Hiller Centrifugal Mercury Clutch
Just to build on the comments made on the Hiller start-up the Hiller has a centrifugal mercury clutch. A mercury bag is thrown out in a rotating assembly when the engine RPM builds up and engages the transmission which drives the MR & TR's directly. It is common for the clutch to catch straight away on start-up and see motion in the MR blades. The Franklin is then warmed up at 1700 RPM before opening out to its 3100 flying RPM.
p.s. we may be doing another run-up tomorrow morning Thurs 25th Feb.