[EricVanWyk]

This distinction confused me as well.

Lets say that you have a fan that can create 100 N thrust. (I'm pulling a number because it makes the math easier.

If you choose to use it to add horizontal thrust, you get the full 100N benefit.

If you choose to use it to add vertical thrust, you get 4 or 5N benefit, because it has to be transferred through the wheels.

The only advantage of a fan that pushes you into the floor (that I can see) is that the force can be applied in any horizontal direction that you need (if you have crab drive OR you are being pushed.

A horizontal fan on a gimbal knocks the socks off of a vertical fan. Can anyone give me a good reason to use a vertical fan instead of a horizontal + gimbal?

Therefore, I'm unsure why the lesser solution is illegal while the better solution is legal.

I've made a few gross assumptions here, but I'm very interested in hearing other thoughts.

EDIT: I forgot about suction devices. This is a special case, and I understand its illegality. My above thoughts only apply to fans pushing, rather than ground force suction fans.