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
Thank you, Jose! That was a real gem!
Right now, I don't have access to a 3D printer or a lathe, but I'll see what I can source from the university labs when the time comes.
I'll give a read to your site as soon as possible.
Georacer, I've made and calibrated many pitot tubes, it's not so hard as you think.
Here below a photo of all the pieces needed to make a 8mm dia model from BasicAirData site
Free drawings , calculations ,3D models and assembly process videos to make your own pitot or better to make your personalized pitot..For some info look here and there. No need for complex machining as some parts can be 3D printed.
I'll be very happy to answer your questions on this blog.
Yeah, I expected that answer.
The sad part is that I'm primarily an Electrical Engineer, not a Mechanical one or a machinist. I've never worked with a lathe.
But even if I could, why would the larger probe (say 20cm) that I would build be any better than the 3DR one? You can tell I'm not an expert in pitot technology...
Georacer, are you 'DIY' enough to maybe try make one yourself? You can easily make separate static and dynamic tubes, as I indicated in the blog - to do a combined Pitot Static tube would best be tackled with a small lathe - can be done without but messy and tricky then. If you have access to some metalworking equipment I could guide you... I do not know of any supplier selling calibrated tubes in the sub 100 Euro range.
Joe
The Nampilot
The pitot technology might have become quite light and reliable, but does this apply to the low-end DIYDroner?
I wanted to find something better/more advanced than the simple $10 probe, which comes with the 3DR airspeed kit. However, the next step in price was at the $600-$1000 range, such as the retail version of the pitot probe of the Penguin professional UAV.
At one point I stumbled upon this site, but they refuse to reply to my mails.
Is there any solution in the sub-100 or sub-50 USD range? I live in Greece, so a dealer in Europe would be ideal.
@ Simon, well, yes, I thought it was obvious, esp from the United Sensor Corp website that these devices are in the sub 150gram region - the larger diameter brass Pitot-Static tube in my blog is only 76grams.
Nampilot
@Nampilot: I think the aircraft's weight does matter, if the sensor itself is 1kg it's of no use for me. I do have some basic understanding of these things, used to fly real aircraft when I was younger. So 220 usd is not too much if it gets me reliable airspeed readings. Thanks for the info.
@Simon:
The smallest KEIL probes, types KA or KB from United Sensor Corp work easily down to 1.2m/s airspeeds, and up to Mach 0.3. The aircraft weight does not really have anything to do with it, but the speeds you fly at must be in the probe's calibrated range. The KEIL probes covers anything hobbyists might fly..Note that the probe is only a 'dynamic' sensors probe - you still need a source of static pressure. Also, not mentioned in the blog, simply leaving the static port vented to the fuselage internals is probably the worst you can do - the fuselage pressurises more or less with prop speed, airspeed, etc, leading to poor height control with wind and prop settings, etc.
The KEIL probe of the KB series - very small- the head is 1/8 inch diameter, and very light - they also lighten ones wallet a lot! Around $220.00 US....But you get a lot for the money. However, it really is an overkill for the application we are generally interested in.
Joe
The Nampilot
Hi Nampilot. Thanks alot! the putting the dynamic proobe into a venturi seems to be a neat trick! Do you know if these kiel probes are available for small (2kg) aircrafts at an affordable price ?
Thanks Nampilot , Great info!
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