Germany's Secret Air Force
The title on this one intrigued me because I've run into Germany's Secret Air Force before when I was checking up on Babylon Berlin.
There were two kinds of radial engines used in early airplanes. There was the kind where the engine block is bolted to the airframe and the crankshaft rotates and turns the propeller. Then there is the Rotary engine where the crankshaft is bolted to the airframe and the entire engine block rotates and turns the propeller. There aren't too many of those around anymore, so I was intrigued to see some tinkering being done on a Rotary engine at the 5:30 mark.