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Flutter Wiggle Shimmy Ever See This?

BH1166

BH1166

Well Known Member
Was feeling a vibe/shake. Repeatable around 150KTS TAS, goes away at less. My 6A has old style pants, no nose wheel gear leg intersection fairing ( have one to install now). Nose wheel has the Antisplat mod ( sealed bearing). Wheel is Cleveland, tire is round, no flat spots or severe out of balance noticeable free spinning by hand. Tire does rotate when flying, but not real fast.

Any idea on a solution? Not a big fan of a different wheel pant ( newer Vans standard). Here a link to my YouTube of it in action.

https://youtu.be/OoXG0XgEvD0?feature=shared
 
While the "excitement" is most visible on the wheel pant, I'd still do a quick check to determine the driver. Remove the NW pant and fly/film again. Will be tough to see without the pant but maybe your calibrated feet will feel a difference.

Tell (the now controversial) Uncle Reamus hello.
 
It reminds me of a wind instrument. Blow just right and a note is produced.

Just a WAG - Perhaps air is entering the leg fairing and exiting the top since there is no intersection fairing installed. You could tape off the top next time you are out puttering around and test it out to see if that is the cause.

I’m curious if you could raise the wheel off the ground and take a high powered leaf blower and reproduce the vibration.
 
Have you raised the nose off the ground with no weight on it to check the "play" in the nose gear rubber mounts? It looks like you may need to add a washer to take some play out. (On the RV10 you have to do this around 500 hours. Not sure the RV6 is the same???)
 
it would be interesting to tuft that nose pant with yarn and see what's actually going on with the airflow around there.
 
Here's a pic of the instruction page in the RV10 regarding that washer to add later to take out play. Again, not sure the 6 is the same, but in case it is, I'd check here first.
 

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Freemasm: no wheel pant ride is smooth

Webb: Will get intersection fairing on a try vs tape. The blower test sounds interesting

Rvanstory: yep, checked leg play, none…thx

D Rat: hmmm, good idea

Thanks
 
Wobble Nose Gear

A friend has a Grumman Cheetah we spend DAYS on trying to find the source of a vibration that was speed related. Finally we lifted the nose and found the nose wheel would rotate very easily and the bearings were simply too loose. We snugged up the bearing preload to an appropriate level and flew again. Problem now gone.
The tire was rotating while in flight. If you look at the video you will see roughly a third of the tire in the slipstream while the other two thirds is shielded by the pant. 140 mph air is trying to rotate the wheel. The wheel was not perfectly balanced and ergo, the vibration.
Ed
 
Put a dot of white paint or chalk on the tire to see if it is rotating during flight. 150 knots may be the right speed and rate of rotation to trigger a harmonic…
 
There is a RV6A owner on the field and he reported the same thing. He added weight to modify the CG of the affecting wheel pant and the problem went away. Not sure if the CG is moved fwd or aft.

He also said the aluminum bracket that holds the wheelpants on the -6 is much thinner than on subsequent -7 and -8 models.
 
look closely at the nose wheel

if you play the video above in 1440P and zoom in to the nose wheel you can see if rotating at a high rate of speed. Not sure if this is normal or not for the rv6, have you balanced your wheels?
 
Another spot to check is play at the upper gear attach bolt. Grab the prop or engine and move the front of the plane side to side. There should be no movement or clicking where the front gear enters the motor mount.

Repaired 3 of these so far. The last one had shimmy during high taxi speeds, and nothing else was off.
 
It's not flutter and it's not shimmy (which are instabilities), it's a forced vibration. Now to find the forcing function/cause as others have suggested.
 
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Thank you.

Edclee: the antisplat mod, you can’t over tighten, add friction, thanks.

The tire is moving…. you all have that correct, original video better quality than YT. I just have added a white dot(tire crayon) thanks Kyle.

I agree, get that tire spinning fast enough, out of balance….would expect stuff to happen. I have the intersection fairing on as of today. I removed pivot SP? nut, flat, and bellevue washers, cleaned, re-installed all; but with a new set of bellevue washers thanks Ray. I Set breakout to spec. Camera mounted, hope to fly and tell tomorrow . With the new spring washers, I hear I need to recheck breakout in 20 hours or so. CI is February.
 
I think it is interesting the main wheel is shaking a bit too. I would not expect that. I would put the white stripe on the main wheels too and see if they are moving. You can tap or hold the brakes at specific times and see if they stop.
 
