Pitot tube, also known as "airspeed tube" or "wind speed tube", in English is Pitot tube.

Pitot tube is a tubular device that measures the total pressure and static pressure of an airflow to determine its velocity. Named after the invention of H. Pitto from France. Strictly speaking, pitot tubes only measure the total pressure of airflow, also known as total pressure tubes; Only those that measure both total pressure and static pressure simultaneously are called anemometers, but they are commonly referred to as pitot tubes. The structure of the pitot tube is shown in the figure, with a hemispherical head and a double layered sleeve at the back. When measuring speed, align the head with the incoming flow, and the small hole (total pressure hole) at the center of the head feels the total pressure p 0 of the incoming flow, which is transmitted to the pressure gauge through the inner tube. A row of holes (static pressure holes) are uniformly opened on the outer sleeve wall about 3-8D behind the head to sense the incoming static pressure p, which is also transmitted to the pressure gauge through the outer sleeve. For incompressible flow, according to the Bernoulli equation, the wind speed is a detailed introduction to the pitot tube technique, where ρ is the airflow density. For compressible flow, after measuring p 0 and p, the Mach number of the airflow can be calculated based on the energy equation, and then the velocity can be determined. However, in supersonic flow, an off body shock wave appears at the head of the pitot tube, and the total pressure hole senses the total pressure behind the wave, making it difficult to accurately measure the incoming static pressure. Therefore, the pitot tube is no longer applicable. The total pressure hole has a certain area and senses the average pressure near the stagnation point, which is slightly lower than the total pressure. The static pressure sensed by the static pressure hole also has certain errors, and other factors such as manufacturing and installation may also have errors. Therefore, when calculating the flow rate, a correction factor Zeta should be added, which is a detailed introduction to the pitot tube technology. The Zeta value is generally within the range of 0.98-1.05, and is determined by calibration in a known velocity airflow or calibration with a standard pitot tube. Pitot tubes have a simple structure, are easy to use, and have a wide range of applications. Pitot tubes, also known as airspeed tubes, are often installed at the head or leading edge of aircraft wings to measure the relative air speed during flight.

Airspeed tubes, also known as airflow direction sensors or flow angle sensors, are connected to precision potentiometers (or synchronizers or parsers) to provide an electrical signal representing the direction of airflow relative to the longitudinal axis of the atmospheric data truss.

The airspeed tube is an extremely important measuring tool on an aircraft. Its installation position must be in an area outside the aircraft where the airflow is less affected by the aircraft, usually in front of the nose, vertical tail or wing tip. For safety reasons, an aircraft is usually equipped with two or more airspeed tubes. Some aircraft have two small airspeed tubes on either side of the fuselage. The US stealth fighter F-117 is equipped with four omnidirectional atmospheric data probes in front of the nose, so the aircraft can not only measure atmospheric dynamic pressure and static pressure, but also measure the aircraft's sideslip angle and angle of attack. Some aircraft have several small blades installed on the outside of the airspeed tube, which can also serve a similar purpose; Vertically installed blades are used to measure the aircraft's sideslip angle, while horizontally installed blades can measure the aircraft's angle of attack. To prevent the small hole at the front end of the airspeed tube from freezing and blocking during flight, the airspeed tube on most aircraft is equipped with an electric heating device..

The principle of measuring aircraft speed with a airspeed tube is as follows: when the aircraft is flying forward, the airflow rushes into the airspeed tube, and a sensor at the end of the tube senses the impact force of the airflow, known as dynamic pressure. The faster the plane flies, the greater the dynamic pressure. If we compare the static pressure and dynamic pressure of the air at rest, we can know how fast the incoming air is, that is, how fast the airplane is flying. The tool for comparing two types of pressure is a hollow circular box with a corrugated surface made of two thin metal sheets, called a bellows. This box is sealed, but there is a tube connected to the airspeed tube. If the aircraft speed is fast, the dynamic pressure increases, and the pressure inside the bellows increases, causing the bellows to bulge. A device consisting of a small lever and gears can measure the deformation of the membrane box and display it with a pointer, which is the simplest aircraft airspeed gauge.

Modern airspeed tubes have many small holes around the tube in addition to the front opening, and another tube is used to measure the static atmospheric pressure inside the airspeed gauge, which is called static pressure. The deformation size of the airspeed gauge inner membrane box is determined by the difference between the static pressure outside the membrane box and the dynamic pressure inside the membrane box.

The static pressure measured by the airspeed tube can also be used as a calculation parameter for the altimeter. If the membrane box is sealed, the pressure inside will always remain equivalent to the pressure of the ground air. In this way, when the aircraft flies into the air and the altitude increases, the static pressure measured by the airspeed tube decreases, and the bellows will bulge. Measuring the deformation of the bellows can determine the altitude of the aircraft. This type of altimeter is called a barometric altimeter.

The static pressure measured by the airspeed tube can also be used to create a 'takeoff and landing speedometer', which measures the speed of the aircraft's altitude change (climb rate). There is also a membrane box in the table, but the pressure inside the membrane box is not measured based on the dynamic pressure measured by the airspeed tube, but is measured through a specialized tube with a small hole at the outlet. The small hole size on this tube is specially designed to limit the speed of pressure changes inside the membrane box. If the airplane rises quickly, the pressure inside the bellows is constrained by the small holes and cannot decrease quickly, while the pressure outside the bellows can quickly reach the same pressure as the outside atmosphere due to the static pressure holes on the airspeed tube, causing the bellows to bulge. By measuring the deformation of the membrane box, the speed of the aircraft's ascent can be calculated. When the plane descends, the situation is exactly the opposite. The pressure outside the membrane box increases rapidly, while the pressure inside the membrane box can only slowly increase. As a result, the membrane box sinks, driving the pointer to display the negative climb rate, that is, the descent rate. After the plane flies flat, the pressure inside and outside the capsule gradually becomes equal, the capsule returns to its normal shape, and the takeoff and landing speedometer indicates zero.

The speed measured by the airspeed tube is not the actual speed of the aircraft relative to the ground, but only the speed relative to the atmosphere, so it is called airspeed. If there is wind, the speed of the aircraft relative to the ground (known as ground speed) should also be added to the wind speed (downwind flight) or subtracted from the wind speed (upwind flight). In addition, the principle of airspeed tube speed measurement utilizes dynamic pressure, which is related to atmospheric density. At the same relative airflow velocity, if the atmospheric density is low, the dynamic pressure will be small, and the deformation of the membrane box in the airspeed indicator will be small. So for the same airspeed, the indicated value at high altitude is smaller than at low altitude. This airspeed is generally referred to as' gauge speed '. Modern airspeed indicators have two pointers, one relatively thin and the other relatively wide. The wide pointer indicates the 'gauge speed', while the thin one indicates the airspeed at ground atmospheric pressure after various corrections, which is called the 'real speed'.

Pitot tubes are not only used to measure aircraft speed, but also serve multiple other functions. In scientific research, production, teaching, environmental protection, tunnel and mine ventilation, and energy management departments, pitot tubes are commonly used to measure the airflow velocity in ventilation ducts, industrial pipelines, and furnace flues. After conversion, the flow rate is determined, and the water flow velocity in pipelines can also be measured. Using a pitot tube for speed measurement and flow determination has a reliable theoretical basis, is easy to use, accurate, and is a classic and widely used measurement method. In addition, it can also be used to measure the pressure of fluids.