Xavier
If you’ve read any of my ramblings about vibration you’ll know what my first question will be.
What frequency, 1/rev or 2/rev??
I know I’m repeating myself yet again but without knowing which frequency we’re talking about discussions about vibration tend to be meaningless.
If it’s 1/rev one of the things I recommend when training guys to track and balance rotors using a dynamic balancer is, before taking any vibration data, fly 5 to 10 minutes of crank and bank. This is because I’ve found that often any 1/rev vibration data recorded during the first flight of the day will not be the same as data taken later under the same conditions.
My gut thinking on this is that a rotor in the hanger with the blades unsupported tends to “”creep” out of shape causing either the CofG to move slightly or the blades to twist differently. Because our rotors are incredibly sensitive to tracking changes I think it’s probably more a problem of twisting than CofG shift but that’s just my gut feeling. Looking back over all the rotors I’ve balanced in the past, I get the impression that this seems to happen more with composite blades than with extruded blades, but that’s just a feeling I’ve no statistical data to confirm that.
If it’s 2/rev then I’d guess it’s more a question of friction in the control system. Again if you read some of the stuff I’ve written (there are even some videos that show) that 2/rev stick shake is controlled more by friction damping than anything else. Although it’s a bit surprising that friction in the control system/linkages would increase.
So yes stick vibration often decreases (gets weaker) after flying for some time.
Mike G