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Old 09-24-2021, 10:16 AM   #17 (permalink)
extrapilot
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Join Date: Aug 2010
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Id suggest you look up Phase Lag as an official term. Wikipedia is a typical hit, and they summarize it as: “the angular difference between the point at which a control input to a rotor blade occurs and the point of maximum displacement of the blade in response to that control input.’

In the video, the entire discussion is focused on why it is that the control inputs are out of phase with the behavior of the rotor. In multiple instances, he discusses precession as a physics principle, and then nearer the, end, asserts ‘That’s it. That’s why helicopter blades operate 90deg out of phase.’ He titled the video- ‘Are helicopeters gyroscopes?’

The phenomenon he is talking about has nothing to do with precession- and it is fair to say his ignorance of this is why he doesn’t refer to it as phase lag. Im not going to refer to phase lag as precession- any more than I would call addition ‘multiplication.’ In that world, you can say, OK, well, 2+2=4, and 2*2=4, so, the analogy for that video title becomes ‘Is addition multiplication?’ and concluding after testing 2+2 and 2*2 that yes indeed, they are equivalent. When I tell him- the S76 has a phase lag of 57deg, and the BO105 is something like 78deg (from memory), and all gyros precess at 90deg by a core law of the universe, how can he not see a problem with his assertions? If he types 3*3 on a calculator, and he believes it will output ‘6’- but it outputs ‘9’, you would think a guy who named his channel ‘Smarter Every Day’ might want to revise his errors.

Phase lag isnt some esoteric concept that makes no practical difference to you as an RC pilot. So while Id hope that anyone interested in learning about how things work would care about the error for that reason alone- there is a practical reason to understand the error. Rigid-head machines- 3-bladed heads etc will have significantly less than 90deg phase lag. Not 1-2 deg less. Some of you are flying with 10deg error or more. And that is literally analogous to rigging a head with the swash drivers rotated 10deg off for a teetering head. You put in pure forward cyclic, you get some roll with that. Pure roll, you get some pitch with that. That is a ‘couple.’ And nothing good comes from it. The FBL sees a pitch command, and the machine rolls a bit with the pitch output. So, it puts in opposite roll, and that takes out some of the pitch. So it adds more pitch, and that reverses the roll error etc. On hard stops, you will see the rotor wobble hard. This is easy to fix- adjust the phase lag in the FBL. But if you believe lag is precession, and lag by law is ALWAYS 90deg, there is no reason to alter lag unless you are compensating for a rigging problem on a 5-blade head etc where maybe the links can only permit 45deg offset. So you think you need 45deg more lag to be good- to get to 90. And now your 80deg rotor is setup with 90deg lag, and you have problems again…
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