A frame with four legs is not self-stable in flight at all. The thrust on all four legs must be identical to prevent rotations, and rotations are not self-cance

A frame with four legs is not self-stable in flight at all. The thrust on all four legs must be identical to prevent rotations, and rotations are not self-cancelling. Quadcopters must be computer controlled because the control loop must be very tight. Humans cannot fly them without electronic attitude control. It is certainly no easier than the gimbal control of a rocket, which is similar to balancing a stick on your hand. We give that control problem to undergrads as homework.