The Art of the Helicopter John Watkinson - Karatunov.net

eing more visible to ground personnel than the spinning rotor. The Enstrom F-28
series has a conspicuous D-ring.
The fast spinning tail rotor is actually very hard to see under many lighting conditions
and a significant number of ground personnel have literally walked into one, often with
fatal results. The problem is that the human visual system cannot respond to light
changes above the critical flicker frequency which is at about 50 Hz. Unfortunately
the blade-passing frequency of most tail rotors is beyond this. The result is that when
looking at a spinning tail rotor, there is literally nothing to focus on and the eye tends to
see only the background beyond the rotor, especially if this is more brightly illuminated
than the rotor. A further problem is that when a hovering helicopter yaws, the tail rotor
may move laterally at some speed, too fast for someone on the ground to move clear.
Unfortunately most helicopters don’t have rear view mirrors and the pilot may be
unable to see a person near the tail.
There have been too many tragedies due to these effects, and these can be avoided by
some simple rules. Ground personnel should never approach a hovering helicopter or
one on the ground with turning blades unless the captain has indicated that it is allowed.
If a conventional helicopter must be approached, it should only be from directly ahead,
in full view of the captain. Helicopter pilots should avoid initiating rapid yaws in a low
hover, as this gives an unnoticed person on the ground no chance of escape and increases
the risk of striking the tail rotor on obstacles. It is good practice to paint both dark
and bright patches on the tips of the tail rotor blades so that some contrast will be
available whatever the background. Painting the patches at a different radius on each
blade causes a spiralling or flickering effect that is more noticeable. In the case of a
multi-bladed tail rotor, the flicker frequency can be lowered into the visible range by
painting all of the blades a dark colour except for one which should be as brightly
painted as possible. Another useful safety feature is to have a tail plane-mounted light
to illuminate the tail rotor.
An increasingly relevant drawback of the conventional tail rotor is that it seems to
generate a lot of noise. Although the tip speed is typically about the same as that of the
main rotor, the tail rotor turns at higher RPM and so the blade-passing frequency is
higher. This in itself doesn’t make more noise, but human hearing is more sensitive to
the increased frequency and so it seems louder. Another problem is that the tail rotor
often works in the disturbed wake of the main rotor and impulse noise will be created
when a main rotor vortex passes through the tail because this causes rapid variations
in local angle of attack.
From some directions the tail rotor may be the noisiest part of the machine. Society is
quite reasonably becoming less tolerant of noise, and there is no reason for the aviation
community to expect special treatment. In military applications helicopter noise may
also be an issue. The helicopter excels at inserting and retrieving special forces, but
covert missions are likely to be compromised by excessive noise.
Helicopter designers have explored various ways of countering the main rotor torque
in a way that reduces or eliminates one or more of these problems. These techniques
give a noise reduction and a safety advantage, but currently at an increased cost. The
fenestron system uses a fan enclosed in a short duct in much the same location as a
conventional tail rotor. The NOTAR system (NO TAil Rotor) uses a combination of a
sideways-lifting tail boom and air jets at the end of the boom. There is a fan inside the
hull providing air for boundary layer control over the boom and for the tip jets. These
systems will be considered in later sections of this chapter.
The tail rotor needs power and control. The power is generally delivered from the
main gearbox by a light shaft supported by regularly spaced bearings to prevent
whirling. Flight loads can cause the tail boom to flex and the drive shaft must be
The tail 167