FWIW, what Bruce is talking about is literally the propeller pitch angle motors used on vintage aircraft. After WWII the surplus market was flooded with old aircraft parts and hams use them to turn beams on top of their towers. The ones hams look for were made by Curtis from the late 1930s to the mid 1950s.
Attached a web photo of what one looks like. Powerful suckers to turns the blades on the front of a P51, so a dinky 40m yagi in New Mexican wind ain't nuthin'.
I might head up to the swap tomorrow morning, LMK if anyone else wants to go. I would not be up there very long and honestly I'm not sure if they'll cancel or not.