CENTRE OF PRESSURE (CP)
The sum of all aerodynamic forces on an aerofoil may be represented by a single vector acting at a particular point on the chord line, called the centre of pressure. This vector is referred to as the Total Reaction. The Total Reaction is tilted back with respect to RAF. The TR may be resolved into two components one perpendicular and one parallel to RAF. The perpendicular component is called Lift and the Parallel component is called Drag. It can also be seen from distribution of pressure over aerofoil figure that as the angle of attack is increased, the height and the size of the upper surface suction peak increases and it moves further forward. This indicates that the total reaction increases in magnitude and the centre of pressure (through which the TR seems to act) moves forward towards the leading edge with increasing angle of attack up to the point of stall. Beyond stalling angle of attack, the magnitude of the vector reduces rapidly and it simultaneously moves backwards towards the trailing edge
Movement of Centre of Pressure.
The location of the CP is a function of camber and section lift co-efficient, both the resultant force and its position varying with angle of attack (Fig below). As the angle of attack is increased the magnitude of the force increases and the CP moves forward. When the stall is reached the force decreases abruptly and the CP generally moves back along the chord. With a cambered aerofoil, the CP movement over the normal working range of angles of attack is between 20% – 30% of the chord aft of the leading edge. With a symmetrical aerofoil, there is virtually no CP movement over the working range of angles of attack at subsonic speeds
CP Movement on a Flat Plate. On a flat plate the CP lies well behind the leading edge and any increase in angle of attack tends to move the CP further back. This creates a nose-down pitching tendency with increasing angle of attack on a flat plate. Thus, CP movement on a flat plate in subsonic flow is stable.
On a cambered aerofoil, on the other hand, the CP moves forward with increasing AOA, further accentuating the ‘nose up’ pitching tendency. Thus, CP movement on an aerofoil (up to stalling AOA) is unstable.
Reduction in CP Movement. As movement of CP depends upon camber, any reduction in camber will reduce movement of CP. Two common methods of reducing camber and thereby reducing CP movement are:
- By providing convexity to the under surface of the aerofoil.
- By reflexing the trailing edge of the aerofoil upwards. This, however has the disadvantage of reducing lift and increasing drag.