The concept of vertical takeoff and landing (VTOL) using tilt rotors is old and has been working reasonably well for decades (Osprey).
Obviously that is a military project where cost don’t matter and things are done just because they can be done. However, lately it seems like there is a lot of push again. Airbus Helicopters are working on a helicopter that can achieve higher speeds and better fuel economy by having airfoils and tilt rotors.
Agusta Westland are already close to production with their tilt rotor plane/helicopter going back to 2003. It’s aiming the corporate market.
In the smaller GA, there are also interesting developments. With electric engines this concept becomes a lot more realistic to implement because you can decouple the heavy and large combustion engines from the propellers and drive them with small electric engines. Also you can have a large number of propellers.
The next incarnation of the Terrafugia is supposed to be built around that serial hybrid concept:
The proposed Trifan project also makes use of this concept: http://www.xtiaircraft.com/trifan-600/
It makes a lot of sense in my opinion. Also for drones because you no longer need runways which makes them a lot more versatile. It could well be that tilt rotors with serial hybrids is the next big trend in aviation.
I wonder what happens in case one engine dies on take off or which speed is needed for an emergency landing in “plane-config”.
On both the Terrafugia and the Trifan, CG must be very critical, much more than on a helicopter because the propellers are so small. In the Trifan, you have to use different power on the rear fan to balance the aircraft during VTOL operation, that looks difficult to me.
What happens if your engine in a R44 fails during takeoff or landing? You’re pretty much toast. So far Terrafugia and Trifan are only computer drawings. If they are subject to the same regulations as existing aircraft, then they need to provide a 61kt Vso with their airfoils only.
I think the Osprey has a shaft running across joining up the two rotors, no?
achimha wrote:
What happens if your engine in a R44 fails during takeoff or landing?
The possibility is only half since one engine only (okay it’s piston driven vs turbine….) I guess the critical Phase will also be longer for the tilt rotors until they are in the final cruising configuration (and then how about Vstall and are the Rotors featherable?). And you don’t have the “Starfighter-option”: Engine failure – eject 
Peter wrote:
I think the Osprey has a shaft running across joining up the two rotors, no?
That is extremely hard to do, the new designs will use a serial hybrid with electric motors for sure. There was a GA design using two Mercedes V8 diesel engines in the back of the airplane (Thielert Centurion 4.0, certified and officially still available) and two shafts for propellers. They never got the shaft/gears to work and the company went bankrupt. It was created by the founder of the German Pilot und Flugzeug (PuF) magazine.
There are huge advantages when you can put the liquid cooled combustion engine in the fuselage and don’t have to hang it in the airflow. Obviously it can be done using mechanical shafts, same like boats do it but you have to choose out of three disadvantages:
Could be. I certainly like the idea. Whether we ever get a capable machine at a reasonable cost is another question. To elaborate:
I really like helicopters. Not only they are fun, I can land it in my backyard. Literally. But they don’t have the speed or range. And are quite pricy. There is the “hybrid” approach where you fly the helicopter to an airport (which can be rather distant given the speed of a helicopter and rather direct track compared to a car) where you switch to a fixed wing. But that entails owning two machines (depreciation, maintenance etc.), starting them and shutting them down (preflight check etc.) and you don’t have the benefit of a helicopter on the other side. While a car is slower moving, drivers don’t bother that much with checking the technical state (that is at least my impression) and can actually beat a helicopter on shorter trips. Especially when densely build up areas are concerned because of limited landing options. OTOH mountainous terrain is good for helicopters.
I’m keeping an eye on the AW609 and I can see the appeal for offshore missions, for example. But it will cost about five times as much as a Meridian and offer similar level of performance. That is hell of a price to pay for VTOL. Just like a helicopter, it won’t make sense for longer trips unless you really need VTOL, like on an oil rig.
PS: It must have interesting OEI characteristic. 
I don’t know if they used similar solution as is on Osprey, which caused some headaches IIRC.
achimha wrote:
What happens if your engine in a R44 fails during takeoff or landing?
You can initiate autorotation even from a low altitude. Of course, some machines are better at this than others. PS: And the certification standard calls for better crash protection than in fixed wing, because of the nature of an emergency landing of a helicopter (the seats can take, I think, 26 g).
I was not a heli pilot even in a previous life but there is an envelope outside which you will crash if an engine fails totally. It is basically to do with being high enough to get smashed up if you plummet, but not high enough and with no forward speed to perform autorotation. If an engine fails at say 2m you will smash it up but “should” walk away. But if it fails on a Robinson at say 30m and you are hovering, you cannot autorotate. The envelope depends on the heli type (rotor inertia etc).
XTI Aircraft have raised $11 million (indications of interest) out of a $50 million crowdfunding effort for their VTOL design.
It is quite a large bird and I think they plan to price at $25 million so not a GA product but still pretty cool.
https://www.startengine.com/startup/xti
