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What is a Pitot Tube?

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  • Written By: Vasanth S.
  • Edited By: R. Halprin
  • Last Modified Date: 18 November 2016
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A pitot tube is an instrument that is mounted on the wing of an airplane. It is used to determine the dynamic pressure of the air flow and subsequently, the airspeed of the plane. The pitot tube was invented by the French hydraulic engineer Henri Pitot in the early 18th century. It was initially used to determine the speed of water in a river.

The pitot tube is used to determine the difference between static, dynamic, and total pressure of a fluid. A fluid is basically a liquid or a gas that flows. For example, the water flowing in a river or the oxygen molecules moving in the wind are considered fluids.

A typical pitot tube consists of a device that is situated in the path of the fluid. As the fluid flows through the tube, a pressure is created which is measured. The tube doesn't have an outlet so the fluid comes to a rest, and its stagnation pressure, which is also called the total pressure, is determined.

In order to calculate the airspeed of a plane, the static pressure of the fluid must also be determined. Static pressure is the pressure created from the moving fluid. This is obtained through a pitot-static tube, which consists of a second tube with holes on the sides.

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Once the static pressure is determined, it is subtracted from the stagnation pressure. This calculation provides the dynamic pressure of the fluid. With this information, the airspeed of the plane can be determined.

The pitot tube was first utilized by Henri Pitot to measure the speed of the Seine River. He studied the flow of water at various depths. His findings seemed to disprove the notion that water flowed faster at deeper depths.

In the 19th century, Henry Darcy modified the pitot tube. Darcy was a French engineer that focused his research on hydraulics. His design of the pitot tube is the current model used in most applications.

Typically, the pitot tube is attached to the wing or the fuselage of the plane. It needs to be checked on a regular basis to ensure it is working properly. Any malfunction can cause an incorrect reading of the airspeed.

One cause of an improperly functioning pitot is ice formation. Icing is a serious problem that can result in damage to the instruments on the plane. Regular deicing procedures are required to ensure that the pitot is correctly measuring the pressures associated with the airflow around the plane.

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Emilski
Post 4

I'm not sure I completely understand how the pitot tube flow measurement happens. I understand the concepts, but an having trouble seeing how it could be done. Now, I'm sure there is nothing to it with electronics and such, but how was it done originally in the water or in early airplanes?

I would have to assume that there was some type of meter that could record the fastest water flow or air speed, but how could it be done? I'm having trouble even thinking of possible ways. As for air travel, did the pitot tube only become popular once electronics were used, or was there a way to send pitot tube readings to the cockpit through some sort of analog dial reading?

cardsfan27
Post 3

@titans62 - I just looked up a picture, and instantly recognized the pitot tube. I have seen these on a lot of planes, and I always wondered what they were. I assumed it was some type of navigational equipment or part of the engine.

It's hard to estimate the size, since I've never gotten an up close looks, but from pictures and experience, I would say they are about a foot long. They are sort of an "L" shape that hooks to the middle of the wing.

On some of the pictures, it showed the pitot tube on fighter jets, and it was usually coming out of the nose, but on commercial planes it was on the wing. Is this to make the plane more aerodynamic, or is there some sort of special purpose that the pitot tube needs to be on the front of a fighter jet?

TreeMan
Post 2

I'm curious if pitot tubes are still the main tool used to determine air speed of a plane. Since we have GPS now, it seems like that would be more common since it wouldn't be affected by ice. I'm sure pitot tubes would still be common on planes as a backup.

I'm really curious about how pitot tube design has changed over the years. How big are they in general, and have they gotten smaller or more sophisticated over the years?

In a worst case scenario where a pitot tube gets damaged or malfunctions, is there some way for a pilot to figure out how fast the plane is moving?

titans62
Post 1

I had no idea there was so much that went into figuring out the speed of a plane. I guess on a car, it is the rotation of the wheels that determines speed, so there would have to be some sort of different measurement for a plane.

It's amazing how an invention from the 1700s to measure water speed could be applied to one of the biggest technological innovations of the 20th century.

I've been on quite a few planes, but I have never seen anything that looks like what I imagine a pitot tube would look like. Could someone describe one to me?

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