V. Measurement of Tunnel Speed

We define the tunnel speed to be the mean speed in the empty tunnel at the section which will be occupied by the model during testing. It will then generally be assumed that this speed is not altered by the presence of the model. It is usually inconvenient to measure pressures, and hence velocities, by placing a pitot - static tube in the working section during a test, since this tube would interfere with the flow past the model. Thus the pressure readings are taken at points upstream of the working section. However, it cannot be assumed that the velocity in the working section is the same as it is further upstream. It is therefore necessary to carry out an initial calibration of the tunnel. This consists in a careful exploration, by means of a travelling pitot static tube, of the velocity distribution in the section normally occupied by the model, for various readings of the upstream pressure tappings. A calibration chart can then be prepared, giving mean speed at the working section in terms of these readings. The cali­bration must cover the entire speed range of the tunnel. The assump­tion is then made that the calibration is still valid when the model is present.

The tunnel speed is measured in terms of the difference between a total head reading and a static pressure reading. The static pressure is usually read by means of an orifice in the wall of the working section, well upstream of the model. This orifice is called the tunnel speed hole. The total head is usually measured upstream of the contraction. Provided that it is made downstream of any gauzes or honeycombs it will be very little different from the value in the working section. If it is slightly different, it will not matter, since it is not used directly to calculate speed, but as part of a calibration. The difference between the total head and static pressure readings is then used to read off the tunnel speed from the calibration chart, which gives tunnel speed in terms of the measured pressure differ­ence.

Since the total head measurement is made in a place upstream of the contraction, the speed there, and hence the dynamic pressure, is very low, so that the total head is almost the same as the static pressure. It may be sufficiently accurate to measure the latter instead of the former. This may be done by means of an orifice in the tunnel wall, which interferes with the flow less than would a pitot tube.

With some tunnels, it may be possible to measure the tunnel speed by means of one tapping only, which may be compared directly with the atmosphere. In a closed section, straight through tunnel, the total head in the working section is equal to the total head in the tunnel room, and this is equal to the static pressure in the tunnel room. Thus it is only necessary to compare the tunnel static pressure tapping with the room static pressure. In an open section, return circuit tunnel, the static pressure in the working section is the same as in the room. Thus it is only necessary to compare the tunnel total head tapping with the room static pressure.

VI. Pitot – Static Tube

The most important and widely used tool for measurement of air speed is the pitot – static head. If we know the value of total pressure p₀ and static pressure p, we know dynamic pressure, and the value of air speed

, (2)

The pitot – static head consists of two metal tubes fixed to some exposed part of the aeroplane and facing directly into the airflow. One of these, the pitot (or pressure) tube has an open end facing the wind, while the other tube, called the static, is closed at the end but pierced with small holes or slits farther back. Sometimes the pitot tube is a short distance below the static (as in Figure 7), but on modern aircraft the tubes are more usually concentric, the pitot tube forming the centre and being surrounded by the static (Figure 8). The other end of each tube is connected by tubing to the air speed indicator in the cockpit.

Pitot – static head

Figure 7

Old types of air speed indicator consisted of two chambers separated by a rubber, metal, or oiled-silk diaphragm, one tube being connected to each chamber, which was otherwise airtight. In a later development the pitot tube is connected to the inside of a capsule (a flat circular box of corrugated metal such as is used in an altimeter or aneroid barometer), while the static tube is con­nected to the casing of the instrument. In each type the principle is the same: when there is no air speed the normal atmospheric pressure will be com­municated to both sides of the diaphragm or capsule, but when the air blows up against the open pitot tube the extra pressure due to the air velocity plus the normal atmospheric pressure will act on one side, while the static tube still con­veys the ordinary atmospheric pressure to the other.

Concentric pitot – static head

Figure 8

The difference in pressure causes a deflection of the diaphragm or capsule, which, by suitable gearing, makes a pointer move round the scale, which is marked off in air speeds. Nowadays the pitot static tube may be connected across a pressure transducer, a device that gives an electrical signal proportional to the pressure difference.

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