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Tail shake

O

oy-rvc

Member
During certain maneuvers I have noticed a rattling/banging noise coming from the back. I had a Gopro camara pointing backward and found that the noise is coming from vibrating HS/tail. As the video of this wingover shows


the tail is shaking shortly at sek 1 and again longer at sek 7/8. The noise is loud enough to hear through headset/cabin noise and can be heard in the video. I have multiple times looked for what could be loose in the back not realizing it was the tail shaking.

It seams to be related to high AOA and/or some disturbed air flowing from the wing and gear and hitting the HS.. I know the maneuver in the video wasn’t flown very clean but nevertheless revealed the problem.

I think this tail shaking puts unnecessary stress on the tail and want to look for a solution. I contacted Vans support about this and they essentially pointed me to some threads here at vansairforce about strakes that some builders have installed over the leading edge of the wings to smooth the airflow at high AOA. Maybe I’m not following here enough but didn’t realize the connection between the noises and tail shake until now.

Reading through the links/info from Vans support I have a few questions which I think is better to adresse here:

There doesn’t seam to be a lot of feedback from people having installed these strakes. Do the they fix the problem?

My impression is that tailshake is mostly reported in a regular stall or 3 point landing. Have others also experienced in aerobatics or other situations?

Where to buy these strakes? Updated information on who sells these? The information I got doesn’t seam to be up to date.

Thanks for any help on this...
 
What is your trim tab position? When in level flight, how far from centered is your trim tab? It seems that possibly the chord line of the trim tab in normal flight is causing an accelerated stall. If your horizontal stab is just a little for off the level mark, it would be fixed with trim. The more it’s off, the more trim required.

It’s just a guess. I hope you find out though.. because makes me very nervous.
 
Strakes for the -8 have been discussed (and sometimes installed) for decades, and reading all of the material in that time, I came to the conclusion (you are, of course, free to form your own), that EITHEWR strakes or proper upper intersection fairings will reduce or do away with tail shake at high AoA. I have never flown my RV-8 without the upper intersection fairing,s nad have no shake. I have never had strakes. I have flown an RV-8 without either, and experienced the shake. Those with strakes report that it does away with the shake if they didn’t have upper intersection fairings.

My experience says that upper intersection fairings are good for several knots in speed, so it makes little sense to NOT install them, and then the starkes become superfluous in regards to the tail shake.

Paul
 
You are at high AoA and seem to experience a rather high energy wing root stall affecting the tail which is not uncommon for the RV-8. Having flown several RV-8 some have this behaviour some not. The root cause seems to be the shape of the intersection fairing between the gear leg and the fuselage. The "strake" partially eliminates this problem. I installed them on the first RV-8 I built (back in 2012) and did not need it for my second RV-8.
 
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When you hear a rattling/banging noise, I think you should look at the entire empennage structure more closely.

I put the strakes on my plane after noticing an "earlier stall" then expected, coming from tail. The sensation is a vibration and stick shake, not an an audible noise. With strakes on I get a lower stall break and it comes from the wings as expected.

Have you done the SERVICE BULLETIN 00036 - REV 3 inspection?
 
oy-rvc said:
During certain maneuvers I have noticed a rattling/banging noise coming from the back. I had a Gopro camara pointing backward and found that the noise is coming from vibrating HS/tail. As the video of this wingover shows


the tail is shaking shortly at sek 1 and again longer at sek 7/8. The noise is loud enough to hear through headset/cabin noise and can be heard in the video. I have multiple times looked for what could be loose in the back not realizing it was the tail shaking.

It seams to be related to high AOA and/or some disturbed air flowing from the wing and gear and hitting the HS.. I know the maneuver in the video wasn’t flown very clean but nevertheless revealed the problem.

I think this tail shaking puts unnecessary stress on the tail and want to look for a solution. I contacted Vans support about this and they essentially pointed me to some threads here at vansairforce about strakes that some builders have installed over the leading edge of the wings to smooth the airflow at high AOA. Maybe I’m not following here enough but didn’t realize the connection between the noises and tail shake until now.

Reading through the links/info from Vans support I have a few questions which I think is better to adresse here:

There doesn’t seam to be a lot of feedback from people having installed these strakes. Do the they fix the problem?

My impression is that tailshake is mostly reported in a regular stall or 3 point landing. Have others also experienced in aerobatics or other situations?

Where to buy these strakes? Updated information on who sells these? The information I got doesn’t seam to be up to date.

Thanks for any help on this...
Click to expand...
Here’s a tip, don’t look back there when spinning! 🫣
 
Every RV model will do this to some degree if it gets into the particular flight regime and AOA that induces it.
It is caused by the tail being in the airflow that is shedding off the wing, which is why it requires a specific flight situation to occur.
It is particularly prominent in the RV – 10 during power on stalls.
The RV-8 does seem to have its own particular occurrence, presumed to be because of the proximity of the gear legs to the wing leading edge.
 
