The Airspeed Indicator

The bush plane indicator utilizes the difference between ram air and static air to mechanically compute airpseed. The pitot makes use of dynamic ram air. The static port measures static air pressure. Both are linked though the gauge itself, with a sealed aeneroid capsule separating the two bush plane . The capsule, or bellows, is an aeneroid capsule sealed at standard pressure. As the higher ram air pressure depresses the capsule a very simple, but very exact mechanical linkage shifts the airspeed needle to display the correct bush plane .
Of course, blocking one or both ports makes it impossible to measure the difference. On the pitot side the pitot has a heater to combat freeze up. On walk around you need to check that the pitot heat is functioning by turning the pitot heat on and touching the pitot tube. You also will have to check that nothing like debris or a bug has entered the tube opening. A pitot tube cover, made of red vinyl so that you don’t forget to remove it, helps with non-ice obstructions.
The static port can get plugged by insects or debris, or if ice forms over the top of it. There is normally an alternate static port to be used in the case of emergency. Some planes will have two static ports.
The stationing of the pitot is set for thegreatest possible operation. It has got to do the joban attitude spectrum from normal flight to a stall attitude and still furnish ram air pressure for the system to work. It’s placement is, therefore, important.
The static port is situated out of the airflow and away from form generated turbulence so that it can measure static air pressure as flawlessly as possible.
A plugged static port will cause a low reading air speed indicator.
As mentioned, the static port has an alternate port for emergencies in order to mitigate this difficulty. A second static port also helps avoid errors due to slipping (when you’re in a slip that exposes the static port to ram air pressure the airspeed will read low incorrectly). Errors also happen if the density of air fluctuates from what the capsule was calibrated for, as a result of icing or water, or from a contaminated or clogged pitot. Do not try to suck an obstruction out of the static port – they are very fragile.
The density of air malfunction can be handled by calibration.
A clogged pitot turns the air speed indicator into an altimeter, meaning going up increases indicated speed and diving means a slowdown in indicated speed, even in a high speed dive. This is because the ASI aeneroid capsule will compress as the plane goes up and puff up as it descends, seeing that there is no ram pressure from the pitot to counter it. Of course, indicated bush plane with a clogged pitot will be random.
The air speed indicator is the only one of the three pitot static three pack that makes use of both static and ram air pressure.


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