The pitot tube is used to measure aircraft velocity relative to the air around the aircraft. They are L-shaped metal assemblies mounted on each side of the aircraft's nose approximately at one quarter of the fuselage height, right below the forward edge of the windshield.

The pitot tube is around 10 inches (25 centimeters) long with a 1/2 inch (1 centimeter) diameter. Several small holes are drilled around the outside of the tube and a center hole is drilled down the axis of the tube. The outside holes are connected to one side of a device called a pressure transducer. The center hole in the tube is kept separate from the outside holes and is connected to the other side of the transducer. The transducer measures the difference in pressure in the two groups of tubes by measuring the strain in a thin element using an electronic strain gauge.

The pitot tube is mounted on the aircraft so that the center tube is always pointed in the direction of travel and the outside holes are perpendicular to the center tube. Since the outside holes are perpendicular to the direction of travel, these tubes are pressurized by the local static air pressure with practically no effect from the movement of the aircraft. The center tube, however, is pointed in the direction of travel and is pressurized by the total of the local static air pressure and air pressure coming from the movement of the aircraft (air velocity). The pressure transducer measures the difference in total and static pressure.

Working Principles

Using Bernoulli's equation, aircraft valocity (airspeed) can easily be measured. This is illustrated by the following figure:

It can be seen that with the difference in pressures measured and knowing the local value of air density (from pressure and temperature measurements) aircraft velocity can be calculated.There are, however, some practical limitations, which one has to keep in mind:

• If aircraft velocity is low, the difference in pressures is very small and hard to accurately measure with the transducer. Errors in the instrument could be greater than the measurement! This means that pitot tubes don't work very well for very low velocities.
• If velocity is very high (supersonic), then the assumptions of Bernoulli's equation are violated and the measurement is wrong again. At the front of the tube, a shock wave appears that will change the total pressure. There are corrections for the shock wave that can be applied to use pitot tubes for high speed aircraft.

In practice air density correction (i.e. dividing by 'r') is not always used, airspeed is calculated as if the aircraft were flying at sea level, using a sea level air density value. This means that in high altitudes, where the air is less dense, the result is a much smaller velocity value than it is in reality. This velocity value is called 'indicated airspeed'. It is a very important input for pilots: a slow aircraft in dense air can behave the same as a faster aircraft in less dense air, and while this latter aircraft flies faster relative to the ground, both aircrafts can have the same indicated airspeed value.

Calculated Airspeeds

Based on the reasons stated above, there are a number of different calculated airspeeds available for the F-15E pilot. These airspeeds are forming a serie with ever increasing values, and are listed below (in any given moment of flight the later the airspeed comes in the following list, the greater value it shows).

Indicated Airspeed: This is the airspeed value coming from the pitot tubes by using the Bernoulli equation desribed above with sea level air density value. Indicated airspeed is measured in KIAS (= Knots Indicated Airspeed).

Calibrated Airspeed ( CAS): This is indicated airspeed, corrected for system installation error (error measured during system calibration). Calibrated airspeed is measured in KCAS (= Knots Calibrated Airspeed).

Equivalent Airspeed: This is calibrated airspeed, corrected for compressibility errors (errors caused by the air being compressed in the pitot tube during measurement).

True Airspeed: This is equivalent airspeed, corrected for air density (depending on altitude). This is the velocity value coming from the Bernoulli equation on the figure above.

The natural progression between these airspeeds can be memorized by using the mnemonic ICE-T, coming from the first letters of their names. Note, that there is a fifth velocity value which is available for the pilot, this is called Ground Speed and it represents the aircraft's velocity relative to the ground (i.e. the speed of the aircraft's shadow on a horizontal, flat surface). However this value is not an 'airspeed' in the sense that it is not measured by the pitot tubes, but by using the EGI system instead.

In the F-15E, calibrated airspeed ( CAS) is displayed on the left side of the HUD in a rectangle (see figure below), right above the 'alpha' angle of attack readout, as a primary airspeed value. An other (secondary) velocity value can be displayed as well, right below the 'alpha' angle of attack readout. This can be either True Airspeed (marked with a letter T in front of the numeric value) or Ground Speed (marked with a letter G in front of the numeric value), depending on the pilot's choice.

Pitot Heating

The pitot tubes can be heated to prevent them from freezing at higher altitudes. They can be covered when on the ground, the cover protecting them from physical harm. Note, that the covers are made of heat resistant material, which can withstand the heat generated by the pitot tube heaters in case heating is accidentally turned on while the covers are still on.

The switch for the pitot tube heating is located on the environment control systems ( ECS) panel at the forward end of the pilot's right console. When engaged, the switch sends an electric current to a heating coil in the root of the pitot probe assembly. The pitot tube's metal body conducts the heat to the entire surface and makes it impossible for ice to accumulate.

Sources

• Tom Benson, Glenn Research Center, NASA, 2004
• The Strike Eagle Forum

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