The Art of the Helicopter John Watkinson - Karatunov.net

70 The Art of the Helicopter
reduced by the vertical velocity. This tends to reduce the angle of attack and hence the
rotor thrust. As a result the vertical velocity reaches a value where the rotor thrust once
more balances the weight and the velocity then remains constant.
Figure 3.7(b) shows that if the collective control is lowered, the machine accelerates
downwards, but the change in relative airflow is such that the rotor thrust is increased.
The vertical velocity again stabilizes at a value where the thrust balances the weight.
Thus the effect of the collective control in the hover is primarily to control the vertical
velocity.
In most powered flight conditions, but particularly when hovering, the rotor is working
in the descending airflow it has itself caused. This self-inflicted problem is called
inflow. When inflow is present, there is a vertical component of airflow vectorially to
be added to the local blade velocity in order to find the direction of the RAF.
The pilot controls the pitch angle using the collective lever and this, along with the
velocity of inflow through the rotor, determines the angle of attack. As a result there is
no one correct position for the collective pitch lever and the pilot will need constantly to
adapt the collective pitch setting for changes in speed, fuel burned and altitude. As there
is no correct position for the collective lever, there is no spring centring. Instead there
may be a friction mechanism that can be adjusted to hold the lever at the last position
without unduly adding to the force needed to move it. This allows the pilot to release
the lever for short periods in order to operate secondary controls.
3.6 Forces on the blades
In a helicopter having a hingeless rotor, the displacement between the area where lift is
generated and the hub where the load is applied causes upward bending stresses that
increase dramatically towards the rotor head. The underside of the blade is placed in
tension, and the upper surface is placed in compression. The rotation of the blades
produces a tension along their length that also increases towards the rotor head. The
stress caused by this tension is of the same order as the stresses due to bending, and the
stress on the underside of the blade is roughly doubled, although the compression on
the upper surface is relieved. The drag of the blades causes a rearward bending stress
that is constant in the hover.
There is a tendency for the blade rotation to force the blade to twist to a pitch angle of
zero. Figure 3.8(a) shows that when the blade is set to zero pitch, the leading and trailing
edges of the blade are further from the feathering axis than when pitch is applied. In
order to set the blades to a working pitch, some of the mass of the blade must be
brought closer to the shaft axis, and work will need to be done against the rotational
forces that will be reflected as a reaction seen by the control system.
The effect can be reduced by the use of so-called Chinese weights that are placed on a
rod at right angles to the chord line as shown in Figure 3.8(b). These will tend to move
away from the rotor shaft axis as a result of rotation. The term comes from the tales
told by sailors to the gullible alleging the orthogonality of certain parts of the anatomy
of the oriental female at a time long before the development of either the helicopter
or political correctness.
A less colourful alternative is to incorporate a spring into the collective linkage that is
adjusted to supply a steady force to hold the blades at a typical positive pitch, relieving
the pilot of the need to apply a constant force to the collective lever. When the blades are
not turning this spring may be powerful enough to force the blades to maximum pitch
and so the collective lever is fitted with a lock that the pilot can apply prior to shutting