Dave Anders reportedly had a flutter problem in his highly optimized RV-4 fastback. Supposedly at 280 MPH, one of his wheel pants fluttered.

Don't recall the details, hope I got this much correct...
 
Butch
You can also balance the wheel pants.

Would check off ground and see how level they
Are flying.

My .02

Boomer
 
BH1166 said:
What method/how does one balance a wheel pant? Thanks Bruce
Click to expand...

Thousands of these airplanes are flying without balanced wheel pants, and don’t have the issue that yours is exhibiting.
One big difference between yours and many of those other airplanes is you have installed different wheel bearings that allow a very free rotation of the wheel. Most others have some rotational friction that doesn’t allow the free stream air to rotate the wheel.
My suggestion is that you focus on the source of the problem and not secondary solutions that might fix the problem.
 
I know of a builder that had the front tire rotate due to improper torque and it did exactly this. Tightened it up to spec which stops the free wheeling and problem solved. If your set only allows it to freely turn you may have to change that.
 
Amazing to see when you zoom in. I think I can see the wheel spinning. Please close the loop and let us know the outcome.
 
Check wheel balance

Butch - I have the same setup (antisplat bearing mod) as you. A few years ago, I installed a new tire and did not balance it. At 140 KIAS, mine would shake similar to yours. I balanced the wheel and the shimmy went away.
I used a motorcycle wheel balancer (horizontal shaft with centering cones that tighten against the bearings).
At first, I could not get the wheel to balance repeatedly. I finally realized that the centering cones were not against the wheel bearings - they were against the bearing spacer, and the spacer was not centered.
 
I was going to pull my front wheel off my 9A this weekend and send down for the bearing mod.

Is this mod too good and induces another potential problem to deal with?

I cant imagine so but I thought I would ask.

The friction in the stock bearing is excessive as it is and throw in our winter temperatures here in Canada just makes it even worse.
Thanks
Tim
 
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Tim, most don’t have this mod, a lot do. Its solves someone’s problems, but I see as a safety mod, maybe. I purchased my RV this way. With or without, plane performs no differently from my seat. Sounds like your need is different than what I thought most purchasers needs are. Im no help…sorry.
 
NEW VIDEO - Spinning—-ALOT

Ok, she spins like a top ! Fast forward after 15 seconds or so to 3:00 minutes…

Guess time to make a choice: 1: balance nose wheel, spins, but spins smoothly, no shaking - hopefully 2: revert to original spec wheel bearing combination, tighten so not able to spin fast or at.

https://youtu.be/KdYa_2A_D8k
 
Tim,
I have the AS wheel bearing mod. It works great for me and I dont have any inflight bouncing. If I did I would much rather balance or replace the tire than mess with the wheel bearings. My belief is a poorly greased or cold wheel bearing is greater cause for worry than a bouncy at a specific speed nose wheel.


riobison said:
I was going to pull my front wheel off my 9A this weekend and send down for the bearing mod.

Is this mod too good and induces another potential problem to deal with?

I cant imagine so but I thought I would ask.

The friction in the stock bearing is excessive as it is and throw in our winter temperatures here in Canada just makes it even worse.
Thanks
Tim
Click to expand...
 
Another option besides modifying your nose wheel for different bearings would be to upgrade to the improved nose wheel axle.
https://store.vansaircraft.com/rv-6a-7a-8a-9a-matco-nose-wheel-axle-kit-7a-9a-axle-kit.html
Unfortunately, it’s on backorder right now.


This Axel allows for adjusting the preload on the bearings with the length adjustment of the axle, and then fully torquing the axle bolt to clamp the axle between both sides of the fork.

This allows for a much freer rotation of the wheel, but there can still be higher rotational friction than desired because of the integral rubber seals on the bearings depending on where in the tolerance range the machining of the opening in the wheel is.
The way to improve this is to reduce the diameter of one of the two seal ribs of each bearing, using a fine sanding disk or just on some sandpaper on a flat surface
 
My seals had two lips that contacted the wheel. I carefully used a razor blade to trim off one of the lips and reduced the friction from the seal perfectly. This isn’t helpful for the OP but if someone is trying to reduce friction, I throw it out there.
 
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It is obvious to me that the issue is totally due to pilot error. :rolleyes::D:D Just kidding Butch! lol I'm just jealous that you were flying today while I had to be at work...

