I know there have been several other threads about this, but I couldn't find answers to my specific questions, so as much as I hate to stir this up again, I'm hoping someone will take pity on a newbie and talk me through this. I would like to say that I have no doubts that these vehicles work, and I'm simply trying to understand the principles involved. First, what I think I understand so far: At first I couldn't understand how the propeller could deliver enough thrust to overcome the friction on the wheels, if the friction was providing the power. But then I realized that the propeller doesn't move as far through the air as the wheels travel across the ground since there is a wind. So for the same amount of power, the propeller can apply more force. So once at wind speed, thepropeller can accelerate the cart to faster-than-wind speeds. It can continue to accelerate until the headwind is strong enough to balance out the additional thrust, where it then reaches a constant, faster-than-wind speed. But what confuses me now is this: If you assume that the wind delivers its maximum amount of power to the cart when it travels at wind speed, as happens with a sailboat, you have a problem. At faster than wind speeds, the wheels have even more power, since they are moving faster. But at this point the wind should be applying negative power in the form of a headwind. I thought maybe the negative power from the headwind might simply balanced out the additional frictional power from the wheels, so that the cart as a whole has the same amount of power as it does at windspeed. But in order for this to be the case, the drag would have to be constant and equal to the frictional force. But drag changes with velocity, so this can't be. So my assumption that the wind is delivering max power to the cart at wind speed might be wrong, or else I am wrong about something else. How does the cart harvest additional power from the wind at faster-than-wind speeds? One other question: I read in one explanation that the propeller can act as a windmill to drive the wheels at slower-than-wind speeds, and then be turned by the wheels at faster-than-wind speeds. But after playing with a fan for a few seconds, I realized that if this were the case, the thrust provided by the prop in the second stage would be in the wrong direction. Could someone clarify how the prop acts at slower-than-wind speeds? Thanks in advance for your patience, and I apologize if I've made any particularly annoying mistakes in my first attempts.