I find that grass tends to limit terminal velocity of takeoff roll out in MPH, I see this real time using EagleTree telemetry. it appears to me to relate to wheel size and length of grass. Any wheel under 2.75" is very hard in grass and critical to length of grass, angle of attack, no matter what the model.

Heres some examples for me: The BVM Kincat Ive not yet been to a grass feild it wont take off from (BEST in class) I can take off of rotary mowed dry grass with a BVM Bobcat XL with 2.75 main wheels and set @ high positive angle of attack This same feild in the morning hours with dew creates enough drag to limit takeoff speed I cannot lift off, I need to bring out the KingCat before noon at times. I have a Cermark Viper Jet with 2.5" of wheels that wont take off grass mowed with the rotary mower, my home feild has 600' concrete, so I takeoff concrete with the ViperJet and land in grass with no grass issues and wheels/struts. I firmly believe if I got to 3" wheels in this model it would do fine as its only about 7MPH short of normal takeoff speeds from concrete versus grass.........