Airspeed Indicator Design Question

[bbrady1]

I am building an airspeed indicator that will be mounted on the roof of my car. The design is based on a water manometer. See the attached schematic. I will be able to determine the airspeed based on the weight indicated on the digital scale.

My question is in regard to the dynamic response of this system to gusts/turbulence. How fast the system responds is dependent on the diameter of the cross tube in the schematic. If the diameter is too small, abrupt changes in airspeed will not be picked up. If it is too large, I may never get a steady reading on the scale.

The "Diameter effects" picture shows how the system would respond to a typical gust. True airspeed appears in blue, and the other colors show the indicated airspeed for different cross tube diameters (reported in meters). What diameter cross tube should I select?

 

[boneh3ad]

Typically a Pitot tube's frequency response is dependent on both the diameter of the tube and the length of tubing between the pressure port and the sensing element, or more correctly the volume of that whole space.

The smaller the volume, the faster the response. This means minimizing diameter and tubing length as well as the large dead volumes you have in your design.

You also generally have your static port separate from your stagnation port, as the pressure inside your tube if designed properly will be at stagnation pressure. Your design would theoretically measure zero at all times.