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pulls to the left when taxiing

kritsher

Well Known Member
My relatively new (6 hrs) RV-12 seems to pull to the left quite a bit during taxiing.

First, I thought it was weathervaning from the wind until I realized it was always in the same direction.

Then I thought the left brake pad was catching, but I can roll that wheel by hand no problem. The brake assembly has a little play and I can see daylight between the disk and the pads.

Then I thought one of the brake cylinders wasn't free to rotate where it's attached to the pedal, causing it to maybe stick occasionally in the engaged position, so I loosened the bolts there by one or two castellations.

I'm running out of ideas. Any thoughts on how to troubleshoot this?

Thanks!
 
My relatively new (6 hrs) RV-12 seems to pull to the left quite a bit during taxiing.

First, I thought it was weathervaning from the wind until I realized it was always in the same direction.

Then I thought the left brake pad was catching, but I can roll that wheel by hand no problem. The brake assembly has a little play and I can see daylight between the disk and the pads.

Then I thought one of the brake cylinders wasn't free to rotate where it's attached to the pedal, causing it to maybe stick occasionally in the engaged position, so I loosened the bolts there by one or two castellations.

I'm running out of ideas. Any thoughts on how to troubleshoot this?

Thanks!

Since it sounds like you checked everything related to a possibly dragging brake or wheel friction... did you check axle alignment and shim if needed, when installing the gear legs and axles?
 
Ken,

Check out today's thread titled, RV12 Rudder Pedal Extensions by RV12Roger. My RV-8A would come to a stop when I tried to do a 180 and back taxi. I found out that my clodhoppers were applying the brakes. I made extensions out of alum. angles and bolted them to the bottom of my pedals and it worked. It is something to look at. Good hunting. If you would like to see a picture of my setup send your email address to jpthornton at frontier.net.

Jim
 
Thanks for the suggestions, guys.

Scott, I did check the axle alignment during the build, but it feels like I should re-check it. Are there any tricks out there for checking axle alignment without jacking up the plane and taking off the wheels?
 
Mine sometime pulls right or left. I attribute it to friction in the nose wheel castor because I can usually stop it by tapping the brakes until it centers
 
Thanks for the suggestions, guys.

Scott, I did check the axle alignment during the build, but it feels like I should re-check it. Are there any tricks out there for checking axle alignment without jacking up the plane and taking off the wheels?

Not a trick really, but you could get a good idea of the situation with a couple of straight edges (4 ft carpenters levels). Lightly clamp one to the outside of each tire with them parallel to the ground. Drop a couple plumb lines from the belly center line and measure front and back to see if the values are all close to the same. Checking 2 ft for and aft of the axle should be more than enough distance to detect if things are off very far.

Your next question would likely be how much is acceptable. I don't know the answer to that. You could calculate the value based on what the KAI suggests, but I think the best thing to start with would be just verifying if there is a difference between left and right.
 
Using Scott's technique, it appears that the right wheel is about 1/4 degree toe-out and the left wheel is about 1/4 degree toe-in. One quarter degree does not seem like a lot (given that the smallest shim is 1/2 degree). On the other hand, if both main wheels are angled off to the right, it makes sense that the nose would want to go left.

So, two questions:
1. Does anyone think that 1/4 degree could cause a significant tracking problem? and if so:
2. Is it better to shim the left wheel 1/2 degree so that both wheels are a tiny bit toe out, or is it better to shim the right wheel 1/2 degree to that both wheels are a tiny bit toe-in?
 
It seems to me that no matter how many degrees the main wheels are out of alignment, the nose wheel has much more affect. For instance, if the main wheel is pointed 1 degree to the left and the nose wheel is pointed 15 degrees to the right, the plane is not going to pull to the left. The direction that the nose wheel is pointed needs to be known before looking for other causes.
A GoPro camera would come in handy for this situation.
Joe Gores
 
My airplane always pulls to the left during taxi -and needs either right brake tap or a fairly consistent right brake application to keep it on the center line. Other RV-12 pilots in my group have reported something similar.

I have attributed it to P-factor. I need about 2200 rpm to taxi level, and - some of KPWT taxiways climb uphill quite a bit - needing 2500-2600 pm - and figured that was the way it is.

We checked the nose wheel fork pull pressure values, and have also played with the nose tire pressures, but other than that have done nothing about toe-in or toe-out or any of the rest of this stuff.

Bob Bogash
N737G
 
Pulling to the left

With these very light aircraft, when flying solo, the extra weight on the left main gear will cause more tire friction and a tendency to turn left.
 
A few thoughts....

