Landing gear vibration

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Pascal
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Landing gear vibration

Post by Pascal »

Taking off I notice a vibration coming from the front landing gear. My mechanic says when he lowers the tail he can wiggle the front landing gear leg quite a bit.

He's supposed to call Diamond in London but that might take weeks as he is in the habit of taking his sweet time doing things.

Has anyone else experienced the same problem? DA20-C1 1999.

Thanks
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Derek
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Re: Landing gear vibration

Post by Derek »

I had a similar/same thing develop last summer. Upon takeoff I noticed more rattle and shimmy coming from the front nose gear. My mechanic replaced the bearings for the nose landing gear this spring and the shimmy is gone.

He also replaced the front tire and tube at a crazy price. What’s up with that? Our front tire is so much more expensive than any other GA tire.

Also my fuel pump had to be replaced for $3k or so although I didn’t notice any problem with the old one. I’m sure it wasn’t functioning correctly but I’m saying it didn’t affect noticeable performance one way or another. It seems like an opportunity for improvement that we have to pay big money for small things like fuel pumps when ours are much less reliable and robust compared to an automotive fuel pump.
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Pascal
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Re: Landing gear vibration

Post by Pascal »

Hi Derek,

Thanks for the quick response. My mechanic says the airplane is still safe to fly so I'll be able to travel to the Bagotville QC airshow to see an F35 up close.

The price of parts is higher on these metric birds.

I want a better panel, and I'm not sure it makes sense to plunk 50k$ in avionics on this plane. I might sell it and switch to a light ifr plane with auto pilot.
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CWS
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Re: Landing gear vibration

Post by CWS »

Have you tried tightening the bolt that holds the castoring arm onto the gear leg? If it’s too loose the wheel will shimmy.
The maintenance manual gives a description of how much the bolt must be tightened.
The nose wheel should not turn on this bolt too easily when you drop the tail.
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Re: Landing gear vibration

Post by CWS »

There you go

Maintenance Practices
6. Adjust the Nose Landing Steering Friction Steering friction prevents nose wheel shimmy. A. Equipment
Refer to Chapter 07-10-00 for the equipment required for jacking the aircraft. B. Procedure
DA20-C1 AMM
Detail Steps/Work Items
Key Items/References
1.
Jack the aircraft.
Refer to Chapter 07-10-00.
2.
Remove the nose-wheel fairing.
3.
Inspect condition of belleville springs, washers, spacer and stop plate.
Replace as required.
4.
Check the NLG friction by application of force to NLG wheel axle in both directions along the axle.
5.
If required, adjust the nose-wheel fork pivot nut:
- Remove the cotter pin (if required) - Adjust the nut
- Install a new cotter pin.
The nose-wheel must just caster when you apply a force of 6.75 - 11.25 lb. (30 - 50 N) acting in the direction of the nose wheel axle. Refer to Figure 206
If the cotter pin hole and the nut castellation do not align after adjustment of steering friction install/remove one AN960-616L thin washer. Washer is optional and (if installed) located in between the two AN960-616 washers. It may be necessary to tighten the nut to the next castellation if alignment is not obtained. Refer to Figure 205.
6.
Apply corrosion protection to the stud, nut and washers only.
Use CRC SP-400 or CRC Corrosion shell. Do not contaminate tire or fiberglass.
7.
Install the nose-wheel fairing.
8.
Remove the trestle from the front fuselage.
9.
Remove the aircraft from jacks.
Refer to Chapter 07-10-00.
7. Nose Wheel Balancing
Always have a new nose-wheel balanced before installation.
Page 208 32-20-00 12 Jun 12
DA201-C1 Rev 20
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Pascal
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Re: Landing gear vibration

Post by Pascal »

Thanks, I will forward that to my mechanic.
CWS wrote: Sat Jun 22, 2019 8:21 pm There you go

