During hovering, airflow over
the rotor blades is produced by rotation of the rotor system. Here is a picture
showing a typical helicopter rotor system:
Blade speed near the main rotor
shaft is much less because the distance traveled at the smaller radius is
relatively small. At point "A", half way from the rotor shaft to the blade tip,
the blade speed is only TBS knots which is one-half the tip speed. Speed at any
point on the blades varies with the radius or distance from the center of the
main rotor shaft. An extreme airspeed differential between the blade tip and
root is the result. The lift differential between the blade root and tip is even
larger because lift varies as the square of the speed. Therefore, when speed is
doubled, lift is increased four times. This means that the lift at point "A"
would be only one-fourth as much as lift at the blade tip (assuming the airfoil
shape and angle of attack are the same at both points).
Because of the potential lift
differential along the blade resulting primarily from speed variation, blades
are designed with a twist. Blade twist provides a higher pitch angle at the root
where speed is low and lower pitch angles nearer the tip where speed is higher.
This design helps distribute the lift more evenly along the blade. It increases
both the induced air velocity and the blade loading near the inboard section of
This picture compares the lift
of a twisted and untwisted blade:
Note that the twisted blade
generates more lift near the root and less lift at the tip than the untwisted