The Art of the Helicopter John Watkinson - Karatunov.net

The tail rotor designer is faced with permanent compromise. Chapter 3 showed that
the least power is needed if a rotor has a large diameter and a low, uniform, induced
velocity. Profile power is reduced if the tip speed is low. Unfortunately the solidity will
have to go up and the resulting tail rotor will be very heavy. To make matters worse,
the large tail rotor will have to be mounted further aft to maintain clearance with the
main rotor and this will require a longer and heavier tail boom. Ground clearance may
also be an issue. There are further problems with large, slow, tail rotors. With a very
low tip speed the translational speed will be limited by the growth of the reverse flow
area. In the hover if the induced velocity is too low the tail rotor can enter a vortex ring
condition during a yaw.
In fact minimizing tail rotor power is not the goal. A more useful goal is to minimize
the ratio of the total power absorbed by both rotors to the overall machinery weight.
A highly efficient but heavy tail rotor may need more total power as the main rotor
has to work harder to lift it. As a result the designer will settle for a tail rotor diameter
that contains the weight and clearance problems, and a tip speed similar to that of the
main rotor so that the same advance ratio is obtained. As a result the only room for
variation is in the solidity and taper. As a design evolves, the solidity can be adapted
by changing the number of blades. For example, the Super Puma has four tail blades,
whereas the original Puma had five. In some helicopters the high tip speed tail rotor
was replaced in a later model with a slower version to reduce noise. This may need
more blades to increase the solidity.
5.4 Tail rotor location
The tail rotor and the main rotor are both actuator discs and both produce thrust by
virtue of the induced velocity they impart to their respective inflows. The main and tail
rotors cannot be considered independently because of their proximity. They can and
do affect each other by interaction of inflow to a degree that varies considerably from
one regime of flight to another.
In the hover a low mounted tail rotor will actually draw air in from the edge of the
main rotor. This will result in an increase in the induced velocity experienced by the
main rotor and it will need more power. This might amount to 10–20 kW in a mediumsized
helicopter. If the tail rotor is mounted higher the effect is largely eliminated and
some power can be saved.
Figure 5.4(a) shows that the increased pressure below the main rotor disc and the
reduced pressure above causes air to flow in a toroidal path around the edge of the disc.
This is the mechanism of tip loss. The tail rotor operates in this region and there is a
strong interaction. When the tail rotor turns in the same direction as the main rotor
vortices (b) the relative airspeed of the tail blades is reduced and the available thrust
is limited. When the tail rotor turns against the main rotor vortex (c) the performance
is considerably enhanced because of the square-law connection between thrust and
speed. There appears to be no detrimental aerodynamic effect of this direction, and so
it is now considered to be the only direction to employ.
This phenomenon was understood relatively late in the history of the helicopter with
the result that many machines were designed with the tail rotor going the ‘wrong’ way.
In many cases in later models the tail rotor direction was reversed to universal acclaim.
Oddly enough many model helicopters are still designed (if that is the word) with the
wrong tail rotor direction. The author has modified a number of models to have the
‘right’ rotation and can vouch for the improvement in these cases also.
The tail 173