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?
@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 I had originally made a nice brass dual ported tube that stuck out the nose, but it lasted exactly 1 flight.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.
@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.
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.
You're right- GPS alone can tell you the wind. That's a good solution for wind estimation.
The reason I got into plotting this data was to see if my pitot was calibrated. It seemed obvious that you could instantaneously determine wind in a given direction by: pitot - forward_gps_velocity. So if the plotted results fo a few circles weren't symmetric around 0 or didn't match the wind (using your GPS only method) you know the pitot isn't calibrated right.
I tried the usual methods of applying the known pressures and working the formulas, and even driving in a car with the wing stuck out the window. But both seemed to be error prone, while the method I show here seems to be accurate.
I haven't worked on the wind problem yet, but I think your data makes the problem look fairly tractable. I am a fan of forming a wind estimate in the air because I know that in my local environment we VERY commonly have low level wind shear, and the wind at 100 feet can be quite different than the wind on the ground. I think the simplest solution is to intentionally fly a few fairly large circles at constant airspeed and track the max and min gps speed and the headings at which they occurred. Wind speed will then be half the difference between the min and max and bearing would be the average of the heading for max wind and 180 degrees + the heading for min wind. You can also make smaller constant airspeed circles and track the drift.
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
The reason I got into plotting this data was to see if my pitot was calibrated. It seemed obvious that you could instantaneously determine wind in a given direction by: pitot - forward_gps_velocity. So if the plotted results fo a few circles weren't symmetric around 0 or didn't match the wind (using your GPS only method) you know the pitot isn't calibrated right.
I tried the usual methods of applying the known pressures and working the formulas, and even driving in a car with the wing stuck out the window. But both seemed to be error prone, while the method I show here seems to be accurate.
I haven't worked on the wind problem yet, but I think your data makes the problem look fairly tractable. I am a fan of forming a wind estimate in the air because I know that in my local environment we VERY commonly have low level wind shear, and the wind at 100 feet can be quite different than the wind on the ground. I think the simplest solution is to intentionally fly a few fairly large circles at constant airspeed and track the max and min gps speed and the headings at which they occurred. Wind speed will then be half the difference between the min and max and bearing would be the average of the heading for max wind and 180 degrees + the heading for min wind. You can also make smaller constant airspeed circles and track the drift.