The point of the altimeter static ports is to make sure that the pressure inside the avionics bay (where the altimeter pressure transducers are) is as close to ambient pressure as possible. Careful placement of the ports is required - they should not be drilled too close to the nose cone, or a transition, or any other airframe discontinuity since we need laminar airflow over the port. Turbulent airflow will give a false pressure.

The rule of thumb for ports is:

1/4” diameter port for every 100 cubic inches of bay volume.

Bay volume = π x R2 x L

where:     π  = 3.14

                R = bay radius (inches)

                L = bay length (inches) - note: this is the altimeter bay length

It is not recommended to use just one port - a minimum of three are recommended (spaced angularly at 120 degrees) and on bigger airframes, possibly a 4th. Where multiple ports are used, each port can be smaller than the “single port” diameter, as long as the total cross-sectional area is the same. Cross sectional area is calculated as: A = π x R2 where R = port radius in inches.

A 1/4” diameter port has a cross-sectional area of 0.05 sq inches. So for every 100 cubic inches of bay volume, one needs a port of 0.05 sq ins. For example, a 6” ID bay which is 20” long has a volume of 565.2 cu ins. It therefore needs a overall port cross section of 565.2/100 x 0.05 = 0.282 cu ins. If one uses 3 ports, each port would have c/s area of (0.282/3) = 0.094 sq ins which equates to a diameter of 0.34”. If using 4 ports, each port would need to be 0.3” diameter.

Do NOT confuse these static ports with vent holes for the main airframe sections. These are to allow expanding air to escape during flight, so as not to cause premature separation. A 1/4” hole for each airframe section is generally enough.