>>13351300You are looking at the machine against the ground. This is a case of looking at the machine against the air.
This makes you think the propeller is being pushed by the air. It is not. The air the propeller is pushing is being pushed by the wind. The propeller is going in the opposite direction that what it would go if it were pushed by the air.
Think of an airplane with a tail wind. The airplane is pushing against the wind and the wind is moving over the ground and the speed of the plane is the vector addition of the two.
Now then think of a sailboat being pushed by the wind. The wind is pushing the sailboat over the water. The wind give the energy to move the sailboat.
Now put them together. The wind that is moving with respect to the ground is pushing against the thrust of the machines propellers that is then pushing against the machine that is pushing against the ground that is driving the propellers to push against the wind that pushes against it...
Now think of the limits. First., if there is no initial push on the cart there is no initial thrust from the propeller that the wind is pushing against. the wind would move the propeller in the oppostite direction but the gears would move the cart towards the wind. Second, the machine can only go as fast as the energy delivered as a differential of the wind to the ground. The speed of the machine is limited in the same way as the sailboat by the force of the wind with respect to the ground or water.
In the middle of these limits is a sweet spot where the wind against the thrust pushes hard enough for the cart to push against the ground to drive the propellers and create the thrust that the wind can push against. As long as your ratio of wheels to propeller is right in the advantage of the thrust, the cart moves forward.
Get it?
