What Is a Pitot Tube?

Pitot tube, also known as "airspeed tube", "wind speed tube", English name is Pitot tube. Pitot tube is a tubular device that measures the total pressure and static pressure of airflow to determine the speed of airflow. It was named after H. Pito of France. Strictly speaking, the pitot tube only measures the total pressure of the airflow, also known as the total pressure tube; the one that measures the total pressure and the static pressure at the same time is called the wind speed tube, but it is customary to call the wind speed tube a pitot tube.

Pitot tube, also known as "airspeed tube", "wind speed tube", English name is Pitot tube. Pitot tube is a tubular device that measures the total pressure and static pressure of airflow to determine the speed of airflow. It was named after H. Pito of France. Strictly speaking, the pitot tube only measures the total pressure of the airflow, also known as the total pressure tube; the one that measures the total pressure and the static pressure at the same time is called the wind speed tube, but it is customary to call the wind speed tube a pitot tube.

Chinese name

Pitot tube

Foreign name

pitot tube

nickname

Airspeed tube

Types of

Instrument for measuring pressure

inventor

Henry Pito

Invention time

Early eighteenth century

Pitot tube principle

Pitot tube is a device for measuring the total pressure (see pressure) of airflow. It was invented by French engineer H. Pito in the 18th century, hence the name. It is more difficult to directly measure the velocity of the airflow experimentally, but the pressure of the airflow can be easily measured with a manometer. It is mainly used to measure the speed of the aircraft, and it also has a variety of other functions. Therefore, the pitot tube can be used to measure the pressure, and then Bernoulli's theorem can be used to calculate the velocity of the airflow. Pitot tube consists of a double-layered sleeve with a round head (see picture), the diameter of the outer sleeve is D, and a total pressure hole connected to the inner sleeve is opened at the center O of the round head, which is connected to one end of the manometer. The diameter is 0.3 to 0.6D. A row of static pressure holes perpendicular to the outer tube wall is uniformly opened in the circumferential direction at the position C at the outer surface of the outer tube at a distance of about 3 to 8D from the O, and the other end of the manometer is connected, and the pitot tube is placed at a steady airflow at the desired speed. In the middle, the axis of the tube is aligned with the direction of the air flow, and the leading edge of the tube faces the incoming flow. When the airflow approaches the point O, its velocity gradually decreases, and the stagnation to the point O is zero. Therefore, the total pressure P is measured at point O. Secondly, because the pipe is very thin, point C is sufficiently far from point O, so the speed and pressure at point C have basically recovered to the same value as the co-current velocity V and pressure P . Static pressure. For low-velocity flow (the fluid can be considered approximately incompressible), the formula for determining the flow rate by Bernoulli's theorem is:

According to the total pressure and the static pressure difference P ₀ -P measured by the manometer, and the density of the fluid, the velocity of the airflow can be obtained according to formula (1).

The following relation holds for subsonic flow:

Where Ma , c , and T are the Mach number, sound velocity, and temperature of the incoming flow, respectively; is the specific heat ratio; and R is the gas constant. The total pressure and the static pressure are measured by a manometer, and the temperature of the flow is measured by a temperature measuring instrument, so the flow velocity V can be obtained according to formulas (2) and (3).

For supersonic flow, in vitro shock waves (see shock waves) are generated at the head of the pitot tube, so the following relationship holds:

Where p ₂₀ is the total pressure at the stagnation point after the shock. Further, the velocity V can be obtained _.

In the case of high subsonic flow and supersonic flow, it is not accurate to measure the static pressure using the static pressure orifice because of various interference factors. At this time, other methods are often used to measure static pressure.

Because the pressure measuring hole has a certain area, and due to the interference of the pole and the manufacturing, the pressure difference measured by the pressure gauge will not be exactly P ₀ -P , so it is usually multiplied by the root of formula (1) A correction factor close to 1 (between 0.98 and 1.05). The value can be obtained by calibration with the standard pitot tube. For some special types of flow, such as low Reynolds number flow and thin gas flow, where viscosity plays a major role, the conventional pitot tube calculation formula must be appropriately modified to accurately calculate the flow rate.

Pitot app

Airspeed tubes are extremely important measurement tools on aircraft. Its installation position must be outside the aircraft in the area where the airflow is less affected by the aircraft, usually directly in front of the nose, in front of the vertical tail or wing tip. At the same time, for the sake of insurance, one aircraft is usually equipped with more than two airspeed tubes. Some aircraft have two small airspeed tubes on both sides of the fuselage. The American stealth fighter F-117 is equipped with four omnidirectional atmospheric data probes at the forefront of the nose. Therefore, the aircraft can measure not only high aerodynamic pressure and static pressure but also the sideslip angle and angle of attack of the aircraft. There are also several small blades on the outside of the airspeed tube on some airplanes, which can also play a similar role; vertical installation is used to measure the aircraft's sideslip angle, and horizontally installed blades can measure the aircraft's angle of attack. The small hole at the front end is frozen and blocked during flight. Generally, the airspeed tube on the aircraft has an electric heating device.

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

The static pressure measured by the airspeed tube can also be made into an "elevator speed table", which measures the speed of the change in the aircraft's altitude (climbing rate). There is also a capsule inside the table, but the pressure inside the capsule is not measured based on the dynamic pressure measured by the airspeed tube but through a special tube with a small hole at the outlet. The size of the small hole in this tube is specially designed to limit the speed of the pressure change in the capsule. If the aircraft rises quickly, the air pressure inside the capsule cannot be reduced quickly due to the small holes, and the pressure outside the capsule can reach the pressure equivalent to the outside atmosphere quickly due to the static pressure hole on the airspeed tube. The capsule bulged. By measuring the deformation of the capsule, you can calculate how fast the airplane is rising. When the plane descended, the situation was reversed. The pressure outside the capsule increases sharply, but the air pressure inside the capsule can only rise slowly, so the capsule sinks, driving the pointer, showing a negative rate of climb, that is, a rate of decline. After the plane leveled, the air pressure inside and outside the capsule gradually became equal, the capsule returned to its normal shape, and the lift speed indicator indicated 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 (called ground speed) should also be added to the wind speed (flying downwind) or subtracted from the wind speed (flying upwind). In addition, the airspeed tube speed measurement principle uses dynamic pressure, which is related to atmospheric density. At the same relative air velocity, if the atmospheric density is low, the dynamic pressure will be small, and the deformation of the capsule in the airspeed indicator will be small. Therefore, the same airspeed indicates a lower value at high altitude than at low altitude. This airspeed is commonly referred to as "speed". Modern airspeed indicators have two hands, one thinner and one wider. The wide pointer indicates "gauge speed", while the thin one indicates the airspeed equivalent to the surface atmospheric pressure after various corrections, called "real speed".

In addition to measuring aircraft speed, pitot tube also has many other functions. In scientific research, production, teaching, environmental protection, and tunneling, mine ventilation, and energy management departments, pitot tubes are commonly used to measure the air velocity in ventilation ducts, industrial ducts, and furnace flues, and the flow is determined by conversion. speed. Using a pitot tube to measure the speed and determine the flow has a reliable theoretical basis, and is convenient and accurate to use. It is a classic and extensive measurement method. It can also be used to measure the pressure of a fluid.

Pitot tube reference

1. terms of: Wang Wu a "Encyclopedia of China", 74 (second edition) physics entries: Fluid Mechanics: Encyclopedia of China Publishing House, 2009-07: 380.