As long as you have a rotor you will have severe speed limitations.
You perhaps can bump it up 20 MPH or so but the wave of the future
IMHO is tilt rotor.

The old tail sitter problems can be over come with modern electronics.

http://www.rotaryeng.net/VTOL.html

Paul Lamar

I'll underscore what Paul says in that, if you really want to increase the
speed of a VTVL craft helicopter or otherwise, you have to get rid of the
classic rotor during horizontal flight. DARPA is already funding work in
rotor retraction, high speed craft. There is already one patent filed on the
retraction method with I'm sure many more duplications to come. The tilt
rotor has already done this of course with many more examples showing up in
UAV's.

Any new design must exceed 250 kts cruise speed which are the already
demonstrated speeds of the Sikorsky twin rotor X2 and Eurocopter's X3
helicopters respectively.

Look in the near future for UAV's with retractable rotors with or without
tilting wings.

IMHO this is where you want to put your creative energies.

Cheers,

Brian Feeney
alaia design