The Art of the Helicopter John Watkinson - Karatunov.net

3
Introduction to helicopter
dynamics
3.1 Creating and controlling lift
The basic principle of wings, rotors and airscrews is that they accelerate a mass of
air and that the resultant lift or thrust is the Newtonian reaction to that acceleration.
Producing sufficient lift to permit a helicopter to fly is only a matter of having enough
power to accelerate the air mass without excessive weight. A practical helicopter must,
however, be able to control that lift precisely or it would be dangerous.
Any structure placed in a flow of air for the purpose of generating lift is called an
aerofoil or airfoil. Aeroplanes and helicopters can fly when there is a wind blowing and
can climb or lose height in the process. In helicopters the rotor blades have airspeed
due to their rotation. Thus there can be situations where the airfoil moves through the
air, or where the air moves past the airfoil. All that matters from the point of view
of generating lift is the relative velocity and the direction from which it appears to
be approaching the airfoil. Figure 3.1(a) shows that this is known as a relative airflow
(RAF). RAF is a vector quantity as it has speed and direction. Figure 3.1 further shows
that a flat plate will produce lift if it is slightly inclined to the RAFat an angle of attack.
When the airfoil changes the direction of air flowing by, this represents a change of
velocity and so is classed as acceleration. The reaction to that acceleration will point
in the opposite direction.
Figure 3.1(b) shows how the direction of the reaction is found. For the time being,
the air is assumed to be inviscid (having no viscosity). The relative airspeed does not
change, only the direction, so the velocity vector V is changed to V ′ . The acceleration
to change V to V ′ must have been in the direction of the vector Va so the reaction must
be in the opposite direction. It can be seen that this is at right angles to the average
airflow direction. The blade reaction is the only actual force present. In fixed-wing
aircraft studies, the reaction is traditionally resolved into two components: that which
is measured at right angles to the RAFand called the lift, and that which is measured
along the direction of the RAFand called the induced drag. In cruise lift will be vertical
and drag will be horizontal.
Figure 3.1(c) shows that the lift can be increased by increasing the angle of attack
since this has the effect of increasing the change of velocity. In the aeroplane, the
angle of attack is controlled by alteration of the attitude of the entire machine with the
elevators. In a helicopter the blades are mounted on bearings in the rotor head that