From: http://en.wikipedia.org/wiki/Hiller_VZ-1_Pawnee http://en.wikipedia.org/wiki/Hiller_VZ-1_Pawnee "Due to aerodynamic effects in the duct within which the

From: http://en.wikipedia.org/wiki/Hiller_VZ-1_Pawnee http://en.wikipedia.org/wiki/Hiller_VZ-1_Pawnee "Due to aerodynamic effects in the duct within which the propellers rotated, the platform was dynamically stable, even though the pilot and center of gravity of the platform were fairly high up. In testing, the prototypes flew well enough, but the U.S. Army judged them to be impractical as combat vehicles as they were small, limited in speed and only barely flew out of the ground cushion effect." Seems like it wouldn't be so great for going over mountains and such.