Does stick shake get weaker with flight time?

XXavier

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An ELA gyro that I use to fly has a fairly strong stick shake at takeoff, but after that, the stick vibration seems to get gradually weaker, finally stabilizing –after half an hour or so– at a much lower level than its original strength. Of course, it’s only a personal, subjective observation, but I’m quite sure that it’s actually so…

Could there be an explanation for that...? Any comments…?
 
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Brian Jackson

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Can you measure it to be sure it isn't your hand getting acclimated to the vibration? I'm interested in learning more about this.
 

XXavier

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Can you measure it to be sure it isn't your hand getting acclimated to the vibration? I'm interested in learning more about this.

I've not measured it, but letting the stick loose while in trimmed flight, it oscillates with a visibly smaller amplitude (half as before or so...) after 20-30 minutes flying...
 

Brian Jackson

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I've not measured it, but letting the stick loose while in trimmed flight, it oscillates with a visibly smaller amplitude (half as before or so...) after 20-30 minutes flying...
This is puzzling. I'm not an expert on gyros (or a pilot right now) but is it possible there is friction in the system that lessens as the pivoting parts heat and expand?
 

Resasi

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One thing that changes with flight is weight which decreases with fuel burn, and lessens wing/disc loading?
 

XXavier

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One thing that changes with flight is weight which decreases with fuel burn, and lessens wing/disc loading?

But a flight time of 30 minutes means a fuel burn of no more than ten liters, and that's only 8 kilo...
 

XXavier

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This is puzzling. I'm not an expert on gyros (or a pilot right now) but is it possible there is friction in the system that lessens as the pivoting parts heat and expand?

Perhaps it has something to do with a small re-arrangement of the blades and hub bar, due to the tension caused by the centrifugal force while in flight. I have often heard, when the blades are turning slowly (or already stopped) after a flight, a distinct and strong 'clonk' coming from the blades (Averso extruded aluminum blades). I mentioned that to the manufacturer, who told me that it does sometimes happen, but has no importance. The bolts are duly torqued at 4,7 Kgf - meter.
 
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Doug Riley

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The pivots may be wearing a little. With a some clearance in the pivots, the movement of the pushrods may be taken up within this clearance, thus never reaching the stick.

Some pilots even WANT play for this reason.

You can check for movement of the pushrods using a Go Pro video camera (high tech), or by turning around and looking at them or their shadows (low tech).
 

Resasi

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But a flight time of 30 minutes means a fuel burn of no more than ten liters, and that's only 8 kilo...
True.

It was simply the first thing I thought of.
 

Illini85

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No expert here but it makes me wonder about heating and expansion closing tolerances in bearings or maybe bushings. perhaps? SWAG for sure.
 

Mike G

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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
 

XXavier

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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

It's mainly 2/rev, I think. Stick shake is almost absent when I let the gyro sink almost vertically with the engine at idle...
 

MAK

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Do you tie the rotor blades down when in storage? As Mark said, can take a while for it then to settle in to flight.
 

GyrOZprey

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It is a well known quirk of composite rotors ...exhibiting this phenomena! It has been long observed in our carbon-on carbon-spar TAG rotors ... more "shake" (even the smoothest - best balanced rotor seems to have an exaggerated "heartbeat") for the first 15 minutes of flight .... amplified in winter temps, at fly-ins when the rotor is tethered down to nose-clip parked overnight!
...I have rotor tip supports that hold the rotor level to a slightly coned-up position - to help the composite rotor remember it's "happy-place"! ...when my TAG's are parked overnight ...& days between missions!
It REALLY does seem to help eliminate those initial rough15 minutes of stretching out & rotor finding the memory happy-place where it is smooth!
 

Brian Jackson

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It is a well known quirk of composite rotors ...exhibiting this phenomena! It has been long observed in our carbon-on carbon-spar TAG rotors ... more "shake" (even the smoothest - best balanced rotor seems to have an exaggerated "heartbeat") for the first 15 minutes of flight .... amplified in winter temps, at fly-ins when the rotor is tethered down to nose-clip parked overnight!
...I have rotor tip supports that hold the rotor level to a slightly coned-up position - to help the composite rotor remember it's "happy-place"! ...when my TAG's are parked overnight ...& days between missions!
It REALLY does seem to help eliminate those initial rough15 minutes of stretching out & rotor finding the memory happy-place where it is smooth!
This is news to me. I have a never-flown Gyro-Tech carbon fiber rotor & hub that has been stored assembled and hanging from teeter bolt the past couple of years in an indoor controlled environment during construction of my gyro. Is this a good practice or should they be stored boxed and flat? The flex memory description makes sense, but I've been hopeful to eventually hangar it with blades on for convenience. Thanks.
 
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