Khi máy bay tăng tốc để tiếp cận siêu âm tốc độ các phương trình của chuyển động mô tả sự thay đổi dòng chảy trong nhân vật. Để dự đoán hành vi của phần cánh tại các khu vực trên, cận âm và siêu âm, các phương trình dòng chảy nén được yêu cầu. cơ bản định nghĩa | 102 Aeronautical Engineer s Data Book the principle of moments the following expression can be derived for kCP XAC kCP - c CMac CL cos a CD sin a Assuming that cos a 1 and CD sin a 0 gives 1 -v kCP c _ Cmac Cl Supersonic conditions As an aircraft is accelerated to approach supersonic speed the equations of motion which describe the flow change in character. In order to predict the behaviour of airfoil sections in upper subsonic and supersonic regions compressible flow equations are required. Basic definitions M Mach number M Free stream Mach number Mc Critical Mach number . the value of which results in flow of M 1 at some location on the airfoil surface. Figure shows approximate forms of the pressure distribution on a two-dimensional airfoil around the critical region. Owing to the complex non-linear form of the equations of motion which describe high speed flow two popular simplifications are used the small perturbation approximation and the so-called exact approximation. Supersonic effects on drag In the supersonic region induced drag due to lift increases in relation to the parameter Vm2 - 1 function of the plan form geometry of the wing. Supersonic effects on aerodynamic centre Figure shows the location of wing aerodynamic centre for several values of tip chord root chord ratio y . These are empirically based results which can be used as a rule of thumb . Basic aerodynamics 103 M1 local x c Fig. Variation of pressure deterioration 2-D airfoil Wing loading semi-ellipse assumption The simplest general loading condition assumption for symmetric flight is that of the semiellipse. The equivalent equations for lift downwash and induced drag become For lift VK0ns L P V replacing L by CL 1 2pV 2S gives K0 ClVS ns 104 Aeronautical Engineer s Data Book p tanALE tanALE p Fig. Wing aerodynamic centre location subsonic supersonic flight. Originally published in The AIAA Aerospace Engineers Design Guide 4th Edition. Copyright .