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Rudder Flutter

Made structure repairs to horizontal stabilizer of RV-6A per Vans service bulletin and repositioned empennage forward 1 inch to comply with plans. Now encounter rudder flutter at 175 mph. Had never flown above 175 mph prior to repairs so don’t know if rudder flutter was already there, but ops manual has the published VNE. Called Vans and they had no other reports of rudder flutter. Any ideas?
 
Wow I'm surprised they didn't hang up the phone when you mentioned the F word. I asked them for some moment numbers on the -7A rudder and that one got brushed under the rug.

P.S. Welcome aboard!
 
RV-8A aerodynamic tail vibration

Years ago, there was some discussion about the nosegear causing the tail to vibrate. Can’t find any info on it. Is it archived?
 
Made structure repairs to horizontal stabilizer of RV-6A per Vans service bulletin and repositioned empennage forward 1 inch to comply with plans.

It would be helpful if you further explained “Repositioned empennage fwd 1 inch”.
To do this it would require major structural changes to the fuselage and/or emp attachment.
 
Vne is not necessarily the number on the airspeed indicator. On RVs, it's true airspeed. Flying out of Colorado, the indicated airspeed will be lower than true airspeed, and that needs to be taken into account.

Also, it's not likely that you had full-fledged rudder flutter because you survived. Flutter is usually destructive, almost explosively so. Can you describe what happened and what the flight conditions were more thoroughly? Did the post-flight inspection reveal any damage?

Dave
 
Made structure repairs to horizontal stabilizer of RV-6A per Vans service bulletin and repositioned empennage forward 1 inch to comply with plans. Now encounter rudder flutter at 175 mph. Had never flown above 175 mph prior to repairs so don’t know if rudder flutter was already there, but ops manual has the published VNE. Called Vans and they had no other reports of rudder flutter. Any ideas?

You repositioned the empennage forward by 1"? I'd love to see pictures of the before and after with the empennage fairing removed.
 
Like others I am interested in what and why you did to the empennage.

But....


PLEASE....


MAKE SURE THE JAM NUTS ON THE ROD ENDS THAT FORM THE RUDDER HINGES ARE TIGHT. TIGHT.
 
Vne is not necessarily the number on the airspeed indicator. On RVs, it's true airspeed. Flying out of Colorado, the indicated airspeed will be lower than true airspeed, and that needs to be taken into account.

Hi Dave,

Is this right, TAS? Section 15 of the builders manual specifies Vne at 230 MPH IAS (200 kt IAS) for the -7
 
It would be helpful if you further explained “Repositioned empennage fwd 1 inch”.
To do this it would require major structural changes to the fuselage and/or emp attachment.

And if doing so, most likely made tail not stiffly tied in and would make it very likely to have aero issue, or as OP described as “flutter”.

Why was the empennage moved forward 1”? I must have missed that in the SB.
 
Had a buddy install a tail beacon x on his RV4 and when completed, had more what I would call, rudder oscillation during flight between the speeds of 140 to 160 knots. Reversed everything and still had the oscillation. Had another experienced builder look and they determined the rudder skin was flexing way too much. He put On 3 or 4 pieces of angle with 2 sided tape On each side of the rudder to see if that rectified the oscillation and it did. He ordered a new rudder from Vans. Not sure why it showed up after his work but it did. Not sure if Vans changed the thickness of their rudder skin but hopefully his new rudder will work flawlessly.
 
Yes TAS

Bill - this has been addressed many times here on VAF and in Van’s docs. I’m sure someone will cite the exact references. But, yes VNE for the 7 is TAS, NOT indicated airspeed.
 
What’s that old saying.....This thread is worthless without pics!!

We are all trying to figure out how you moved your empennage forward???
 
Rudder flutter

Fourth owner, 20 year old Oregon built airplane, no builder logs. Hard to read builders mind, but our A&P thinks that builder failed to trim fuselage skin so that empennage could contact rear bulkhead. Photo shows three and four 1/4 inch spacers used to attach to fuselage. Need help uploading pictures. Recovery just requires slowing down. No visible damage. Oscillation felt in rudder pedals and yaw fishtailing.
 

