và trung tâm của hệ số áp lực = - (CM / CL) như trước đây. Một loạt các kết quả các bài kiểm tra trên siêu âm aerofoil phần được xuất bản bởi A. Ferri * phục vụ để so sánh với lý thuyết. Các thiết lập được chọn ở đây là cho một phần đối xứng aerofoil bi-lồi của t / c = 0,1 đặt trong một luồng không khí của Mach số 2,13. Tỷ lệ này đã được thay đổi từ -10 "đến 28" và cũng có thể vẽ trên đồ thị của hình. 6,48 là. | 358 Aerodynamics for Engineering Students Fig. SD 2 _lpF2 2 4 óx 6CD -Pv2c and integrating gives after substituting for u and L Cp 4 a2 ặ Jm2 -1L 3 J Now by geometry and since Ê0 is small e0 2 t c giving _ 3 4 r 2 . 4 fM2 - 1 L 3 cJ J The lift drag ratio is a maximum when by division DỊL a j t c 21 a is a minimum and this occurs when 3 c a Then 1 _ a _ 1 _ 73 c _ D max a2 a2 la 4 Í t c For a 10 thick section L Dimsa 4 at a Moment coefficient and kcp Directly from previous work . taking the moment of 6L about the leading edge M CM pV2c2 - Jo c 4a yjM2 -1 -2a y M2- Compressible flow 359 Fig. and the centre of pressure coefficient - CmICl as before. A series of results of tests on supersonic aerofoil sections published by A. Ferri serve to compare with the theory. The set chosen here is for a symmetrical bi-convex aerofoil section of t c set in an aừ flow of Mach number . The incidence was varied from 10 to 28 and also plotted on the graphs of Fig. are the theoretical values of Eqns and . Examination of Fig. shows the close approximation of the theoretical values to the experimental results. The lift coefficient varies linearly with incidence but at some slightly smaller value than that predicted. No significant reduction in Cl as is common at high incidences in low-speed tests was found even with incidence 20 . The measured drag values are all slightly higher than predicted which is understandable since the theory accounts for wave drag only. The difference between the two may be attributed to skin-friction drag or more generally to the presence of viscosity and the behaviour of the boundary layer. It is unwise however to expect the excellent agreement of these particular results to extend to more general aerofoil sections - or indeed to other Mach numbers for the same section as severe limitations on the use of the theory appear at extreme Mach numbers. Nevertheless these and other published data amply .
