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Aviation History
1913
1913 - 0744.PDF
HYDRO-AEROPLANES. SKIMMERS AND THEIR LONGITUDINAL STABILITY. By J. E. STEELE B.Sc SKIMMERS and hydro-aeroplanes are of such growing interest to the naval architect, that even the following short notes on the longi tudinal stability of such craft may not be out of place here. The investigation is limited to the consideration of the longitudinal stability of the machine—classing skimmer and hydro-aeroplane under the one heading—when wholly or in part water-borne. This is the case all the time with the skimmer, but only part of the time with the hydro-aeroplane. When the latter leaves the water and is altogether air-borne, it is an aeroplane pure and simple, and has pasted l*yond the sphere of the naval architect into that of the aeronautical designer, and, therefore, beyond the scope of the present paper. Before discussing the various classes of machine which come under one or other of the headings of Skimmer or Hydro-Aeroplane, it would be well, perhaps, to define the terms used in connection with the stability which it is proposed to investigate. The usual term in aeronautics for movements in the plane ol symmetry of the machine is " longitudinal or symmetrical dynamical stability." To the naval architect, however, the term "dynamical stability" conveys quite another meaning—viz., the work done in inclining a vessel to a given angle. It is proposed, therefore, for the purpose of this paper, to substitute the expression "longitudinal or symmetrical kinstual stability." This stability may be defined as follows ;— Suppose a machine to be in steady motion in the plane of symmetry, which contains the centre of gravity and the line of flight, and the external forces acting on the machine to be in equilibrium. If now the machine be tilted either up or down in this plane, the forces will no longer be in equilibrium, but will Constitute a longitudinal righting or upsetting couple, as the case may I*. If a righting couple acts, the body will return to its original position of equilibrium, and will probably oscillate about We will now consider each type in detail, together with some of the problems to which they give rise. First comes the skimmer, a type of craft whose displacement at high speeds is very much less than the weight of the vessel, and which, as its name indicates, skims over the surface of the water that position. If it returns to the original position without oscillation, or if any oscillations set up gradually die out, then it is kinetically stable. If, on the other hand, the oscillations get larger with time it is kinetically unstable. A machine may have automatic stability—that is, stability attained by the use of moving parts such as gyrostats, pendulums, Sec, or may be stable in its design. This inherent stability is the only kind dealt with here, ami any displacement from a position of equilibrium, due to nn alteration of the rear elevating plane, or an increase in the propeller thrust, of Course produces unbalance of the forces ; but this is quickly followed by dynamical equilibrium under the new rigimc. When an aeroplane is struck by gusts of wind, its behaviom depends to a great extent on its inherent stability. If it be inherently stable, then oscillations set up by the gust will be quickly damped out, and the aeroplane will revert to its state of steady motion. If, however, the gusts be periodic and synchronise with the free oscillations of the aeroplane, the results may be disastrous. If the machine lie kinetically stable—that is to say, if its free oscillations have a modulus of decay—then the theory of forced oscillations shows that the forced oscillations will not exceed a certain limit. Lateral or asymmetrical stability is not dealt with here, as this is mainly of importance when the machine is altogether in the air, and beyond the range of this paper. A machine may be a double-lifting system, one in which there are two lifting surfaces or sets of superposed surfaces, one forward *nd the other one aft. Or it may be a single-lifting system, in which case the auxiliary surfaces, such as tail planes, must be neutral—that is, parallel to the direction in which the wind blows on them. Three types are dealt with in which there is a gradual evolution from the .skimmer on the surface of the water, 77a the machine Htsigned to fly with its tail on that medium, to the machine which rises from and alights on the water but is otherwise an aeroplane. * Paper read at the Spring Meetings of the Fifty-fourth Session of the Iiutuulion of Naval Architects, March 13th, 1913. rather than ploughs its way through it. " Miranda IV," designed by Sir John I. Thornycroft with phenomenal success, is taken as an example of this type of boat, " Miranda IV." This vessel (Fig. 1) is 26 ft. long by 6 ft. broad, and is 2 ft. 6 in. in depth. The fore end is moulded to the usual form of a high speed motor boat. As amidships is approached her lines deviate from the ordinary, and are modified to enable the vessel to skim at high speeds, but this modification is as small as possible in order that she may be driven with ease at speeds below the skimming phase. When running at skimming speeds only that portion from A to B, and again aft of C, are water-borne, and as this is the condition which affects the present paper, only those parts of the hull need be considered. As will be seen from the drawings, the portion of the bottom from A to B forms a dihedral angle, rounded at the apex, and increasing in magnitude as we go aft. Aft of C the bottom may be taken as being fiat. When running within her skimming phase (Fig. 2) the forces acting on the beat are the weight W, the propeller thrust T, the reaction (RF) of the water on the "forward plane " A B, and (RA) that on the " after plane" C D. These reactions act at the centre of pressures of their respective planes, and knowing W, T, RF, and RA in magnitude, position, and direction, it is easy to find the same three things with respect to the remaining force acting on the boat, that being the reaction (Rw) of the wind on the out-of-water portion. The reactions Rw, RF, and RA combine to give the common resultant R, which must pass through O, the meeting point of the remaining forces W and T. From what follows it will be seen that an increase in the propeller thrust will cause the boat to rise forward. The three forces R, T, and W are in equilibrium (Fig. 3), and T acts at a constant angle 9 to the boat, the angle between R and T being therefore constant and equal to 90° - 0. The point A, then, lies on the circumference of the segment of a circle containing the angle 900- 0. It will be seen from this that if the thrust of the propeller be increased as shown, the boat will be tilted up forward. Strictly speaking, the above can only hold if T, W, and R all pass through 770
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