The Art of the Helicopter John Watkinson - Karatunov.net

7.17 Airflow-sensing devices
The helicopter differs from the fixed-wing aircraft in that it can move in virtually any
direction in three dimensions and at very low speeds. This makes airspeed measurement
difficult. The conventional ASI with its pitot head is only accurate when the relative
airflow approaches within 10–15 ◦ of the straight-ahead position. At greater angles, the
pressure caused by flow of the rotor downwash around the hull may result in major
errors, especially at low speeds where the dynamic pressures due to airspeed are very
low. Helicopter pilots learn to treat ASI readings with suspicion or even amusement at
low speeds especially when hovering in cross- or tail-winds.
With a conventional ASI, it is impossible to establish if a zero-airspeed condition
exists, yet this is important to helicopter operations because of the possibility of entering
a vortex-ring condition. Another requirement for accurate low airspeed information is
in systems that control automatic stabilators. Special sensors have been developed for
helicopters to overcome some of these difficulties.
Figure 7.22 shows a system that can measure horizontal airspeed in any direction.
This consists of a rotating assembly with pitot heads on each end of an arm. The
assembly can be mounted above the rotor to turn with it, or on the hull and turned
by a motor. The dynamic pressure will only be constant if the horizontal airspeed
is zero. In all other cases there will be a sinusoidally varying difference in dynamic
pressure between the two pitots. The amplitude will represent the airspeed and the
phase with respect to the rotational phase of the arm will represent the direction. A twodimensional
indicator may be used to display the fore-and-aft and lateral components
of airspeed.
Figure 7.23 shows an alternative system consisting of a small finned swivelling body
that will align itself with the downwash. This contains a pitot to measure the downwash
velocity. From the downwash velocity and the angle of the body it is possible to compute
the horizontal component of the downwash and this is the airspeed.
The slip string is a simple piece of string attached to the centre of the windscreen.
If the machine is being flown without slip, the string will align with the airflow and
take on a vertical attitude. If there is any slip the string will turn. Despite its apparent
crudity, the slip string is a useful device because it does not suffer from the problems
of the pendulum-based slip indicator. A machine flown with the slip string centred will
have lower drag than one flown with the slip ball centred. This may be significant if
power is limited. The machine will also fly along its heading, making navigation easier.
Fig. 7.22 A rotating airspeed indicator that can measure airspeed in any direction down to zero. The difference
between the dynamic pressures at the two pitot heads determines the airspeed. The phase of the dynamic
pressure signals with respect to the rotation reveals the direction.
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