The Art of the Helicopter John Watkinson - Karatunov.net

276 The Art of the Helicopter
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Fig. 7.12 Dynamic pressure rises as the square of the airspeed and instrument manufacturers use various
techniques to linearize the scale to some extent. This may include the provision of stops to make the return
spring progressively stiffer as speed increases.
range of flying speeds. In a light helicopter the stiffer spring may come into operation
at about 100 mph. Pointer movement from 70 to 80 mph may be twice that from 100
to 110 mph. A further consequence of the square law is that the dynamic pressure at
low speed is very small indeed and comparable to the friction in the linkages. The ASI
is unreliable at low speeds because of the low dynamic pressures involved and because
of the effect of downwash. Thus an ASI generally has no calibrations below 20 mph.
According to the country of origin ASIs will be found calibrated in mph, knots and
kph, some have dual scales like car speedometers.
The reading of the ASI is also proportional to air density, so it will underread at
high altitude. The scale on the ASI displays indicated air speed (IAS) proportional to
density and the square of the speed. Since the rotors behave proportionally to these
parameters, IAS is the best way of displaying speed from the point of view of the
airframe. Clearly IAS is not the actual speed through the air, and for navigational
purposes it needs compensating for ambient density to give true air speed or TAS.
Density is affected by altitude and temperature. At 10 000 feet the IAS must be increased
by around 17% to compensate for density change. An increase of 10 ◦ C raises TAS
about 2%. If TAS, windspeed and direction are known, the groundspeed (GS) can be
calculated.
In real ASIs the scale markings are not perfect, and frequently the position of the pitot
head with respect to the hull causes errors at certain airspeeds. A calibration table is
supplied so that IAS can be corrected by looking it up on the table. The result is called
rectified or calibrated airspeed (RAS or CAS) and is the value the IAS would have
read if it were free of error. In a helicopter airspeed is relatively slow and uncertainty
about windspeed causes more error than the ASI. In this case it is probably not worth
correcting the ASI for navigational purposes.
7.10 Airspeed and altitude sensing
Airspeed and altitude are both sensed by measuring pressure, one dynamic and one
static. Both of these processes are often combined in a single unit. As was seen above,
pressure is measured using the deflection of a corrugated capsule. The amount of
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