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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?
 
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?
 
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
 
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
 
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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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A partially plugged static port might cause the ASI to read high while you are descending. Paint Job?
 
Check all wheel and gear fairings. Perhaps the are not aligned properly. Fly it with wheel pants off, then try it with gear fairings and pants off
 
My understanding was that some prestall buffet on the 8 came from the wing LE to fuselage intersection fairing and or the gear fairing. Haven’t some people installed strange little vortilon gizmos there to fix that and lower stall speed? I would review the elevator balancing and I would check everything upstream of the tail, namely the wing root and gear leg intersection.
 
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Hi guys, I flew Tony's 8 before and after paint and I flew it yesterday after Tony's in flight shudder to evaluate things. I can confirm that it's an angle of attack issue. I was able to recreate the shudder at 67 knots, clean in level flight. The HS is shaking violently exactly as it would due to main wing flow separation proceeding a stall but the rest of the aircraft is not exhibiting stall characteristics. The controls are solid, attitude is sensible, the aircraft is not close to stall. Upon further inspection today I noticed that the rubber wing to fuselage intersection fairing is not correct. A different plastic channel is in place that leaves a 1/8 " gap between the wing and fuse. Maybe that's a factor?? I couldn't replicate the issue with full flaps. I did a few full stall three point landings with no issues. Tony however had the buffet just before touchdown on 3 separate full flap wheel landings.
 
AOA issue

If it’s an AOA issue then shouldn’t you be able to replicate the result at various speeds with a corresponding change in Gz?

Hi guys, I flew Tony's 8 before and after paint and I flew it yesterday after Tony's in flight shudder to evaluate things. I can confirm that it's an angle of attack issue. I was able to recreate the shudder at 67 knots, clean in level flight. The HS is shaking violently exactly as it would due to main wing flow separation proceeding a stall but the rest of the aircraft is not exhibiting stall characteristics. The controls are solid, attitude is sensible, the aircraft is not close to stall. Upon further inspection today I noticed that the rubber wing to fuselage intersection fairing is not correct. A different plastic channel is in place that leaves a 1/8 " gap between the wing and fuse. Maybe that's a factor?? I couldn't replicate the issue with full flaps. I did a few full stall three point landings with no issues. Tony however had the buffet just before touchdown on 3 separate full flap wheel landings.
 
... The controls are solid, attitude is sensible, the aircraft is not close to stall. Upon further inspection today I noticed that the rubber wing to fuselage intersection fairing is not correct. A different plastic channel is in place that leaves a 1/8 " gap between the wing and fuse. Maybe that's a factor?? ...

The pre-stall buffet that we all know and love can be removed using a "strake" just above the intersection of the front of the wing and the fuselage. This strake is changing/smoothing airflow in the exact region of the rubber wing to fuselage intersection fairing, so I would have to say "yes", this 1/8" gap might be causing this problem.

Sounds like an order to Van's or McMaster-Carr for the correct rubber channel is in order. Or perhaps get the paint shop to look more closely to see if they have the original rubber channel laying around.
 
What's your paint scheme? Are there any span wise stripes with significant paint edges in the first 25% of chord that may be tripping the flow?
 
Hi guys, I flew Tony's 8 before and after paint and I flew it yesterday after Tony's in flight shudder to evaluate things. I can confirm that it's an angle of attack issue. I was able to recreate the shudder at 67 knots, clean in level flight. The HS is shaking violently exactly as it would due to main wing flow separation proceeding a stall but the rest of the aircraft is not exhibiting stall characteristics. The controls are solid, attitude is sensible, the aircraft is not close to stall. Upon further inspection today I noticed that the rubber wing to fuselage intersection fairing is not correct. A different plastic channel is in place that leaves a 1/8 " gap between the wing and fuse. Maybe that's a factor?? I couldn't replicate the issue with full flaps. I did a few full stall three point landings with no issues. Tony however had the buffet just before touchdown on 3 separate full flap wheel landings.
The gap between the wing intersection fairing and the fuselage is a strong candidate for this buffet. Perhaps some of the high pressure air below the wing is leaking into the wing, and squirting out that gap, disrupting the airflow over the inner wing. This disrupted airflow could then hit the tail.

Try applying some 2" wide vinyl tape to temporarily cover that gap, or install the proper rubber strip.
 
What's your paint scheme? Are there any span wise stripes with significant paint edges in the first 25% of chord that may be tripping the flow?

