The Art of the Helicopter John Watkinson - Karatunov.net

Introduction to helicopter dynamics 87
rolling couple. As was seen in Chapter 2, the rotor is gyroscopic and so the cyclic pitch
change is arranged to occur 90 ◦ ahead of the required result to allow for precession in
the rotor.
Figure 3.20(b) shows that the application of cyclic control tilts the thrust vector,
and that there will be a resultant of the thrust vector and gravity which is primarily
horizontal but also has a slightly downward component. This is because the vertical
component of the rotor thrust becomes smaller when the disc is tilted. As a result the
helicopter will accelerate sideways and lose height. To compensate, the pilot slightly
raises the collective lever so that the vertical component of the thrust remains the same.
3.19 Basic manoeuvres
From the hover, the helicopter can be accelerated into translational flight by using
cyclic pitch to tilt the disc. The greater the acceleration required, the more extreme will
be the disc tilt and the greater the application of collective pitch. Figure 3.21(a) shows
a helicopter at maximum horizontal acceleration. If done at low height, the blades can
get very near the ground. Figure 3.21(b) shows that at constant speed and height the
thrust necessary to balance the drag of the pure helicopter comes from the horizontal
component of the inclined rotor thrust. The vertical component of the rotor thrust must
balance the weight of the machine. Clearly the rotor thrust must be greater than in the
hovering condition. The diagram simplifies the situation in that it assumes the drag of
the fuselage acts at the rotor hub, which is not actually the case. In practice the drag
acts below the hub to produce a couple which would depress the nose. There are two
ways of dealing with that: using a rotor head which can apply an opposing couple from
the blades and/or using a tail plane to produce a down thrust.
Note that the tip path plane may be inclined forwards with respect to the hull so that
the balance of forces is obtained with the hull level. In this condition the drag will be
minimized and the occupants will be most comfortable. A consequence is that when
the machine comes to the hover it will sit with a tail down attitude. The length of the
undercarriage legs or the angle of the skids is often arranged such that the machine can
settle on the ground very nearly in this attitude.
Tilting the disc accelerates the machine along, but bringing the disc back to the
horizontal only removes the acceleration, and the machine will continue along, slowing
only because of drag. In order to stop, the disc must be tilted the opposite way to
obtain acceleration in the opposite sense to reduce the velocity. Figure 3.21(c) shows
a helicopter with maximum deceleration in a manoeuvre known as a quickstop. Note
that the inflow has reversed so that a quickstop is actually a form of autorotation. Note
that the tail boom is very low, and could strike the ground.
Figure 3.21(d) shows a turn at speed. The turn changes the direction but not the
speed, so it is a change of velocity or acceleration. The acceleration must be towards
the centre of the turn, and it is necessary to obtain a resultant sideways force to cause
that acceleration. This is done by banking with lateral movement of the cyclic stick.
Collective will need to be increased to maintain the vertical component of thrust equal
to the weight. The machine will then fly around as if on the surface of a cone until
the desired heading is reached, when the bank will be taken off and the collective
reduced again. Note that in a 60 ◦ bank the thrust vector has to be twice that needed
to hover in order to maintain height. The ratio of manoeuvre thrust to hover thrust
is called the load factor. Not all machines can deliver that much power, and not all
transmissions can accept it. However, some large transport helicopters can carry their