Influid mechanics the termstatic pressure refers to a term inBernoulli's equation written words asstatic pressure +dynamic pressure = total pressure. Sincepressure measurements at any single point in a fluid always give the static pressure value, the 'static' is often dropped.In the design and operation ofaircraft,static pressure is theair pressure in the aircraft'sstatic pressure system.

The concept ofpressure is central to the study of fluids. A pressure can be identified for every point in a body of fluid, regardless of whether the fluid is in motion. Pressure can bemeasured using ananeroid,Bourdon tube, mercury column, or various other methods.

The concepts oftotal pressure anddynamic pressure arise fromBernoulli's equation and are significant in the study of all fluid flows. These two pressures are not pressures in the usual sense - they cannot be measured using apressure sensor. To avoid potential ambiguity when referring topressure in fluid dynamics, many authors use the termstatic pressure to distinguish it fromtotal pressure anddynamic pressure; the termstatic pressure is identical to the termpressure, and can be identified for every point in a fluid flow field. The static pressure can be measured in flowing fluid using a hole or tube which is perpendicular to the flow.

InAerodynamics, L.J. Clancy^[1] writes: "To distinguish it from the total and dynamic pressures, the actual pressure of the fluid, which is associated not with its motion but with its state, is often referred to as the static pressure, but where the term pressure alone is used it refers to this static pressure."

Bernoulli's equation is foundational to the dynamics ofincompressible fluids. In many fluid flow situations of interest, changes in elevation are insignificant and can be ignored. With this simplification, Bernoulli's equation for incompressible flows can be expressed as^[2][3][4]

${\displaystyle P+\frac{1}{2}\rho v^2=P_0,}$

where:

• ${\displaystyle P}$ is static pressure,
• ${\displaystyle \frac{1}{2}\rho v^2}$ isdynamic pressure, usually denoted by${\displaystyle q}$,
• ${\displaystyle \rho }$ is thedensity of the fluid,
• ${\displaystyle v}$ is theflow velocity, and
• ${\displaystyle P_0}$ is total pressure which is constant along anystreamline. It is also known as thestagnation pressure.

Every point in a steadily flowing fluid, regardless of the fluid speed at that point, has its own static pressure${\displaystyle P}$, dynamic pressure${\displaystyle q}$, and total pressure${\displaystyle P_0}$. Static pressure and dynamic pressure are likely to vary significantly throughout the fluid but total pressure is constant along each streamline. Inirrotational flow, total pressure is the same on all streamlines and is therefore constant throughout the flow.^[5]

The simplified form of Bernoulli's equation can be summarised in the following memorable word equation:^[6][7][8]

static pressure + dynamic pressure = total pressure.

This simplified form of Bernoulli's equation is fundamental to an understanding of the design and operation of ships, low speed aircraft, and airspeed indicators for low speed aircraft – that is aircraft whose maximum speed will be less than about 30% of thespeed of sound.

As a consequence of the widespread understanding of the termstatic pressure in relation to Bernoulli's equation, many authors^[9] in the field of fluid dynamics also usestatic pressure rather thanpressure in applications not directly related toBernoulli's equation.

TheBritish Standards Institution, in its Standard^[10]Glossary of Aeronautical Terms, gives the following definition:

4412Static pressure The pressure at a point on a body moving with the fluid.

An aircraft'sstatic pressure system is the key input to itsaltimeter and, along with thepitot pressure system, also drives theairspeed indicator.^[11]

The static pressure system is open to the aircraft's exterior through a small opening called thestatic port, which allows sensing the ambient atmospheric pressure at thealtitude at which the aircraft is flying. In flight, the air pressure varies slightly at different positions around the aircraft's exterior, so designers must select the static ports' locations carefully. Wherever they are located, the air pressure that the ports observe will generally be affected by the aircraft's instantaneousangle of attack.^[12] The difference between that observed pressure and the actual atmospheric pressure (at altitude) causes a smallposition error in the instruments' indicated altitude and airspeed.^[13][14] A designer's objective in locating the static port is to minimize the resulting position error across the aircraft's operating range of weight and airspeed.

Many authors describe the atmospheric pressure at the altitude at which the aircraft is flying as thefreestream static pressure. At least one author takes a different approach in order to avoid a need for the expressionfreestream static pressure. Gracey has written "The static pressure is the atmospheric pressure at the flight level of the aircraft".^[15][16] Gracey then refers to the air pressure at any point close to the aircraft as thelocal static pressure.

The term(hydro)static pressure is sometimes used influid statics to refer to the pressure of a fluid at a nominated depth in the fluid. In fluid statics the fluid is stationary everywhere and the concepts of dynamic pressure and total pressure are not applicable. Consequently, there is little risk of ambiguity in using the termpressure, but some authors^[17] choose to usestatic pressure in some situations.

• Pascal's law
• Stagnation pressure
• Standard conditions for temperature and pressure

Aircraft design and operation

• Gracey, William (1958),Measurement of static pressure on aircraft(PDF), Langley Research Center: NACA, TR-1364, retrieved2023-04-07.
• Gracey, William (1980),Measurement of aircraft speed and altitude(PDF), Langley Research Center: NASA, RP-1046, retrieved2023-04-07.
• Gracey, William (1981),Measurement of Aircraft Speed and Altitude, New York: John Wiley & Sons,ISBN 978-0-471-08511-9
• Kermode, A.C. (1972)Mechanics of Flight, Longman Group Limited, LondonISBN 0-582-23740-8
• Lombardo, D.A.,Aircraft Systems, 2nd edition, McGraw-Hill (1999), New YorkISBN 0-07-038605-6

Fluid dynamics

• L. J. Clancy (1975),Aerodynamics, Pitman Publishing Limited, LondonISBN 0-273-01120-0
• Streeter, V.L. (1966),Fluid Mechanics, McGraw-Hill, New York

• Aerodynamics
• Aircraft instruments
• Fluid dynamics

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