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  #1  
Old 03-18-2021, 09:41 AM
MacBoat MacBoat is offline
 
Join Date: May 2020
Location: Lawrence, Ks
Posts: 9
Default Trouble taxiing straight

I recently purchased an RV-12iS E-LSA from the builder and am having difficulty keeping straight when taxiing or taking off. Most of the time, but not always, the airplane pulls to the left. I have ensured both main tires are at the same proper air inflation and all three tires look good. Nothing seems out of kilter from eyeing it. The problem doesn't manifest itself nearly as much when landing.

I've flown a couple of different RV-12s and have not had this problem; all I'd have to do to turn or keep straight is to tap the appropriate brake. However, on my airplane I have to at times "ride" the brake. It's especially egregious during takeoff, and is a bit unnerving. If I don't have the airplane perfectly straight when going to full power, I will invariably have to overuse the brake, usually the right side, to keep it straight.

It's hard to get this behavior consistently, though. At times, it pulls to the left instead. At other times, it seems to be perfectly fine going straight only; when turning though, it will usually start all over again pulling to one side or the other when trying to straighten out.

One other data point is that to make it stop pulling to one side and straighten out while taxiing, if I press fairly hard on the appropriate brake, it will overcompensate (as expected) then I can use the other brake to feather it back straight.

What should I be looking for to troubleshoot this issue? I'm not exactly sure where to start just yet so thought I'd ask before randomly, and likely inefficiently, jumping in. Thanks for getting through this rather wordy post!
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  #2  
Old 03-18-2021, 09:53 AM
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Mel Mel is online now
 
Join Date: Mar 2005
Location: Dallas area
Posts: 10,986
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Sounds like you might have dragging brakes. Sometimes a master cylinder may not return to the released position when you get off the pedal. If this is the case, adding compression springs to the master cylinder shaft will solve the problem.
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Mel Asberry, DAR since the last century. Over 1,000 certifications accomplished.
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  #3  
Old 03-18-2021, 09:54 AM
seagull seagull is online now
 
Join Date: Dec 2019
Location: San Bernardino
Posts: 283
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Has your plane had the nose leg replaced?
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  #4  
Old 03-18-2021, 09:57 AM
MacBoat MacBoat is offline
 
Join Date: May 2020
Location: Lawrence, Ks
Posts: 9
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Yes, good point, the nose leg SB has been complied with.

And Mel I'll look into your master cylinder compression springs suggestion.
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  #5  
Old 03-18-2021, 09:59 AM
JFCRV12 JFCRV12 is offline
 
Join Date: May 2015
Location: Liberty Township OH
Posts: 193
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In addition to the brake dragging, maybe check break out force on the nosewheel?
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  #6  
Old 03-18-2021, 10:48 AM
seagull seagull is online now
 
Join Date: Dec 2019
Location: San Bernardino
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Quote:
Originally Posted by MacBoat View Post
Yes, good point, the nose leg SB has been complied with.

And Mel I'll look into your master cylinder compression springs suggestion.
There is a known issue with the replacement nose legs. The bends are not to spec. The part that bolts to the firewall should be parallel to the bottom vertical axle that the fork pivots on. I have seen many as far out as 3.5 degrees forward of vertical. I have personally worked on several and checked replacements. Out the (5) I have had my hands on (2) were straight enough to use.

What happens when the leg leans forward is that the fork wants to caster to one side or the other. When the leg is vertical or back the fork (wheel) will prefer to center.

If you have wheel pants you can stand back to the side and notice if the nose wheel pant is pointing nose high / tail low. That is an indication the leg is angled forward. My first leg replacement was 3.5 degrees forward, the rear of my pant was 1.5" low. I made shims to level the pant not realizing the leg was the issue.

I also noticed that when the leg is angled forward there is more tenancy for the wheel to shimmy if you lower the nose too early when landing.

I was able to subside the shimmy somewhat by setting the break-away torque to 23-25 pounds, this also helped to keep it straighter during taxi. The real solution is a straight leg.
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Last edited by seagull : 03-18-2021 at 10:51 AM.
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  #7  
Old 03-18-2021, 10:54 AM
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rvbuilder2002 rvbuilder2002 is offline
 
Join Date: Jul 2005
Location: Hubbard Oregon
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Quote:
Originally Posted by Mel View Post
Sounds like you might have dragging brakes. Sometimes a master cylinder may not return to the released position when you get off the pedal. If this is the case, adding compression springs to the master cylinder shaft will solve the problem.
Quote:
Originally Posted by JFCRV12 View Post
In addition to the brake dragging, maybe check break out force on the nosewheel?
To me it sounds like it could be a combination of both of these.
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  #8  
Old 03-18-2021, 12:10 PM
printbr printbr is offline
 
Join Date: Oct 2019
Location: Richland, WA
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Since it is inconsistent it could be as simple as a cross wind. While taxiing and there is a wind blowing at the side of the tail it will push the nose the opposite direction and you will have to drag a brake to keep it straight.

When taking off the Rotax produces quite a bit of torque and the airplane is light so you need to get on the right rudder early. When I line up on the runway I usually taxi forward a bit to make sure the front wheel has straightened out. Then get your toes off the brakes and at the bottom of the rudder pedals and as you increase power; bring in the right rudder. You are now flying/steering with the tail as the prop pushes air across the rudder. With a castering nose wheel that the RV's have, it is more like flying a tail dragger than a steerable nose wheel. And it seems to take a bit more right rudder to keep it straight than a Cessna does. And as mentioned above a cross wind will affect rudder inputs as well.
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Last edited by printbr : 03-18-2021 at 12:30 PM.
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  #9  
Old 03-18-2021, 12:49 PM
JFCRV12 JFCRV12 is offline
 
Join Date: May 2015
Location: Liberty Township OH
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Quote:
Originally Posted by seagull View Post
There is a known issue with the replacement nose legs. The bends are not to spec.
This is a new one for me. I have the new leg sitting in a crate waiting to put on in the next few weeks. I only have 130hrs on the original leg and all seems fine.

Is there a way to see if the bends on new leg aren't to spec before starting install? I sure hope mine is a good one since they're backordered forever.
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  #10  
Old 03-18-2021, 01:14 PM
NinerBikes NinerBikes is offline
 
Join Date: Dec 2018
Location: Granada Hills
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Quote:
Originally Posted by JFCRV12 View Post
This is a new one for me. I have the new leg sitting in a crate waiting to put on in the next few weeks. I only have 130hrs on the original leg and all seems fine.

Is there a way to see if the bends on new leg aren't to spec before starting install? I sure hope mine is a good one since they're backordered forever.
Probably with a level on the round bar stock, I would think.... place in a vice, so it doesn't move, and put one of those digital levels on both the fork section of tubing and the mount section of tubing, without moving. Take pictures of the relative same degrees, or how much it changes, top to bottom. Compare to what it is on your original front leg, in degrees. They should be close to identical, old and new. Maybe ask Van's what the specs were on the original front landing gear, they don't seem to have the wobble problem.

If the bottom of the landing gear is at an angle such that the bottom of the landing gear leg is IN FRONT of the top portion 3 inches above, with a forward rake, then when aligned perfectly straight, the fork and wheel is trying to LIFT the whole front of the airplane, and the fork will try to divert to the side and turn, due to gravity pushing the front end down. As soon as the wheel gets the least bit offset, it wanders even further away from rolling straight ahead. It's pretty imperative that that final section of tube that the fork pivots on is perfectly plumb, straight up and down, if you don't want the front end to wander.
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Last edited by NinerBikes : 03-18-2021 at 06:01 PM.
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