Speed and altitude measurement

So far when we have used the term ‘speed’ we have meant the relative speed between the aircraft and the air. This quantity is known as the ‘true air speed’.

The most common way of measuring the air speed is to use pressure differ­ences generated by the motion. Such a pressure difference can be obtained by taking one tapping at a stagnation point, where the air is brought to rest relative to the aircraft, and a second tapping at a point on the surface of the aircraft where the local pressure is equal to that in the surrounding atmosphere (Fig. 7.2). Bernoulli’s equation (Chapter 1) tells us that this pressure difference will be equal to – pV2 (where p is the air density and V is the speed of the air

Pitot probe (one each side) —

flow is stopped

pressure

Fig. 7.2 Air speed measurement

‘Indicated’ air speed is derived from the difference between pitot and static pressures stream). Thus, provided we know the density, we can calculate the speed of the air stream from the measured pressure difference.

In principle it would be possible to find the local density by measuring atmospheric temperature and pressure, but for historical and practical reasons this is not normally done. Instead the density is assumed to be the sea level value in the standard atmosphere (1.226 kg/m3). The air speed calculated from the measured pressure difference, using this constant density value, is called the Equivalent Air Speed (EAS).

As we have seen, the real value of the density will vary with location, weather and altitude so that the equivalent air speed will be coincident with the true air speed only at sea level under standard conditions. As the height increases so the actual density reduces and the equivalent air speed falls below the true air speed.

Although this may appear at first sight to be a grave disadvantage, as far as the practical task of flying the aircraft is concerned this is not so. For example the pilot needs to know when he is in danger of stalling the aircraft. In Chapter 1 we saw that the aerodynamic forces acting on the aircraft are proportional to the dynamic pressure (-pV2). If the aircraft is slowed, the lift is kept equal to the aircraft weight by increasing the angle of attack to compensate for the loss of dynamic pressure. Since the dynamic pressure and equivalent air speed are directly related, the stalling angle of attack will occur at a particular equivalent air speed rather than true air speed.

If the pilot had to work in terms of true air speed, the air speed reading at stall would depend both on the height and the weather conditions at the time; not very convenient for a pilot trying to make quick decisions!

The detailed flow-field around the aircraft will be changed by such factors as the attitude of the aircraft and whether such devices as flaps are deployed. These changes in the flow-field will have some influence on the two pressure tappings used to measure the air speed. This will mean that there will be errors (called position errors) in the equivalent air speed presented to the pilot on his Air Speed Indicator (ASI). The actual ASI reading therefore differs slightly from the equivalent air speed and is known as the Indicated Air Speed (IAS).

Fortunately these position errors will at least be the same for a given set of flight conditions. Thus, although the indicated air speed shown on the ASI differs slightly from the position-error-free equivalent air speed, stalling will always occur at the same ASI reading, which is all the pilot requires.

From the point of view of navigation the indicated air speed given by the ASI is of limited use, although in simple light aircraft this may be the only avail­able information regarding speed. In this case the pilot will have to estimate the actual speed relative to the ground from his knowledge of altitude and prevailing wind speed. In more complex aircraft a variety of navigational aids is available which are either based on ground-based transmitters GPS or, for inertial navigators, may be self-contained within the aircraft.

In Chapter 5 we saw that the Mach number is of great importance at high speeds, and this will become even more apparent in Chapters 8 and 9. In such aircraft a Mach meter is fitted.

The other important quantity that the pilot needs to know is the altitude of the aircraft. Traditionally this too is derived from a pressure measurement. This time it is the static, or local atmospheric pressure which is required. As we saw at the beginning of the chapter, this static pressure will vary with height. The pressure measurement is not all that is needed to obtain the true height because the local static pressure will depend on the local weather conditions as well as the height. The altimeter is thus calibrated assuming that the atmosphere has the characteristics defined for the International Standard Atmosphere (ISA) (Fig. 7.1).

The reading obtained on the altimeter with this ISA assumption is known as the pressure height. As far as the pilot is concerned the main problem occurs during landing when the pressure height may not correspond to the actual height of the airfield at which the landing is to be made. For this reason the altimeter reading can be adjusted so that the correct indication will be obtained at the airfield. This is done by the pilot immediately prior to landing in response to information supplied by the controllers on the ground. Because the altitude is derived from the static pressure measurement, it too is subject to position error.

On most military and commercial aircraft other means of altitude measure­ment are generally supplied in addition to the pressure altimeter. These radio altimeters are based on the reflection of radio waves from the ground and are not subject to the errors detailed above. GPS systems are also now used.

The instruments we have described above are used by the pilot to give him information relating to the aerodynamic performance of the aircraft, and are known as primary flight instruments. Another instrument which falls into this category is the rate of climb indicator. Yet another is the artificial horizon which gives the pilot information about the attitude of the aircraft with respect to the ground. This instrument relies on a gyroscope to provide a stable refer­ence. Figure 10.2 shows the instrument panel of a typical modern light aircraft.