Seriously though, I have the same bearing mod with no issues that I am aware of. I don't know if my tire spins like yours in flight but I do not notice any vibration. I did have Alan's team balance and round the tire when they modded the front wheel bearing and I'm still running that tire.

chad
 
I have the same issue with my AS nose wheel, only at higher airspeeds during descent close to Vne.
 
This is all really interesting. I don't recall hearing about this before.

For me I don't like that much drag even when set up properly. I hold the nose wheel up as long as I can and still, occasionally when the nose wheel is lowered slowly there is still a light chirp. For me to i'd like to remove the extra drag and possible associated risk to that gear.

Obvious this ASplat mod has very little friction, maybe too little. The standard friction on our main gear or even the nose gear on other manufactures nose wheels wont rotate in flight. Certainly nothing I've owned, flown or at least I've never noticed it.

Sounds like some options such as with Vans, AS with the Matco upgrade and trimming the seals down.

Tim
 
Well, no wheel pant, no shaking noticed/felt. I’ll remove and fly till solution is identified and available. For grins, I’ll see if someone locally can balance it for me( not purchasing a balancer).

Scott, is this axel upgrade the standard now in kits or a fix for concerns about all things nose wheel on 6-9A ? Looks like moving from the AS wheel requires a new wheel since my Cleveland wheel has been modified, bearing pressed in. FYI, the mod was done 5/2012, 900 hours ago. This is guess, or can the new axel work on my modified wheel?

Tim, you have options today about things, upgrade axel, AS mod.

FYI, I sent Alan at AS an email asking his thoughts and directed him to this post. I appreciate all helpful posts. Thank You
 
BH1166 said:
Scott, is this axel upgrade the standard now in kits or a fix for concerns about all things nose wheel on 6-9A ? Looks like moving from the AS wheel requires a new wheel since my Cleveland wheel has been modified, bearing pressed in. FYI, the mod was done 5/2012, 900 hours ago. This is guess, or can the new axel work on my modified wheel?
Click to expand...

The upgraded axle has to be used with the standard bearings.
I mentioned it in your thread asan alternative option for anyone that might be considering the AS mod.
I'm not sure if it is now standard in the finish kit but I think it is.
 
6A nose wheel shake

Here’s a temporary fix for AS bearing mod until they come up with something permanent. First nose wheel mod 2018 ( true & balance) lasted 400hrs until the tire went out of round and got the shakes. This new tire ( NOT true but balanced) starts shaking about 180 mph. Here’s what I did to put a little drag on the wheel. Hose clamp around the spacer and a piece of baffling material.
No shaking.
 

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Thanks Rich. A non aviator friend recommended something similar. Your pictures help. Going to try it since YOU did.
 
Yes, I thought of something similar like some baffling lightly rubbing on the tire. Another option for sure.

Its probably not an issue as long as everything is balanced and true. But if its not, then it becomes a pita.

I bought one of those tire balancers off Amazon and it was junk. Had to buy different bearings as it came with junk and some more machining. So, double the money to get it up and running. But now I have one.

Tim
 
rich said:
Here’s a temporary fix for AS bearing mod until they come up with something permanent. First nose wheel mod 2018 ( true & balance) lasted 400hrs until the tire went out of round and got the shakes. This new tire ( NOT true but balanced) starts shaking about 180 mph. Here’s what I did to put a little drag on the wheel. Hose clamp around the spacer and a piece of baffling material.
No shaking.
Click to expand...

Does the AS bearing mod. Use the standard kit supplied mushroom shaped bearing inserts?
If so, I wasn’t aware of that.
If that is the case, then the upgrade axle assembly could likely be used, but it would not likely provide the ability to adjust the rotational friction of the wheel.
 
Interesting observations of the nosewheel spinning. Nice catch.

On Comanches there is a rubber pad in the wheel wells to stop wheel rotation when the gear is up. A piece of baffle material could be fabricated at the openings of the wheelpant to add some drang to prevent the tire from spinning.
 
rocketbob said:
Interesting observations of the nosewheel spinning. Nice catch.

On Comanches there is a rubber pad in the wheel wells to stop wheel rotation when the gear is up. A piece of baffle material could be fabricated at the openings of the wheelpant to add some drang to prevent the tire from spinning.
Click to expand...

Along those lines -- add a nylon scrub brush to the aft wheel pant opening. Serves two purposes: Stop the wheel spin, and clean the dirt/mud off the tire
 
Update

Well I found a flat spot on left of center, when viewing rotation from the side. I changed tire. Never say never….. purchased a motorcycle wheel balancer, tired my best to balance. Heavy at the stem, so 1oz 180 degrees got things real close.

