control speeds
1. Vmcg & Vmca are affected by changes in altitude/temperature as these affect the amount of thrust being prodused by the engines. The less the thrust being produced, the lower the control speed.
2. Vmcg is a higher speed than Vmca as when on the ground the aircraft rotates around its main undercarriage which is aft of the cg. This gives a shorter rudder arm (flight mechanics), it is therefore required that Vmcg is higher to allow directional control to be maintained.
Hope that helps!