Hamilton Standard Hydromatic props used a hydraulically actuatted cam which achieves the desired picth control. Not sure if the propellor governorn had any electrics involved.
No, the governor didn't, but the oil pumps were electrically actuated, though. As you stated, the hydromatic props used differential oil pressure against a piston attached to a moving cam, which altered the blade angle.
To feather the props, a button was pressed in the cockpit, which was held down by a coil until the prop had feathered. Simultaneously a solenoid relay switched power to the feathering motor and electric oil pump fro the a/c battery. Oli was pumped from a separate reservoir to the governor, where the pressure build-up would open a transfer valve, which would shut out the governor, then pass through the distributor valve in the hub and act against the inside of the piston to change the blade angle. ( I had to look this up.)
I've never worked on a Curtiss electric prop, but brushes and slip rings certainly make sense.
Interestingly, Alec Harvey-Bailey in the book Rolls Royce- The pursuit of excellence offers this:
"The Vulture, not a Royce design, was a 24 cyliner x engine based on Kestrel bore and stroke. Investigations showed that there had been master rod bolt failures, allowing the flailing assembly to almost cut the engine in half and in some cases to prevent feathering of the propeller."
The following is probably of interest too:
"The problem of the x layout is how to get four pistons to drive one crankpin. On the Eagle XVI, Royce had chosen two pairs of fork and blade rods, running side by side, but the Vulture had a master rod and three articulated rods on one big end. This was to keep the engine short and light. There was one articulated rod on the master rod and two articulated rods on the big end cap. To accommodate this arrangement the split line for the big end had to be angled to the master rod.
There was so little space that Royce's practise of using long ductile nickel steel bolts, which could be tightened to plastic deformation, was abandoned. The assembly was held together by two longish bolts and two very short bolts, which had to be made in high brinell nickel chrome steel. In such bolts the elastic limit and yield point are very close and in the case of the short bolts the permissable extension was very small and had to be precisely controlled.
Primary failure occurred in the short bolts. Because the rod and cap were located by both saw tooth serrations and fitting bolts, there was a quarrel between the two causing stepping of the bolts at the joint face. There had been problems in tightening the short bolts, which had caused variations in tension. The two factors led to the bolts breaking, but it was found also that at the t/off rpm of 3,000, the bolt loadings were imprudently high.
The issue was tackled by re-working the engines. This included new bolts of a slightly easier fit, concentrating location in the serrations and to avoid stepping during assembly. The bolt tightening operation was revised and meticulously followed. Cyril Lovesey pointed out that at 3,000 rpm plus 6 lb boost at t/off, the engine was running throttled and the same power could be achieved by raising the boost to plus 9 lb and reducing the rpm to 2850. This gave much more conservative bolt loadings. Operations were restarted with an engine life of 80 hours, rising to 120 hours as reliability proved satisfactory.
This dealt with the immediate problem, but Hives was faced with a much bigger decision - what toi do with the engine. The Vultures would only be useful if its power development could continue, but this would entail a new master rod design as well as other changes to cope with various problems, which were also present; a major task in the context of the vast Merlin programme.
...The Manchester had great potential and the answer lay in installing four Merlins..." "...Hives took the decision, which was accepted by the ministry, to stop the Vulture."
I fear this might have been covered in another thread...
