I'm running a kk2.0 + 4 20A Multistar ESCs + 4 EMax GF 2215-20 motors + 4 Slow Fly Props
After about a foot off the ground, the entire quadcopter starts wobbling like crazy (no auto-level). Any ideas?
I'll add some video if needed.
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Sounds like ground effects. When a plane or helicopter is close to the surface, the aerodynamics change. This distance is usually considered to be the same as wingspan, or rotor diameter for a helicopter. The lift/drag ratios change, the thrust efficiency changes, and the balance can be affected because you are moving on or off a bubble of pressurized air. Also the turbulence can go nuts, placing high demands on the control systems. Slower or unoptimized systems are going to struggle through this zone. This effect is also dependent on the surface you are above, with the effect maximized on smooth hard surfaces. If the craft has the same behavior at the same height when hovering over grass or very bumpy surfaces that it does off a smooth piece of plywood on concrete, then the problem is something else. If the transition occurs at a lower height on grass, than you'll have found the problem. Check out: http://www.skybrary.aero/index.php/Ground_Effect#The_Extent_of_Ground_Effect Here is a forum where they had this problem, and were able to reduce it via control settings. http://forums.openpilot.org/topic/7596-quad-unstable-in-ground-effect/ and if you want a formal discussion: http://www.seas.upenn.edu/~dmel/ISER2012_powers.pdf
Make sure you have set PIDs for your frame correctly. Here is a video which explains behavior of quadrotor under various PID settings which will help you tune gains for your frame. Also Make sure your frame is well balanced with CG at center of board. ESCs must to be calibrated so that all 4 motors perform equally. Unbalanced propellers can cause significant vibration hence make sure propeller are well balanced. Always fly quadrotor to height of more than 1m and then check behavior so that ground effects are negligible.