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.
Joe
The Nampilot.
Comments
@u4eake:
For Pusher prop , keep the tube a minimum of 1.5 wing MAC away from the fuselage.
For traktor prop, at least 2.5 prop diameters from the prop centerline, which is probably also the fuse centerline.
If mounted under the wing, at least 1/3 MAC below the wing under surface, with the probe tip at least level with the wing tip, or forward of it is preferred.
@Simon:
That is not easy. Commercial Pitot-Static probes are calibrated in a wind tunnel, and have a polynomial curve fit by which you compensate the pressures according to AOA and yaw. There is no simple probe that works at large AOA and yaw angles. However, at low airspeeds the errors are low for probes such as the larger diameter one in the pictures - around 5% at 15deg AOA at 20m/s is typical. And 15deg AOA is large..
High accuracy probes have multiple dynamic pressure ports, spaced over the surface of a sphere at the probe tip, performing in a manner similar to the static holes spaced around the tube circumference. However, in this case , each hole is fed to a separate differential sensor and the sensors outputs are averaged, and subject to the wind tunnel derived calibration curves. Such probes are capable of 0.5% to 0.2% errors over the range 5m/s to 150m/s at AOA up to 20degrees...
The KIEL probe is of different priciple, using a probe in a venturi, and is very good indeed - take a look here:
http://www.unitedsensorcorp.com/kiel.html
Joe
The Nampilot
Thank you very much I am very glad you took the time to enlighten us about pitot tube design and placement.Any thing you have to ad would be grate.
Hi Nampilot. this is very helpful. I am interested in more information. I am flying an aircraft that is capable of flying high AoAs. Any information what to do to keep the readings of the dynamic port as accurate as possible ?
Cheers
-S
Very interesting! If mounting on the wing leading edge, how far do you think it should be from the fuselage to not be disturbed by it?
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