I remember seeing pictures of the predator with the inverted v-tail. I built a small balsa glider with the tail on bottom and it flies really well. There's no concern about a landing gear on it. I can see where ground clearance would be the big concern. If anybody knows anything about yaw effects coming from below rather than above in a real aircraft, that might be interesting. One of my friends said that a downward tail makes a plane unstable/counteracts dihedral in the wings. Not sure if that's got anything to it though.