Strobe Lit Aircraft Propeller

You could put these under and on the sides of the cowling, and sync them so the viewer sees a propeller that's apparently spinning quickly so it's obvious that it's moving, but not so fast that it's just a blur.

The strobes would be aimed at the sides and bottom of the prop, relatively out of the pilots line of site but at the areas most likely to have people walk into them. The strobes would also somewhat illuminate the surrounding area so people facing away from the airplane might get some warning.

It would be easier to do than putting lights on the blades and might do a better job.