The Pitot Tube was invented in 1884 by French engineer Gabriel Voisin. He wanted to improve the accuracy of his invention, the barometer.
This device measures atmospheric pressure and temperature. This article will discuss pitot tubes and how they work to provide precise airspeed measurements.
What Is A Pitot Tube?
A “pitot” is defined as either a) noun meaning a small tube or b) verb meaning to blow into a large tube. A pitot tube is an instrument used to sense changes in pressure.
A pitot tube basically measures changes in pressure. An example would be the needle on a barometer.
The needle moves up or down depending upon the amount of pressure exerted on the surface of the metal ball.
In aviation, the term “pitot tube” refers specifically to an instrument mounted on the nose of an aircraft that measures airspeed and/or true airspeed.
A pitot tube is an instrument that measures wind speed and direction at any given altitude. It consists of two main components: a probe and a sight glass or pitot.
The probe part of the pitot tube is what actually gives it its name ‘pitot’.
How Does A Pitot Tube Work?
A pitot tube consists of two thin metal plates attached to a glass or plastic tube.
When the plane flies through the air, the air flows between these plates and pushes them apart.
This creates a vacuum inside the tube. As the air passes through the tube, it cools down and its density decreases.
The lower the density, the faster the air moves through the tube.
By measuring the speed of the air passing through the tube, the airspeed of the plane can be calculated.
The probe has openings on both sides through which the wind passes. These openings act as the pitot holes.
When the wind hits these pits, a small amount of air enters each hole.
From there, this air mixes with the surrounding atmosphere to form a volume of total air.
This mix allows for the measurement of pressure and velocity. As air moves past the pitot probes, it deforms the outer wall.
This causes an imbalance in the flow rate of air entering each pitot hole.
This imbalance makes the total volume of air measured smaller than the actual volume of air.
Since all this happens at a constant height, the difference in air pressure is multiplied by the length of the tube.
Why Are Pitot Tubes Used In Aircraft Instrumentation?
Pitot tubes have been used since the first airplane flight. Most airplanes today still contain pitot tubes.
Although newer technology exists, most older planes require their usage.
Pitot tubes are also crucial for safe navigation. Airplanes flying over land must know their position relative to the ground.
For this reason, we need to get readings from our pitot tubes , so we can plot out our location. We can then adjust the course or elevation accordingly.
Pitot tubes help us know when turbulence occurs. Turbulence occurs when winds move fast enough that the air cannot stay calm and smooth.
When this happens, a wing may stall. Without pitot tubes, pilots would not know if the wings were stalling until it happened too late!
To take full advantage of using a pitot tube, we must monitor airspeed and direction.
With pitot tubes, the wind speed and direction are only known upfront where the wind meets the aircraft.
What we really want is to measure the wind speeds as the air travels along the fuselage.
This is why there is sometimes a pitot rod instead of a pitot tube. The pitot rod measures the wind speed as it travels along with the aircraft.
However, reading it requires some skill because the wind will not always be directly in front of the nose. Instead, the wind must pass behind the nose.
This increases the time needed to read the airflow data and reduces the accuracy of the reading.
Even though the pitot rod is much quicker, it cannot correct for changes in pitch angle.
So even though the pitot rod reads more accurately, its readings will never make sense unless the plane is level.
To make matters worse, pitot rods often have pitot hole sizes too large for the pitot tube to adequately sample.
How Do I Determine The Size Of My Pitot Tube?
Most airplanes and helicopters have two pitot tubes (or pitot rods). One of them should be set for differential pressure measurements.
The other should be set for static pressure readings.
Aircraft makers tend to have slightly different sizes for pitot tubes. You can look at the specifications provided by the manufacturer to see what size you need.
But it’s hard to guess how many inches an inch equals with exactness. You simply estimate from the specification.
If you’re doing maintenance on an older machine, you must check both sides of the plane.
It’s common to find that one side has more flow than the other. Then you’ll probably need to replace both pits.
What Else Do I Need To Know About Pitot Tubes?
Because pitot tubes operate through the skin of the aircraft, you need to understand the structural soundness of your aircraft to prevent injury.
Be sure to avoid any areas where excessive stress could occur.
Any high-pressure area such as seams between panels or rivets should receive extra care and attention.
Be diligent to clean all surfaces inside the pitot tubes regularly. Use mild detergent rather than harsh chemical cleaners and let dry well before reusing.
