I remain completely baffled by the notion of ‘hybrid aircraft’. If you pose the question “What are the advantages of hybrid aircraft” through the interweb the principal response is ‘improved fuel efficiency’ with figures ranging from 5% to 40%. Mention is also made of the similarities between hybrid aircraft and hybrid road vehicles. Ignoring hydrogen (and the hopeless impracticality of this ‘fuel’ ever achieving any significant penetration into sustainable transport) for the moment my main thought is that for road going vehicles the hybrid may well offer a significant improvement in fuel efficiency on the basis that a large proportion of this type of vehicle spend a large proportion of their time in stop-go traffic. A hybrid system can go along way to level the fuel demand on the ICE allowing it to operate only at its most efficient speed whenever it is actually on and thus leave the stop-go bit to the hugely more efficient electric motor and batteries. But even then a 10% improvement is a realistic figure and, realistically, simply not worth the massive extra complexity - and cost (including servicing costs) - of introducing an ICE into an EV. Besides which all the research done on the subject indicates that hybrid drivers rarely actually ever plug them in (assuming they even have this capability).
In an aircraft, aside from takeoff it pretty much by definition sits at a fixed RPM for the entire duration of the flight thereby giving little opportunity for the hybrid drivetrain to do its thing.
I am therefore left with the conclusion that the whole idea is a complete white elephant and, on a more cynical note, is just a bunch of deluded (at best) individuals making a fortune on ill-conceived government/tax-payer-funded green-wash.
Someone please explain why I am wrong.