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Old Nov 30, 2005, 07:21 PM
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DIY Pitot tube

I'm interested in building a pitot tube to measure onboard speed. I'm unfamiliar on how to implement the mechanics of such a device, from what I've gathered so far it requires a differential pressure sensor to compare dynamic (forward air pressure) and static (air pressure at right angle to the tube). I assume it requires 2 tubes, one facing forward and away from any prop wash, and one at right angles to the air flow. Has anyone found a good air pressure sensor to use for this application and any formulas I could use. I plan to implement a pic based system to measure the speed. I read that true airspeed, one must also take account of temperature as well. Any pictures of the physical setup would be appreciated.

Kin


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Old Dec 01, 2005, 01:05 AM
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Kin, I have successfully measured airspeed using a Freescale MPX5010 differential pressure sensor and a PIC. I do not yet have all the information and formulas on my website for this yet, but I do have altitude information posted. Some of the formulas are relevant for both calculations.

Look at my Altimeter Links page and go to the bottom one - Wahiduddin's page. He has a lot of good info on density altitude calculations that are relevant to airspeed measurements.

Sorry I don't have more info for you - I plan to post a tutorial for airspeed theory that goes into as much detail as my altitude tutorial, but I just haven't had the time.

I'm sure others on this forum will have some helpful advice for you as well.


Luke
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Old Dec 01, 2005, 08:27 AM
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Hi Luke,

Was it the MPXV5010DP sensor, that one looks like the smallest they carry with dual ports for the pitot. If I were to measure altitude as well, I would require one more sensor, or do you think there is a way to combine them. I think not but best to bring up the subject. Thanks for the help, this at least gets me started on the design.

Kin

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Old Dec 01, 2005, 11:55 AM
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Kin,

Yes, the V5010DP will work - I used the 5010DP (no "v") which is slightly larger in size but they're the same.

You're right, if you want to measure altitude as well you will need another sensor - an absolute sensor I use the MPX4115 as you can see on my site. However, the static port of the differential sensor and the single port of the absolute sensor could be tied together with the same tubing (assuming you want to use tubing). This is because they will both be measuring the same thing: ambient pressure.

For this reason it is possible to do altitude and airspeed using two absolute sensors and no diff sensor. One measures static pressure, the other measures pitot pressure. The static pressure is used for altitude calculations. Then in code you take the difference between that and the pitot pressure to arrive at airspeed.

However, you're still using two sensors - also you'll have a slight bit more of code and depending on the resolution you want for your altimeter, you may end up with more circuitry. For that reason I personally feel it's just easier to do as you suggest - use a diff sensor for airspeed and a separate absolute sensor for altitude. They're about the same price so you won't be saving anything either way (at least if you go with Freescale sensors).

Speaking of price - Freescale has a samples program and you can probably get both sensors for free. I know I have. It seems to me they also are willing to ship overseas, if that's an issue - but it looks like you're located in Michigan.


Luke
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Old Dec 02, 2005, 09:49 AM
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Thanks, I like the idea of using the same pitot tube and use of dual sensors. Resolution is a concern so I will go with a differential and a absolute to start out with. They have the same footprint as well which is nice. I have both on order. The pic I will be using has 10bits of resolution, do you think this will be enough?

As far as physically putting together the system, I assume you can use surgical tubing, I will test this on a stryker so I will put the pitot near the front. As far as the static tube, how would I impliment that part of it, I've know it needs to be next to the pitot for accuracy, but do you bend it at a right angle to the wind. I also assume both static and pitot pressure must use the same size diameter tubing.

One of the members pm me and said they use a tube within a tube and just have a hole on the side for the static pressure and have the end capped off, but this seems more complex to implemement.

Kin

http://www.embeddedtronics.com/

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Originally Posted by LukeZ
Kin,

Yes, the V5010DP will work - I used the 5010DP (no "v") which is slightly larger in size but they're the same.

You're right, if you want to measure altitude as well you will need another sensor - an absolute sensor I use the MPX4115 as you can see on my site. However, the static port of the differential sensor and the single port of the absolute sensor could be tied together with the same tubing (assuming you want to use tubing). This is because they will both be measuring the same thing: ambient pressure.

For this reason it is possible to do altitude and airspeed using two absolute sensors and no diff sensor. One measures static pressure, the other measures pitot pressure. The static pressure is used for altitude calculations. Then in code you take the difference between that and the pitot pressure to arrive at airspeed.

