Would this concept of Autogyro/Gyroplane be efficient or practical?

Question

First, i'm quite ignorant on aerospace engineering, so I hope I don't make too many incorrect assumptions.

If you don't know what an Autogyro/Gyroplane is, here is the Wikipedia article of it.

• The concept:

Basically, the idea would be to have two (or less/more) rotors on opposite sides of the aircraft, however, the shaft of the rotors would be connected to smaller propellers that would be exposed to the jet of air from the aircraft's engine.

The idea would be to take advantage of this jet of air generated by the engines to partially increase the speed of rotation of the rotors, since in the case of a Gyrocopter/Autogyro, they need the aircraft to move in a direction to be able to glide (of course, there are types of Autogyros that are connected to the engine).

I don't think that would be enough to make the aircraft take off vertically, but maybe it could produce more lift for heavier aircraft.

And to be honest, I think this rotor would spin so fast that you would have to add an electromagnetic brake, and well, if you have an electromagnetic brake, you have an electromagnetic motor. What could also change the speed of the rotors to more or less revolutions. I know if the goal is vertical takeoff, then hook the rotors to the engine right away with a transmission or something. But in my (ignorant) view, this would increase complexity, weight and maintenance costs.

And for the little I know, I can imagine that it wouldn't be that efficient since the thrust would be spent to spin the rotors, but I don't know if this waste would be small or big, or even if it would be enough to make it rotate fast enough to generate any significant lifting power.

• Why the question:

I ask this because one of the few VTOL planes that almost reached the civilian market was the "Fairey Rotodyne".

According to what was said about this aircraft, the rotors (which worked as in an Autogyro/Gyrocopter after the vertical take-off) were able to eliminate 40% of the aircraft's total weight during flight; making it an interesting option.

However, the way this hybrid plane took off vertically was pumping fuel to the tip of the propellers (turning it into a Tip-jet), which added to the complexity of the aircraft.

And that was just one of the reasons that it didn't attract many customers: when turning the tips of the propellers into rockets, an absurd noise was generated (and if that noise could be louder than the two engines in the wings, it was certainly something that could not be ignored).

Answer 1

There's no sensible reason to use ram air turbines to power the rotors on a craft like a gyroplane. The rotor is already powered directly by the airflow -- essentially "gliding" in and upward flow through the rotor disk. Adding turbines in the airstream, with shafts and gears to drive the rotors, only inserts inefficiencies -- friction and gear losses, and aerodynamic losses in the turbines.

If you want to shorten the takeoff of a gyroplane, the simplest way to do it (dating back to the 1930s at least, perhaps the 1920s) is a prerotator powered by either a takeoff from the main propulsion engine or a pretty small auxiliary engine or electric motor -- I've seen this done with a spring driven motor -- dedicated to this task. If you also have collective pitch control on the rotor and enough power in the prerotator, this can even be used for zero-roll "jump" takeoff (the ultimate STOL, no headwind needed) by storing momentum in a slightly overspeed rotor that's converted to lift by a sharp change in collective pitch (flat, to positive to "jump" then back to gyroplane-normal slightly negative to drive autorotation).

A prerotator requires little power; once the takeoff roll begins or some forward speed is developed after the jump, the airstream powers the rotor as with a simple autogyro.