Static pressure

Lore & Background

In fluid mechanics, the term static pressure arises from Bernoulli's equation, where it is distinguished from dynamic pressure and total pressure. While pressure measurements at any single point in a fluid always give the static pressure value, the term 'static' is often dropped for brevity. The British Standards Institution defines static pressure as 'the pressure at a point on a body moving with the fluid.' Many authors use the term static pressure to avoid ambiguity when referring to pressure in fluid dynamics, as total pressure and dynamic pressure cannot be measured using a pressure sensor.

In aircraft design, the static pressure system is open to the exterior through a static port, which allows sensing the ambient atmospheric pressure at the aircraft's altitude. The location of the static port is carefully chosen because air pressure varies slightly at different positions around the aircraft's exterior, and the observed pressure is affected by the aircraft's instantaneous angle of attack. The difference between the observed pressure and the actual atmospheric pressure causes a small position error in the indicated altitude and airspeed. Designers aim to minimize this error across the aircraft's operating range.

Some authors describe the atmospheric pressure at the aircraft's altitude as the freestream static pressure, while others, such as Gracey, define static pressure as the atmospheric pressure at the flight level and refer to the pressure near the aircraft as local static pressure. In fluid statics, the term (hydro)static pressure is sometimes used to refer to the pressure of a fluid at a nominated depth, though the concepts of dynamic and total pressure do not apply in stationary fluids.

Reader's Guide

Static pressure is fundamental to the understanding of fluid dynamics and the operation of low-speed aircraft. The simplified form of Bernoulli's equation—static pressure plus dynamic pressure equals total pressure—is essential for designing airspeed indicators for aircraft whose maximum speed is less than about 30% of the speed of sound. In aircraft, the static pressure system provides the key input to the altimeter and, along with the pitot system, drives the airspeed indicator.

The careful placement of static ports on an aircraft's exterior is critical to minimize position errors in altitude and airspeed indications across the aircraft's operating range. The concept also appears in fluid statics, where (hydro)static pressure describes the pressure at a depth in a stationary fluid. Because static pressure is identical to the ordinary concept of pressure, it is widely used in contexts beyond Bernoulli's equation, ensuring consistent terminology across aeronautical and fluid dynamics applications.