Pitot Tube

A Pitot tube is an instrument used to measure the velocity of a flowing fluid. Pitot tubes are employed in anemometers within wind tunnels and on aircraft in flight to calculate airspeed. They are also used to measure fluid flow rates.

It was invented in the early 18th century by French engineer Henri Pitot. In the mid-19th century, it was refined by French engineer Henry Darcy.

Principle of Operation of the Pitot Tube

The Pitot tube consists of two hollow tubes that measure pressure at different points along a pipe. One tube measures the stagnation or total pressure, while the other measures only the static pressure, typically that of the pipe wall. These hollow tubes can be mounted separately onto a pipe or assembled together within a single housing together.

The Pitot tube is aligned parallel to and opposite the direction of the fluid flow, namely air, according to work principle.

As the fluid passes through the Pitot tube, it measures the resultant pressure formed by the dynamic pressure due to the relative motion between the vehicle and the fluid, and the static pressure exerted on all surfaces of the fluid.

This total pressure causes the expansion of a sensitive diaphragm within the system, while the static pressure entering through static ports causes the diaphragm to contract proportionally to the static pressure. The difference between these two movements is known as dynamic pressure.

The dynamic pressure is reflected on the airspeed indicator scale. The Pitot tube provides Indicated Airspeed (IAS), which is calculated from the difference between static and dynamic air pressure. IAS is calibrated to Corrected Airspeed (CAS) using an aircraft-specific correction table. However, at high speeds, air compresses in front of the Pitot tube, yielding Equivalent Airspeed (EAS). Because temperature and air density vary with altitude, the final calculated airspeed, known as True Airspeed (TAS), is adjusted for the specific altitude. In computerized systems, TAS is computed and displayed to the pilot as direct.

The equation representing system operation:

Total pressure (p_t) = Static pressure (p_s) + Dynamic pressure (1/2rhoV²)

During ground operations, Pitot tubes must be protected from external factors such as insects. They are therefore covered with protective caps. If the cap is left on or the Pitot tube becomes clogged, the airspeed indicator may show zero, leading to accidents due to erroneous wrong sensor data.

On 11 November 2018, a Bombardier Challenger 604 business jet departing from the United Arab Emirates bound for Istanbul crashed in Iran with no survivors. Subsequent reports indicated that the aircraft entered an abnormal condition at 38,000 feet (12,600 meters) while flying at zero airspeed. The cause of the zero airspeed was determined to be a faulty instrument reading, prompting the pilot to raise the nose in an attempt to counter an apparent speed increase—when in fact no such increase had occurred. This maneuver caused the aircraft to enter a stall while airborne. The report emphasized that the airspeed indicators provided incorrect readings due to a clogged Pitot tube and inadequate or incomplete pre-flight maintenance.

Advantages of the Pitot Tube

• The Pitot tube is portable and contains no moving parts.
• It is economical.
• It causes minimal permanent pressure loss.
• The Pitot tube can be easily integrated into existing systems.

Disadvantages of the Pitot Tube

• Foreign particles in the fluid can easily clog the Pitot tube and disrupt normal readings.
• Less accurate.
• Narrow range.
• Short range.
• Low sensitivity.

In a typical F1 car, due to the many shapes that alter the speed of airflow as it passes through, teams can position Pitot tubes at various locations to obtain different data sets.