Importance of Coefficients (Lift and Drag)

Aerodynamic coefficients plays an important role in performance analysis as well as  design of aeroplanes.Coefficient of lift is defined as Lift divided by dynamic pressure, $L/{q_\infty }S$ where as drag coefficient is defined as Drag divided by dynamic pressure, $D/{q_\infty }S$ .

Coefficient of lift,maximum or ${C_{L,\max }}$ is the determining factor for stalling velocity of aircraft.The higher is the ${C_{L,\max }}$ the lower is the stalling velocity. $${V_{stall}} = \sqrt {\frac{{2W}}{{{\rho _\infty }S{C_{L,\max }}}}}$$ However,${C_{L,\max }}$ can be increased by the use of mechanical devices like high-lift devices.High lift devices include flaps,slats and slots on the wing.Air plane flying at given altitude with maximum thrust ${T_{\max }}$, the maximum value of ${V_\infty }$ ,corresponds to flight at ${C_{D,\min }}$. $${V_{\max }} = \sqrt {\frac{{2{T_{\max }}}}{{{\rho _\infty }S{C_{D,\min }}}}}$$ The actual value of ${C_L}$ and ${C_D}$ for a given body shape just depend on the orientation of body in the flow,that is angle of attack.

Therefore,aerodynamic coefficients are important engineering quantities for determining the stalling velocity by ${C_{L,\max }}$ and maximum velocity by ${C_{D,\min }}$ .

Lift to drag ratio is the measure of aerodynamic efficiency of aircraft .It is given as  $$\frac{L}{D} = \frac{{{q_\infty }S{C_L}}}{{{q_\infty }S{C_D}}} = \frac{{{C_L}}}{{{C_D}}}$$ The importance of aerodynamic efficiency $L/D$ is that at a given velocity ,$L/D$ should be as high as possible,the higher $L/D$ is more efficient is the body.