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To add a picture just select the paperclip icon on the top row of the message box, browse for the picture you want to load from your computer, select upload. That's it.
New feature here, I had the same problem at first.
 

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It might be prudent to NOT fly that airplane again, until you figure this out. I’m trying to visualize the 3 & 4, 1/4” spacers and why/where they are attached. Structural deficiencies in this area is not to be ignored. Original RV4 & 6 elevator and rudder skins were .016 thick. Newer ones are .020.
 
Dr Fernand, the components in the photos are the rearmost horizontal stabilizer spar attach points. They have nothing to do with the vertical stabilizer or rudder. What we really need to see is the forward vertical stabilizer attach point above the front horizontal stabilizer spar.
 
So reading between the lines a bit here, and looking at your pictures, it looks like the previous owner shimmed the horizontal tail aft with those aluminum spacer blocks, and something similar at the forward spar attachment for the horizontal stabilizer to the fuselage.

It looks like you have now removed those spacer blocks and installed the horizontal tail where it is supposed to be. Presumably also fixing the forward spar attachment. Hopefully you have a set of the plans and are doing this work in accordance with the plans.

I hope those phillips-head screws holding the horizontal tail onto the rear attachment lugs are just temporary or something? Those must be AN bolts attaching the tail surfaces on, at all bolted assembly points.

Are we to assume that through this process of moving the horizontal tail forward, the fin attachment stayed the same? That is, the fin spar bolts to the aft fuselage bulkhead, and an attachment bracket joins the forward fin spar to the forward stabilizer spar, with spacer shims as needed? There must have been some significant shims or spacers at the forward fin spar attachment previously, which have now been removed? What does that assembly look like now?

The key things to inspect, related to your initial question about the appearance of rudder flutter, would be (1) the stiffness of the attachment of the forward fin spar to the forward horizontal stabilizer spar, and the forward horizontal stabilizer spar attachment to the fuselage; (2) the stiffness of the rear fin spar attachment to the aft fuselage bulkhead; and (3) the correct adjustment and then TIGHTEN the jam nuts that secure the rudder hinge fittings (rod-end bearings) into the rudder spar.

In this context, what I mean by "stiffness" is the absence of any freeplay, motion, or flexing of attachment points. Everything should be rigidly attached. With AN bolts.

I would not fly this airplane again until you get to the bottom of this.
 
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This is all VERY scary. Please do not fly this aircraft until it has been gone over thoroughly by an inspector familiar with RVs or at the very least, an experienced RV builder.

If this was done to the tail, I can only imagine what else has beed done.
 
With the HS shifted forward thru the shim, I will be very interested in seeing the front spar attachment point, I suspect that makes for another ohhh moment.
 
Thank you for the pics. So when you moved the VS forward, were there matching shims at the forward mount, or did you have to move the bulkhead?

Also, the lower rudder attach point. How did that change??

Any builders in your area that could stop by and take a look?
 
Rudder flutter

Not having made any repairs, I don’t remember relationship of VS to HS. All repairs conducted by A&P IA who rebuilt and replaced several components around the inboard HS including correcting Vans technical notice on annual inspection for HS cracks. I don’t see how anything there effects rudder airflow. I will check rudder installation. No rudder or VS work was done. As to construction, we are all shaking our heads at what builder was thinking. Stick elevator push pull tube was held in place by no more than two threads on horn bolts. Why this didn’t set off alarms, we can’t answer. Airplane was down for maintenance for two years. All changes well documented by IA making repairs. Several before pics from VS area.
 

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Please Listen

This is all VERY scary. Please do not fly this aircraft until it has been gone over thoroughly by an inspector familiar with RVs or at the very least, an experienced RV builder.

If this was done to the tail, I can only imagine what else has beed done.

