And if you have a sensitive butt, great ears, delicate moving fingers and whiffle dust it can be done by hand.
I have no time in 4 engine aircraft, mine is all twin engine. A few times in over 30 years of flying I have a gotten them perfect for over a minute. Can get it almost perfect often but engine quality, control slop, fuel flow regulation, wither normally aspirated or turbo or supercharged makes a different also.
It is a great pleasure when you get it. Much easier to flip a switch & I like that technology. I use it when I got it. he he he
Jets with tail-mounted engines tend to have the same beat frequency vibration, although not so strong as on a prop-driven aircraft. And most modern jet engines have 2 or 3 spools each turning at different RPMs, so as a practical matter you can never get everything perfectly synchronized.
But we try to find that sweet spot of minimal wah-wah. Some aircraft have auto RPM sync systems, other don’t. Couple lack of auto-sync with sloppy autothrottles that are constantly jockeying power/RPM and you’ve got a recipe for a really unpleasant ride in the last few rows of coach.
I can see where controlling the phase of a particular master blade on each prop would be useful for piston engines, because that would correspond to the same crankshaft angle (and therefore a particular engine piston being at a designated point in its stroke) for all 2/3/4 engines.
But on a turboprop, there isn’t really a “master” blade, at least not one that corresponds to the same position of everything else in the engine on every revolution. The prop is connected (through a gear reduction unit) to a power extraction turbine at the back of the engine. The main compressor/turbine spindle could be at any angle relative to the prop, and the power extraction turbine could be just about anywhere too, depending on the drive ratio.
In any event, is it actually helpful to have one blade from all 2/3/4 props passing 12:00 at the exact same time? Wouldn’t this have all of the props’ vibrations reinforcing each other, like 2/3/4 guys beating on a drum at the exact same time? It seems like it would make more sense to have the synchrophase mechanism deliberately keep all of the props slightly out of phase with each other by a particular angle. Example, if you’ve got a pair of four-bladed props, it seems like it would be best to have the two of them set to be 45 degrees out of phase with each other. Got a quartet of four-bladed props? Set them all 22.5 degrees out of phase with each other.
Or is there some other consideration I’m not seeing?
The complexity of the synchronizing mechanism might play a role. For a plane with x engines, it has to be able to detect x propeller positions and adjust the speed of x-1 engines. The detector could be as simple as a cam on the prop shaft. Even with a four-bladed prop, it might be simpler to put just one cam lobe on the shaft (synchrophasing) than four. If you wanted them synchronized but out-of-phase (45 degrees or 22.5 degrees, as you suggest) you’d need to manufacture the shafts specific to each engine position, differing by the position of the cam lobe relative to the propeller. Once you put a #2 shaft into an engine, it would have to be a #2 engine. The airline would have to keep a lot more spare engines around if you couldn’t just swap them into any position.