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Aviation History
1919
1919 - 1666.PDF
DECEMBER 25, 1919 NOTE.—All communications should be addressed to the Model Editor. Calculating Areas THE calculation illustrated diagrammatically by Fig. 1 relates to the problem of finding the correct disposition of surfaces without resorting to a trial flight. In power-driven machines the trial and error method may easily result in a disintegrated plant, and it is therefore advisable to adopt a rnofe certain method of ensuring that the centres of pressure and gravity are coincident, and thus obviating the necessity of making several small surfaces in order to obtain one of the correct area. It is assumed that the machine is complete I have ehecked this formula against many machines, and it has proved itself to be remarkably accurate. A com pressed-air model recently constructed by the writer flew at the first attempt and the area of its elevator had been calculated by means of this formula. With the ordinary flying stick it is so very easy to vary the position of the main plane that it would be unttecessai'y to adopt this method, but as larger machines are contemplated it becomes increasingly difficult to have to move the main plane, once it is fixed. There is another simple formula relating to hydroplanes X' *d 2L tf/,er+jc* Area of Smaller Ptane Q/stence Bushes '\t/jnctino J, Hooks & 4 tol Gatio- G-earCasi"3 re Sw.G Front ElevBtion Stops with the main plane fixed in position. The machine should be poised by suspending it by a fine line until it balances in a longitudinal direction ; this gives the position of the centre of gravity which should be marked on the fuselage. Now, knowing the area of the main plane and the position of the centre of gravity, it is really only a simple proportion sum to determine the area of the small surface (an aj proximate weight for which should be fixed to the fuselage when finding the position of the centre of gravity). The formula for calculating the area of the smaller surface is :— Ad 2 L Where x = area of surface to be found d = distance of centre of gravity of machine to the centre of pressure of the main plane L = distance of centre of gravity of machine to the centre of pressure of the elevator. The centre of pressure of the main plane can safely be assumed (with cambered surfaces) to lie at a point two-fifths of the chord from the leading edge, and a line drawn trans versely through this point is the line on which the pressure affects the plane. The coefficient 2 in the formula is a constant to allow for the positive angle made by the elevator, since a canard machine is being considered. The angle of incidence of the elevator should be nearly twice that of the main plane. To work an example, suppose that A measures 300 sq. ins., L = 40 ins., and d — 5 ins., 300 X S . then x will equal \ v ^ =18-6 sq. ins. which many do not seem to be cognisant of. I refer to the calculation of the flotational capacity needed to support a given weight. The machine exclusive of floats should be cos foi/ Z X 40 weighed and an ounce or'so allowed for the weight of them- Suppose the model weighs 8 oz. x 3 oz. for floats =11 oz., the capacity of the floats are calculated in the following manner. A cubic foot of water — 1,728 cubic ins., weighs 1668
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