The Art of the Helicopter John Watkinson - Karatunov.net

78 The Art of the Helicopter
will be found in axial blowers where the root chord may be several times the tip chord.
Figure 3.12(b) shows a better lift distribution resulting from a combination of twist
and taper. Blade taper has the further advantage that the various vibration modes of
the blade will not be harmonically related. This will be considered in section 3.26.
In practical helicopters the blades cannot extend all the way to the mast because
of the need to provide a rotor head mechanism. A further consideration is that, in
the hover, the extreme roots of the blades simply provide a downwash onto the hull
resulting in a download, negating some of the rotor lift. Section 3.20 will show that,
in forward flight, the blade root of the retreating blade encounters reverse airflow. As
a result blades often have a significant root cut. Ideal twist for hovering will not be
ideal for forward flight or autorotation and a compromise is invariably necessary. In
the proprotors of a tilt-rotor helicopter more twist will be possible because the rotors
work with predominantly axial flow.
3.14 Swirl
In a real rotor, the downwash does not just move downwards, but it also has a rotational
component of motion known as swirl. Intuitively it is clear that the turning rotor must
turn the air with it to some extent. Figure 3.10 showed that the reaction on a lifting blade
is somewhat aft of the rotor shaft axis. It must follow that the direction in which the
momentum of the inflow increases is in the opposite direction. The vertical component
is the inflow velocity, which produces lift, whereas the horizontal component creates
swirl which represents wasted power.
Some of the swirl is due to profile drag and some is due to induced drag. In the absence
of profile drag, the blade reaction is still slightly behind the vertical and this must result
in a small loss when compared with an ideal actuator that only causes vertical inflow.
The main source of swirl is the result of profile drag adding to the rotational momentum
of the air. In an ideal actuator the air leaves with increased vertical momentum only. In
a real rotor the amount of swirl gives an indication of the figure of merit. The higher
this is, the lower the swirl will be. In a twisted blade, swirl will be greatest near the root
where the pitch angle is greater.
The torque delivered to the rotor shaft can be divided into two parts, first the torque
needed to drive an ideal rotor which only produces an increase in vertical momentum
sufficient to create the required thrust, and second an additional torque which is the
reaction to creating swirl and tip vortices. With a single rotor, swirl energy is lost forever,
but in contra-rotating rotors, the swirl of the second rotor can cancel the swirl of the
first. Unfortunately this does not reduce profile drag, it only reduces induced drag.
In the single rotor helicopter, the main effect of swirl is that the downwash on the hull
is not vertical, which adds to the general air of asymmetry surrounding the helicopter.
The effects will be considered in Chapter 5.
3.15 Vertical autorotation
The direction of the reaction on an airfoil depends upon the angle of attack. When
hovering, inflow causes the resultant to be tilted back and opposes the engine thrust.
If, at a suitable height the collective pitch is lowered, the machine starts to fall, the
inflow reverses and the relative airflow has an upward component. Figure 3.13(a) shows
that the principle of the rotor blade in autorotation is no different to powered flight.
It continues to accelerate air into a new direction. However, as the air approaches