Posted by Tj Bordelon on October 17, 2009 at 4:06pm
I just added a pitot to my ezstar (http://bordelon.net/ezstar) and did a flight. Naturally the plots of the airspeed sensor and GPS velocity (projected onto the X axis... straight thru the nose) did not seem to match. But should they? I figured any wind would skew the results, so how do you know you're calibrated?I first tried to make a "manometer" with a tube of water but that seemed to just make a mess and was off by 50% in the end.I finally just flew with the manometer guess, and made 10 or so circles in the air. On landing, I took the data and scaled things to match as best as I could. Here's the result:
So they don't match. But there was a slight breeze that day. I then had an idea! What about plotting the wind speed (pitot - gps) vs bearing. I dumped a bunch of points to the plot and got this:
Mind you it wasn't symmetric about 0, so I played with the scale to make it so. I believe this was the best and most accurate thing I did to calibrate. Because once I did this, I then went and flew in 20 mph winds... and guess what! I got this:
I also verified that the direction of maximum/minimum "wind" was the exact bearing and speed of the wind I measured just before takeoff.So what's the problem? Well, I talked to a few guys I used to work with on a UAV project and this just confused them. So I figure I"d ask you guys out the'e what you do to make sure your pitot is calibrated, and if any of you have a good method for wind estimation. Does my technique look like what you do?
There is no fuselage on a flying wing :) And EPP is all but "airproof", so static reading in the payload compartement are all over the place. Plus, by regulating airspeed of 9 m/s instead of 11 m/s (both of which looking perfectly fine for this type of aircraft), I nearly double the endurance and increase the range by 50%. So precise airspeed measurement (sub-m/s or about 1mph) is critical in that case.
@abey - I've got a bit of cheating going on in that my static port is simply ambient, in fuselage. I had originally made a nice brass dual ported tube that stuck out the nose, but it lasted exactly 1 flight. But surprisingly the difference in two identical flights didn't seem noticeably different. The plots wound up with the same "sine wave" pattern and uncertainty of maybe 5 mph. Maybe it is possible to get down to 1mph accuracy but for my needs it probably isn't worth the extra work.
I'm convinced I'm within 5mph at all times, up to maybe 60 mph flight speed ( I'm still amazed the EZSTAR can go this fast!)
@Ryan - Cool! I'm going to have to compare this method to mine and see how the data compares.
Unfortunately (for the static reading), I deal with an EPP flying wing (a swift II), so i don't have has much flexibility in term of picking up static port around the fuselage (or lack thereof).
@abey - you can get close to "purely" static, but it requires some plumbing. On my (full scale) sailplane I have 4 static ports, two on each side of the fuselage. They are at points behind the wing where there is little disruption of flow. The 4 ports are plumbed together in a manifold which allows cross flow between the ports in case the fuselage is in a slight slip, etc., and the output of the mainifold is an average of the 4 and has little variation with airspeed or attitude. Another method is to have a probe extending forward into clear air with ports on the side. You will often see probes on full scale aircraft that gather both pitot and static pressure.
Actually, its not that simple in my case, because the static pressure reading is never "purely" static. Close to cruise speed, the static pressure is contaminated by dynamic flow, which requires inflight offset and gain estimation. I do this by taking measurement at various power setting around cruise speed. I use a reading at rest to linearly bring to bottom part of the calibration curve to zero.
That's pretty much exactly what we do for wind estimation and pitot calibration. However, a simple scaling from the pressure sensor ADC reading to the dynamic pressure value will not do. At least an offset and a scaling is necessary, and this wont properly account for slow speed (about 0 to 6 m/s).
OH-- the nice thing about the pitot method is that you don't need the constant airspeed, which seems to be difficult on my ezstar for some reason. Heck, the data wasn't even taken with a constant turn. It was flying a triangle of waypoints and ascending and descending, yet the results still show the wind direction and bearing.
Comments
I'm convinced I'm within 5mph at all times, up to maybe 60 mph flight speed ( I'm still amazed the EZSTAR can go this fast!)
@Ryan - Cool! I'm going to have to compare this method to mine and see how the data compares.
This is useful if you don't have the equipment to output pressures! And looks like you have the data to pull this off. Good luck
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