Wether you are planning to install just mains or trike retracts on scale model until now you didn't have much choice. Some models usually in the 60 size or below would have mechanical retracts these being operated by rods connected to a single servo. Pretty reliable if set up properly but in the event of a jam the servo would drag down your receiver battery and most likely cause a "brown out" or worse. Unless you didn't have a weight problem and added a battery just for the retracts.