Читать книгу Flight Theory and Aerodynamics - Joseph R. Badick - Страница 64

If a symmetrically shaped object is placed in a moving airstream (Figure 2.8), the flow pattern will be as shown. Some airflow will pass over the object and some will flow beneath it, but at the point at the nose of the object, the flow will be stopped completely. This point is called the stagnation point. Since the air velocity at this point is zero, the dynamic pressure is also zero. The stagnation pressure is, therefore, all static pressure and must be equal to the total pressure, H, of the airstream.

In Figure 2.9, the free stream values of velocity and pressure are used to measure the indicated airspeed of an aircraft. The pitot tube is shown as the total pressure port and must be pointed into the relative wind for accurate readings. Static pressure, sometimes referred to as ambient pressure, is the pressure around the aircraft whether it is moving or at rest, and is the same “local” pressure your body experiences. When in motion, the air entering the pitot tube comes to a complete stop and thus the static pressure in the tube is equal to the total free stream pressure, H. This pressure is ducted into a diaphragm inside the airspeed indicator.

Figure 2.8 Flow around a symmetrical object.

Figure 2.9 Schematic of a pitot–static airspeed indicator.

Source: U.S. Department of Transportation Federal Aviation Administration (2012c).

The static pressure port(s) can be made as a part of the point tube or it can be at a distance from the pitot tube on the side of the aircraft. It should be located at a point where the local air velocity is exactly equal to the airplane velocity. The static port is made so that none of the velocity enters the port. The port measures only static pressure, and none of the dynamic pressure. The static pressure is ducted into the chamber surrounding the diaphragm within the inside of the airspeed indicator.

Now we have static pressure inside the diaphragm that is equal to total pressure (H), and then static pressure measured from the static port outside the diaphragm. The difference between the pressure inside the diaphragm and outside the diaphragm is the differential pressure that deflects the flexible diaphragm that is geared to the airspeed pointer. The static pressures cancel each other, thus the dynamic pressure is left and indicated on the airspeed indicator. The airspeed indicator measures differential pressure. The airspeed indicator is calibrated to read airspeed.

Figure 2.10 Air data computer and pitot–static sensing.

Source: U.S. Department of Transportation Federal Aviation Administration (2012a).

Figure 2.10 shows a modern pitot–static system associated with an air data computer (ADC). Though the concept with this solid state device is the same as in traditional pitot–static systems, the signal sent to modern “glass” instruments is digital and often has the ability to generate trend vectors.