| Examining the Airspeed Indicator |
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How the airspeed indicator works
If we take a minute and look at the depiction of the airspeed indicator, we see a similarity between the altimeter and the airspeed indicator. As you can see, they both have a flexible capsule that is able to expand and contract with changes in air pressure. In the case of the airspeed indicator, it is not the static (outside pressure) that causes the capsule to expand, it is the "Total Pressure". Let's examine the pressures acting on the airspeed indicator a little closer. The air that surrounds us is static pressure. If an object is moved through the air, the force that is generating against it by means of the surrounding air is called dynamic pressure. Dynamic pressure is present any time an object is in motion. When an aircraft moves through the air, it has dynamic pressure due to its movement and also static pressure due to the surrounding air pressure. When these two pressures are combined, they produce Total Pressure. |
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When the aircraft moves forward, air enters the pitot tube. The force due to movement associated with this air, combines to expand the capsule. As the capsule expands, it causes the linkage rod to move which then causes the
the gears in the indicator face to rotate. This in turn causes the hands of the indicator to move to show the airspeed of the aircraft. What really causes the airspeed indicator to move is the change in dynamic pressure. (pressure
due to movement). Since static pressure is contained both inside and on the outside of the expandable capsule, the only thing that can change the pressure inside the capsule is the addition of dynamic pressure. As the aircraft accelerates, more dynamic pressure is introduced into the expandable capsule. This causes the airspeed indicator to register an increase in airspeed. As the aircraft slows, less dynamic pressure is generated and the capsule begins to contract. This results in the airspeed indicator registering a reduction in airspeed. Errors in Airspeed Indicator The greatest and most dangerous error you will find associated with the airspeed indicator is a blockage of the pitot tube or static port. |
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