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Trouble taxiing straight

MacBoat

Member
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!
 
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.
 
Yes, good point, the nose leg SB has been complied with.

And Mel I'll look into your master cylinder compression springs suggestion.
 
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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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.

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.
 
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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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.
 
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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Jig the leg in a vice or on a work bench to hold it steady. Zero a digital level on the firewall side tube then compare the lower tube. If you already have it on the plane you can zero the level on the canopy longeron then compare on the lower leg tube plus 90 degrees..

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I just realized that this was posted on St Patrick's Day. Driving home from the hangar late last night, I got stuck behind someone who appeared to have trouble keeping his car driving straight. I certainly hope the same reason is not to blame for a zig-zag taxi.
 
While investigating a vibration after installing the new nose gear. I took this video to watch my nose wheel rotation. Note the main wheels stop at about the same time. I pretty sure this is because of the bearing preload. Anyway...good way to check for a dragging brake....or you could just jack up the airplane and turn it. Not as much fun though. GoPro...wonderful too. BTY, my RV-12 tracks straighter after the nose gear change. I chock this up to getting the nose fork break away torque right.

https://youtu.be/P5N8C4_f8eE
 
NinerBikes; 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 said:
my front wheel on my motorcycle has forword rake in the pivot shaft. works fine,no shimmy,steers straight. not to be argumentative but i do not understand your statement.
 
my front wheel on my motorcycle has forword rake in the pivot shaft. works fine,no shimmy,steers straight. not to be argumentative but i do not understand your statement.

Go ride various mountain bikes from steep XC headsets to slack headset down hill mountain bikes while pedaling on level ground trying to ride with no hands, and you will.
 
The steering pivot on a motorcycle / bicycle is on the centerline of axle. On our planes the pivot is significantly forward of the axle.
 
so the forward rake my rv-3 tail wheel has is correct or wrong?
or the pivot shaft should be aligned straight up and down?
 
Thanks for all of the informative replies! Sounds like I have at least three things to investigate: dragging brake(s); break out force on the nosewheel; bends on the nose wheel out of spec.

Regarding both crosswind and lining up straight on takeoff, I've been sensitive to observing those conditions while monitoring this issue and have not found them to be the cause. I'm anxious to get this issue corrected once I locate someone, as I hate wearing down the brakes and also just the uncomfortable feeling of having to keep the airplane straight at speed, especially during takeoff.

This also gives me the opportunity to fabricate jacks for the airplane, which has been on my (long getting longer) list of things to do with the airplane.
 
Related question. I've installed the main gear and nose wheel on my 12iS, per the KAI, but had not yet hung the engine. When I push the fuselage (on all 3 wheels) I find that I cannot steer it unless I physically push the nose wheel in the desired direction and then it tracks that position. Do you think this is normal behavior or might I have set the nose wheel too tight? I used an electronic 'fish scale device' to set it per the KAI. I'm assuming that with the weight of the powerplant assembly the nose wheel would steer without having to manually turn it. Thoughts on this?
 
Related question. I've installed the main gear and nose wheel on my 12iS, per the KAI, but had not yet hung the engine. When I push the fuselage (on all 3 wheels) I find that I cannot steer it unless I physically push the nose wheel in the desired direction and then it tracks that position. Do you think this is normal behavior or might I have set the nose wheel too tight? I used an electronic 'fish scale device' to set it per the KAI. I'm assuming that with the weight of the powerplant assembly the nose wheel would steer without having to manually turn it. Thoughts on this?

Hanging engine may help, but these planes steer best by pulling. Can pull at base of prop (I understand it's not there yet ;)) or build the tow bar and use that.
 
Vans has a RV12 tow bar for $105. Might just purchase that for maneuvering it around in my garage/hangar. If the nose wheel is too tight I can adjust later in the construction.
 
What I do to back the plane up is sit very gently with my wide b*tt on the stabilator to lift the front wheel off the ground, grab the vertical with my hand, and very carefully back up once the front wheel is off of the ground. YMMV, it's been working for me.
 
Since my tailcone is not yet permanently installed, I can't push down on the tail. However, the wide b**t is certainly not a problem.:)
 
Not Recommended......

What I do to back the plane up is sit very gently with my wide b*tt on the stabilator to lift the front wheel off the ground, grab the vertical with my hand, and very carefully back up once the front wheel is off of the ground. YMMV, it's been working for me.

