User Guide

3: The Angle at Which the Wing Meets the Airflow
This is generally known as the Angle of Attack and up to a point which varies with the wing
design, the greater the angle of attack the more lift the wing generates (and the more power
is required to drive it through the air at a given speed). All of the helicopter’s main flying
controls work by changing the pitch angle of the main or tail rotor blades.
GROUND SCHOOL
3
Diagram 6.2: Rotor pitch angle/angle of attack in still air
If the rotor were operating in still air, pitch angle and angle of attack would be identical, but
this situation exists only in the first few seconds as the rotor spins up [diagram 6.2]. Once
the rotor is spinning it sets up a constant air current (the rotor downwash) through the rotor
disc. This means that the effective angle of attack is less than the blade pitch angle –
though not much less because the rotor’s speed is generally much higher than the speed
of the air current down through the disc [diagram 6.3].
If there is an air current across the disc (as there is when you are hovering in a wind or
moving over the ground at any significant speed) this also changes the effective angle of
attack (and airspeed) of the rotor blades [diagram 6.4]. Blades advancing into the wind
have a higher angle of attack (and higher airspeed), and generate more lift than the
retreating blades. At the same time, the effect of the downwash air current is reduced
because you are constantly moving into undisturbed air.
The net result is that the rotor generates more lift altogether (‘Translational Lift’), and more
lift on the advancing than on the retreating side of the disc, so there is a slight tendency to
roll (a ‘rolling moment’) around the wind axis – an imaginary line through the center of the
helicopter drawn in the direction of the airflow [diagram 6.5].