If you happen to be assembling one of the H9 warbirds that have rotating gear (P40, F4U, or F6F), you might look at these.

No screwing around with a servo linkage or building in all the stuff that air retracts require. Put 'em in where the mechanicals go, plug 'em into the RX, and fly.

https://smartweb.ipower.com/lado/pro...0&productID=19