How Air Speed Indicators Work

[mcouch]

The title here may not be descriptive enough...I know how pitot-static systems work: difference in total pressure and static pressure yields dynamic pressure which is equal to 1/2*rho*v^2. My question is, why is it that the dial on the air speed indicator's gauge has linear markings? If I am measuring pressure to get velocity, but pressure increases with the square of the velocity, I would expect the markings on the dial to be very close together in the low range and then get gradually farther apart in the high range.

Clearly, there is a mechanical system inside the gauge that handles this, but it must be more than a torsion spring system because springs have linear expansion rates (kx). What mechanically is happening inside the gauge here?

 

[Navin A S]

The air speed indicator gauge works by using a combination of mechanical and pneumatic components. The mechanical component is a type of torsion spring that uses a rotating arm to convert the pressure differential into rotary motion. This rotary motion is then used to drive the pointer on the dial. The spring itself is designed to have a non-linear characteristic, which allows it to respond differently at different levels of pressure. At low pressure (low speeds) the pointer will move slowly, as the spring is relatively stiff and requires more torque to move it. As pressure increases (speed increases), the spring becomes less stiff and the pointer moves faster. This allows for a linear indication of airspeed on the dial.