Please Listen to what Mel is saying, an AP/AI may not have the expertise necessary to complete the requirements to make your aircraft airworthy. Find someone familiar with RVs that can get you safely back in the air.
 
Someone went to a lot of work to build it wrong.

Consider the photo below. It shows four 1/4" spacer blocks behind each stab attach bar. Now look at the forward vertical spar attach point...no spacers.

In order for that to work, one of two possibilities must be true. Either the forward HS spar was moved forward in the HS skins, or the forward VS spar was moved rearward in the VS skins. Given the location of the HS spar on the top longerons in other photos, aft of the bulkhead, it seems to be the latter...the dimension in red was reduced in the VS (note, illustration from RV-8A drawings, but very similar.)

So now we have the Big Question. If the correction removed the spacer blocks and moved the HS forward, is there now a 1" shim at the forward VS attachment to the forward HS spar?
 

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In addition to the points that DanH pointed out, the attachment point of the forward spar of the horizontal tail to the fuselage must have been moved aft, so that the loads are no longer directly positioned above the fuselage bulkhead F-610B. This puts loads into the longerons that should not.

In the process of removing the shims and re-installing the horizontal tail farther forward, hopefully the attachment of the forward spar has been relocated to its proper place. And hopefully done in accordance with the plans. Hopefully the incidence angle of the horizontal tail is correct. So it is possible that the horizontal tail is all correct now. The only damage being the extra holes in the longerons.

** Except for the phillips-head screws holding the tail on. Kind of erodes confidence in the IA/A&P that did that.(unless that was just temporary for alignment and real AN bolts were later installed)**

In that case, the primary problem is the one that DanH pointed out. The fin forward spar must have been modified to interface to the incorrectly located horizontal tail. That modification would possibly have changed the torsional stiffness of the fin, which would reduce the flutter speed.
 
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The screws appear to be AN525, they have the same shear and tensile strength as an AN bolt of the same size. They also have similar shank tolerances. I’m not advocating or defending the use of these, I would certainly stick with the hardware specified in the plans. Probably not a concern from a structural standpoint though.
 
Fuselage extended?

Is that 1" at the end of the fuselage normal for an RV-6A?

Based on the limited info, seems like a mistake/error was made somewhere, and then a series of other changes were made to correct it. Finding the original error and then working to fix it might be the best solution.

I agree that getting some experienced RV eyes on this will probably save you a lot of time and money. There are some experienced RV builders very close to you I'm sure.

RV-6A VS Mount N160JH 20200804.png
 
Please head the advice

Please, Please, Please, DO NOT fly this airplane until it had been thoroughly inspected by someone who will recognize a mistake or modification. Every inch of this airplane needs to be scrutinized.

Vic
 
The fin forward spar must have been modified to interface to the incorrectly located horizontal tail. That modification would possibly have changed the torsional stiffness of the fin, which would reduce the flutter speed.

My thought too.

Doc, how about a photo of the new VS front spar attach?
 
Rudder flutter

As I read your comments my takeaway is that there is a structural problem and or an aerodynamic problem. Structurally, the reinstallation of the rudder might be corrected by insuring attach bolts are tight (stiffened). The 6 tail uses 1/16th and isn’t mass balanced, corrections made to the 7. Aerodynamically, there may be some flow separation that has shifted slightly and is now exciting an oscillation in the rudder. Addition of mass on the rudder might change the excitation speed. A vortex generator type addition might change the aerodynamics and could be attached with tape so that it can be moved around.
 
As I read your comments my takeaway is that there is a structural problem and or an aerodynamic problem. Structurally, the reinstallation of the rudder might be corrected by insuring attach bolts are tight (stiffened). The 6 tail uses 1/16th and isn’t mass balanced, corrections made to the 7. Aerodynamically, there may be some flow separation that has shifted slightly and is now exciting an oscillation in the rudder. Addition of mass on the rudder might change the excitation speed. A vortex generator type addition might change the aerodynamics and could be attached with tape so that it can be moved around.