Maintenance Practices
6. Adjust the Nose Landing Steering Friction Steering friction prevents nose wheel shimmy. A. Equipment
Refer to Chapter 07-10-00 for the equipment required for jacking the aircraft. B. Procedure
DA20-C1 AMM
Detail Steps/Work Items
Key Items/References
1.
Jack the aircraft.
Refer to Chapter 07-10-00.
2.
Remove the nose-wheel fairing.
3.
Inspect condition of belleville springs, washers, spacer and stop plate.
Replace as required.
4.
Check the NLG friction by application of force to NLG wheel axle in both directions along the axle.
5.
If required, adjust the nose-wheel fork pivot nut:
- Remove the cotter pin (if required) - Adjust the nut
- Install a new cotter pin.
The nose-wheel must just caster when you apply a force of 6.75 - 11.25 lb. (30 - 50 N) acting in the direction of the nose wheel axle. Refer to Figure 206
If the cotter pin hole and the nut castellation do not align after adjustment of steering friction install/remove one AN960-616L thin washer. Washer is optional and (if installed) located in between the two AN960-616 washers. It may be necessary to tighten the nut to the next castellation if alignment is not obtained. Refer to Figure 205.
6.
Apply corrosion protection to the stud, nut and washers only.
Use CRC SP-400 or CRC Corrosion shell. Do not contaminate tire or fiberglass.
7.
Install the nose-wheel fairing.
8.
Remove the trestle from the front fuselage.
9.
Remove the aircraft from jacks.
Refer to Chapter 07-10-00.
7. Nose Wheel Balancing
Always have a new nose-wheel balanced before installation.
Page 208 32-20-00 12 Jun 12
DA201-C1 Rev 20
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Re: Landing gear vibration

Post by RMarkSampson »

Pascal,
That NLG shimmy can be any one of a few things, or a combination of several. I've done this dance with my 1999 DA-20 on several occasions. I do have a few previous posts from those events.

viewtopic.php?f=7&t=5434#p58829

Per my recollection, here were some of my cures:
1. Nose Wheel Caster Friction. I've gone from too loose to too tight. Too loose and you will get the shimmy. Too tight and that very expensive nose wheel will wear through its tire tread all the quicker. I use a digital fishing scale to hook to the tire to pull it left/right and set my friction point. You can't be exact based on where the cotter pin lines up on the nut but having a few extra thin washers does allow you to experiment and find what combination gives you the best friction that is within Diamond's spec. I keep mine toward the higher end of that friction range.

2. Tire worn and no longer round. It may be simply time to replace your NLG tire. I did have a tire that simply got so out of balance that it caused the shimmy. Replacing the tire fixed the problem. It may not be simply a function of looking at the tread left on the tire to determine when to replace. I have determined that the NLG tire should be replaced around 250 hours. Depending on how much she is flown, I have replaced my NLG tire around every two years, or even sooner.

3. Tire balancing. I also bought a fairly inexpensive tire balancer used for motorcycles. Put the NLG tire with axle on the balancer and give it a slight spin. After the spin slows the tire will rock back and forth and the heavy spot will ultimately end up on the bottom. Placing a few tire weights with thick double stick tape into those little cubby holes on the opposite side of the tire rim heavy side improved the balance of the tire - and helps keeps the tire from shimmying.

4. I replaced the bushings that hold the NLG to the plane. I also replaced the pivot pin that fits into those bushings. The holes that are drilled into that pivot pin got were elongated and allowed for some increased side-to-side play that contributed to the shimmy.

5. We have discussed the NLG pucks before - and I've replaced the pucks a few years ago. I also built the jig that allows me to do that maintenance so I am ready to do that maintenance again. I'm not sure how much the pucks contributes to a side-to-side shimmy but the puck stack does get less elastic with age. Believe Diamond says every 1000 hours and I am probably approaching that number of hours since I last changed them out.

6. I've replaced the NLG bearings on the axle. That was also not a shimmy issue but one of those bearings did fail creating a very vocal problem. I cross-referenced the Diamond supplied bearings to a fairly inexpensive brand. I spoke to a bearing expert who recommended a German company's bearing - $35 per bearing vice $12 but when I rotated the tire using those new bearings it was silky smooth. You could feel the quality. I now have a spare set in the hangar so I don't need to chase around town if another bearing fails.

7. I land nose up as much as I can - and fly the NLG until the speed drops. I have others that fly my aircraft so I can't attest to everyone doing that but I do believe using a bit more runway to ensure a nice touchdown on the mains and keeping the NLG off the pavement as long as possible helps reduce the return of the shimmies.

Good Luck - sometimes you got to be persistent and tackle each one of these possible causes before you get the shimmies under control.
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