Sigh, went out to a field today. Even with the much better tune that lets me hover well, if I start to descend quickly it loses control, flops around in the sky, and I have to punch out with throttle to get more control. It flies great every where except descending vertically.
How fast are you trying to descend? More than a few meters/second? A wing won’t be stable going backwards, so you’ll need enough thrust from the motors to maintain attitude. How much that is will depend very strongly on the airfoil and vehicle geometry.
I’m not aware of anyone who has tried to optimize a tailsitter design for fast vertical descents, but it’s certainly an interesting problem. Are you able to tune a quadcopter to descend as fast as you want?
What I’m calling a quick descent is more accurately a “reasonable descent”. In my mind the whole point of building this VTOL plane was to be able to take off and land in areas that are smaller than a typical RC field.
Let’s say I cruise around in FBWA for a while, use most of my battery, come home, and transition to qmodes at 200ft altitude. I don’t have a lot of battery reserve left so I don’t want to spend a lot of time hovering(which takes the most battery) while i descend at a snail’s pace. I haven’t checked my datalog but my descent speed I’m trying to get to is basically 2-3 times a final landing speed. It’s totally possible that my tune is still just very inadequate. I’ll report with more findings after I do some more digging
I just ran my RF8 simulation of my Stryker quadcopter tailsitter, and it is stable in vertical descent at 9 kmh = 8 fps = 2.5 m/sec. So a 200 foot descent would take about 25 seconds. I’m not sure if it can go faster.
This is such a cool platform you have @cyber_06_wolf! I’m curious if you have made more progress PID tuning your hover. I think @kd0aij is correct, in that it’s hard to descend quickly (> ~10 ft/s) in an airframe with such large elevons.
As of Arduplane 4.1 (still in development), you will be able to independently adjust control gains at low and high throttle levels. https://ardupilot.org/plane/docs/guide-tailsitter.html#pid-gain-scheduling