Consider using a lightweight wheel for the foot. This will minimize the longitudinal force and torque on the leg. Wheels on the rear legs may necessitate a wheel at the front since the high friction at the front skid and the low friction at the rear wheels may cause the helicopter to twist sideways? If the 2 rear wheels were on a common axle then 1/ there will be less chance of the helicopter twisting & 2/ it would help brace the rear supporting (in a forward facing view, the 2 legs and axle will look like a 'D' lying on its flat side). The axle will be in tension.