You may have heard of wind carts that claim to travel downwind faster than the wind itself. I used to believe this was impossible, until someone explained that the source of power isn't the velocity of the wind itself, but the difference in velocity between the wind and the ground. This project will adapt that principle to aerial navigation, exploiting the difference in velocity between an air current and the relatively slow air surrounding it. The windplane will take the form of an electric propeller plane connected by a tether to a flying wind turbine. This arrangement will be able to fly both upwind and downwind for free, as long as there's a sufficient wind velocity difference between the two parts.