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RV7A Nose Gear Resonance - Anyone??

BillL

Well Known Member
I was just following a friends 7A this weekend and the nose gear was vibrating fore and aft as a unit. It apparently does that continuously at that airspeed. It was at at cruise ~140KTAS 6500 ft, 2400 RPM. The owner did not know this but has a strong vibration. It does not vibrate at 2400 rpm WOT, 9500 ft. More investigation is underway to inspect the attachment and mount for cracks.

Although I searched VAF, I could find no threads or posts about this.

Has this been encountered by anyone? and what was the root cause?

To be clear, the nose wheel is not pivoting, and there is no lateral motion.
 
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Two potential causes -

The nose wheel is being rotated by the slip stream and it is out of balance.

The engine / prop is not very well balance and at certain RPM's it is exciting a resonance in the nose gear leg.

If the first one is not true, then I would suggest a dynamic prop balance (actually I suggest it to any one that hasn't ever had one done, whether they seem to have a problem or not).
 
Some suggestions I have seen on nose wheel RV's.

Make sure the attach bolt near firewall is tight, check it twice a year.
- If bolt is loose the nose leg moves all over.
Wheel rolling force needs to be small.

Break out force need to be 20-25 lbs.


Not sure how this will effect vibration in the air, but I have found making sure the above is correct solves most nose wheel issues.
 
The nose wheel is being rotated by the slip stream and it is out of balance.

Good call Scott. Fits the conditions. It's likely to have a different RPM at different airspeeds, If one of those RPMs matches the natural frequency of the leg assembly, it resonates.

The engine / prop is not very well balance and at certain RPM's it is exciting a resonance in the nose gear leg.

Doesn't seem to fit. At one airspeed it vibrates, another it doesn't, but they are both at the same RPM.
 
Two potential causes -

The nose wheel is being rotated by the slip stream and it is out of balance.

The engine / prop is not very well balance and at certain RPM's it is exciting a resonance in the nose gear leg.

If the first one is not true, then I would suggest a dynamic prop balance (actually I suggest it to any one that hasn't ever had one done, whether they seem to have a problem or not).

Thanks for posting Scott, I was hoping you would share some insight here . . .

Spinning nose wheel . . . the owner and I were just discussing the possibility as you posted. It would certainly fit the variables, and it was not balanced with the last tire replacement.

Side question(s), though, it seems like we would not want the tire spinning for 2000 hrs.? Wouldn't that add some drag too? Is it really OK in a practical sense? What (else) could possibly go wrong? :rolleyes:

Some suggestions I have seen on nose wheel RV's.

Make sure the attach bolt near firewall is tight, check it twice a year.
- If bolt is loose the nose leg moves all over.
Wheel rolling force needs to be small.

Break out force need to be 20-25 lbs.


Not sure how this will effect vibration in the air, but I have found making sure the above is correct solves most nose wheel issues.

Experience is king! That seems like a high number, but assume it drops much lower once in motion.

Also, assume you tested lower breakout forces and it did not work?

Retainer bolt high on the list after hours of vibration.
 
Experience is king! That seems like a high number, but assume it drops much lower once in motion.

Also, assume you tested lower breakout forces and it did not work?

Retainer bolt high on the list after hours of vibration.

That # comes from the Van's build manual, so I can't take credit.
 
Doesn't seem to fit. At one airspeed it vibrates, another it doesn't, but they are both at the same RPM.

Valid point... I read right past that fact. My mind immediately jumped to the variable being different RPM's.

That being the case, an out of balance nose wheel seems a likely cause.
 
Also consider replacing the spec'd AN bolt with a close tolerance NAS bolt. I had some shimmy during roll-out after landing at certain speeds and the close tolerance bolt fixed it.
 
You didn't specify if he has a nose wheel fairing installed. If he does, remove it and attempt to recreate the vibration. If he doesn't, see if he can find one to borrow for a few flights. If its not an out of balance wheel, it's likely just hitting a resonance with the vortex shedding off the gear at certain airspeeds.
 
