The orbital family of ICE engine of 1972 has seen a lot of ongoing innovation and crossover variations with other rotary engines (like the Wankel) in past decades - such engine variants have had a major place in UAV (and other) drones.

* https://en.wikipedia.org/wiki/Sarich_orbital_engine

* https://www.orbitaluav.com/

* https://www.aieuk.com/225acs-wankel-rotary-engine/