If pressure drop at top wing side,density of air must also decrease because density depend on pressure and temperature. Same apply on the bottom side where density must increase.
So why density effetcs caluculate only from Mach 0.3 above?
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A flow can be considered incompressible if the variation in density is small. As you said, density is proportional to pressure and the inverse of temperature. The static pressure at sea level in the standard atmosphere is 101300 Pa. The dynamic pressure at Mach 0.3 (roughly 100 m/s) is
$p_{dyn} = \frac{\rho} {2} V^2 = \frac{1225\;\textrm{kg/m}^3}{2}(100\;\textrm{ m/s})^2 = 6125\;\textrm{Pa}$
So the highest total pressure you can get would be only 6 % higher than the ambient pressure, and the lowest one depends on the shape of the airfoil, but is typically of the same magnitude.
For most applications, this variation of ~5 % is small enough to be negligible.