Jean,
There are differences between the RV-6 and RV-7 but they weren't made to correct a problem. The RV-7 has a much higher Vne than the RV-6. There are 2000+ RV-6's flying with the same rudder you have, without any flutter problems or modifications like VG's or counter balance to make them problem free. Making changes like those to solve your issue would just be masking (perhaps temporarily) a bigger issue.

I am willing to help you work through this issue. I sent you a private message via VAF forums. Please reply to that if you desire any assistance.
 
In addition to the points that DanH pointed out, the attachment point of the forward spar of the horizontal tail to the fuselage must have been moved aft, so that the loads are no longer directly positioned above the fuselage bulkhead F-810B. This puts loads into the longerons that should not.

In the process of removing the shims and re-installing the horizontal tail farther forward, hopefully the attachment of the forward spar has been relocated to its proper place. And hopefully done in accordance with the plans. Hopefully the incidence angle of the horizontal tail is correct. So it is possible that the horizontal tail is all correct now. The only damage being the extra holes in the longerons.

** Except for the phillips-head screws holding the tail on. Kind of erodes confidence in the IA/A&P that did that.(unless that was just temporary for alignment and real AN bolts were later installed)**

In that case, the primary problem is the one that DanH pointed out. The fin forward spar must have been modified to interface to the incorrectly located horizontal tail. That modification would possibly have changed the torsional stiffness of the fin, which would reduce the flutter speed.

***Disregard this comment. I mistakenly thought the spacers were the final condition not "corrected"***
Steve, the 1" off-set for the vertical loads transferred from the HS to the fuse will add a couple to that joint and be twisting the HS spar as part of the loading. That would create some vertical displacement to the joint and to the spar along with a small change in incidence. This would be my concern with the change made. I wonder what Scott is going to recommend - - likely correct the whole geometry for the aft fuse to make sure the loading is like the original design. It seems to me that is necessary.
 
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any chance there is a mix of RV-3 parts in there? (I don't know the 3 vs 4/6 dims, but as Dan pointed out, something is very wonky) :confused:
 
Hi. New to the forum? Welcome!
You should know that some very knowledgable folks have responded to your post, at least 2 DARs, at least 2 shop owners that have EXTENSIVE experience in building and maintaining RV models, a number of concerned builders and a couple responses from the FACTORY. All are trying to caution you about what they have seen & read re: possible structural integrity issues in your aircraft. Please, get a second opinion/inspection from a RV knowledge specific person. The biggest concern we have is for your safety!

As I read your comments my takeaway is that there is a structural problem and or an aerodynamic problem. Structurally, the reinstallation of the rudder might be corrected by insuring attach bolts are tight (stiffened). The 6 tail uses 1/16th and isn’t mass balanced, corrections made to the 7. Aerodynamically, there may be some flow separation that has shifted slightly and is now exciting an oscillation in the rudder. Addition of mass on the rudder might change the excitation speed. A vortex generator type addition might change the aerodynamics and could be attached with tape so that it can be moved around.

By this post, I get the impression you are convinced the the group is raising flags on a lesser (structural) issue, that maybe an aerodynamic band aid (vortex generators) might solve the symptom. Instead, check the rudder and vert rear spar hinges for exact alignment, this, if severely out of alignment can cause that symptom as well. Which again points to a - structural review - because hinges out of alignment usually lead to spar cracks amongst other things. & something is causing those hinges to be out of alignment all of a sudden.
 
This is the perfect example of how the FAA gives you all the rope you need to hang yourself.
 
Colorado RVs

Made structure repairs to horizontal stabilizer of RV-6A per Vans service bulletin and repositioned empennage forward 1 inch to comply with plans. Now encounter rudder flutter at 175 mph. Had never flown above 175 mph prior to repairs so don’t know if rudder flutter was already there, but ops manual has the published VNE. Called Vans and they had no other reports of rudder flutter. Any ideas?

