Pitot tube

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Aircraft use pitot tubes to measure airspeed. The example, from an Airbus A380, combines a pitot tube (pencil point shape) with a static port and an angle-of-attack vane (black). Air-flow, relative to this device, is right to left.
Types of pitot tubes
Pitot tube on Kamov Ka-26 helicopter

A pitot (/ˈpt/ PEE-toh) tube is a pressure measurement instrument used to measure fluid flow velocity. The pitot tube was invented by the French engineer Henri Pitot in the early 18th century[1] and was modified to its modern form in the mid-19th century by French scientist Henry Darcy.[2] It is widely used to determine the airspeed of an aircraft, water speed of a boat, and to measure liquid, air and gas velocities in industrial applications. The pitot tube is used to measure the local velocity at a given point in the flow stream and not the average velocity in the pipe or conduit.[3]

Theory of operation[edit]

The basic pitot tube consists of a tube pointing directly into the fluid flow. As this tube contains fluid, a pressure can be measured; the moving fluid is brought to rest (stagnates) as there is no outlet to allow flow to continue. This pressure is the stagnation pressure of the fluid, also known as the total pressure or (particularly in aviation) the pitot pressure.

The measured stagnation pressure cannot itself be used to determine the fluid velocity (airspeed in aviation). However, Bernoulli's equation states:

Stagnation pressure = static pressure + dynamic pressure

Which can also be written

p_t = p_s + \left(\frac{\rho V^2}{2}\right)

Solving that for velocity we get:

V = \sqrt{\frac{2 (p_t - p_s)}{\rho}}

NOTE: The above equation applies ONLY to fluids that can be treated as incompressible. Liquids are treated as incompressible under almost all conditions. Gases under certain conditions can be approximated as incompressible. See Compressibility.

where:

  • V is fluid velocity;
  • p_t is stagnation or total pressure;
  • p_s is static pressure;
  • and \rho is fluid density.

The value for the pressure drop p_2p_1 or \Delta p due to \Delta h, the reading on the manometer:

\Delta p = \rho g \Delta h

Where:

  • \rho is the density of the fluid in the manometer
  • \Delta h is the manometer reading

The dynamic pressure, then, is the difference between the stagnation pressure and the static 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 diaphragm 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 separate pitot and static ports, a pitot-static tube (also called a Prandtl 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.[4]

Operation[edit]

Pitot tubes on aircraft commonly have heating elements called pitot heat to prevent the tube from becoming clogged with ice. The failure of these systems can have catastrophic consequences, as in the case of Austral Líneas Aéreas Flight 2553, Birgenair Flight 301 (investigators suspected that some kind of insect could have created a nest inside the pitot tube: the prime suspect is a species called the black and yellow mud dauber wasp), Northwest Airlines Flight 6231, Aeroperú Flight 603 (blocked static port), and of one X-31.[5] The French air safety authority BEA said that pitot tube icing was a contributing factor in the crash of Air France Flight 447 into the Atlantic Ocean.[6] In 2008 Air Caraïbes reported two incidents of pitot tube icing malfunctions on its A330s.[7]

Industry applications[edit]

Pitot tube from an F/A-18

In industry, the velocities being measured are often those flowing in ducts and tubing where measurements by an anemometer would be difficult to obtain. In these kinds of measurements, the most practical instrument to use is the pitot tube. The pitot tube can be inserted through a small hole in the duct with the pitot connected to a U-tube water gauge or some other differential pressure gauge for determining the velocity inside the ducted wind tunnel. One use of this technique is to determine the volume of air that is being delivered to a conditioned space.

The fluid flow rate in a duct can then be estimated from:

Volume flow rate (cubic feet per minute) = duct area (square feet) × velocity (feet per minute)
Volume flow rate (cubic meters per second) = duct area (square meters) × velocity (meters per second)

In aviation, airspeed is typically measured in knots.

See also[edit]

References[edit]

Notes

  1. ^ Pitot, Henri (1732). "Description d'une machine pour mesurer la vitesse des eaux courantes et le sillage des vaisseaux" (PDF). Histoire de l'Académie royale des sciences avec les mémoires de mathématique et de physique tirés des registres de cette Académie: 363–376. Retrieved 2009-06-19. 
  2. ^ Darcy, Henry (1858). "Note relative à quelques modifications à introduire dans le tube de Pitot" (PDF). Annales des Ponts et Chaussées: 351–359. Retrieved 2009-07-31. 
  3. ^ Geankoplis, C.J. (2003). Transport processes and separation process principles (includes unit operations) (4th ed.). New Jersey: Prentice Hall. 
  4. ^ "How Aircraft Instruments Work." Popular Science, March 1944, pp. 116.
  5. ^ NASA Dryden news releases. (1995)
  6. ^ "Training flaws exposed in Rio-Paris crash report". Reuters. 5 July 2012. Retrieved 5 October 2012. 
  7. ^ Daly, Kieran (11 June 2009). "Air Caraibes Atlantique memo details pitot icing incidents". Flight International. Retrieved 19 February 2012. 

Bibliography

  • Kermode, A.C. (1996) [1972]. Mechanics of Flight. Barnard, R.H. (Ed.) and Philpott, D.R. (Ed.) (10th ed.). Prentice Hall. pp. 63–67. ISBN 0-582-23740-8. 
  • Pratt, Jeremy M. (2005) [1997]. The Private Pilot's Licence Course: Principles of Flight, Aircraft General Knowledge, Flight Performance and Planning (3rd ed.). gen108–gen111. ISBN 1-874783-23-3. 
  • Tietjens, O.G. (1934). Applied Hudro- and Aeromechanics, based on lectures of L. Prandtl, Ph.D. Dove Publications, Inc. pp. 226–239. ISBN 0-486-60375-X. 
  • Saleh, J.M. (2002). Fluid Flow Handbook. McGraw-Hill Professional. 

External links[edit]