Expert: Paul Soderman - 10/8/2011

Question
Sir,
Profile drag contains form drag and skin friction drag. Profile drag increases with velocity ( as seen in Drag polar),which part of profile drag increases with velocity, form or skin friction?

On contrary,  Profile drag remains constant with M ( mach number) for subsonic range Why is this so?

Answer
Sangeeth
In a plot of drag versus airspeed, the initial part of the curve will descend to a minimum as drag due to lift decreases and then will increase as profile drag increases (see http://en.wikipedia.org/wiki/Drag_%28physics%29).  Profile drag contains friction plus pressure drag, both of which increase with velocity. In fact, S. Hoerner (Fluid-Dynamic Drag) shows that pressure drag coefficient Cdpr is proportional to friction drag coefficient Cf for typical airfoil section with maximum thickness at 30% chord:  Cdpr/(2Cf) = 60(t/c)^4 where t/c is the thickness ratio of the airfoil.  The total profile drag coefficient is in Hoerner's notation:  Cds/(2Cf) = 1 + 2t/c + 60(t/c)^4.  Total profile drag is simply profile drag coefficient times dynamic pressure times reference area.  It is important to note that drag coefficient decreases with Reynolds number as the flow transitions from laminar to turbulent flow.

Drag increases with Mach number in the same way as it does with airspeed.  At some point compressibility effects become important and another drag term develops.  I am not sure what drag polar versus Mach number or portion thereof you are seeing.
Paul