We want to use APM to control a glider which should be launched by a weather balloon and climb up to 30 km altitude. The problem is to achieve best gliding performance to return against strong headwinds of up to 400 km/h. The stronger the headwind the higher the airspeed for best glide. The headwind's speed at all altitudes is known from ascent.
Our question now is: Is it possible to set and autonomously change the target airspeed during flight (without radio connect of course) and therefore control the pitch down angle of a motorless glider?
As far as we understand APM, the whole system is based on the idea of a more or less constant airspeed which is controlled by the motor. A glider's "motor" is the pitch down angle. The relationship between airspeed and pitch is known so I guess we could as well try to control the pitch angle (which seems to be the same problem). We absolutely want to avoid having to develop our own control system for the target speed.
Does anyone have experience with this or have a solution?
Many thanks for your help
I suspect you're right that it won't work out the box for a glider. I think you'd need to try and convince one of the developers (like Tridge) to try and modify the code to make it work for a glider.
Many thanks. So I'd better forward this to Tridge.
Yes, APM should be able to do this. There are a few caveats though.
First off you should use the upcoming 2.75 release as we have added a lot of changes to auto-adjust the tuning for pressure altitude. We've tested it up to 37km in the simulator. If you used the 2.74 release then the tuning would be stuck at one setting, and with that altitude it would probably spiral out of control as it would try to do far too tight turns when above 10km.
I also suggest you get the new MS4525DO airspeed sensor which is temperature compensates. I hope that will be in the store soon.
I'd also suggest you use a PX4 or Pixhawk, as it has a SD card for logging. You'll run out of log space with an APM2.
For a RTL mission you may also want a new feature being developed by Michael Day to support "rally points". It will choose the closest rally point to return to, rather than just home. This will allow you to add rally points that cope with different wind conditions for the balloon.
Regarding airspeed control, if you have a good airspeed sensor (like the MS4525DO) then it will indeed try to maintain constant equivalent airspeed if you are asking it to fly at constant altitude. Note that this is not constant true airspeed. At 30km the true airspeed will be around 10x as high as the equivalent airspeed, so for navigation purposes the glider will be going 10x as fast as the airspeed you set.
However for a RTL from 30km up the plane will not be trying to maintain constant altitude - it will setup a target glide slope, and try to follow that. So it will be trying to descend at a rate that leaves it at the right final altitude for the RTL. That means it will put it's nose down as needed to keep that glide slope.
It is a bit trickier than that though. The glide slope is currently setup when it enters RTL, so if you set it for RTL when it was launched from the ground the glide slope would be all wrong. I'd suggest you instead setup a mission that triggers when the balloon is released, and then have two waypoints, one at a high altitude (maybe 1km or more?) above the RTL point, then a 2nd at a lower altitude within visual range, and then landing if you want it.
That may require a small code change for the initial triggering.
I'd also suggest you setup the SITL simulator to test the behaviour, and setup different wind speeds and see how it reacts.
I do have the L/D performance data of our glider and the derived approximating function parameters, which really work fine. Furthermore all wind data. Please contact me in Skype to give you some data files. Please see also
http://de.wikipedia.org/wiki/Geschwindigkeitspolare (the german version is better and includes the approximation function)
Nevertheless I still think that the problem of not being able to force APM to fly a given target speed which can change every few seconds is significant for the application of APM in a glider.
I'm using Throttle to Pitch Mix KFF_THR2PTCH (there is no servo or ESC connected to throttle header) in my aero-towed glider. Know it's just a rudimentary stop-gap method to maintain airspeed with elevator only. Look forward to 2.75