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Compressibility effects on drag can be calculated through the flight Mach number. For flights that are not close to being supersonic the drag polar for the aircraft can be written as
Also, the dynamic pressure can be expressed as
The variation of the profile drag with the free stream Mach number is shown on the left.
M = 1 is the sound barrier where there is an enormous rise in drag. Note that drag drops in supersonic flow
Mcr is the critical Mach number explained in the section on the lift effect. However, note the critical Mach number does not effect an increase in drag
Mdiv is the divergence Mach number - that is where the drag coefficient starts to increase and must be accounted for in the calculations
Usually, the divergence Mach number is located above the critical Mach number. It is where the general aviation aircraft will fly to avoid the increase in fuel consumption.. Note that the engines will have to overcome this drag in level flight
The effect on skin friction drag coefficient is shown here. Unlike the figure on top, the scale on the bottom includes up to hypersonic flow. The plot is the ratio of the local skin friction drag coefficient at the flight Mach number to the local skin friction drag coefficient for incompressible flow
Laminar flow does not see much change
Turbulent flow sees a lot of decrease in the coefficient. It is difficult to identify the reason why without experimental illustration.
Swept back Wings
Planes flying at high speed will have wings swept back.
This decreases the effective Mach number on the wing (the Mach number perpendicular to the swept line)
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