Stagnation pressure Static pressure = Dynamic pressure

If instead of connecting the static pressure vent and the pitot tube to two sep­arate pressure-measuring devices, we connect them across one device which measures the difference in pressure then, from the above expression, we can see that we will obtain a measurement of the dynamic pressure. Thus we have a simple means of measuring the dynamic pressure, and if only we could find a way of measuring or assessing the density, we could determine the flow speed, but more of this later; let us first concentrate on the measurement of dynamic pressure, which we will show is actually just as important to the pilot as the speed.

The pressure difference measuring device used on aircraft consists of either a diaphragm or a capsule (similar to the type used in an aneroid barometer). The stagnation pressure is applied to one side, and the static pressure is applied to the other. The resulting deflection of the diaphragm can then either be amplified through a series of levers to cause a dial pointer to move, as on older mechanical devices, or can be used to produce a proportional electrical output to be fed into an appropriate electronic circuit. This instrument thus gives a reading that is proportional to the dynamic pressure, but as we shall see, it forms the basis of the air speed indicator.

The pitot tube and static pressure hole are located at a suitable convenient position on the aircraft. The location of the static tapping is very important because it is essential to choose a position where the local static pressure is about the same as that in the free stream away from the aircraft. We need, therefore, to find a place where the flow speed is about the same as that in the free stream, and also is not too sensitive to change in the direction that the air­craft is pointing. The pitot and static holes are normally heated to avoid icing at low temperatures.

As an alternative to using separate pitot and static tubes, it is possible to use a combined device called the pitot-static tube which is illustrated in Fig. 2.5. The pitot-static tube consists of two concentric tubes. The inner one is simply a pitot tube, but the outer one is sealed at the front and has small holes in the

Static

tube

Fig 2.5 Concentric pitot-static tube

side to sense the static pressure. The pitot-static tube is a very convenient device, and by mounting it on the wingtips or the nose it can be arranged so that it is well clear of interference from the flow around the aircraft. Pitot – static tubes are always used for accurate speed measurement on prototypes, but for civil and private aircraft separate pitot and static tappings are normally used. Pitot-static tubes are frequently used in wind tunnels.