Dynamic & Static Pressures
As aircraft depend on the flow of air over their wings to generate lift there is a need to measure that air flow so that the speed of the aircraft through the air can be indicated. The same applies for indication of altitude and vertical speed.
The aircraft pitot static system measures this air flow and static pressure for presentation to the pilot. This page takes a look at how this is accomplished.
Static pressure
Air pressure is measured by two sensors: the static port and pitot tube. On some aircraft these are integrated in one probe and others have more than one static port to equalize pressure differences around the fuselage. These pressure differences occur due to slip / skid by the aircraft and/or turbulence.
Air molecules
The air exerts a pressure equally in all directions at any point in the atmosphere, this is called static pressure. This pressure results from the weight of the air molecules above that point and it decreases with a gain in altitude. This is how an altimeter operates.
The name implies that there is no involvement with the movement of the aircraft through the air, the measurement is done with a port (hole) facing at a right angle (90°) to the airflow, usually at the fuselage but sometimes included in the pitot tube.
Dual ports
Most experimental aircraft have only one static port but ideal would be two on either side of the fuselage. These two ports are then connected with a flexible hose and routed to the instrument panel.
Dynamic pressure
As air has mass (from the molecules) and air in motion has dynamic (kinetic) energy which is converted to pressure the moment a body tries to stop or slow it down. This is called dynamic pressure. Usually measured in a tube (pitot) and it includes static pressure at that point too.
Speed
Dynamic pressure is mathematically expressed as: 1/2 ρ V2, as we know from aerodynamics. It shows clearly that dynamic pressure relies on two things: air density (rho) and aircraft speed (V).
Air density
Density depends on a number of factors: altitude, humidity, temperature and ambient air pressure. These factors will result in a number of different speeds: IAS, CAS, EAS and TAS; see aircraft speeds for more detail.
Note: for good aircraft and engine performance you need thick air, ie. cold, low humidity air.
Pitot pressure
Total pressure as measured in a pitot tube (named after Henry Pitot) consists of static and dynamic pressure. The airspeed indicator has connections to both pitot and static lines measuring static and total pressure to indicate dynamic pressure on the instrument (ASI).
Formula
In short: pitot pressure - static pressure = dynamic pressure and this shows up to the pilot as speed on the airspeed indicator.
Altitude and vertical speed indicators use the static port and measure static pressure only.
