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Aviation History
1928
1928 - 0809.PDF
ATJOXTST 30, 1928 65 THE AIRCRAFT ENGINEER SUPPLEMENT TO FLIGHT FACTORS IN SEAPLANE FLOAT DESIGN By WM. MUNRO (Concluded from page 59.) Getting Off Speed for Floats of Similar Form From the stipulations made in Froude's Law of Comparison, we can estimate what the taking-off speed of the new machine should be. Remembering that " the speeds of the vessels must be proportional to the square root of their linear dimen- sions," we have— And as " the ratio of the linear dimensions will be the cube root of the ratio of displacements," we have— L_ 3 /W I * V w A71 /« I so that, given the weights \V and w of the two machines and the taking-off speed S of the machine used as a base, then the W1 "•'taking-off speed of the new machine should be — S X ir Problems of Resistance. Subdividing the total water-resistance in the accepted manner, we have :— (1) Frictional or skin resistance. (2) Eddy-making resistance. (3) Wave resistance. As might be expected, frictional resistance is at low speeds a high percentage of the total resistance. The resistance due to eddy-formation is small in a clean design with fine lines aft, but a marked increase will occur if a projection of any kind is fitted to the tail of the float. Against the aerodynamic advantage of fine lines aft, must be placed the necessity for maintaining enough buoyancy aft to give suitable righting moments.* It is, however, with wave-making resistance that the real problem arises, and in particular with the occurrence of wave- interference. The phenomenon of wave-interference—that is to say, the interference of succeeding crests and troughs of the bow transverse system with the stern wave—is of a very complex nature. There are two separate and distinct series of waves caused by the motion of a vessel through the water :— (1) At the bow. (2) At the stern. Each of these series of waves consists of (1) a series of diverging waves, the crests of which slope aft; and (2) a series of transverse waves, whose crests are nearly perpendi- cular to the middle line of the ship. " The transverse waves show themselves along the sides of the vessel by crests and troughs. The lengths of these waves [i.e., the distance from one crest to another) bears a definite relation to the speed of the vessel." Experiments made by Froude show that residuary resist- ance varied in a vessel which always had the same fore and after bodies, but had varying lengths of parallel middle body inserted, thus varying the total length. '' These variations in residuary resistance for varying lengths are attributed to the interference of the bow and stern transverse series of waves. " When the crests of the bow-wave series coincide with the crests of the stern-wave series, the residuary resistance is at a maximum. " When the crests of the bow-wave series coincide with the troughs of the stern-wave series, the residuary resistance is at a minimum." As previously mentioned, the interference between port and starboard float wave-systems adds further complications. * See " Seaplane Stability Calculations," AIRCRAFT ENGINEER, Feb. 23rd and March 29th, 1928. As the speed of the vessel increases, the bow and stern wave systems are affected differently, and the positim>s\oi the wave crests relative to the vessel defy accurate computation by any means other than tank-tests. It is in consideration of the bow-wave system likely to be formed that the angle of entrance compels attention. An angle of 34° has proved satisfactory in practice. (See Fig. 2.) Another problem for investigation may be mentioned, •which, (while not of such practical importance as the co- relation of float length, wave-length, and speed involved in the study of wave-interference) is yet by no means lacking in interest. This is the effect of depth of water on speed. It may seem early in the day to compare our present sea- planes and flving-boats to ships of the line, but in view of the development of these craft the following excerpt from " Manual of Seamanship " may l>e of interest. " In Destroyers and Torpedo Boats it has been found that considerable differences in speed can be obtained for the same power in different depths of water. The German Scout Bremen, of 3,000 tons with 11,000 i.h.p., obtained 22£ knots in 14 fathoms, whilst in 35£ fathoms of water she obtained the same speed with 9,750 i.h.p. In torpedo craft, differences of speed of as much as 5 knots have been noted. "' The difference is caused by the formation of a solitary wave of translation which moves forward with the ship. A certain depth of water is more favourable to the formation of this wave, and in such a depth a larger resistance is ex- perienced." One notes that a new tank for the testing of seaplane models is provided for in the Air Estimates, and looks forward with interest to the advance in knowledge of wave-inter- ference and kindred problems which is bound to accrue. The remaining problem of interest in the design of sea- plane floats is the aerodynamic one of step resistance, and the considerations involved may best be indicated by the follow- ing notes supplied by Mr. G. H. Dowty, A.F.R.Ae.S., who has been interested in this problem for some time. The value of any such scheme, would, of course, depend on the simplicity and efficiency of the designed components, which, in practice, might be difficult to attain. " The steps used on flying-boats and seaplane floats, while essential to the ease and speed of ' unsticking ' from the •water, are a great handicap in flight, and at least one-fifth, if not more, of the total aerodynamic resistance of floats can be directly attributed to the step. " Particularly in the case where high speeds are attained, the presence of even a small step incurs a large loss of power. " Taking for an example an actual case of a fast seaplane having a step of total back area of 0 • 65 square feet, the step, at a velocity of 220 m.p.h. will absorb 54 h.p."' The proposal favoured by Mr. Dowty as a means to reduc- ing the step drag was Collapse of the Step ut High Speeds " The float is built up without the step, this being added afterwards as a separate unit. "" The step is made in two portions, one either side of the keel. The front ends of the step are made fast to the float and the rear ends are held away from the float at the required distance by suitable supports. These supports, when col- lapse is required, can be released to allow the step to lie up flush with the sides of the float."' To obviate further controls in the cockpit, an automatic means of operation was suggested, this being carried out by a control device on the lines of the Savage-Bramson anti- stall gear, whereby operations might be carried out at a pre- determined flying speed, or alternatively " by a control bperated by pressure derived from a breather on the float top and working in the slip-stream : at high velocities the pressure produced to be applied through a diaphragm, plunger or similar means to operate a quick release." While on the subject of " step," it is of interest to note that the aft step of the new flying-boat Calcutta appears, in published photographs, to be '" faired-in," and to speculate whether or not this has a definite bearing on the latest ideas for the design of flying-boat planing-bottom : That is to say, relying on one step only at take-off. 748*
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