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Aviation History
1928
1928 - 0671.PDF
JTTLY 19, 1928 sy THE AIRCRAFT ENGINEER SUPPLEMENT TOFLIGHT Model. Load on water is adjusted for any speed and true for one trim only. Trim of model at high speeds depends mainly on water forces. No material damping against pitching. Experiments usually made in smooth water, occasionally in rough. Full Size. Load on Water varies with wind force and trim of hull. Trim at all high speeds is de- termined by the air struc- ture. Pitching is damped to some extent by the wings and tail system of planes. Normally the water surface is broken into waves varying up to two or three feet in height. A useful figure to note here is that in comparing similar floats the area of the wetted surface may be taken as pro- portional to the two-thirds power of the displacement, i.e., W2'3. Having obtained from the tank test the total water resist- ance, and ignoring the limitations stated, the total air resistance of remainder of m/c is calculated and the two resistances added. From this the total effective thrust and E.H.P. required to maintain the speed on the water is calculated, and a curve is plotted of E.H.P. on a base of speeds from V = O to V = VI. On the same base the effective horse-power available curve is plotted off, and if the machine is to " unstick" in a calm, these curves must not intersect, i.e., the E.H.P. required must not be more than E.H.P. available between these speeds. Floats of Similar Form. It is quite often convenient and desirable to construct a flotation system geometrically similar to one previously designed for a machine the weight of which was in the same neighbourhood as the projected machine, built for generally similar purposes, and with the floats set at the same angle relative to the wing chord. In such a case (assuming that the original design was satis- factory in water-performance), Froude's Law of Comparison can again be used, and the " design" of the float shape becomes a matter of proportioning up. The cube root of the ratio of the total buoyancy of the new float to the total buoyancy of the old float is taken as the ratio of the linear dimensions. If we take for an example the float used in the writer's previous article on " Seaplane Stability Calculations," we have Weight of machine ==4,854 lbs. Total buoyancy of float ... = 4,630 lbs. Length offloat -=20 -7 ft. Breadth of float = 34-76 in. Depth of float = 30-25 in. Supposing now a set of floats to be required for a similar machine with a weight of 5,500 lbs., the reserve buoyancy to be the same in both cases, say, 90 per cent. Then the length, breadth, depth, and all linear dimensions given in the table of offsets will be found for the new floats by using a multiplier which is the cube root of the ratio of the two total volumes, i.e., Total volume of original float = 4,854 + 90 per cent, of 4,854 2 ' X. Total volume of required float = 5,500 + 90 per cent, of 5,500 __ 2 ~~~ ~~ 3 /Y.-. The multiplier — A/ — = say, «. The length of required float will be 20-7 X a ft. The breadth of required float will be 34 • 76 X a in. The depth of required float will be 30-25 X a in. Similarly, the ratio of waterplane areas can be immediately assessed as the square of the cube root of the ratio of total volumes, i.e.— Taking the area of waterplane of original float as 41-25 sq. ft., then the area of waterplane of new float will be 41-25 X a2/:i This obviates the necessity of a detailed calculation of the waterplane area, which must be known when checking-out the static stability of the flotation system. The position of the load-water-line below top of float would also be fixed by multiplying the existing dimensions in the original float by the multiplier a. This, of course, applies only when the reserve buoyancy is the same in both cases. Had a reserve buoyancc of 100 per cent, been required in the new design, then a geometrically similar shape would be arrived at by taking the total volume of required float as 5,500 lbs. + 100 per cent. = say, Yx and a different multiplier would be used derived from X For proportioning up. the procedure would be as before but the position of the load water line would have to be determined by " trial and error," and the fore and aft position of the Centre of Buoyancy would have to be calculated. With equal reserve buoyancy in the old and new floats, the fore and aft position of the Centre of Buoyancy would be found in the same manner as all other linear dimensions, that is. the distance from the step in inches on the old float would be multiplied by the value «. (To be concluded.) TECHNICAL LITERATURE. SUMMARIES OF AERONAUTICAL RESEARCHCOMMITTEE REPORTS. These Reports are published by His Majesty's Stationery Office, London, and may be purchased directly from H.M. Stationery Office at the following addresses : Adastral House, Kingsway, W.C. 2; 28, Abingdon Street, London, S.W.I; York Street, Manchester; 1, St. Andrew's Crescent, Cardiff; or 120, George Street, Edinburgh; or through any book- seller. THE IMPORTANCE OF "' STREAMLINING " IN RELATION TO PERFORMANCE. By Professor B. M. Jones, M.A., A.F.C. R. & M. No. 1115 (Ae. 288). (15 pages and 2 diagrams.) September, 1927. Price 9d. net. The author is of opinion that more attention should be directed to researchestowards improving performance than has been given in the past. With this idea in mind, he makes a general examination of the factors governing per-formance, and came to the conclusion that the most hopeful line of attack is that directed towards obtaining a better streamline flow about the aeroplane,than is apparently obtained at present. Other factors, such as the improve- ment of airscrew efficiency by the use of variable-pitch screws and the reduc-tion of induced drag by means of increased aspect ratio or better plan form may be important, but they do not show promise of improvement of the same,order of magnitude as that which may be gained by improved streamlining, and, moreover, their realisation is not being held back so much by lack ofaerodynamic knowledge as by structural and other difficulties. The improvement of streamlining, on the other hand, whilst holding outpromise of very great improvement of performance, is also definitely retarded by lack of aerodynamic data relating to the kinds of shape which give riseto streamline flow ; almost our only reliable inf orniation on this matter relates to the simplest forms acting in air undisturbed by the screw slipstream. It therefore appears that it is towards the accumulation of experimentaland, if possible, theoretical information, relating to the obtaining of good streamline motion around all the parts of an aeroplane, that attention shouldnow be directed. The present note deals solely with the empirical side of such an investigation.The performances of certain aeroplanes are compared by an empirical formula with the skin friction resistance of a flat plate, and the comparisonsuggests that great improvement is possible. Certain suggestions for experi- ments are made, including body-air«crew interference, excrescences, and thecooling of air-cooled engines.
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