The Art of the Helicopter John Watkinson - Karatunov.net

288 The Art of the Helicopter
Fig. 7.23 A swivelling pitot head aligns itself with the rotor downwash. From the angle it makes to the vertical
the airspeed can be calculated.
In a fancy version of the slip string, a small weathervane on the nose of the machine
having an angular sensor drives a slip meter on the instrument panel.
7.18RADAR sensors
The principle of radio direction and ranging (RADAR) is simply the analysis of
reflected radio waves. In active RADAR, the radio signals are generated at the analysis
equipment, whereas in passive RADAR signals may come from other sources.
Radio signals are electromagnetic waves and these have the characteristic the way they
interact with objects is a function of the relative size of the object and the wavelength.
Very long wavelengths simply diffract around objects returning very little energy to the
transmitter, whereas short wavelengths are reflected more efficiently, hence the use of
short wavelengths in RADAR.
Figure 7.24 shows that two variables can be extracted from returned radio signals.
At (a) a pulse is transmitted and the time taken for the reflection to arrive will allow
the distance to the target to be computed. At (b) a continuous signal is transmitted and
any relative velocity between the target and the transmitter will cause the frequency of
the return to be shifted by the Doppler effect.
The RADAR altimeter is based on the first principle. Despite its name, it is not an
altimeter. The transmitter and receiver are directed downwards and the signal reflected
from the earth is used to compute height above ground, not altitude. This is extremely
useful for terrain avoidance and for landing in poor conditions and may be used as the
actual height input to an altitude hold system.
RADAR altimeters have some limitations. When flying over a tree canopy, the reading
may be anywhere between ground level and treetop height. When flying with an
underslung load, the RADAR altimeter may measure the length of the load cable.
Figure 7.25 shows how a Doppler RADAR works. The received signal is amplified
and multiplied by the transmitted signal in a mixer. Any difference in frequency between
the transmitted and received signals will be output by the mixer. This frequency is
proportional to the axial component of relative velocity between the transmitter and the
target. Figure 7.26(a) shows that as a practical matter, an airborne Doppler RADAR
must use a transmitted beam which is angled downwards. The forward motion of the
helicopter is not in the same direction as the beam. The system will measure the actual