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Aviation History
1913
1913 - 0121.PDF
FEBRUARY I, 1913. l/ycmj HYDRO-AEROPLANES. A Comparison of the Different Float Systems. AT the Monaco meeting the single float Curtiss type system probably proved itself the best. It must be remembered, however, that the Curtiss machine was a very light one; and one which attains a high speed when running awash and " unsticks" (decoller) very easily. At rest, i.e., when floating on calm water, the machine leans to one side or the other according to the distribution of weight on it (see Fig. 10), one or another of the balancers being thus called upon to support some weight in consequence. Once the machine has any way on it, the lifting effect of the planes and the movement naturally combine to steady the machine so that it sometimes ran for quite a distance without dipping either balancer into the water. The Henry Farman biplane began with three floats, one aft under the tail and two under the main planes, but discarded the former towards the close of the meeting. The Maurice Farman was much improved when the aft float was removed and the forward floats brought closer together ; but it must not be forgotten that their longitudinal flotational stability was much lessened in consequence. The Sanschez-Besa was at first unmanageable owing to her floats being placed very widely apart ; being quite out of control when landing in consequence. If the machine was canted the least bit to one side when she struck the water, she pivoted on the more immersed float, and in spite of any amount of helm the machine swung sharply round ; it was greatly improved when the floats were shifted forward a little and also brought nt arer together. In order to have good BY V. E. JOHNSON, M.A. Continued I'roni fog* 99.) or lowered to the extent of at least three feet, either indepemlently or together at the will of the pilot. In order to make any float skim over the surface of the water, it is evidently a great advantage to have as little bulk beneath the surface as possible; it is also important that the float should commence to urise itself out of the water at a low speed, say 8 m.p.h., and as the speed increases skim or glide over the surface with the least possible resistance. It is also evident that this skimming should be as steady as possible both laterally as well as longitudinally. Increasing the number of floats helps to steady the machine, but greatly increases frietiotml resistance while in the water and head resistance whilst flying. Placing the floats as far apart as constructional possibilities permit also steadies the machine on calm waters, but under certain conditions undoubtedly has the opposite effect on rough. The lower the centre of propulsion (always supposed above the water) the easier can the machine bfl made to hydroplane, since there is less necessity to counteract the "nose-digging" effect of the propeller thrust. I have already stated that the speed at which skimming commences increases with the float length ; this, however true it may be so far as the hydro plane principle is concerned, can be much modified by the lift of the aeroplane surfaces. Also if the floats be made too short t In formation of large waves is assisted which permit of the after part of the float to fall and greatly increase the angle of the planes, thui making it more difficult for the float to rise to the surface ami Fig. 10. Fig. 11. Fig. 10.—Showing how a machine with only one central float leans to one side or the other owing to weight distribution, &c. Fig. 11.—Lateral oscillations produced by the waves on a machine whose floats are some distance apart. stability whilst hydroplaning over the surface of the water and to prevent the machine being upset by a gust of wind, one is led into putting the floats as far apart as possible. This is all right in tranquil waters, such as rivers and small lakes, but on the sea the further the floats are apart there is always the chance of one being on the top of a wave whilst the other is in a hollow (see Fig. 11), and vice versa. This may and does produce great concussions, which are very damaging to the solidity of the machine. It is therefore advantageous in this respect to bring the floats closer together, thus arriving at a design having a single central float or two floats fairly close together. By so doing you have, of course, greatly reduced your lateral flotational stability. In the case of rivers and small lakes where the waves are, comparatively speaking, insignificant, any recognised method can really be adopted with success. Another important factor is the plan form of the floats, according to the state of the water, calm or rough, on which the machine has to carry out its evolutions. Short floats enable a quicker "get off," but on the other hand, receive more violent shocks from the waves, necessitating the placing of some sort of spring or buffer between them and the machine. Long floats do not receive such violent shocks when cutting through the waves, especially if pointed in front. We thus see that the chief distinction between machines destined for rough waters and those for rough seas, is that the first can with possible advantage be provided with short floats rather far apart, whilst the second should have long floats closer together, or a single central one. The great difficulty being that in rough water you want good flotational stability both laterally and longitudinally, which (laterally) is exactly what this system does not give you, unless wing tip floats of considerable flotational capacity are fitted on the machine, these latter should undoubtedly be capable of being raised skim. To overcome these the weight must be reduced to a minimum ; but sufficient, not to say ample, dotation must be provided ; in my opinion, not less than twice that of the total weigh! of the machine. And if one large central float be used, this float should be divided into at least several watertight compartments. Boat-formed hydro-aeroplanes have less resistance at moderate speeds, 15-20 m.p.h., than those of the old Curtiss type ; in the last named the resistance increases very rapidly at first with increase in velocity, provided of course the float be not hydroplaning or skimming; but, if owing either to increased lifting surface or increased speed skimming has commenced, then the resistance rises very slowly, in the case of the hydroplane proper only increasing at a slightly faster rate than the speed. Froude established (in the case of hydroplanes) that the angle of the plane to the lines of motion should be such that resistance due to surface friction should equal the resistance due to gravity, i.e., in other words the horizontal component necessary to balance the weight of the vessel inclined at the same angle as the supporting surface. Now this inclination is about 1 in 17, so that the total resistance amounts to approximately one-eighth of the weight or displacement of the float. This resistance varies, however, with the value of the surface friction, which again varies with the kind of surface and still more with the length of rubbing surface. The angle of inclhwtton is not therefore constant, still a one-eighth basis serves as a very fair guide in all preliminary experiments. The stepped hydro plane has proved itself the most successful in actual practice. Float Construction. The French floats are generally constructed as follows : the floats ox fuselage hulls always consist of a framework of wood covered with thick veneer wood, i.e., plywood ; this veneer wood consists of 3 layers 121
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