Flybuddy2

Flybuddy2

Well Known Member
I've read all of the shimmy posts and have a couple of questions:

1) tire pressure--my experience in various diff steering nosewheel aircraft (RVs, Grummans, Velocity) has been lower tire pressure seemed to lessen nosewheel shimmy BUT most posts here advocate higher tire pressures. Anyone have experience with higher pressure reducing shimmy?

2) I've noticed shims and stiffeners mentioned--how would those work or are they for mains only??

3) putting lead in forward portion of nosewheel fairing--anyone do that and how much weight used??

I've been fighting a marginal shimmy for a while. I've changed tires/tubes and balanced--have the Matco setup, have 36 lbs breakout force, played with pressures, pulled the downtube, put in a larger bolt and retorqued, etc, etc. I'm running out of things to try--any info appreciated...thx
 
I balanced the main pants

and that was beneficial. They are mounted in the middle. The nose pant geometry is much different. i don't see a benefit to balancing the nose pant. I reduced the nose tire pressure fairly low; on a normal landing the spin force on touch down ripped the valve stem. I keep the nose tire pressure on the high side now. I like the optional axel, too. I have a belly cam that shows all 3 wheels. I can never tell without looking which wheel is actually the offender. Since I balanced the main pants and bought the better quality nose tire (Matco I think) I have had no problems. Most of the Shin tires are not very round, too egg shaped for me.

Dale
RV6a 1555 hrs
 
3rd wheel

You might want to try putting that wheel in the back just below the rudder.:eek::D

sorry couldn't resist

bird
 
Just fixed one like that yesterday, I replaced the tire/tube, balanced the assy, replaced the disc spring washers and reset the breakout force.. all is wonderful again and owner is a happy camper :D.
 
Walt said:
Just fixed one like that yesterday, I replaced the tire/tube, balanced the assy, replaced the disc spring washers and reset the breakout force.. all is wonderful again and owner is a happy camper :D.
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Might be time for new washers---what did you set the breakout force at??
I saw one post a while back where someone had it in the 40s, my next door neighbor has a 6A and he doesn't check it, he just keeps tightening till it stops shimmying. I've got 36 and his is tighter than mine. I wonder at what point it will not self align in the air?
 
Flybuddy2 said:
Might be time for new washers---what did you set the breakout force at??
I saw one post a while back where someone had it in the 40s, my next door neighbor has a 6A and he doesn't check it, he just keeps tightening till it stops shimmying. I've got 36 and his is tighter than mine. I wonder at what point it will not self align in the air?
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Plans called for 22 lbs. side force, with a fish scale pulling at the axle location. Mine was around 25 lbs. If it doesn't self align, it will act somewhat like a rudder.

L.Adamson
 
Be sure and have someone witness the wheel that is shimmying. You might find that it is not the nose wheel.
 
Need opinion too. Couple weeks ago I was at unfamiliar airport and slipped hard to lose altitude. After touch down I hold the nose up then relaxed a bit and as soon as it touched the runway the vibration started. Next second I lifted the nosewheel again and after it lowered itself at slow speed the vibration repeated but was lighter and then went away fast. Somebody happened watched and he said it was visible. Never experienced that again, checked nosegear attach points everything by the book. What it could be? Cocked nosewheel on touchdown after forward slip?
 
Vlad said:
Need opinion too. Couple weeks ago I was at unfamiliar airport and slipped hard to lose altitude. After touch down I hold the nose up then relaxed a bit and as soon as it touched the runway the vibration started. Next second I lifted the nosewheel again and after it lowered itself at slow speed the vibration repeated but was lighter and then went away fast. Somebody happened watched and he said it was visible. Never experienced that again, checked nosegear attach points everything by the book. What it could be? Cocked nosewheel on touchdown after forward slip?
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Quite a number of RVs have experienced minor shimmy while still on the rollout, or severe shimmy on landing, if the nosewheel comes down too quick. Severe shimmy feels like a flat tire. Seems like the best answer is to keep the nose as light as possible during the rollout...........in addition to breakout force, balance, etc.

L.Adamson
 
Thanks Larry. Looks like it's the case and I experienced it only once after more than 100 landings.
 
We keep the nose up till it just about won't stay up any longer and keep the breakout and tire pressure per van's specs and haven't had any issues...
 
MauiLvrs said:
We keep the nose up till it just about won't stay up any longer and keep the breakout and tire pressure per van's specs and haven't had any issues...
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That's the million dollar question. Why do some have issues and others don't. Got lots of RVs and Mustang IIs where I live, some have issues and others don't. One guy with a 9A has terrible pilot technique never keeping nose wheel off and 3 pointing most landings. He doesn't mess much with breakout or tire pressure and has zero shimmy issues. it's the same design.....why?
 