The fit of the rim inside the fork gives only 1/8th” clearance. I’ve always thought that was too close for comfort, but nothing I can do about it (Cleveland Wheel ). That said , I am unable to fit clamp on bushing, as the worm gear is too wide. Bummer.

Another friction idea was a brush inside rear pant area,, placing drag on tire. Didn’t like the thought of it ever coming loose, at rear it would get sent up into the pant…yikes. So, I took piece of Macfarlane baffling and pull riveted to forward underside of pant opening, applying rub/pressure to tread. Wx stinks today and tomorrow. Camera mounted, hope to test mod in flight soon. I appreciate everyone’s thoughts on dealing with my challenge. Looks like a redo of wheel and bearings, etc, $3-$400


Thanks
 
rvbuilder2002 said:
My bet is that it is more out of balance than you think, and that above 150 kts it is spinning faster than you think.
Click to expand...

Yep, I was flying with a friend, RV7A and noticed the same thing 145-150kts. He had been chasing a vibration for months.

He white striped the wheel and it was the issue. No sure how to fix the issue, but I would consider increasing the rolling resistance of the bearings, along with being really sure the wheel is balanced.
 
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RESOLVED: See post 👆🏻#41 for what actions last taken. Seems to have stopped the inflight rotation of nose wheel. New tire, balanced better than out of the box ( not perfect) , and drag flap on tire. Flew today, video link attached, no white dot seen ( yes I put one on new tire ) . My conclusion right or wrong, the flat spot made tire waay out of balance, flat spot caught a bit more wind, equals a big wobbly mess. Thanks for everyone’s input. Good AS mod, but keep your nose tire in good shape. Cheers

IMG_9797.jpeg
 
A very good outcome and glad that you found the problem. There might be many others flying around with it. I have never used a rubber flap, but always tighten the standard axle nut (standard kit parts) to give the nose wheel about half a rotation before it stops when pulling it through by hand.
 
BH1166 said:
So, I took piece of Macfarlane baffling and pull riveted to forward underside of pant opening, applying rub/pressure to tread.
Click to expand...
Thanks for sharing how you were able to resolve the spinning issue (y)

Would you be able to describe how hard it is to spin your tire now or how hard the baffle seal is pressing against it?
 
Pulled my wheel off, sent it off to Allan at Anti Splat to get a new tire and the bearing mod. Wow, what a difference. So smooooooth and spins freely, plane pulls around easier and taxis with less power. No vibration in flight at 163 kts. No idea if it's spinning or not as I can't see it so not important.

Great mod and well worth it. Side benefit is 1 less set of bearings to clean and re-pack..

Thanks

Tim
 
Many times I read here, also in this thread, I had to adjust the breakout force on the nosewheel ... See the instructions attached, this is how I have my setup and many friends of mine too. It just works!
 

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While flying VERY close formation in my RV8, the other airplane reported that my LEFT gear was occilicing a couple of inches left to right at certain speeds (mostly fast). The right pant was not moving at any speed or flight condition.
Video was taken from below the plane and after landing we put on our thinking caps to try to figure out the problem. BTW, I had no indication there was a problem inflight. There was no vibration that I could feel or any other clue.
All the obvious solutions were tried including wheel balance and bearings and checking gear alignment, etc etc etc. We found nothing less than perfect.
Multiple formation test flights, complete with video were performed to test for onset speeds, yaw changes etc etc etc
After trying everything in the book, I came upon a possible cause and cure solution. My theory was that the prop blast was affecting the airflow under the fuselage (due to the swirling of the airflow from the prop aft) and to the LEFT. The airflow was therefore higher on the inboard side of the left wheel pant relative to the outboard side. The pant is basically an airfoil mounted vertically. The extra airflow on the inboard side was "flying" the gear leg inboard until the gear leg pushed it back outboard, Rinse and repeat. The yarn tuffs were then applied and another flight test proved the theory. I fabricated a balsa wood "spoiler" and stuck it on the inboard side of the wheel pant and went flying. No more vibration!
I then started a flight test process that involved smaller and smaller spoilers until I ended up with one about 3 inches long and 1/2 inch tall.
A permanent spoiler made from micro buildup was applied and a paint touch up. You have to take a second glance to see the fix.
Fixed it!
 
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