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Landing gear alignment

Posted By:
Duane Reetz
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#1 Posted: 2/1/2010 10:04:43

I have a Piper Colt converted to a PA20 with the Univar kit.  The wheels are have toe in and camber (the top of the wheels are wider than the bottom).  Since I am recovering the fuselage I would like to fix this.  Can I heat and bend the gear legs?  Any ideas or cautions would be greatly appreciated.  Thanks.



Joanne Palmer
Young Eagles Pilot or VolunteerHomebuilder or Craftsman
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#2 Posted: 2/1/2010 10:42:39

The wheels and tires REQUIRE toe-in so do NOT adjust that.   The toe in makes tha aircraft more stable due to differences in wheel/tire shimmy on rollout or high speed taxi.  You need between 1/2 to 1 degree of toe-in. 

The camber can be misleading.  I don't know how the Piper 20 LG works during it deflection, but from what I can gather from looking at a picture, it seems to have a bungee loop shock system like the Cubs.  This means that the gear leg pivots at the fuselage somewhat and the wheel plane swings through an arc.  This means that as the wheel "strokes" you get camber changes.  This is NOT uncommon for spring legged gear like the Cessnas, Cirrus, and RV series.  So in order to find out what you have you need to jack the airplane off its wheels and see how much camber you start with and then where you end up with the airplane at max weight.  Those two dimensions are the controlling dimensions.  You will likely start with wheels having top out camber gong to top IN camber as the aircraft  LG strokes throughout its travel.   This also presumes that your bungee system is giving correct deflections to reach the MAX weight static point in the gear travel.   If the biggest issue is excess tire wear, then my suggestion is to live with it. 

Since you have a conversion, who knows how it was set up or why.  BUT, if it travels down the runway fine at high speed like for takeoff or landing, I would leave it alone. 



Julian Smith
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#3 Posted: 2/5/2010 08:51:39

Tail wheel airplanes should have NO toe-in at all, as far as camber on your plane that is how the gear is attached to the airframe.

If the gear has tow-in and the airplane starts to the right the weight is shifting to the left gear if it is towed-in it will tend to make the plane turn harder to the right NOT good on a tail wheel air plane. Since this is a converted plane I would check the gear toe-in through the full travel of the gear.

Julian Smith

EAA tec-counselor



Duane Reetz
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#4 Posted: 2/5/2010 11:35:35

Thank you for the replies.  On a converted Piper PA 22/20  the original front landing gear mounts are left in place and become the rear landing gear mounts.  Toe and camber are affected by the position of the new front mounts that are welded on.  More toe in = less camber and vise versa.  The conversion was done 18 years ago (alignment by me) and there is a little toe in and too much camber. 

Are there any suggestions or comments about heating and bending the landing gear tubes near the axles to correct this?

 



Dave Stadt
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#5 Posted: 2/5/2010 19:01:34
Julian Smith wrote:

 

Tail wheel airplanes should have NO toe-in at all, as far as camber on your plane that is how the gear is attached to the airframe.

If the gear has tow-in and the airplane starts to the right the weight is shifting to the left gear if it is towed-in it will tend to make the plane turn harder to the right NOT good on a tail wheel air plane. Since this is a converted plane I would check the gear toe-in through the full travel of the gear.

Julian Smith

EAA tec-counselor

 

Might be true for some airplanes but certainly not all.  Maintenance manual for my taildragger requires a minimum of 1/8 and a maximum 1/4 inch toe in on each wheel.  Neutral or even a small amount of toe out can cause very difficult handling.  Best to go by the book or by  what others with identical aircraft have found to work.  Generalities don't always work.    



Justin Daugherty
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#6 Posted: 2/6/2010 14:36:41
Julian Smith wrote:

 

Tail wheel airplanes should have NO toe-in at all, as far as camber on your plane that is how the gear is attached to the airframe.

If the gear has tow-in and the airplane starts to the right the weight is shifting to the left gear if it is towed-in it will tend to make the plane turn harder to the right NOT good on a tail wheel air plane. Since this is a converted plane I would check the gear toe-in through the full travel of the gear.

Julian Smith

EAA tec-counselor

 

Can you please expand on that some more? When I first read that, I was thinking it maybe had to do with when the tail wheeled aircraft lifts its tail off the ground, the rotation would increase the toe in? That is if you adjust the toe in with the aircraft static with all 3 wheels on the ground.


Why is this not an issue with a tricycle geared aircraft?




Joanne Palmer
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#7 Posted: 2/8/2010 14:54:11

Duane:

 

I took a look at the landing gear leg picture on the Univar website and I offer the following comments:

 

The geometry of this part is essentially a tripod truss.  The materials are most likely sized for these tubing pieces to be in a specific geometric relationship.  Heating and bending would change that relationship.  The part would then need to be re-analyzed and then "approved" by a DER.  I am presuming that since you have a certificated airplane that has been converted by an STC then the STC holder would have to re-approve the leg.  In other words this is a MAJOR change as defined in the FARs.  Hence the re-approval requirement.

 

I wouldn't do this UNLESS there is a serious controllability issue while doing landing rollout or takeoff.  It just ISN'T worth the trouble and the potential downside. 

 

I'm not a tech counselor, but I've been in aircraft engineering for 3 decades both as a designer and a stress engineer.  And I wouldn't do this without some serious controllability issue first.  



Jeff Snell
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#8 Posted: 2/8/2010 18:13:00

Hi Duane,a maintenance manual for a PA-20 will have specs for toe-in and camber.But,if it tracks straight down runway and is not prematurely wearing tires it is fine the way it is. Tire scuffing will indicate where  you would need to make adjustments.This information will be in a maintenance manual.Good luck