While reading this question, I found that the answers did not address the point I had come for. On planes with electronic propeller synchronization, how is it actually accomplished? I know why propeller synchronization is useful but I am curious about the mechanisms behind how it works.
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Consider finding some way to update the existing question to bump it up to the top of stack and emphasize that you are interested in the actual mechanism? Oops I assumed it was your own question, guess it wasn't. Still, it seems like your question is pretty much a duplicate, not sure what's the best thing to do here. – quiet flyer Apr 22 '20 at 13:58
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You could offer a bounty-- – quiet flyer Apr 22 '20 at 14:01
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You could just edit line 2 to say something like "How is it achieved? I.e., what is the actual mechanism?" Then leave a comment to emphasize that this part of the question has not yet been answered. – quiet flyer Apr 22 '20 at 14:07
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Or if no one objects and votes to close, maybe just leave your question up and maybe you'll get the answers you are looking for. – quiet flyer Apr 22 '20 at 14:08
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4@quietflyer I don't think it is a duplicate at all. The other question only asks what prop sync is and how it is used from a pilot perspective. It does not ask about the actual mechanism behind it. – Bianfable Apr 22 '20 at 15:07
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@quietflyer "How is it achieved?" -> You synchronize RPM of the engines. But it does not ask about how that synchronization works from a technical perspective. – Bianfable Apr 22 '20 at 15:10
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@Bianfable so you are saying "by synchronizing the engines" is a sufficient answer to line 2, "How is the synchronization of the props achieved?" Or vice versa, if line 2 is interpreted to read "How is synchronization of the engines achieved", then a sufficient answer is "by synchronizing the props"? The original question was obviously asking for more than that and I don't see how the present question is not a duplicate. After all, similar terse answers would seem to be equally suited (or unsuited) to the present question as well. But, I'll save my close votes for something else... – quiet flyer Apr 22 '20 at 15:20
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1I don't think this is a dupe. The solution isn't just to sync the RPM's, but also to get blades in the right relative phase, which requires more than just matching RPM. (The C-130 engines ran at 100% RPM the whole time, but blade position mattered, a lot.) VTLO. – Ralph J Apr 22 '20 at 22:44
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2@RalphJ That sounds like a great answer in the making! That is the type of stuff I want to know. – dalearn Apr 22 '20 at 22:57
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3Does this answer your question? What is propeller / engine sync and how does it work? – SSumner Apr 23 '20 at 00:33
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@SSumner Other comments and the question explain why this is not a dupe. – Manu H Apr 23 '20 at 04:43
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@dalearn: Feel free to refine the scope. I don't know anything about that C-130 system. I've deleted my answer so you don't have to worry about invalidating it with any edits. – Apr 23 '20 at 14:02
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@ymb1 Hate to see that good answer wasted. Hope you consider re-posting it if is still applicable once the question is stabilized, as long as community consensus is that question is not a duplicate. Also, why not post it to the older question too, it sure seems applicable there. Don't think there is any rule against posting the same answer to two questions. – quiet flyer Apr 23 '20 at 17:25
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Is the point of some of the comments meant to be that "prop synchronization" inherently implies synchronization of the propeller blade positions, not just engine rpm? Wikipedia calls that function a "synchrophaser". ( https://en.wikipedia.org/wiki/Propeller_synchronization ) A question that is specific to how that works would arguably not be a duplicate. – quiet flyer Apr 23 '20 at 17:31
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I fly a large 4-engine turboprop. It uses constant speed hydraulic props, which automatically change blade angle to maintain 1020 rpm. The sync box (which I was unable to find a picture of) takes inputs from each engine's tachometer generator (where it gets the RPM signal) and a pulse generator, which is a magnetic pickup right behind the prop. There is a magnet on the number 1 blade of each prop. With this information, the solid state logic components inside the sync box can slightly change blade angle by sending power to the speed derivative servomotor. This is different from the answer that @davidinnes provided, but I suspect that he's correct for the type of propellers he's familiar with. The sync system on the P-3 also takes inputs through a potentiometer attached to the power levers, and can start increasing or decreasing blade angles if you change your power setting quickly. It works faster than the hydromechanical system and lessens RPM overshoots and undershoots as the prop works to get back to 100% rpm.
In this picture you can see syncrophasing in action. The props are all slightly out of phase with each other, which reduces noise and vibration. #2 and 4 look pretty close, but it's far from a perfect system. You can definitely tell the difference from the cockpit when it's working well. Again as @davidinnes mentioned, you can adjust the system to the change where the point of minimum (or maximum) noise is in the cabin. We call it "running the buzz" up and down the tube, and it's a great way to annoy a navigator that you don't like
James Ford
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By modulating the fuel flow to the slave engine, ie the one which is to synchronise to the "master" engine, Note. Limited authority of fuel modulation and not permitted in Take off or approach.
One writer alluded to synchrophasing to not only reduce beating bit alos reduce noise, You can as I have done move the point of minimum noise around/along the cabin by small changes in phase angle. Actual angle (relative position of the prop blades) depends on number of blades per engine, Easy with two engines, more than 2 ,,, its more of an art
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Might as well post that answer here too, as it addresses line 2 -- https://aviation.stackexchange.com/questions/14263/what-is-propeller-engine-sync-and-how-does-it-work – quiet flyer May 12 '20 at 14:12
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I've been told that the flight engineer's position in a B-17 was such that he could look out a window and shine a strobe light at the prop blades on one side of the plane. The strobe was slaved to one of the engines and the engineer could use the strobe light to tweak all the other engines into sync manually.
niels nielsen
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