A pretty good explaination of prop efficiency was made in this thread by Oryx.

http://www.rcuniverse.com/forum/m_43...tm.htm#4345826

For Speedboy's Patriot to reach the vaunted 200 mph, it will require around 6 1/2 hp. Since the engine is not going to provide that amount, gravity has to provide the rest. Since you know the terminal velocity goal, it is a simple matter to determine the weight required.

And gliders do use ballast to gain speed.