However, you're still using two sensors - also you'll have a slight bit more of code and depending on the resolution you want for your altimeter, you may end up with more circuitry. For that reason I personally feel it's just easier to do as you suggest - use a diff sensor for airspeed and a separate absolute sensor for altitude. They're about the same price so you won't be saving anything either way (at least if you go with Freescale sensors).

Speaking of price - Freescale has a samples program and you can probably get both sensors for free. I know I have. It seems to me they also are willing to ship overseas, if that's an issue - but it looks like you're located in Michigan.


Luke
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Old Dec 02, 2005, 01:29 PM
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Quote:
For this reason it is possible to do altitude and airspeed using two absolute sensors and no diff sensor.
This is possible, but much better to use differential for airspeed. Unless the two absolute sensors track each other perfectly, you can get significant errors.
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Old Dec 02, 2005, 03:22 PM
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Jeff - I agree with you, I was just pointing out the possibility.

Kin, surgical tubing or model RC fuel tubing is fine. I'm not sure that if you use two separate tubes that it really matters all that much if they are of the same diameter, but it wouldn't hurt. At any rate the nipples on the sensors are going to be the same size so it makes sense to use similar diameter tubing.

There is nothing that says you have to have the static port near the pitot tube. On many full sized aircraft they are in completely different locations. Single engine Cessnas I have seen typically put the static port on the side of the fuse near the aft of the craft, close to the tail. This is to get as far away from the turbulence of propwash as possible. However, in many instances the static port will be built into the pitot tube (then it's called a pitot-static tube). The reason is, I suppose, because the pitot tube typically points out into undisturbed oncoming air, and this is the kind of air you want for the static port as well. However, you are right, the pitot tube points into the oncoming air, whereas the static port must exit perpendicular to the oncoming ari (90 degrees). A tube within a tube can be used. This is sometimes hard to visualize, but this site has a picture that gives the general idea.

Or, you could use two seperate tubes instead of a tube within a tube. I've attached some pics below that hopefully help. So you have several options.

Now, as for your resolution - 10 bits isn't going to be much when it comes to altitude, but it should be ok for airspeed. If you want to read the nightmare adventures of increasing resolution for altitude, I'd point you to this thread, which chronicles my adventures in this very area, and even includes schematics. It's been a while since I posted but I'm back on the project and I intend to revive the discussion there soon.

If you don't intend to go very high, 10 bits may be fine - although the altitude sensor will work to very high altitudes, you could just scale the output so that your A/D converter only sees the first 1000 feet or so. Then you'd have a resolution of 1 foot. But if you want to measure higher than that and maintain small bits of resolution, then you're going to need a better A/D (I'm using a 12 bit) or dynamic scaling (which I'm also doing - I want ~1 foot resolution up to 45,000 feet). Either way you're in for a bit of work.


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Old Dec 02, 2005, 09:02 PM
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LukeZ, did you buy that pitot-static tube hardware, or make it yourself? Are there any sources of information for designing/building pitot tubes for RC aircraft?

-Brandon
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Old Dec 02, 2005, 11:07 PM
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Brandon, I'm sorry I gave the wrong impression: I didn't make those pitot tubes, or even know who did. I just found the images by searching the net, and posted them for illustrative purposes. At any rate they're for full sized-aircraft so probably would be too heavy for most of our purposes.

I don't know of any sources for designing/building your own, specifically for RC aircraft use - and if anyone does I'd be just as interested in seeing it. Maybe someone will post here.

But it's my impression that most people just make these themselves out of metal tubing that you can get at the hobby shop - brass or aluminum or steel even if you wanted. The pitot tube is easy: just point the tube into the wind. The static tube is pretty much just as easy: seal off the end of the tube that faces into the oncoming airstream, and drill small holes along the shaft. Attach some surgical tubing between the tubes and your sensors, and there you go.

The tube that actually is a bit tricky to make, as I understand it, is the one used in variometers. And if anyone has more info on that I'd be real interested to see it as well.


