Dynamic & Static Pressures
As aircraft depend on the flow of air over and under their wings to generate lift. There is a need to measure that airflow so that the speed through the air can be shown to the pilot on the instrument panel. The same applies also for indication of altitude and vertical speed.
With this information the pilot can keep the aircraft flying at the correct speed and land at the slowest speed possible without overrunning the runway.
The pitot static system measures this airflow and static/dynamic pressure for presentation to the pilot on several of the basic flight instruments. This page takes a look at how this is accomplished.
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.
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.
Most experimental aircraft have only one static port but ideally would be two on either side of the fuselage. These two ports are then connected with a flexible hose and from a tee fitting routed to the instrument panel. The advantage is that when the aircraft slips or flies in turbulence, no air pressure is measured in the static port.
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.
Dynamic pressure is mathematically expressed as: 1/2 ρ V2, as we all know from aerodynamics. It shows clearly that dynamic pressure relies on two things: air density (rho, ρ) and aircraft speed (V).
Density itself depends on a number of factors: altitude, humidity, temperature and ambient air pressure. All of these factors together will result in a number of different speeds available for the aircraft: IAS (indicated), CAS (corrected), EAS (equivalent) and TAS (true); see aircraft speeds for more detail on this.
Note: for the most optimum aircraft and engine performance you need thick air, ie. cold, low humidity air. Which is in general experienced during winter time!
The total air pressure as measured in a pitot tube (named after Henry Pitot) consists of these variables: static and dynamic pressure. The airspeed indicator has connections to both pitot and static lines and thus is able to mechanically subtract static pressure to indicate dynamic pressure on the instrument face (ASI).
In short: pitot pressure - static pressure = dynamic pressure and this shows up to the pilot as speed on the airspeed indicator.
Both altitude and vertical speed indicators use the static port and measure static pressure and the rate of chance only.
During the preflight inspection the pilot must make sure that the pitot tube is not blocked, either by the cover or insects in the tube. For instrument flights the pitot heater must be checked for correct operation and warmup.