Hand-turned radials.
Hi Guys,
Forgive me, but I'm not sure some of what has been said on this subject is correct.
If it really was necessary to hand-turn radials to clear hydraulic lock, then just imagine trying to do it on a Short Stirling or a Blackburn Beverley! You'd have to be twelve feet tall, or else erect a scaffoling platform first! Anyway, you probably couldn't physically hand turn an engine that size.
Whenever I have seen film footage of airmen hand-turning a big radial, it has always looked to me more like they were 'blowing-out' an engine that had perhaps been flooded during a botched start-up and from which the plugs had been removed to facilitate the 'blowing-out' process.
If hydraulic lock was indeed the issue, then it would also be true of inverted engines, like the Gypsy Major and the Cirrus Minor and not just radials. Yes, you do hand turn them, but not to clear hydraulic lock. It's for 'sucking-in' fuel into your cylinders prior either, to throwing the left magneto and hand swinging the prop, (or firing the cartridge starter if it's something like a military spec Chipmunk).
Furthermore, if hydraulic lock really was present, then you would be physically unable to turn even a small engine; you'd have to call the mechanics to it to remove the ignition plug(s) and drain off the oil first.
Anyway, you would only get hydraulic lock if your lower cylinders had very badly worn rings and/or bores. And long before they reached that state, you would have oily plugs and a high oil consumption. You would have constant bad starting and misfiring and vibration on startup, which would warn you about the state of your engine long before hydraulic lock became a possibility.
I would submit that since a radial requires two whole revolutions in order for every cylinder to complete its cycle of suck-squeeze-bang-blow, the prime purpose of hand-turning the engine is purely for 'sucking-in' and nothing else.
With a big aeroplane, however, like a Beverley, either the batteries would be big enough to do the sucking in before the engine started, or perhaps a trolley-acc would be used to take the strain off the batteries at start-up.
Well that's my view anyway.
Broomstick.