Ironflight said:
Strakes for the -8 have been discussed (and sometimes installed) for decades, and reading all of the material in that time, I came to the conclusion (you are, of course, free to form your own), that EITHEWR strakes or proper upper intersection fairings will reduce or do away with tail shake at high AoA. I have never flown my RV-8 without the upper intersection fairing,s nad have no shake. I have never had strakes. I have flown an RV-8 without either, and experienced the shake. Those with strakes report that it does away with the shake if they didn’t have upper intersection fairings.

My experience says that upper intersection fairings are good for several knots in speed, so it makes little sense to NOT install them, and then the starkes become superfluous in regards to the tail shake.

Paul
Click to expand...

I have the intersection fairings installed since the beginning of flying my RV8. If they were missing I would agree then it would be the obvious place to start. Its difficult to 100% know the correct shape since I recall I had to do a clayform and then fiberglass on top.

They look like this:


intersection.jpg
 
michjor said:
When you hear a rattling/banging noise, I think you should look at the entire empennage structure more closely.

I put the strakes on my plane after noticing an "earlier stall" then expected, coming from tail. The sensation is a vibration and stick shake, not an an audible noise. With strakes on I get a lower stall break and it comes from the wings as expected.

Have you done the SERVICE BULLETIN 00036 - REV 3 inspection?
Click to expand...
I agree. What worries me more is the rattling/banging noise. I have currently taken the empenage fairing off to do a close inspection of the tail including the SBs related to cracks also SB 00036.
 
When I tested my RV-8 I noticed a tailshake and buffet at high AoA. On three-point landings the lift suddenly decreased and so the airplane just dropped onto the runway. All this especially with forward C/G and I believe it is early separation on the inner wing. I too read the threads here on VAF and thought I give it a try with the strakes. So I fabricated temporary strakes and did two flights with same weight and same C/G on the same day. The indicated stall speed dropped by 4-5 kts, with and without flaps - very significant. So I fabricated real ones and they are on ever since. The tail shake is not gone but significantly reduced and three-point landings are easier. No penalties on the cruise speed. Be aware that these are my observations on my airplane. It is probably very dependent on the shape of your upper wheel fairings

N6xbgRrh.jpg
 
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oy-rvc said:
I agree. What worries me more is the rattling/banging noise. I have currently taken the empennage fairing off to do a close inspection of the tail including the SBs related to cracks also SB 00036.
Click to expand...
Christian
Looking back over your post history, I see you have been flying this plane since completed August 2010.
Had you never experienced this in the past? If not, what has changed with you or your bird? Doing aerobatics? Landing different, different maneuvers. Had you left a bucking bar in your tail 14 years ago and acro has identified it?

Something is different that it just now surfaced. Make sure as suggested to look at both SBs, the horizontal spar for cracks and cracks behind the spar for the elevator outboard hinge.

Buffeting is one thing but I would stay on the ground until identifying where the sound is originating from.

All the best. Be safe.
A
 
Bastien said:
You are at high AoA and seem to experience a rather high energy wing root stall affecting the tail which is not uncommon for the RV-8. Having flown several RV-8 some have this behaviour some not. The root cause seems to be the shape of the intersection fairing between the gear leg and the fuselage. The "strake" partially eliminates this problem. I installed them on the first RV-8 I built (back in 2012) and did not need it for my second RV-8.
Click to expand...
Do you recall if you got any noises when you did have this shaking? Some shaking is expected from this type of dirty airflow, but I wonder about the sound… ?
 
AlphaCharlieBravo said:
Looking back over your post history, I see you have been flying this plane since completed August 2010.
Had you never experienced this in the past?
Click to expand...

I have done a little aerobatics from time to time and yes I have heard the sound before, but never did I expect it to come from the tail because of vibration. It’s a fairly short condition - maybe a second long when it happens so never gave it much attention until now when I finally connected the two because I had a camera to see and hear the vibration. For that reason I also don’t recall a certain flight when it started. Had it been something in the front vibrating I would have seen it right away, but looking back at the tail in that right second is a different story.

Xkuzme1 said:
What is your trim tab position? When in level flight, how far from centered is your trim tab? It seems that possibly the chord line of the trim tab in normal flight is causing an accelerated stall. If your horizontal stab is just a little for off the level mark, it would be fixed with trim. The more it’s off, the more trim required.

It’s just a guess. I hope you find out though.. because makes me very nervous.
Click to expand...

I’m pretty sure it’s set up straight so neutral trim is inline with elevator, but will check next time at the airport.
 
The adverse interaction of the vortices of the wing/fuselage intersection and the gear/fuselage intersection at high AoA's can be tamed by fuselage strakes. Both my previous RV-8 and my current RV-8 have those and there is no buffet on landings (including 3-pointers).

The strakes are shown in the pics below. Note that the upper gear leg fairings are different between the two airplanes. Their design makes a difference in the strength and interaction of the two vortices coming from the wing/fuselage intersection and the gear/fuselage intersection. Those vortices affect the HStab and elevator effectiveness in the flare, and tail shake (pre-stall buffet) due to flow separation at high AoA's. The strakes help tame the two vortices and alleviate and delay their adverse effects. A very good design of the upper gear leg fairings could, and do, provide the same benefits that the strakes provide.

