Freesteel Blog » Glider orientation upright compensation
Glider orientation upright compensation
Friday, May 27th, 2016 at 12:07 pm
The datalogger is fitted to the right hand upright of the control frame. According to my shoddy measurements the upright is 176cm long and the basbar is 138cm long. There are two wires from the corners of the control frame to a point on the keel 151cm forward, each of length 206cm. And there are another two wires to a point on the keel 80cm backward, each of length 205cm.
This seems to add up to a vector of (69.0, 42.06344442205491, -156.2492943010685) from the top of the control frame to one corner of the base bar, which is the alignment of the datalogger and its BNO055 orientation sensor strapped to the upright.
I definitely don’t have time for a complete blog record of this, and a lot of it isn’t working, even though I’ve spent no end of time hacking on things, but if I plot the logger vectors that go backwards and perpendicular to the upright on the gps track, I get this picture:
For scale, the red lines are 11m long and the plot rate is 3x on the 100Hz samples, so there are 33 of those lines per 1second of motion.
It’s reasonably smooth and consistent, unlike the accelerometer values, which I think represent a lot of vibrations in the system, if the creaking wicker chair-like noise on the gopro soundtrack is anything to go by. It’s about right, if you think of the airspeed being in line with the keel of the glider (which it is probably not, because of the yawing and side-slipping in turns).
I nearly bought a sonic anemometer second hand off ebay last week, but missed it. I’ve got an alert watch on a new one and have designs on fitting one to the top of the kingpost where it is out of the way and least likely to get damaged by flying into a hedge or the ground.
Regarding the kinematics of the system, every part of the flight is the same as everything else, so I can strip out the simpler bits to see if there is a relationship. In particular, this is a plot that claims to be 10 second long sections of the flight where the bank angle is less than 3degrees.
This doesn’t fit too well, and I can explain it that the calibration value on the device was at 1 out of 3 for most of the flight, in spite of starting off on the ground at 3/3, so it’s telling me it got it wrong.
Anyway, it’s straight enough to move on to this complex plot of some of those flight sections:
Time goes from left to right of about 10 to 20 seconds. The following traces have been displaced close to the zero horizontal white line so you can see their correlations, if any. Green is pitch angle, Yellow is smoothed acceleration in forward direction (approx because it’s not aligned with gravity yet), Red is airspeed, and White is differentiation of windspeed.
What I would hope for is a delayed correlation between pitch (green) and airspeed (red). If you push the nose up and hold it there, the glider will slow down, and then fly at a slower speed. If you pull the bar in and hold it there, the glider will speed up and then fly at a constant higher speed. But of course you’re moving the bar all the time. But you can almost convince yourself of the response; the green line goes down, and the windspeed then starts to rise, and vice versa.
This should be built this into a kinematic model of control and response. It would actually also align with the barometric reading, where the sink rate goes up with a higher speed, so it’s quite 2 dimensional in the vertical plane, with energy equations.
The other correlation should be between the forward acceleration and the derivative of the airspeed. Both the airspeed signals and the acceleration signals are quite full of noise, so they needed to be smoothed (I think I’ve done this incompetently here, but that’s for another fortnight of coding).
Here you would expect a direct correlation in still air, because if the accelerometer says I am accelerating at 1metre per second per second, then over the course of 2 seconds my absolute velocity will increase by 2 metres per second. And, if the air is still, then my measured air speed should also increase by 2 metres per second.
So these two ought to be scaled the same. Except the smoothing flattens out all the humps and troughs, so you really can’t tell what the absolute value of anything is from this anymore.
Some of the bumps seem to line up, but this is not adequate for a kinematic model.
The numbers get hairier and hairier as I try to approach something useful. And that useful thing would be to separate the absolute motion into that which is due to my control and the flight characteristics of the glider, and that which is due to the underlying wind speed and direction.
Accordingly, I should be doing lots of boring flights in completely still air at the start of the day with a working logger where everything is calibrated, rather than having lots of fun in thermic air. This is a job to do in Austria where the hills are high and I should have hours and hours in the campsite not coding on other things.
This work just takes so much time and has hit so many technical failures already that I doubt it could be done by someone who is answerable to a boss.