Basic Flight Aerodynamics ( PART -2)

Pressure over an Airfoil : Pressure over an airfoil usually expressed in terms of the pressure co-efficient. Various parts of the pressure distribution are expressed in the diagram.

overall pressure on airfoil

• Upper Surface:  The upper surface pressure is lower (plotted higher on the usual scale) than the lower surface Cp in this case. But it doesn't have to be.
•  Lower Surface: The lower surface sometimes carries a positive pressure, but at many design conditions is actually pulling the wing downward. In this case, some suction (negative Cp > downward force on lower surface) is present near the mid-chord.
• Pressure Recovery :  This region of the pressure distribution is called the pressure recovery region;The pressure increases from its minimum value to the value at the trailing edge;This area is also known as the region of adverse pressure gradient. As discussed in other sections, the adverse pressure gradient is associated with boundary layer transition and possibly separation, if the gradient is too severe. 
• Trailing Edge Pressure: The pressure at the trailing edge is related to the airfoil thickness and shape near the trailing edge. For thick airfoils the pressure here is slightly positive (the velocity is a bit less than the free-stream velocity). For infinitely thin sections C_p = 0 at the trailing edge. Large positive values of C_p at the trailing edge imply more severe adverse pressure gradients. 
• C_L and C_{p :}The section lift coefficient is related to the C_p by: C_l = int (C_pl - C_pu) dx/c
  (It is the area between the curves.)
•  Stagnation Point : The stagnation point occurs near the leading edge. It is the place at which V = 0. In compressible flow it may be somewhat larger.