ORIGINAL: cfircav8r
...........Remember the CG wants to be ahead of the center of drag. In the air that center is far aft of the CG due to the large tail surfaces, on the ground it is at the wheels. The farther the CG is aft of the wheels the greater the tendancy to swap ends.
Lerning2fly,
See the attached schematic that supports the previous post.
The taller the grass, the higher the drag force on the main wheels, the stronger the tendency of the CG (and rest of the plane aft of it) to relocate itself forward, either with a swing around (ground loop) or a flip over.
While the take-off run lasts, you are trying to counteract that tendency with the rudder, just like when you try to balance a vertical stick on the palm of your hand (which CG tries to relocate below your hand).
You have three destabilizing forces working against you:
1) Any cross wind.
2) The bumps and grass hiting each main wheel at different times.
3) The several forces created by the propeller (inducing a left yaw mainly). The bigger and heavier the worst, as well as the less gradual acceleration.
Best luck in your future take-off's