The Art of the Helicopter John Watkinson - Karatunov.net

62 The Art of the Helicopter
Fig. 3.1 (a) Lift is the reaction to the acceleration of air into a new direction. (b) The direction of acceleration
of the air is derived as shown. The reaction must be in the opposite direction. (c) An increase in the angle of
attack increases the lift. (d) In the helicopter, lift is controlled through twisting the blade about a span-wise
axis;a mechanism called feathering.
allows them to turn about a radial axis as can be seen in Figure 3.1(d). This mechanism
is known as feathering.
When the relative velocity alone is doubled with respect to the case in (b), the vector
Va representing the change ofvelocity is doubled. As the air is moving twice as fast, the
wing accelerates the air in halfthe previous time. Achieving double the velocity in half
the time means that the reaction is increased by a factor of four. In other words, the
reaction is proportional to the square ofthe velocity. In practice four times as much lift
would be quite unnecessary to balance the weight ofthe machine and instead, as the
relative velocity doubled, the angle ofattack would be reduced to about one-quarter
of the previous value. This is the mechanism by which aircraft obtain the same lift over
a range ofairspeeds.