Engineering...Beyond the Classroom

Batteries and an internal combustion Wankel engine drive an electric generator which powers the Siemens electric motor turning the propeller.

Bad writing. The batteries do not power the generator. The engine drives the generator. The batteries can power the electric motor directly.

Will hybrid or electric airplanes be commercially viable in your lifetime?

Electric airplanes are already viable.

On the other hand, airplanes are a poor choice for a series hybrid, because they spend virtually all their time operating at high engine load and engine efficiency (unlike a car where most of the time is spent at very low loads and idling). Thus, the weight of hybridizing is hard to offset with fuel savings. (Airplanes are more weight-sensitive than ground vehicles because each additional increment in lift increases drag by a greater percentage... whereas in a ground vehicle, weight causes a linear increase in rolling drag, which is only one part of the total drag.)

In cruise, the engine required is of the same size, because it must supply continuous power equal to that required for cruise flight. In optimistic theory, one could downsize the engine by 25%, but in practice, an engine operates more efficiently at less than full power (at the roughly the full torque point, which works out to about 65-75% power). An engine also lasts longer at 75% than at 100%.

Non-geared engines deliver 100% of their power to the prop. Interposing a 90% efficient generator and a 90% efficient motor (for 81% efficiency) means that there is a 19% loss that is not incurred with direct drive. So in cruise, the series hybrid would be less efficient. (This efficiency loss is the reason for the Chevy Volt's horribly complicated arrangement for powering the wheels mechanically during cruise [when in charge sustaining mode]). The "fuel savings" in large aircraft (in the Siemens strategy) would come about during a long climb-out on electricity. For this to be a true environmental savings, the electricity would need to generated by solar, hydro, etc.

Given the conservative nature of the aircraft industry, I doubt that viable hybrid airliners will be seen in my lifetime. Boeing thinks maybe 2035, but I doubt it. (Interesting that their plan is the reverse of the Siemens plan: jet fuel for takeoff -where jets are least efficient -- and electric power for cruise... which would require a really impressive battery.)