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Yes. The turbine engine wants to run really fast for more efficiency. And the fan wants to be big and run really slow for more efficiency. This is a mismatch. I

Yes. The turbine engine wants to run really fast for more efficiency. And the fan wants to be big and run really slow for more efficiency. This is a mismatch. If you connect them with an axle (like in a turbofan), you have to make a lot of compromises, lowering efficiency. You can put a gearbox in there but it's heavy, maintenance intensive and expensive. Efficiency of the gearbox is pretty good, but not 100%. Then you have a geared turbofan or a turboprop. Geared turbofans are rare for a reason. With a generator and electric motors, you can have a different amount of fans and turbines, and they don't need to be even close to each other. You don't need the expensive and maintenance prone gearbox. You can optimize the propulsion system more -> more efficiency. Turbines don't scale down very well - they lose efficiency the smaller they get. This means, electric propulsion is promising for small aircraft since you can have one or two turbines but can have more propulsors.

baybal2 7y ago
The big problem with gas turbine gearboxes is that they have to operate at such high RPM that only the best metalworking tech works. Same with driveshafts and any kinds of transmissions. Any off-axis power diversion from turbines is extremely hard. Imagine a driveshaft spinning at 1/10 of the turbine RPMs, it will still require extreme stiffness or it will disintegrate from vibrations. Very high RPM generators are not that efficient too because of eddy current losses. It's not so bad in big industrial turbines spinning at 5-7k RPM or so, but losses mount at 15k+ RPM. This is why there is such a high interest in making superconducting generators for direct drive by turbines. They can be very efficient, and very small — two usually mutually exclusive qualities for an electrical machine.
- baybal2 7y ago
  And even with the best metalworking, materials, and lubricants, high speed gearboxes don't last long, and require frequent inspections for fatigue, and overhauls. P&W 1000G series have 32000hp gearbox rated in excess of the core cycle life and hours, but still apparently needs mandatory defectoscopy.

baybal2 7y ago

The big problem with gas turbine gearboxes is that they have to operate at such high RPM that only the best metalworking tech works. Same with driveshafts and any kinds of transmissions. Any off-axis power diversion from turbines is extremely hard. Imagine a driveshaft spinning at 1/10 of the turbine RPMs, it will still require extreme stiffness or it will disintegrate from vibrations. Very high RPM generators are not that efficient too because of eddy current losses. It's not so bad in big industrial turbines spinning at 5-7k RPM or so, but losses mount at 15k+ RPM. This is why there is such a high interest in making superconducting generators for direct drive by turbines. They can be very efficient, and very small — two usually mutually exclusive qualities for an electrical machine.

baybal2 7y ago
And even with the best metalworking, materials, and lubricants, high speed gearboxes don't last long, and require frequent inspections for fatigue, and overhauls. P&W 1000G series have 32000hp gearbox rated in excess of the core cycle life and hours, but still apparently needs mandatory defectoscopy.