small rotary - small promise (unfortunately)
I doubt we're going to see any aviation Wankel engines any more. This seems a shame, but here is the reality of the Wankel.
THIRSTY!
LOUD!!!
Either of these alone is a deal killer for aviation use, both of them together drives a stake right through the elegant triangular rotor.
And I have hands-on first person experience with a couple of Wankel engines, not just "had one once" but take them apart, rebuild them, and they run properly afterward.
The Wankel makes a lot of horsepower for its size, but horsepower comes from burning fuel, and the Wankel does that very well - it isn't a wonderfully efficient engine compared to a regular four stroke engine, even less so compared to a diesel. Each time the price of fuel goes up, the Wankel's prospects go down.
The Wankel also has a very loud and strident exhaust - that means you must have effective mufflers, and the only way to have effective mufflers is to make them large, restrictive and heavy - not precisely what we want in aviation.
There's yet another problem - tip seal life isn't great. While there are some trick alloys and even ceramic tip seals, they are quite costly and still have a limited service life.
Wankels would probably be best suited for some kind of military drone which is expected to crash or be shot down within a few hundred hours anyway, they do not appear to be the holy grail of engines for light airplanes after all. (Darn - they are so fascinating, too!)
Best Regards,
Echo Mike