Hydraulic props may be self contained or not. The Ham Stand Hydromatic runs from the engine oil. The Aeroproducts is a self contained unit, that sits at the rear of the prop hub. Note the Bearcat and Skyraider that used Aeroproducts have no prop hubs. The BMW 801 engine, according to a BMW manual I translated, used both hydraulic and electrical. This is the text I extracted in translation: The airscrew adjustment system consists of a hydraulic and an electric one, both of which can adjust the airscrew independently of one another. The D model Fw-190s went to the Jumo 213 engines, and as I recall used the Junkers Jumo hydraulic propellors. Most DB engines used the VDM electric prop.