Creating a quadcopter model. The ground contact just doesn't seem to work.

My quad keep getting through the ground "solid" which I created, connected to the world axis through a rigid transform and kept right under the quad. The quad falls and goes through the ground. Please help. Is it something I'm missing? Similar problem: http://de.mathworks.com/matlabcentral/answers/140143-how-do-i-keep-to-solid-objects-from-passing-through-each-other