Jean
Not sure we can help, but I manage a Googe Group of Colorado RV Builders and owners. Several RV6s. If you would like to join, send me an e-mail (see below) from a Google G-mail account with a few bits of info.
Name
City
Phone #
Model
 
Steve, the 1" off-set for the vertical loads transferred from the HS to the fuse will add a couple to that joint and be twisting the HS spar as part of the loading. That would create some vertical displacement to the joint and to the spar along with a small change in incidence. This would be my concern with the change made. I wonder what Scott is going to recommend - - likely correct the whole geometry for the aft fuse to make sure the loading is like the original design. It seems to me that is necessary.

Yes, I agree. But since the IA has moved the horizontal tail back to where it is supposed to be, that problem should be resolved now.
The remaining issues are (1) how is the fin attached? (and how was it attached before);
(2) was the fin modified internally to facilitate its attachment to the incorrectly positioned horizontal tail?;
and
(3) Is the rudder correctly installed?, i.e. hinge alignment correct plus tight jam nuts.
 
An alternative possibility

This is a speculative alternative hypothesis to what might have happened here.

It is possible that the original builder mis-located the F-611A+B bulkhead, and the associated stabilizer attachment bars, by 1" too far forward, but everything else about the build was correct. Remember the -6 wasn't pre-punched, so a builder measurement error is possible. And any carpenter will tell you how easy and common it is to have exactly 1" error. The F-611 bulkheads would have been too narrow to fit in the mis-measured location, so a skin doubler could have been added, or the flanges on the bulkhead adjusted so that the bulkhead fit.

The builder would have discovered this error when it was time to mount the horizontal tail and fin. The 1" shim blocks would then have been added to correctly position the horizontal tail and everything else would have fit correctly.

So now, along comes an IA and says this isn't right, takes the shims out, moves the horizontal tail forward 1", and we don't know what he did to make the fin install.

Yes this is speculation, but it explains some things, like why did the fin fit correctly before, as evidenced by the photos.


Ultimately, someone with good knowledge of RV construction, and a set of plans, needs to look at this airplane.

And none of this readily explains the OP's original question about the rudder flutter. That is very likely misaligned hinge bearings and/or not tight jam nuts on the hinge bearings.
 
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Buyer beware! You get this stuff when you trust the buyer. I had an IA due a prepurchase inspection and he loved the plane. I think the second and third owners were also snuckered. Can’t believe I was sold a lemon on purpose. Who would do that? The airplane was down for two years on the tail rebuild. IA gave us several pages documenting his work, referencing the plans. No one at my airport had any ideas. Vans tech ops didn’t have any. Your collected experience is great. Just surprised that no one had rudder oscillations. We’ll keep trying.
 
Rudder flutter

I imagine that if one experienced rudder flutter, then they may not be around to share the experience. Those things don’t usually end up well for the pilot.

Since we are talking flutter with the RV-6 rudder which shares the same folded edge design as the RV-4 and many rockets, it’s a pretty robust design. Lots of people flying with little issues. If we were talking about the -7and -9 zipped edge rudder, well, that would be another discussion concerning rudder flutter.

I’m amazed when we see these type of posts about a dangerous design change or dangerous build, and many experts who weigh in, yet the poster’s slowness to absorb the warnings. This is the kind of stuff that keeps me up at night. I really appreciate you guys who can detect an accident waiting for a place to happen. I just wished that I understood better the psychology of denial. It’s like a slow motion nightmare. I’m glad this situation appears to be in process of being correctly corrected. Again, thanks to all the folks who weight in on this one. I really appreciate all you folks.
 
Frenchy, don’t “keep trying“ :eek:

Before any further flight you need to:
1/ take up communications with rvbuilder2002, aka Scott. He is an official Van’s Aircraft rep and willing to help you
2/ have your aircraft inspected by an RV knowledgeable person, there are quite a few around

Looking at the pictures also reveals some sub-standard riveting, you really gotta have an RV specialist look your aircraft over.
 
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