Previous RV-8 with Twin Cessna nacelle strakes (and a link to more pics: https://www.smugmug.com/gallery/n-qjnCt7):

1716866807798.png

Current RV-8 with custom carbon fiber strakes:

1716866753295.png

Terry Burch may still be making the carbon fiber strakes shown in the photo above, and below.

1716866697859.png
 
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D Weisgerber said:
I had the tail shake, I got a set of strakes from Terry Burch. They worked well for me.
Click to expand...
RV8JD said:
The adverse interaction of the vortices of the wing/fuselage intersection and the gear/fuselage intersection at high AoA's can be tamed by fuselage strakes. Both my previous RV-8 and my current RV-8 have those and there is no buffet on landings (including 3-pointers).

The strakes are shown in the pics below. Note that the upper gear leg fairings are different between the two airplanes. Their design makes a difference in the strength and interaction of the two vortices coming from the wing/fuselage intersection and the gear/fuselage intersection. Those vortices affect the HStab and elevator effectiveness in the flare, and tail shake (pre-stall buffet) due to flow separation at high AoA's. The strakes help tame the two vortices and alleviate and delay their adverse effects. A very good design of the upper gear leg fairings could, and do, provide the same benefits that the strakes provide.

Previous RV-8 with Twin Cessna nacelle strakes (and a link to more pics: https://www.smugmug.com/gallery/n-qjnCt7):

View attachment 63623

Current RV-8 with custom carbon fiber strakes:

View attachment 63622

Terry Burch may still be making the carbon fiber strakes shown in the second photo above, and below.

View attachment 63621
Click to expand...

Thanks for the info. Can someone share the e-mail (or link if webpage) to contact Terry Burch for the strakes he makes?
 
Following up on my own thread.

I inspected the entire tail structure including for cracks (SBs). I found no cracks anywhere in the tail structure. However, I did find that the 4 x AN3 bolts holding the front horizontal stabilizer down against the fuselage was not entirely tight and that it allowed a slight wiggle if the tail was lifted/pushed up on the horizontal stabilizer. I was not visible by eye, but I could feel a slight “working” with a finger held at the joint between the aluminum angle and fuselage rear deck. The easy turning bolts confirmed it was not entirely tight. This is strange as I know for sure these critical bolts were tightening to correct torque during the build. The pain of getting in the tail to torque these and torque seal was still there on the nuts – so I don’t really understand how these where no longer to correct torque. Have others experienced the same? Anyhow these are now re-torqued. Next annual I will most like pull the entire tail off to do SB 14-01-31 as a preventative measure. I have also checked that no jam-nuts are loose and all joint between HS/elevator and elevator/trimtab look good.

I also did an experiment to tab the HS firmly with a flat hand, making it vibrate to see if I could tell what noises it makes and a rattle seams to orgin mostly from the elevator and trimtab or some combination. It’s difficult to assess if this is the same noise that can be head of the video as these two situations cannot be compared directly.
 
oy-rvc said:
Following up on my own thread.

I inspected the entire tail structure including for cracks (SBs). I found no cracks anywhere in the tail structure. However, I did find that the 4 x AN3 bolts holding the front horizontal stabilizer down against the fuselage was not entirely tight and that it allowed a slight wiggle if the tail was lifted/pushed up on the horizontal stabilizer. I was not visible by eye, but I could feel a slight “working” with a finger held at the joint between the aluminum angle and fuselage rear deck. The easy turning bolts confirmed it was not entirely tight. This is strange as I know for sure these critical bolts were tightening to correct torque during the build. The pain of getting in the tail to torque these and torque seal was still there on the nuts – so I don’t really understand how these where no longer to correct torque. Have others experienced the same? Anyhow these are now re-torqued. Next annual I will most like pull the entire tail off to do SB 14-01-31 as a preventative measure. I have also checked that no jam-nuts are loose and all joint between HS/elevator and elevator/trimtab look good.

I also did an experiment to tab the HS firmly with a flat hand, making it vibrate to see if I could tell what noises it makes and a rattle seams to orgin mostly from the elevator and trimtab or some combination. It’s difficult to assess if this is the same noise that can be head of the video as these two situations cannot be compared directly.
Click to expand...
Glad that you found (and fixed) that problem! Sometimes structural bolts get loose while was not fully demurred when assembled, and those burs collapsed over time, leaving the bolt under torqued. I recently found the three bolts that hold the lower end of the vertical stab spar to the bulkhead and tailwheel socket loose on a long-flying RV-8, and my hypothesis is that the stack of materials had a bur in there at one time. A good reason to put wrenches on critical bolts during inspection, even if they have torque seal!
 
Ralph Inkster said:
Wearing of paint between 2 bolted surfaces can also cause loosening bolts.
& yes, a very good idea to use wrenches to check tail surface attach bolts every CI.
Click to expand...
I agree, though I wouldn’t use the term wearing.
Protective primer and paint coatings are much softer than structural aluminum or steel
That means that under compression load at torqued fasteners, the coating can creep or extrude overtime, which will change the preload on the fastener.
Especially in cases where the coating thickness is excessive.
 
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