Very interesting subject FoxyBob.
You are right about flash duration probably 1/10000 sec here.
I know you will like this ....... we need a few tech details of rotor operation. Pics suggest it is hand powered. Therefore speed will be initially high and decreasing. Also difference between winding main rotor and tail rotor.
I think here you used front curtain sync. In this mode the flash will fire immediately the front curtain is open. For the purpose here lets assume a duration of 1/10,000 sec. So if exposure was say 1/60 sec it will be at least 1/59 sec before second curtain moves. During this time exposure is affected by ambient light.
If rear curtain sync is used, situation is reversed. Initial exposure is ambient and immediately before the rear curtain closes flash will be triggered.
The problem if it is hand powered is trying to have the main rotor slowing sufficiently before starting the tail rotor.
BTW very good natural light shot but rotors a bit fast for good prop blurr.
Don