Why can't you have sail-powered airplanes? So, sailing boats take the kinetic energy of a crosswind and turn it into thrust by using their sails as lifting surfaces, and the discussion I had in the comment section of this question made me wonder why nobody has ever built any (working) sail-powered airplanes.
Basically, imagine a glider, but with sails on it (maybe on both top and bottom of the aircraft, to avoid imbalances in the center of thrust tipping it over) to catch the crosswind and provide forward thrust. Since these vehicles don't exist, there are presumably problems with them that would prevent them from working.
What are these problems? Do the aerodynamics of the sails interfere with the aerodynamics of the wings or something? Even if sails don't provide enough thrust to get the plane off the ground, that wouldn't be a problem for a glider that can't get off the ground by itself, right?
High performance sailboats can sail faster than wind speed in certain conditions. Therefore, your question is not as absurd as it might seem at first glance.
However, the technique that enables sailboats to do so is not available to airplanes, at least not in flight. Sailboats use the speed difference between water and wind. Their keel keeps them from being blown with the wind. A glider in the air has no such second medium to resist being blown away by the wind.
Historically, the Andrée ballon used for artic exploration in 1897 tried to &quot;sail&quot; by combining sails and trailing ropes. However, this did not work in practice.
The only way to extract energy from the wind is when this wind changes. Wind gradients allow albatrosses to soar dynamically, and changing updrafts can be exploited by changing the load factor.