This is an article from the January 2004 issue of Flying Magazine
Gravity Powered Airplane
Hunt Aviation is promoting a gravity-powered, fuel-less airplane. The theoretical airplane would alternate between rising like a hot air balloon--its helium filled structure is like that of an air-ship--and gliding back toward earth, gaining speed and ground in the process. During the dive it generates power via wind turbines. That power can later be used for compressing air into the aircraft, making it heavy enough to loose lift and dive again, continuing the cycle. The chief
the gravity-powered airplane, Robert D. Hunt, says that comparisons
to a perpetual motion machine are off base. His airplane, he claims,
exists in an "open system" in which the missing energy input
is gravity, a force his design harnesses.