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.
Application 2.3
Figure 2.11 indicates the impact of a blockage of the pitot tube while an aircraft is flying at altitude. The resultant effect is that the airspeed indicator acts like an altimeter: as the aircraft climbs in altitude, the airspeed increases, and when the aircraft descends, the airspeed decreases.
In a traditional airspeed indicator with a diaphragm, what is happening in reference to our discussion on q, P, and H in the situation above? What is the impact of the blockage on the other pitot–static instruments? Why?
Figure 2.11 Blocked pitot tube and drain hole.
Source: U.S. Department of Transportation Federal Aviation Administration (2016b).
Indicated Airspeed
Indicated airspeed (IAS) is the direct reading of the airspeed indicator, and is uncorrected for errors related to installation or nonstandard atmospheric density. If there are any errors in the instrument, they may be shown on an instrument error card located near the instrument and/or in the AFM.
Calibrated Airspeed
Calibrated airspeed (CAS) is obtained when the necessary corrections have been made to the IAS for installation error and instrument error. These position errors are especially prevalent at lower airspeeds, and the IAS may be “indicating” slower than the CAS. Most of the time in non‐pressurized aircraft operating at lower altitudes, the CAS can be assumed to be within several knots of the IAS.
Equivalent Airspeed
Equivalent airspeed (EAS) results when the CAS has been corrected for compressibility effects. Figure 2.12 shows a compressibility correction chart. In general, if flying above 10 000 ft and 200 kts., the compressibility correction should be made. Unlike the instrument and position error charts, which vary with different aircraft, this chart is good for any aircraft.
EAS is not a significant factor in airspeed computations when aircraft fly at relatively low speeds and altitudes, but at higher speeds and altitudes, the compressibility correction must be taken into account. For example, if an aircraft is flying at a pressure altitude of 20 000 ft at a CAS of 400 kts., Figure 2.12 indicates a compressibility correction of −17.5 kts. The EAS for this example is 382.5 kts.
Figure 2.12 Compressibility correction chart.
True Airspeed
True airspeed (TAS) is obtained when EAS has been corrected for density ratio, the airspeed indicator measures dynamic pressure and is calibrated for sea level standard day density. As altitude increases, the density ratio decreases and a correction must be made. The correction factor is
