take a look at spring air retracts
www.retracts.com they are good retracts, pretty strong, and the 700 series will take robostruts directly into the retract without having to grind those doggone flats for the set screws robart uses. i hate grinding those set screws
. they use one standard servo to operate these air retracts.
i put spring air retracts in my hangar 9 mustang because the stock retracts were weak. i also have them in my world gs miss america p51 and my pica fw190d9. they have been very reliable.
they seem to take at least 60psi to retract relialy- i usually put about 100 psi in the tank, which is good for at least 6-7 retracts.
ed