Luke
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Old Dec 03, 2005, 12:28 AM
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Hi Luke, looks like I'll be following your footsteps. Interesting where all this is taking me, sure is getting complicated fast. TOTAL ENERGY PROBE article on how to make one for a real glider http://www.wcsa.org/docs/TEprobe.pdf. It had not even occur to me about sink rates and it's effects on altimeter readings. I'm playing catch up at the moment. I find your website very informative, will be making a circuit board. Maybe we can coordinate it. Not sure how much time I can devote to this project, too many on my list, but will eventually see it thru.

Kin

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Old Dec 03, 2005, 11:53 AM
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kfong--
Awesome article.
Thanks!
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Old Dec 04, 2005, 01:01 AM
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Kin, that was a good article about the TE probe, thanks for posting it. Although I'm still not entirely certain how I would implement a vario with a pressure sensor: what would one do for the "flask"? I suppose this could probably be a different thread altogether...

Yes, it does all get very complicated pretty quickly, if you want something that is precise, accurate, and has a wide working span (both in terms of altitude and temperature). However I find that the altimer is much the more difficult circuit to do, airspeed is relatively trivial from a hardware point of view (unless you want to go Mach 3 for some reason). The formulas for airspeed can get pretty complicated though, especially if you want to accurately calculate air density - I guess I really need to post that section of my site soon.

What circuit board are you going to make? If you meant the one on my site I'd just caution that it's fairly basic - in fact it was my first attempt at PCB design. The circuit is not incredibly advanced, and is useful mostly just for testing the principles but probably not much else. The resolution is pretty poor, I think something on the order of 7 feet at sea level, and less as one gets higher, as I did no amplification of the signal. The AD chip I used is a bit expensive also, but you can get free samples from Linear.

I've had your site bookmarked for quite some time now - you have some good circuits and clearly a lot of experience in this area. I did not realize you were also an eZoner until you posted here. If you make a board I'm certain it will be a good one. When my version 2 is completed (who knows when) I'd really like to have it all be done in surface mount components - perhaps we could coordinate. I've never done SMT before and it seems a bit daunting, especially if you were to see the size of my soldering iron tip! (just a cheapo I bought at Radio Snack.) But you seem to have that down pretty good.

Please keep me posted on your progress, I'm very interested to hear about other people's experiences along this same path. You may also want to post over in the other thread that discusses op-amps for scaling of the altimeter output as well, depending.


Luke
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Old Dec 04, 2005, 12:55 PM
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Hi Luke,

I was thinking the flask isn't needed since we are using solid state sensors. I was under the impression that the setup they had were the older style mechanical stuff. The total energy probe does most of the work of giving the sensor a static pressure reading was the way I interpreted it, since it creates a vacuum under certain conditions stabilizing the total static pressure, but I could be wrong.

I'm just at the beginning stages of creating an on board telemetry system so I want to incorporate as much as I can. It will probably be in stages, long term project. Just a lot of other projects and interests that keep me busy. I tend to jump around to what keeps my interest. I'm information gathering at the moment, but if I see a circuit that looks ok, I will usually get a circuit board made to test it out. I haven't had a chance to go through the thread or cover what you have already done.

I do plan to make a quick board, pic base with the sensors soon so what ever I come up with will be thrown in. I don't bother with proto board wiring, faster for me to just make up a circuit board test it and incorporate it later in a final design.

Kin

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Old Dec 04, 2005, 02:43 PM
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Kin, yes I think you're right about the flask. I suppose one would need to use a vacuum sensor of some sort - or differential sensor perhaps as shown in Figure 8 of your document - simply have the dynamic side attached to the static line, and the static side attached to the TE probe. This would essentially be your vario, and would allow you to measure "vacuum" (really, just a difference). The rest of the work would be done in software. But I think I need to do some more reading on this.

At any rate the vario stuff and total energy compensation is not really related to altitude; one loses altitude if they dive or sink either way, and the altimeter will still read that (unless the mass of air you happen to sink in remains the same pressure - but even then, a TE probe isn't going to help you measure your geographic altitude any better). I'm interested in the vario however, because someday I'd like to write a program that allows the plane to autonomously find lift and avoid sink - an automatic thermaler if you like.


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Old Dec 04, 2005, 06:58 PM
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Yes, I'm interested in the TE, or else I could just use a absolute sensor for altitude and be done with it. I like to fly gliders as well and had not thought much of the problem until you brought it up. I should be getting the sensors soon, but need to get my butt in gear so I have pc boards to test them on.

Kin

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