Vertical Shimmy

All these posts seem to be about the nose wheel shimmy in the horizontal. My RV-9A seems to shimmy vertically. I can't feel it in the airplane, but ground observers have noticed varying degrees of vertical excursions at almost all speeds. I think this condition has something to do with the reports of "pogo sticking" by nose wheel airplanes. My tire pressures and breakout is to spec, and all seems secure in the engine mount. I am not using the nose whee pant right now. Any thoughts on this condition?
 
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mkbreezy said:
All these posts seem to be about the nose wheel shimmy in the horizontal. My RV-9A seems to shimmy vertically. I can't feel it in the airplane, but ground observers have noticed varying degrees of vertical excursions at almost all speeds. I think this condition has something to do with the reports of "pogo sticking" by nose wheel airplanes. My tire pressures and breakout is to spec, and all seems secure in the engine mount. I am not using the nose whee pant right now. Any thoughts on this condition?
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Lift the nose off the ground and see if the wheel turns with little drag...... then do a search on nose wheels..............
 
Thx for the thought but I'm way beyond spinning the wheel (and it's not pilot technique)...One thing I do notice that seems wrong is that there is only one belleville washer. That seems odd as I had 2 on the Velocity. I checked some RVs in the neighborhood and a Mustang and they all have 2 Bellevilles--I looked at the plans and it does only show ONE belleville. I'm going to try 2. I've called all over locally and no one has, guess I'm going to have to order from Vans (they call them disc springs).
 
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UPDATE:: Replaced the one Belleville washer with 2 today. I lowered the breakout force from 36 to 27 and ran 28 psi in the tire. Was able to accelerate to 45 knots with no shimmy. I did get a few minor wiggles at 35 knots and 22 knots on the decel but it's much improved. I noticed much more consistency on the breakout readings with 2 Belleville washers as opposed to one.

I did notice a paragraph in the plans which states that lower tire pressures are more beneficial in reducing shimmy.
 
Flybuddy2 said:
Thx for the thought but I'm way beyond spinning the wheel (and it's not pilot technique)...One thing I do notice that seems wrong is that there is only one belleville washer. That seems odd as I had 2 on the Velocity. I checked some RVs in the neighborhood and a Mustang and they all have 2 Bellevilles--I looked at the plans and it does only show ONE belleville. I'm going to try 2. I've called all over locally and no one has, guess I'm going to have to order from Vans (they call them disc springs).
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Welcome to Van's blueprints where you hunt and may never find the answer.

On plans #62 it shows part # U-611. It also shows two of them in place under the fork. But reading the instructions, it does not say that they go cup to cup and they will be almost flat when properly torqued and broken in.

My plans are for a 6A dated 4/16/90
 
Flybuddy2 said:
UPDATE:: Was able to accelerate to 45 knots with no shimmy. I did get a few minor wiggles at 35 knots and 22 knots on the decel but it's much improved.
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There is no need to test the nose wheel past 30 knots. Anything after that and the nose wheel should be off of the ground.
If the wheel started to shimmy at 45 knots, you might have had damage.
 
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gasman said:
There is no need to test the nose wheel past 20 knots. Anything after that and the nose wheel should be off of the ground.
If the wheel started to shimmy at 45 knots, you might have had damage.
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Huh? 20 knots will lift the nose wheel? Must be a very aft loading, or a super size elevator!

I've had various vibrations from the gear at different periods during 1300 hours' time. Tire weight (i.e., wear), air pressure, tire/wheel balance, temperature, etc., all affect what will vibrate when. About the only meaningful damping in the system is the tire internal and external friction, and the belleville washers (in the case of the nose wheel castor). I've chased the vibrations away, only to have them show up at some other speed (wheel rpm). My advice is to avoid the speeds that cause the vibrations, or pass quickly through. Lowering nose wheel pressure is not a great idea, as many flat tires will attest (search for flat tires, nose wheel)

To the person describing the pitch type of vibration on the nose wheel - please search the archives for wheel bearing/axle/friction as was suggested.
 
When I have got 2 people in plane and full fuel, the nose wheel doesn't come off the ground till about 40 knots hence the 45K on the test. As far as lower tire pressure, it's relative. Someone with a lighter engine and composite prop does not need to run as much pressure as someone with more weight on the nose. I only mentioned that since many folks have posted that they add pressure to help reduce shimmy which is simply not accurate.
Also, in the plans I have it shows one Belleville in the schematic but lists two in the parts section.
It is all a learning experience. I wound up installing a locking nose wheel when I owned a Velocity. One other idea that might work for us is adding a vane on the nose wheel fairings (lengthen it). This would allow a higher breakout force and the vane would help straighten the nose in flight.
 
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