An R/C glider with no engine can reach speeds of 392MPH in the right conditions by using a principle called dynamic soaring, as you can see in this clip.
Anyone who's ever rafted or kayaked will probably recognize the fundamentals at play. Air moving along a mountainside causes an eddy right below the ridge on the mountain's leeward side—a tumbling whirlpool of low pressure air. The glider precariously circles between the eddy and main air current before slingshotting out at speeds up to 8x the surrounding air. (There's a flash animation that explains it better.)