Pitot tubes are very small, but they’re easily damaged by sharp objects. A pilot needs to keep his hands off any part of the airplane that enters the pitot tube.
While working outside the cockpit, wear gloves to protect against cuts and jagged glass.
Pitot Tube Safety Considerations
There are many ways that people injure themselves or damage their planes while operating pitot tubes. Here are a few examples:
In a Cessna 140, a student decided to change the pitot tube while sitting on the ground.
He was leaning over to remove the pitot tube when he moved forward suddenly.
His helmet struck the pitot tube causing him to fall backward onto the runway and injure himself.
An instructor was flying a Cessna 172 during training exercises. When landing after practicing instrument approaches, the left-wing touched down first and bounced up.
As the right-wing continued to land, it came into contact with the left-wing. His head hit the pitot tube, breaking it.
During engine maintenance of a Piper PA28 Cherokee, a mechanic was replacing a tire.
He put his foot under the propeller to stabilize the spinning prop so it wouldn’t cause further damage.
Unfortunately, he stepped too close to the pitot tube and cut his leg open.
So take precautions to ensure safety. Never lean into or near pitot tubes. They are fragile and easy to break. Keep away from anything that might strike the wall of the pitot tube.
Keep your gloved hand safely above the pitot tube when you use tools or perform tasks around the pitot tube.
Pitot tubes operate according to Bernoulli’s principle which states that for incompressible fluids flowing past an object, the velocity vector of the fluid changes direction (from upwards) as it passes over the rounded end of the object, like the airfoil section of an airplane wing.
The Pitot tube is used to measure the actual velocity of the air passing over the top surface of each wing.
If the air is not moving at its correct speed, then the airflow pattern becomes disturbed giving information about the current attitude and orientation of the aircraft.
Where Are Pitot Tubes Located?
The pitot tube is located at the front of the windshield. It consists of two metal discs separated by a rubber diaphragm.
Although the pitot tube has no moving parts, some components may be susceptible to mechanical failure.
For example, the rubber diaphragm can tear (the main component), jamming the flow of air through the pitot tube.
Also, corrosion affects the reliability of the pitot tube because it promotes deterioration and reduces the effectiveness of the pitot tube element.
It is important to note that pitot tubes deteriorate with time and cannot always be repaired.
However, there are several preventive measures you can apply to extend the life of the pitot tube and reduce possibility of future failures.
Preventive Measures To Extend The Life Of Your Pitot Tube
Here are some tips to help you extend the life of your pitot tube.
Ensure that all areas inside the cockpit have been cleaned thoroughly before takeoff. This includes cleaning the pitot tube itself, along with surrounding surfaces.
If possible, let the pitot tube dry completely between flights. Do this by removing the cap from the pitot tube and leaving it in place until dry. Once dry, replace the cap.
Keep your airplane free of obstructions such as loose wires, peeling paint, and other foreign objects.
These can cause problems if they interfere with the operation of the pitot tube or if they are ingested by the pitot tube.
Avoid using sharp pilot instruments. Sharp edges on pitot tubes and instruments increase wear rates significantly.
Never force air into a pitot tube once it is installed. Doing so can break the rubber diaphragms.
Do not install pitot tubes on aircraft while the aircraft is being fueled because fuel fumes can corrode the pitot tube.
Before installing a new one, flush the old pitot tube with clean water to remove any residual fuel residue. Flush again just prior to initial flight testing. After that, periodically wash out the pitot tube with fresh water to keep corrosion at bay.
When replacing pitot tubes, ensure that they match the original equipment manufacturer’s specification.
Check the location of the pitot tube every time the aircraft takes off to be sure that it faces forward as designed. If it tilts backward, it could affect the accuracy of the readings.
After landing, check for moisture build-up around the pitot tube. If excessive liquid is present, change the gauge filter assembly to prevent damage to the pitot tube or fuselage skin.
Check the pitot tube frequently during operations to ensure proper functioning. A sudden drop in pressure indicates that the pitot tube may need repairing.
The pitot tube must be properly maintained to ensure its operational integrity.
Many maintenance procedures can be performed throughout the lifetime of an instrument without having to disassemble it.
It is highly recommended to perform periodic inspections and regular maintenance tasks on the pitot tube to avoid potential performance issues such as inaccurate readings due to malfunctioning of the pitot tube.
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