concept

Downwash

The downward-moving air a lifting wing leaves behind it. Since the only way to hold a body up is to push air down, every wing making lift drags a sinking sheet of air in its wake. That matters the moment a second surface flies inside it: the trailing wing meets air already deflected downward, so its effective angle of attack is smaller than its geometric one - same wing shape, same attitude, less lift and more drag. This is the core penalty behind every multi-wing arrangement, from a biplane to a four-winged animal.