pressure patternsDistribution of pressure over an
airfoil section may be a source of an aerodynamic twisting force as well as
lift. A typical example is illustrated by the pressure distribution pattern
developed by this cambered (nonsymmetrical) airfoil:
The upper surface has pressures
distributed which produce the upper surface lift. The lower surface has
pressures distributed which produce the lower surface force. Net lift produced
by the airfoil is the difference between lift on the upper surface and the force
on the lower surface. Net lift is effectively concentrated at a point on the
chord called the centre of pressure
When angle of attack is
increased:
upper surface lift increases
relative to the lower surface force. Since the two vectors are not located at
the same point along the chord line, a twisting force is exerted about the
centre of pressure. Centre of pressure also moves along the chord line when
angle of attack changes, because the two vectors are separated. This
characteristic of nonsymmetrical airfoils results in undesirable control forces
that must be compensated for if the airfoil is used in rotary wing applications.
Pressure patterns for
symmetrical airfoils are distributed differently than for nonsymmetrical
airfoils:
Upper surface lift and lower
surface lift vectors are opposite each other instead of being separated along
the chord line as in the cambered airfoil.
When the angle of attack is
increased to develop positive lift, the vectors remain essentially opposite each
other and the twisting force is not exerted. Centre of pressure remains
relatively constant even when angle of attack is changed. This is a desirable
characteristic for a rotor blade, because it changes angle of attack constantly
during each revolution.
|