Done before on the 300C - Firefly.
https://www.youtube.com/watch?v=oD8TfjDMYT0
http://www.sikorsky.com/Pages/Innova...echnology.aspx
15 minutes flight time, near max weight with the batteries.
Only way this will work is when a new serial/parallel hybrid drivetrain makes it viable, with a battery reservoir for needed power boosts.
The generating engine needs to be VERY light, as does the motor.
The only benefit it will really bring is to remove the gearbox, clutch and linkages. So it has to come in under those weights when you've added a generator/alternator.
So to replace a 3000hp powertrain, you'd need a 3mW electric motor on the hub, and a turbine or similar rated to supply that amount of electrical power.
As for "Why a robinson?" well, it's not feasible trying to replace turbines just yet, but the less efficient pistons are a prime target for the tech as it matures.