I’d doubt that would create any difference due to the boundary layer.
Elevator balance issues would more than likely cause flutter rather than a buffet issue however balance is very important so I’d ensure that was done. I would try to return the aircraft to factory spec….remove any gaps….the airflow is being disturbed pre stall and fly again. Calibrate speed instruments against reliable gps speed after any painting as well.
 
Elevator balance issues would more than likely cause flutter rather than a buffet issue however balance is very important so I’d ensure that was done.

Plus 1...
As a reference I had to add about 6.5 oz of weight to my elevator to balance it, granted the 14 has a larger elevator than 8 and probably requires more weight.
 
Seal it

Gliders often use "Gap Seal" tape for this exact reason. It comes in various widths and is very stretchy, and probably would solve the issue until you get new seal.
 
Elevator balance is for flutter, not to achieve a prefect static balance. If the elevator was built per plans, it should be just fine (as in don’t start cutting up the elevator tips to add lead until you eliminate the other possibilities).

To the OP, you state this issue is new following paint. I suggest a review of things that changed from before to after paint. I’d verify:
- If the tail feathers came off for paint, look to see if the forward HS spar shim was put back.
- If the wings came off, is the rear spar bolt in place?
- As suggested, put some vinyl tape on the wing root to fill the gap you have from the missing rubber trim (while you are waiting to get the new trim from Van’s).
- As previously discussed, the main gear upper intersection fairing have been a source of similar issues (especially if the fairings where not put back on)
- Did you do a before and after W&B (as required)? If your painter got carried away and added a boatload of weight, the tail may have gotten a disporpotialy higher amount than the rest of the plane as it has a lot of surface area. As a data point, I added 17 pounds before and after paint (prime/base/clear), but I did what I could to keep the weight add down.

Carl
 
Carl, Paul and Kevin have many good ideas. Follow their lead and good luck. I would remove the fairing on the HS and check all the bolts.
 
Check all wheel and gear fairings. Perhaps the are not aligned properly. Fly it with wheel pants off, then try it with gear fairings and pants off

Looks like the intersection fairings are also an issue (together with a big gap between wing and fuselage)

See photos. We are adjusting and will test fly tomorrow

Note. My photos seem to be rotated 90deg left
 

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Upon further inspection today I noticed that the rubber wing to fuselage intersection fairing is not correct. A different plastic channel is in place that leaves a 1/8 " gap between the wing and fuse. Maybe that's a factor??

OMG! This is really bad. This is surely your problem. You guys are lucky the plane was safe to fly. The flow leaking through a narrow gap along the wing root not only puts really dirty air onto the horizontal tail, it also produces a huge amount of induced drag.

I watched a super cub go through the fence at the end of a runway because it was newly reassembled after fabric and paint and the owner was too impatient to wait and put the root cuffs back on. That was probably a 3" gap, and the airplane would not accelerate to a high enough speed to fly with all that induced drag.
 
Looks like the intersection fairings are also an issue (together with a big gap between wing and fuselage)

See photos. We are adjusting and will test fly tomorrow

Note. My photos seem to be rotated 90deg left

That doesn't look like the correct material for the seal. You may want to get the right material and use another RV8 as a reference before flying.
 
I agree that it is not the right material, or it might be trimmed off too much.
 

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Wrong material and/or installation

It’s this material that appears to be incorrect and/or installed incorrectly. I wouldn’t be too concerned about the gear leg fairing gap. See post above for the correct material.
 

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I’d doubt that would create any difference due to the boundary layer.
Elevator balance issues would more than likely cause flutter rather than a buffet issue however balance is very important so I’d ensure that was done. I would try to return the aircraft to factory spec….remove any gaps….the airflow is being disturbed pre stall and fly again. Calibrate speed instruments against reliable gps speed after any painting as well.

This isn’t flutter. It’s buffeting. If it was flutter he would either no longer be with us or he would be busy laundering his shorts and no way he would fly the airplane again. Nobody gets flutter and says hmmmm, let’s try that again. It is almost certainly due to the aforementioned gap at the wing root fairing.
 
The left hand wing root seal on my 8 decided to let go one aero session I was doing, close to VNE, the buffering it created was wicked! I didn’t have any new rubber so it temp put 100mph tape on to cover the gap, ugly but upon a test flight the buffeting stopped.
 
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