The Art of the Helicopter John Watkinson - Karatunov.net

Introduction to helicopter dynamics 93
Fig. 3.25 How the cyclic stick might be trimmed at different airspeeds. Inflow roll begins at low airspeed but
has less effect as speed rises, whereas coning roll increases with speed as the coning angle may increase to
produce more thrust.
with airspeed. Figure 3.25 shows how the lateral cyclic control might have to be moved
to the retreating side in order to fly straight at different airspeeds.
Thus in straight flight at constant speed, the cyclic stick must be held slightly towards
the retreating side to counteract the inflow and coning induced rolls as well as forwards
to counter the advancing/retreating induced pitch-up. In some machines the phasing of
the controls is arranged so that forward cyclic automatically creates a degree of lateral
cyclic. However, in the hover this results in the machine not responding precisely in the
direction the stick is pushed.
Like the collective control, there is no one correct setting for the cyclic control. The
cyclic trim control is used to shift the neutral position of the stick, relieving the pilot
of the need to produce continuous control forces.
3.22 Rotor H-force
The retreating blade has the airspeed subtracted from the rotational speed and so has
to operate with an increased angle of attack. The profile drag will be reduced but
the induced drag will increase. Conversely the advancing blade will suffer significantly
more profile drag because of the high relative airspeed, but the induced drag will fall
because the angle of attack has been reduced. The overall effect of profile drag and
induced drag does not balance between the advancing and retreating sides and the
resultant is a rearward acting force called the H-force. Figure 3.26 shows that the
H-force is the reason why the rotor thrust is not precisely at right angles to the tip path
plane. A typical figure is 1 ◦ .
Because of inflow and coning, the blades crossing the nose produce more lift than
the blades crossing the tail and so the induced drag is not the same. The resultant is the
Y-force which acts at right angles to the direction of flight. The H-force is small and
the Y-force is very small with the result that the effect on the magnitude of the thrust
is insignificant.
3.23 Blade stall and compressibility
Stall occurs when air passing over the cambered upper surface of a blade can no longer
accelerate rapidly enough to follow the surface. Airflow breaks away and lift is lost