Airspeed Indicator

None

The airspeed indicator is part of the pitot-static system, a differential pressure system that measures both dynamic air pressure from the pitot tube and static pressure from a static port. Inside the casing of the instrument is a sealed diaphragm that receives both static and dynamic pressure from the pitot tube. Static pressure is also measure from inside the casing but outside of the diaphragm. The static pressures from both inside and outside of the diaphragm cancel each other out, leaving a measurement of total dynamic pressure, or ram air pressure.
As the airplane accelerates, the dynamic pressure from the pitot tube increases, causing the diaphragm to expand. Through mechanical linkage, this measurement of increased airspeed is shown in the airspeed indicator needle.
The instrument reads Indicated Airspeed: IAS is the airspeed read directly from the airspeed indicator.
Along with the altimeter and vertical speed indicator, the airspeed indicator is a member of the pitot-static system of aviation instruments, so named because they operate by measuring pressure in the pitot and static circuits.

This prototype has no external or internal markings and no dial glass.
The internals can be slid out of the case to reveal the two aneroid diaphragms and the driven mechanism.