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  #1  
Old 05-26-2022, 07:38 AM
tdavis1198 tdavis1198 is offline
 
Join Date: Jul 2019
Location: Toronto, Ontario
Posts: 46
Default Excessive Tail Shaking/Buffet

Looking for some ideas from the brain trust. I have a RV-8 (not the original builder) that I purchased in 2020. It was completed in 2019. The builder is very experienced and well known. The plane performed really well until we had it painted. Since then there has been intermittent Tail Shaking first detected on the flight back from the paint shop.

My research has shown that this seems to be a known aerodynamic issue with the RV-8 design where there is pronounced tail buffeting/shaking just prior to stall due to dirty air flowing over the wing roots and down to the tail.

Our testing has found that this occurs at about 67kt clean and the tail shaking is bordering on quite violent when it happens.

My real question is why this issue would suddenly start occurring after paint. We have checked and re-torqued all the attachment bolts in the empennage and everything looks good.

Any ideas?
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  #2  
Old 05-26-2022, 07:40 AM
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Snowflake Snowflake is offline
 
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Did you re-balance the control surfaces after painting?
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  #3  
Old 05-26-2022, 08:17 AM
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DanH DanH is offline
 
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Location: 08A
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Quote:
My research has shown that this seems to be a known aerodynamic issue with the RV-8 design where there is pronounced tail buffeting/shaking just prior to stall due to dirty air flowing over the wing roots and down to the tail.
I'm sure you realize some see the "issue" as a desirable last chance stall warning. Mine exhibits a nice shudder about Vs plus 1. Me like.

(Warning, incoming WAG) Addressing the before vs after paint difference, perhaps surface finish has an effect on boundary layer separation?
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  #4  
Old 05-26-2022, 09:06 AM
tdavis1198 tdavis1198 is offline
 
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Location: Toronto, Ontario
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Quote:
Originally Posted by DanH View Post
I'm sure you realize some see the "issue" as a desirable last chance stall warning. Mine exhibits a nice shudder about Vs plus 1. Me like.

(Warning, incoming WAG) Addressing the before vs after paint difference, perhaps surface finish has an effect on boundary layer separation?
Interesting theory Dan. A couple of things that are weird. Firstly in clean config the shudder is at 67kt indicated so a lot higher than stall speed and secondly its quite violent compared to buffet i am used to on other planes
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  #5  
Old 05-26-2022, 09:11 AM
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Ron RV8 Ron RV8 is offline
 
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Location: Okanagan Valley BC, Canada
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Suggest check:

Individual elevator balance

Play in the trim tab
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  #6  
Old 05-26-2022, 09:38 AM
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Carl Froehlich Carl Froehlich is offline
 
Join Date: Dec 2007
Location: Dogwood Airpark (VA42)
Posts: 3,732
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I suspect this is the issue others have observed associated with the upper gear leg intersection fairings.

What can you tell us on how this intersection fairing is configured on your plane?

Carl

Last edited by Carl Froehlich : 05-26-2022 at 09:42 AM.
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  #7  
Old 05-26-2022, 09:41 AM
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Ironflight Ironflight is offline
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Location: Dayton, NV
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Assuming that your fairing configurations are the same before and after paint, I would find it hard to believe that the flow would change significantly enough to raise the stall warning by close to ten knots (where did you get buffet before paint?).

I’d look at a re-assembly or balance issue, and I’d look REAL close before further flight - and then treat the flight as a test flight! I had a trim tab linkage break once (slow plane, not an RV….), and the result was…exciting! You don’t need that kind of excitement, so proceed with caution….
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Last edited by Ironflight : 05-26-2022 at 09:44 AM.
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  #8  
Old 05-26-2022, 10:08 AM
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DanH DanH is offline
 
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Quote:
Originally Posted by tdavis1198 View Post
Firstly in clean config the shudder is at 67kt indicated so a lot higher than stall speed...
That ain't right.

What Paul said.
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  #9  
Old 05-26-2022, 10:28 AM
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Webb Webb is offline
 
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Trailing edge radius?
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  #10  
Old 05-26-2022, 10:46 AM
Mikeyb Mikeyb is offline
 
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A partially plugged static port might cause the ASI to read high while you are descending. Paint Job?
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