The stabilator attach points are not really designed for this kind of load.

Whereas a horizontal stabilizer is mounted by several solid bolts, a stabilator is mounted by only the hinges.
 
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The stabilator attach points are not really designed for this kind of load.

Whereas a horizontal stabilizer is mounted by several solid bolts, a stabilator is mounted by only the hinges.

Thanks, Mel, I'll find another way, then. A weighted dolly that attaches with a short rope to the eyelet on the bottom then. I generally also push down on the fuselage just in front of the vertical stabilizer too, at the same time, with the palm of my hand.
 
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If my tow bar isn’t available I press down on the fuselage just in front of the vertical stab to get the nosewheel off the ground and point the nose where I want to go before pulling it by the prop near the hub. Sometimes I have to stop and readjust the direction. You can also press down and walk it backwards for small repositioning like moving back into a tie down. When I was a student pilot in the 80s we did it all the time with Cessna 152s.

When using the tow bar I have found it is best to not push on the tow bar when going backwards. Push on the prop near the hub or on the cowling and use the tow bar for steering only. Pulling is a lot easier to control direction since the tow bar is being used for movement and steering.
 
Not sure if you guys that find it difficult to push the plane using the tow bar are using the Van’s aluminum tow bar. If so, it’s great to carry in the plane for maneuvering at a transient airport or using as a gust lock, but for maneuvering into your hangar at your home airport it’s best to spring for a steel tow bar. I use the Bogert 4-RVA to maneuver mine and don’t have any issues backing it into the hangar. Just a thought/suggestion, as this works well for me.
 
I added a grease fitting to the nose fork bushing. No need to remove fork as small drill pulls chips out. I used 1/16” thru drill and then ¼” drill x ¼” deep counterbore. Grease fitting is ¼” drive type.

I thought I had the void between the two pivot bushings packed well with grease. I was surprised how many pumps on grease gun it took to fill the cavity.
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I added a grease fitting to the nose fork bushing. No need to remove fork as small drill pulls chips out. I used 1/16” thru drill and then ¼” drill x ¼” deep counterbore. Grease fitting is ¼” drive type.

I thought I had the void between the two pivot bushings packed well with grease. I was surprised how many pumps on grease gun it took to fill the cavity.
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Great tip. I haven't started my gear yet, so I will incorporate this as I go. Or will that jeopardize my E-LSA cert?
 
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I followed the advice on the grease fitting a couple of years ago. Great idea! Now I don’t have to pull the nosewheel every annual. Like Jim, I was surprised how much grease I pumped before it was full.
 
Tolerance?

What is the acceptable tolerance between the top straight section and the bottom straight section?

My replacement nose leg is bent one degree forward.

Thanks.

Brett
Columbus, IN
 
I believe the thread you referenced is for a different problem that can be fixed in the field. The issue identified in posts #6 and #11 is the bends do not appear to result in parallel planes. I have not seen a number on an acceptable caster angle for the RV-12 nose gear that would result in asking Van's for a replacement part. I am fairly certain they would not recommend bending the gear leg tubing in the field to make the upper and lower sections parallel, but you never know. Caster angle has an effect on steering stability, only Van's can answer the angle vs breakout force design point.

John Salak
RV-12 N896HS
 
Great tip. I haven't started my gear yet, so I will incorporate this as I go. Or will that jeopardize my E-LSA cert?

I am also building under E-LSA. As I understand it, your product must be exactly the same as the S-LSA (factory built) product. If it is not (and they notice), you’d have to go E-AB. Any deviation from the plans would be considered a modification away from the original spec. I’d wait until after getting the Airworthiness Cert., after that you can modify, since it’s still an experimental.
 
What is the acceptable tolerance between the top straight section and the bottom straight section?

My replacement nose leg is bent one degree forward.

Thanks.

Brett
Columbus, IN

Hey Brett, hopefully 1 degree won't be an issue. Please let us know what Van's says so the rest of us that are waiting for the part will know. Once the weight of the aircraft is sitting on the nose gear leg, maybe there will be a slight flex aft that might cancel out the 1 degree forward angle?? If not, maybe a hard landing or two will.. (JUST KIDDING) :)
 
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