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Aviation Words: Pitot

By Ian Brown, EAA 657159

September 2015 - Following our brain teaser item, it’s worth looking at the word pitot. An inventor in the 18th century named Henri Pitot, a Frenchman, invented the basic principle. It consists of inserting a tube with an orifice facing the flow of a liquid or gas. The pressure from that flow, called the dynamic pressure, is introduced to one side of a diaphragm.

Now if the other side of that diaphragm was closed off, the dynamic pressure would be working against the resistance of trying to compress the air on the closed side. Similarly, if one side of the diaphragm was closed off and the other side was exposed to the static air pressure, we’d have a diaphragm moving to compress or decompress the fixed volume of air. So, we have two devices—one to measure altitude and the other to measure airspeed.

In the case of the airspeed indicator, dynamic air pressure working to compress a fixed volume of air on the other side of the diaphragm is avoided by opening the diaphragm on the opposite side to the outside air, and that’s why we have static ports. The flow velocity is proportional to the square root of the dynamic pressure minus the static pressure, but you can forget the mathematics because the design of our altimeters takes that into consideration. Remember, though, that problems with the pitot or static ports can both give incorrect readings of both air speed and altitude.

So what can happen to cause this to go wrong? Well, the first thing is obviously a faulty instrument. Any sort of resistance of movement in either direction of the diaphragm can cause an incorrect reading, but this is relatively rare. Our gauges can go on working for years without problem.

A blocked static port or line will cause the altimeter to stay in the same position as we climb. A partially blocked one will cause a lag in the same instrument. In my own experience, this happened when an insect had laid a series of waxy looking nests in the plastic tube running from the static ports. On takeoff the altitude didn’t change at all. Kinks in the line could also cause this.

For the airspeed indicator it gets a bit more complicated. This indication (whether it’s digital or analogue) comes from a comparison of the dynamic air pressure in the pitot line on one side of the diaphragm with the static pressure at whatever altitude we happen to be flying. As our airspeed increases, the dynamic pressure increases.

If the static port is blocked, our dynamic pressure will be working against a resistance— a volume of air on the other side of the diaphragm that is somewhat compressible, especially if there are long runs of static line, but will nevertheless result in a higher static pressure than actual.

Since the estimate of airspeed is derived from the square root of the dynamic pressure after the static pressure has been subtracted, a higher than normal static pressure (blocked static) would result in a lower than expected estimate of airspeed. That means that if you were flying by your airspeed indicator, you would be correcting for this by flying at a higher airspeed than needed.

Conversely, if you were suffering from a partially blocked static port, you would probably have no real problem on takeoff, since you would start the flight with a correct static side of your altimeter. You might not notice much while flying, especially with a partial blockage, since you would eventually settle to a correct airspeed as the static pressure equalized.

The real problem would come when you approached your landing. As you descend, the landing static pressure would be lower than it should be. Subtracting this from the dynamic pressure would result in a higher displayed airspeed than actual. That would cause you to fly the aircraft slower than desired, and depending on the recent altitude change, this could take you very close to stall speed, especially if you were in the habit of flying the landing closer to the stall speed than the standard 1.3xStall.

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