I was asked on the 'Advances in Airspeed Handling' forum to give more info on this, so here is some practical info on Pitot tubes and sense piping for Airdata measurement systems.
In general, strategically located air inlet ports are required to sense both static and dynamic air systems, feeding said pressures to the relevant sensors.
Static pressure sensing is used to compute the Barometric Altitude, and must be as immune to airflow as possible.
Dynamic pressure is related to the flow of air at the inlet of the dynamic pressure sensor, as induced by the aircraft motion through the air. This pressure is very small for low air speeds ( sub 40m/s, say) and is measured by a differential sensor. Such a sensor has both ports exposed to the static ambient air, with one port having the pressure caused by movement added to the ambient pressure. This way a low pressure sense element can be used to sense the difference between static and dynamic pressures. However, any variation on the static port input side will be seen by the sensor as a change in differential pressure, and a change in calculated airspeed will result.
It is therefore important to ensure that the static pressure sensed does not vary due to aircraft attitude, wind and wind direction, etc. Measured static pressure should only vary as a result of altitude and temperature.
These requirements place considerable constraints on the design and location of the air inlet ports for both airdata sources.
The dynamic inlet port is normally a suitably shaped orifice, facing the oncoming air. This orifice is a hole into a tube, the tip of which is rounded to coax the oncoming air to neatly part without creating vortices at the inlet. At low air speeds the 'rounding' is not critical - a hemisphere the diameter of the tube is acceptable. At high airspeeds the shape is critical, becoming more pointed. The dynamic port is most accurate at an angle directly facing the oncoming air. Pitching or yawing the tube in the oncoming air reduces the resultant pressure with ensuing airspeed changes.
The static pressure ports are normally orifices directly side on the the airflow. These are often combined co axially in a Pitot-Static Tube, with the holes spaced evenly around the circumference of the tube, at least 10 to 15 tube diameters rearwards of the probe tip. To close to the tip results in tip vortices affecting the static pressure at the hole entrances.
Such a tube can be located in the wing tip or in the fuselage nose ( pusher prop) but the static port holes must be at least 40 to 50 tube diameters from the wing leading edge or fuse nose, to not be affected by the airflow.
The picture below shows a working tube, the larger diameter one. The thinner diameter tube static sensing performance is poor due to the static inlet holes being far to close to the tip.
The static inlet holes are spaced around the tube so that the aiflow 'balances' out when the tube is not facing directly into the airflow, eg, when pitching or yawing, or with side winds. Higher pitch/yaw angles do however result in erroneous measurements.
An alternative static air sense port can be located directly on the straight sides of the fuselage, preferably two ports directly opposite each other, and joined in a T.
These pictures show the making of such a tube setup:
This is the long tube, the end of which will be flush with the fuselage sides. left and right. The nick in the middle is where the hole will be, into which the T tube is soldered.
This is the T Tube, with the scalloped end.
These are the two tubes tinned and ready for soldering
Now soldered together:
This T assembly is inserted into the fuselage, from the insight, left or right side first, and then bonded in place with the ends of the tube flush with the left and right side of the fuselage. The T piece is then piped to the static and dynamic sensors.
The tube ends MUST be on a regular surface part of the fuselage, ie, not directly behind or in front of any protrusions, bumps , landing gear, etc. Also not on a tapered part of the fuselage. All these will cause vortices and pressure variations at the tube tips, rendering measurements worthless. The principle relies on a smooth airflow past the tube orifices, and if any side wind is experienced, the air enters one hole and exits the other, with little or no pressure change in the T part of the tube.
This is a picture of my SurVoyeur aircraft fuselage, showing 3 positions where I placed this tube to do measurements to see effects of the chosen position.
Location 1 is no good - was on the tapered part of the fuse and pressure changed with airspeed.
Location 2 is good, on the flat portion, and forward of the landing gear vertical struts.
Location 3 is no good - it is on the flat portion, but the landing gear vertical stut ( only 5mm thick) creates sufficient disturbance to cause significant variation of pressure with airspeed ( of the order of 0.3mbar - 1mbar = approx 8meters .)
If anyone is interested in more detailed info, let me know.