An open-ended tube parallel to the longitudinal axis of the aircraft is used to sense the total pressure (static plus dynamic). This device is a ‘pitot tube’ mounted in a ‘pitot head’.
The open end of the tube faces into the moving airstream, the other end leading to the airspeed capsules in the ASI and Machmeter.
The moving airstream is thus brought to rest in the tube, so generating the extra (dynamic) pressure which together with the static pressure already in the tube provides the required total (pitot) pressure.
A ‘static head’ consists of a tube with its forward end sealed but with holes or slots cut in the sides. These slots do not face into the airflow and therefore, in theory, they sense only the static pressure. In fact, of course, there will be a suction effect and the sensed static pressure will be slightly lower when the aircraft is moving. This pressure supplies the static ‘line’ to the pressure instruments. A pressure sensing system consisting of separate pitot and static heads is shown in Figure 2.1.
The static and pitot sources may be combined in one ‘pressure head’, the static tube surrounding the pitot tube, with separate pressure lines leading to the pressure instruments. Figure 2.2 illustrates an example of this type.