Have you checked both main gear bearings - set to Matco specs? Could be uneven rolling friction. How tight is your nose wheel as far as castoring goes? If set per the plans it may be way too tight - mine was. Is your nose wheel perfectly aligned in the vertical plane?

There are a couple of theories stated above that I don't think could cause your problem. Weight of pilot on one side is balanced out by fuel on the other. Both close to center line - unlikely to cause a noticeable difference. P-factor would only come into play with nose high attitude - seems unlikely you are taxiing around at a high enough speed to get the nose up.

Here is something you could try, to isolate the left turning tendency to either the mains or the nose gear. Loosen the castoring nut way up so there is no friction and the nose wheel swivels freely. The try pushing the airplane ahead on a level surface and see if it tends to go one way or another.
 
We checked the fork pull at about 15-16 lbs on Friday, John - less than the spec - but haven't operated since (I'm involved in a major maintenance episode, that I think will have evolved into an early Annual.)

I set the main wheel bearings VERY carefully (actually RESET them very carefully), but will give them another look.

Actually, the turning while taxiing is not a big issue for me - my Number One Issue currently is wandering during take-off due to the free-castering nose wheel, the P-factor, and maybe poor pilot technique. I've been playing with the fork pull, tire pressures, checked the rudder cables, and worked on the technique - been getting LOTS of advice from fellow pilots.

Hope to resume flying by mid-week.

Bob Bogash
N737G
 
The first few times I took off I made the mistake of not applying enough back pressure on the stick and encountered some wheel-barrowing and tendency to wander in a squirrelly (engineering term?) fashion. Since improving my technique there have been no problems.
 
The taxiways are crowned for water drainage at my airport. Unless my nosewheel is rolling on the painted centerline, the plane is heading downhill.
 
Ken,

I just mounted my gear legs and wheels/tires this week. I found one axle to be straight but the other had some toe out so I ordered some shims from Van's. I would spend some time to get this right based upon my experience with other RVs/Rockets. I believe it has significant impact on the taxiing and landing qualities. It should be set as straight as you can get it.

Yes, there are many things that can affect the handling qualities and cause the airplane to steer one way or the other. However, you have to get the toe set to neutral first or you'll never solve it. I flew for a year with an ill-handling airplane before I took the time to get it fixed. The difference was amazing. It was a totally different airplane after that.
 
Same left pulling problem here. Put in .5 degree shim to correct a left toe-out condition, alignment is now dead nuts on. Tire pressures are identical. Even measured break away torque of wheels - 8 in lbs on both sides. My conclusion is that the cause is having a 180 pound person in the left seat. Doesn’t help there was only 50 lbs of fuel on the right side. A quick back-of-the-envelope calculation says the left wheel sees approximately 35 lbs more weight than the right under this loading. Tomorrow we’ll taxi from the other side and let you know.
 
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Even measured break away torque of wheels - 8 in lbs on both sides.

I don't understand break away torque of wheels... Are you talking about main wheels? The mains should spin pretty freely indicating no bearing preload drag or excessive brake pad drag.
 
If you have the standard MATCO main wheels on your RV-12 they should not spin freely. MATCO specifies a certain bearing pre-load and drag for their wheel bearing assemblies. This is all explained in the MATCO on-line service literature. Note that the MATCO nose wheel uses a different kind of bearing and has a different torquing procedure.

See MATCO literature at: https://static.veracart.com/matco/item_pdfs/2665/document1.pdf
 
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David's right. The reason given is that the Matco bearing grease seals are not supposed to rotate with the wheel. This generates a small amount of rotational drag hence the torque required to turn the wheel. My documentation, for my part number, says 8-10 inch pounds. There is no bearing play, or pre-load, that I can feel and the brake pads are not dragging.
 
before adding shims and stuff, would it be prudent to eliminate some possible causes:

1. crosswind. taxi up and down the same taxiway.
2. extra weight on left side. taxi plane with pilot seated on right side.
3. p-factor. taxi at a moderate speed, get on the taxiway centerline, then throttle back or turn off the engine.
4. brake caliper or wheel drag. measure torque to turn each wheel.

Failing all of those, then the only thing left is the toe-out... ?
 
I would also check the perpendicular component of the nose gear leg. If it's tilted it could influence the direction the castering nose gear tends to point.
 
Never thought to check geometry of nose gear.

In any event, taxied from the right seat today and sure enough, pulled slightly right. It?s about the same when I sit on the left side and it pulls left (after adding the shim). I think at this point my particular aircraft is as good as it?s going to get. I don?t want to overstate the pull. The aircraft is imminently controllable and rolls pretty straight above about 15kts.
 
An update. Provided everything else is straight and you fly solo a lot with partial fuel, increasing the tire pressure in the left tire makes a big difference.
 
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