Pitot Static System

Topics: Sensors

Overview

The pitot static system is a network of pressure-based instruments used to measure airspeed, altitude and vertical speed in aircrafts.

Its key components are:

• Pitot tube: a forward facing tube that captures ram air resulting from the forward motion of the aircraft. This captures the combination of static and dynamic pressure.
• Static ports: openings on the fuselage that measure the ambient atmospheric pressure (static pressure) undisturbed by the aircraft motion.

Airspeed

The pressure in the static ports is the static pressure $p_s$ because they are perpendicular to the direction of flow. The Pitot tube is pressurized by both the static pressure $p_s$ and the air velocity, which together constitute the total pressure $p_t$. Therefore, the dynamic pressure $p_d$ (contribution of the dynamic movement of the aircraft) can be obtained by subtracting the static pressure from the total pressure. Then, Bernoulli's equation can be used to solve for the airspeed.

$$p_s + p_d = p_t \\ \\ p_d = \rho \times \frac{V^2}{2} \\ \\ V = \sqrt{\frac{2(p_t-p_s)}{\rho}}$$

Altitude & Vertical Speed

Because of the inverse relationship between atmospheric pressure and altitude, the static pressure measurement from the static ports are used to determine altitude. The change in static pressure is also used to obtain the vertical speed.

Analog vs. Digital

Earlier analog versions of the pitot-static system used physical hoses and mechanical instruments connected to the analog cockpit indicators. New systems feature electronic pressure sensors connected to flight computers and digital displays. The working principle is still the same, but the tools are different.

Limitations

• The tubes could become clogged or pinched, which results in incorrect measurements
• At very low speeds, the difference between total and static pressure is small. So, the pitot-static tubes don't work well for low velocities.
• At very high velocities, like supersonic, the assumptions of Bernoulli's equations are violated and there is a shock wave at the front of the tube. So, the measurements are inaccurate.

References

1. https://pilotinstitute.com/pitot-static-system/
2. https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/pitot-static-tube-speedometer/