to the actual raising up to a certain extent of the surface of the water when such a body is lifted straight up from it.] Bodies Rolling on the Surface of Water. The first floats which I employed were celluloid balls, with steel spindles stuck through them, and capable, therefore, of revolving. Experiment soon showed that it was easier to drive such through the water when the balls were not revolving than when they were. The balls were not quite half immersed. Similarly it is easier for a chassis wheel to emerge from and quit the water when not revolving than when doing so ; more especially if the wheel be provided with an inflated rubber tyre. In the case of a flat-bottomed hydroplane, inclined at an angle, and with its trailing edge the final part to quit the surface of the water ; this long line of water-skin resistance (presuming the aspect to be pterygoid) is easily cleared away by the blast of air travelling across its surface, and there is not the slightest necessity to try in any way to "ease off" the effect. Further Experiments with Models. One of the experiments tried during the last week was the possibility in a tractor machine of making the tail plane fulfil the double capacity of tail and float, in other words the use of a A neat tractor biplane constructed by Messrs. Holt Bros. 6l8
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