Feedback from the Owner

Thank you all for suggestions and comments. A little history, and information on what I have found so far -

The builder (I am the second owner, purchased 2015, approx. 1290H hrs A&E, 540H hrs were accumulated by me) had the Antisplat front wheel bearing mod done. The front wheel has weights on it, so I am assuming, Allen balanced the tire at that time. It also has the AS Nose Job 1 (nose gear brace).

I replaced the front tire in 2017. I did not balance the tire, and I did not remove the existing weights. My thought being I would see if I noticed any vibrations during T&L. I did not. Sometime after that is when I noticed the airframe vibration, at around 140 k IAS. Unfortunately, I did not document when I first noticed it..... Scott's comment about the wheel spinning and being out of balance makes sense. I removed the weights (1 oz) last night and went for a quick flight. It was not the smoothest air, but it seems the vibration is gone or lessened.

My front wheel breakout is set at 25#. The wheel bearings are OK. I check this yearly. I also check for play in the nose gear leg/socket. I have not checked the tightness of the nose gear leg bolt since purchase. I will do that ASAP, and add it to the yearly inspection list.

The nose gear leg fairing has no pin & hinge in the fairing to hold it together on the back side. There is also no leg to lower cowl intersection fairing. The leg fairing is only held in place by the hose clamp near the wheel fork, and the tightness of the slot in the lower cowl that the fairing passes thru. I wondered if the leg fairing was vibrating, and this is what I felt.... It has been on my list to add the hinge and intersection fairing, per Vans RV7 DWG 45.

Next Steps:
Get the wheel balanced.
Take off the cowl and inspect the leg/motor mount socket and bolt.
Mount a GoPro on the wing tie down and get some video.

Thank you all, for your comments - greatly appreciated!
 
we had quite a similar behaviour on our nose gear.

wasn't really noticeable from the cockpit / in flight but once I started regular formation flying, the nosegear fore/aft motion at certain speeds got better attention from closer and better trained eyes ;-)

also, there always was a certain "excitation" speed during the rollout that had to be "braked through", or otherwise the wheel would start to excite itself fore/aft. never once did we have left/right shimmy or similar.
inflight, the nosewheel would also turn slowly in the airstream.

we have the grove wheel/rim (the one with the aluminum axle spacer tube between the "donuts") on the stock 2nd generation Van's "angled" fork and steel nose leg, btw.

finally tackled all of it during the annual a year ago. Three actions have positively solved all issues:

- we grinded down the axle spacer just a tiny bit, which actually increased friction on the bearings just a nudge. Gone is the turning wheel in the air and the "excitation speed" during rollout!
Before the mod, manually accelerating the wheel, it would continue to turn for several revolutions. Now, the motion stops after maybe 1 full revolution or so.

- We were somehow expecting play up at/near the nosegear bolt based on posts here in the forum, however this actually turned out quite snug even after 10ish years and 1000 landings. Honestly, don't think that was in any way the source of the problem. Nonetheless, since we were "at it" and had expected it to be the bolt, we did replace the standard bolt with a slightly longer (more washers but better shank-in-the hole fit) and close tolerance bolt as described on several of the nosegear threads here in this forum. We even had organized a custom supersize bolt and reamer (both hilariously expensive) yet turned out not needing either....

- Turns out the most likely culprit was the steel sleeve "tube" at the bottom of the engine mount weldment, which was/is actually rather oval than a circle :-( Allowing the gear leg to just slightly move fore/aft while being fixated quite nicely at the upper end/by the bolt (since the gear leg actually is truly round...).
What then removed all play was some thin strips of steel shim "tape" courtesy of swisseagle/Dominic that we fitted and mounted between the forward mating face of the nosegear leg and the bottom engine mount "oval tube".
Not a problem since. And the shim strips stay in place and do not move. But quite a mission to disassemble/reassemble the whole nosegear assembly, including making a new/better/larger bolt access hole in the firewall to swap out bolts (we had assembled originally with the engine mount off the firewall). It would have been easier to make the bolt pass at that time *G*
 
Tire was out of balance

Scott M. “Nailed it” (thanks Scott). Once I balanced the front tire, the vibration in flight disappeared. Bill L. and I flew to Boone Iowa last month, and he did a visual confirmation.
Thank you to all to posted their suggestions.
 
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