I think that a tilt-wing or a tilt-rotor could be viable homebuilt projects, with a lot of ifs, not excluding somebody willing to design and built controllable pitch propellers with flapping and (probably) lead-lag hinges, either real ones or virtual ones (hingeless rotors have a soft-in-bending section in place of mechanical hinge). (Check out the VTOL Wheel for more fun homebuilding ideas; I rather like this one: ). Once one leaves out helicopters and compound helicopters, VTOL becomes hard, and for many of them, I'd want to have a zero-zero ejection seat, not just a ballistic parachute.I'd love to see almost any non-helicopter VTOL approach come to life as viable homebuilt. I have always like the tailsitters, so maybe something along the lines of a scaled down Convair XFY-1 Pogo, perhaps with prone position pilot (so basically standing up for takeoff and landing).
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Something like the Hirth H30ES (102 hp, 45 kg) two-stroke should work for power. I have sketched out ideas for trying to address the obvious safety issues with a 360-degree crash cage around the pilot, prop brake or explosive destruction of the blades in an emergency, and a ballistic chute in the oversized spinner. Even simpler, but less attractive, would be a ballistic chute launching at 45-degrees up and forward with a guard to keep the lines out of the prop. There would still be an envelope in VTOL mode where you'd be too low for the chute, so that would have be dealt with as a crushable structure or, even better, airbags like a Mars lander. ;-)
Neither the tilt-wing nor tilt-rotor configuration is particularly easy, and both have disadvantages. I suspect (which means that I've not done a full-blown analysis) that the structural issues of a tilt-rotor are easier (for one thing, the wing doesn't need to move) but the aerodynamic ones of the tilt-wing are easier; it ends up being a trade study. Bell Helicopter had a lot of experience with tilt-rotors, so the V-22 was a tilt-rotor; had Hiller or LTV been major players, the JVX may have been a tilt-wing.
Bluntly, I would not trust my life in any kind of tail-sitter, especially one powered by a piston engine. Give me a robust, well-proven gas turbine engine any day!