Are there any other types of system that can allow 2 engines to drive a single shaft even at different speed?

Answer 1

A worm gear? I have GOT to see a link for how that is used as a variable speed drive!

One obvious way is electricity, diesel electric and turbo electric power plants have been used on ships for years. They let you add plant as you need it.

Another method is to just gear your gas turbines together, the majority of the turbine is free rotating compared to the output shaft and the turbine just acts as a gas generator, you can have different power settings on the separate turbines and they will all contribute.

Thinking about it, hydraulic should do too, since it's volume rather than rate that transmits the power.

Direct mechanical solutions? None that I know of. I suppose you could use a couple of those CVT gearboxes, but they are only variable within limits. John Deere tractors use a truely continuously variable transmission, anything from 0-2x I think, and very good torque handling, but not compact in aircraft terms.

The electric drive is far too big for aircraft, it's a ship power plant only, and not a very economical one.

Answer 2

What a great question. I've wondered about something similar for years and have found some loosely related ideas. The only way that this can be achieved as far as I have found out is a worm gear or variable speed transmission like the one that is available commercially for some imported cars. One engine turns at 3000 rpm, 1 at 4000, and through a system of gear ratio reduction, the shaft will take power equally from both sources of power. I have also thought of a design for a geared version of a jet engine to allow for greater fuel economy in commercial aircraft. I'm no engineer, but I love to try and come up with simple solutions to complex issues.