Maximum lift to drag ratio decreases as the Mach number increases. How to mitigate the effect of decrease in maximum lift to drag ratio when there is increase in Mach number.
How to mitigate the effect of decrease in maximum lift to drag ratio when there is increase in Mach number.
Maximum lift to drag ratio is given as
\[\begin{array}{l}{(\frac{{{c_l}}}{{{c_d}}})_{\max }} = {({c_f})^{\frac{{ – 1}}{2}}}{({M^2} – 1)^{\frac{{ – 1}}{4}}}\\ = \frac{1}{{\sqrt {{c_f}} {{({M^2} – 1)}^{\frac{{ – 1}}{4}}}}}\end{array}\]
From the above equation we can conclude that maximum lift to drag decreases as Mach number increases. As the designers always want a higher aerodynamic efficient airplanes means a higher (L/D) ratio, at higher Mach number this can lead to decrease in aerodynamic efficiency.
We have a total drag coefficients for the supersonic airfoil as sum of skin friction drag, cf and wave drag cd. The skin friction accounts for a larger cause of drag than the wave drag. Skin friction greatly diminishes lift to drag ratio of the airfoil, so the airplane designers try to reduce the skin friction drag,by encouraging laminar rather than turbulent boundary layer on the airfoil.
