Horex made an upright, four-cylinder, inline aircraft engine.

“Horex schematic drawings”

“Herrmann Reeb with his Horex”

-Towards the end of the 1920s, Horex manufactured "Stuway Archer + Horex" motorcycles.

-The engine designer was Hermann Reeb, who appears standing next to the engine in the photo.

“Horex engine side view”

From Appendix 12: This 1928 aero engine is built by Herrmann Reeb.

-Horex made motorcycles and apparently also larger engines. In the photo below we see Herrmann with his engine.

“Herrmann Reeb together with his interesting engine”