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Aviation History
1927
1927 - 0290.PDF
30 SUPPLEMENT TO FLIGHT APRIL 28, 1927 THE ATRCRAFT ENGINEER THE MARTIN P.M.3 : Photograph of the wind tunnel model. this type of section holds out promise of good L/D combined with a reasonably high maximum lift coefficient. Mr. Martin has designed several machines with wings of this section, and the one under discussion here shows a bi-convex aerofoil section with very sharp " nose." The monoplane type of wing haB been chosen, and as already mentioned, two fuse- lages, each carrying an engine and accommodating passengers and/or goods, are another feature of the design. THE MARTIN P.M.3: Curves of lift, drag, etc., obtained in the Gottingen wind tunnel. The maximum L/D is 18. 5. 2626 The first step in the evolution of the Martin P.M.3 was the testing of a model in the Gottingen wind tunnel. A photograph of this model is given herewith. We have by us a photostat copy of the original Gottingen curves, on which it is stated that the wing area of the model was 0-2385 sq. m. (2-57 sq. ft.), and the lateral axis was through the foremost point of the wing plane. As indicated on the chart giving the wind tunnel results, published herewith, the model was tested with and without under- carriages, and with tail- settings of 0, —10, and -r-10 degrees. The maxi- mum L/D obtained was 17. With corrections for scale this is, Mr. Martin states, increased to 18, a verv good result indeed. Referring to the wind tunnel chart, it will, of course, be realised that this is plotted in the usual German style, with lift coefficient * Ga, plotted against drag coefficient (V. and moment coefficient Cm. The German units being different from ours, and incidentally C = 100 €„,), the co- efficients shown on the chart may be converted to British " absolute " coefficients by dividing them by 200. so that the maximiim lift coefficient of the Martin P.M.3 model is approximately 0 -5, not a very high value, while the minimum drag co- efficient of the model of the whole machine is 0 -01. in " absolute " units. Compared with the general run of commercial aircraft, a maximum L/D of 18 is, of course, extra- ordinarily high, and if the large machine contem- plated can be made to
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