Pitot tube

A Pitot (pronounced pē-tó) tube is a measuring instrument used to measure fluid flow velocity, and more specifically, used to determine airspeed on aircraft. The Pitot tube is named after its inventor, Henri Pitot, and was modified to its modern form by Henry Darcy.

The basic pitot tube simply consists of a tube pointing directly into the fluid flow. As this tube contains air, a pressure can be measured as the moving air is brought to rest. This pressure is the stagnation pressure of the air, also known as the total pressure, or sometimes (particularly in aviation circles) the pitot pressure.

The measured stagnation pressure cannot of itself be used to determine the airspeed. However, since Bernoulli's equation states that

stagnation pressure = static pressure + dynamic pressure

then the dynamic pressure is simply the difference between the static pressure and the stagnation pressure. The static pressure is generally measured using the static ports on the side of the fuselage. The dynamic pressure is then determined using a diaphragm inside an enclosed container. If the air on one side of the diaphragm is at the static pressure, and the other at the stagnation pressure, then the deflection of the diagram is proportional to the dynamic pressure, which can then be used to determine the indicated airspeed of the aircraft. The diaphragm arrangement is typically contained within the airspeed indicator, which converts the dynamic pressure to an airspeed reading by means of mechanical levers.

Instead of static ports, a pitot-static tube may be employed, which has a second tube coaxial with the pitot tube with holes on the sides, outside the direct airflow, to measure the static pressure.

Pitot tubes on aircraft commonly have heating elements to prevent the tube from becoming clogged with ice.

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References