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.