Pitot tubes measure flow based on air pressure and density, using Bernoulli’s principles. They calculate stagnation pressure, requiring a measurement of external static air pressure for velocity calculations. Pitot tubes are used in aircraft to determine airspeed, but factors such as ice buildup can affect accuracy. Henri Pitot invented the pitot tube in 1732, and Henry Darcy modified its design for aircraft use. Pitot tubes have built-in deicing heaters and can be adjusted for unique conditions.
Pitot tube flow is measured based on the pressure of the air passing through it and the established air density of the atmosphere at its particular height and air temperature. These equations are based on Bernoulli’s principles at moderate speeds which must be below the supersonic range. Other factors such as ice buildup or crosswinds can also affect the accuracy of the pitot tube flow. Although Pitot tubes could theoretically be used to measure any fluid flow velocity, they are often incorporated into aircraft today to determine airspeed in flight. Henri Pitot is credited with inventing the pitot tube in 1732 while studying the flow pressure of the Seine River in France, and French scientist Henry Darcy modified its design for aircraft use in the mid-19th century.
As a form of measuring pressure, pitot tubes do not measure average velocity, but, instead, a single point of velocity in the flow. Fluid flow velocity cannot be measured from pitot tube flow alone on aircraft, as they also require a measurement of external static air pressure for velocity calculations. These devices therefore calculate the so-called stagnation pressure, i.e. the pressure exerted by the air as it enters the Pitot tube and exits through the holes connected to the pressure transducer at the other end. Static pressure is calculated from static ports usually mounted on the side of an aircraft’s fuselage, while pitot tube flow is based on a pitot tube which is often mounted on a boom extending from the nose of the aircraft.
With pitot tube flow, the stagnation pressure is calculated by adding the standard atmospheric static pressure to the dynamic pressure exerted inside the pitot tube. At the rear of the pitot tube are a ring of holes and a central outlet port, both of which connect to the pressure transducer. As air exits these holes, the pressure differences are used by the transducer to calculate the dynamic air pressure. Bernoulli’s equation states that static air pressure plus dynamic air pressure equals the total air pressure, which, in this case, is the pitot tube stagnation pressure.
When the stagnation pressure and local air density are known, Bernoulli’s equations can be used to calculate the speed of the aircraft passing through the Pitot tube flow. While this is reliable under ideal conditions, low air velocities often have such small pressure changes in pitot tube flow that the pressure transducer can often fail to calculate them accurately, resulting in erroneous velocity readings. Several fatal aviation accidents involving faulty pitot tube flow readings also occurred when iced over, affecting the airflow, so built-in deicing heaters are now incorporated into pitot tubes to prevent such tragedies in the future. Adjustments can also be made for unique conditions, such as low airspeed or supersonic flight, so the pitot tubes generate accurate readings.
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