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For the tailwheel guys, main gear toe-in or toe-out?

moespeeds

moespeeds

Well Known Member
Friend
I measured my gear, and I'm toe-out 1/4* each side. Grove gear and axles.

In racing cars, toe-out adds instability, and a tendency follow any turning inputs. It makes a car that will want to pitch into the corners, but can be darty on straights.

Toe-in will give you a car that rolls straight, and resists turning inputs.

I would describe my 8 as darty on rollout.

So I can adjust my toe in 1/4* increments using shims from grove. Should I go zero toe, or toe-in? What's everyone else doing?
 

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Positive camber requires toe-in.

There is a cool tool called Gunson Trakrite that is made in England but sold on ebay that will determine proper toe-in/out by rolling a tire over it.
 
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I got my alignment as close to zero as I could.

In addition to resolving your toe out problem, you might want to check your gear leg bolts are tight. At 50 hours my RV-8 started to jump to the right on landing. I figured this was just me. I checked the bolts again and found them loose. After torque the problem was fixed (other than my poor landing technique). It now lands straight - even after the typical series of bounces…..

Carl
 
Gear bolts mod was done when I got the plane a few months back. They were loose.

I do have positive camber but I haven't measured it yet.

So zero toe. Nobody toe's in?
 
Gear

This is a subject that predates the oldest RV by decades. The flat spring gear was designed and patented by Steve Wittman in 1936. The round gear in early 50's.
Camber with zero toe in three point attitude becomes toe out as the tail is raised. Very undesirable.
 
Put enough weight in the plane to simulate typical flying conditions.

Get some greased plates and put the tires on them so they can find their happy place.

Adjust the toe to zero with the tail down.

Ignore speculation about what happens when the tail raises... it won't matter enough to worry about.

YMMV!
 
Tailwheel aircraft.............

As has been said, "0" toe is best. Toe-out is better than toe-in. Think of it this way, if the airplane starts to veer to the left, more weight is transferred to the right wheel. If it is toed-in, that wheel will take you even more to the left. If it is toed-out, it will tend to pull the airplane back to the right. ref: "Landing Gear Design for Light Aircraft" by Ladislao Pazmany.

If you will notice Van recommends "0" toe with the airplane off the ground. With the Wittman style landing gear, by design, any weight on the gear causes a small amount of toe-out.

When I designed the conventional gear for the Moni Motor-glider back in the 1980s, I discovered quickly that any amount of toe-in makes the airplane quite squirrelly on the ground. Once I added a minute amount of toe-out, the airplane became a pussy- cat.
 
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Toe out or toe in?

Hi, Moe,

Mel beat me to it:), but here is an expanded explanation:

You are entirely correct that in racing cars, toe out is desirable to make the turn in more aggressive. The reason for this is that at turn initiation, there is little weight transfer until the entire chassis rotates to give the rear tires the angle of attack (known as "slip angle" in tires), they need to create a lateral force. Toe out makes the inside front grab more aggressively on turn entry, speeding up the rotation of the chassis. However, the opposite is true with a taildragger aircraft: Mainwheel toe OUT creates a stabilizing force.

First, let me explain my background to give a bit more credibility: I am a Mechanical Engineer, with 35 years' experience in engineering and driving of open wheel (Formula 2000 and others) cars, reasonably successfully. Chassis setup was my specialty. Now, why toe out?

As we all know, the tailwheel aircraft is inherently unstable on the ground because the CG is aft of the main wheels. If both wheels have toe in, both wheels will be creating a force towards the centerline of the aircraft when rolling straight ahead. When the aircraft diverges from a straight path (swerves), even slightly, weight is transferred from the inside wheel to the outside wheel, thus the outside wheel will have a greater lateral thrust capability and the inside wheel will have less lateral thrust capability - the net is an increase in force in the direction of the swerve, increasing the swerve rate, and requiring a very authoritative rudder response from the pilot.

Conversely, if the wheels have toe out, both wheels will be creating a force outward, away from the centerline of the aircraft when rolling straight ahead. When the aircraft diverges from a straight path (swerves), weight is transferred from the inside wheel to the outside wheel. Again, the inside wheel loses some lateral force, while the outside gains some lateral force. But here's the key: weight and lateral force capability is still transferred, but the toe out of the outside wheel helps to "steer" the airplane straight.

Note that the airplane is still unstable, but less so with toe out. In my own case, my RV8 had an initial toe IN of 1.25 degrees. I found the airplane to be controllable, but "nervous." Using the VAN's supplied shims, I set toe OUT 0.25 degrees. The "nervous" feeling was greatly reduced.

Now, look at the same analysis for a tricycle aircraft: Toe IN is stabilizing as the CG is ahead of the main landing gear. Think about it…..

As noted above, when transitioning from tail up to tail down on landing, the camber of the wheels will move the wheels towards toe in. For the typical10 degree nose up three-point attitude this will translate to approximately (10/90 x camber angle) or .11 degrees per degree of caster. This argues for even MORE toe out to retain relative stability in the 3-point attitude. At my next condition inspection, I'll increase the toe out to see if this can be documented.

As an aside, by changing the toe in I increased my tire life from 240 landings to 600 landings, and the wear is much more even (both sets of tires were Goodyear Flight Eagle III)
 
gear1 said:
Hi, Moe,

Mel beat me to it:), but here is an expanded explanation:

You are entirely correct that in racing cars, toe out is desirable to make the turn in more aggressive. The reason for this is that at turn initiation, there is little weight transfer until the entire chassis rotates to give the rear tires the angle of attack (known as "slip angle" in tires), they need to create a lateral force. Toe out makes the inside front grab more aggressively on turn entry, speeding up the rotation of the chassis. However, the opposite is true with a taildragger aircraft: Mainwheel toe OUT creates a stabilizing force.

First, let me explain my background to give a bit more credibility: I am a Mechanical Engineer, with 35 years' experience in engineering and driving of open wheel (Formula 2000 and others) cars, reasonably successfully. Chassis setup was my specialty. Now, why toe out?

As we all know, the tailwheel aircraft is inherently unstable on the ground because the CG is aft of the main wheels. If both wheels have toe in, both wheels will be creating a force towards the centerline of the aircraft when rolling straight ahead. When the aircraft diverges from a straight path (swerves), even slightly, weight is transferred from the inside wheel to the outside wheel, thus the outside wheel will have a greater lateral thrust capability and the inside wheel will have less lateral thrust capability - the net is an increase in force in the direction of the swerve, increasing the swerve rate, and requiring a very authoritative rudder response from the pilot.

Conversely, if the wheels have toe out, both wheels will be creating a force outward, away from the centerline of the aircraft when rolling straight ahead. When the aircraft diverges from a straight path (swerves), weight is transferred from the inside wheel to the outside wheel. Again, the inside wheel loses some lateral force, while the outside gains some lateral force. But here's the key: weight and lateral force capability is still transferred, but the toe out of the outside wheel helps to "steer" the airplane straight.

Note that the airplane is still unstable, but less so with toe out. In my own case, my RV8 had an initial toe IN of 1.25 degrees. I found the airplane to be controllable, but "nervous." Using the VAN's supplied shims, I set toe OUT 0.25 degrees. The "nervous" feeling was greatly reduced.

Now, look at the same analysis for a tricycle aircraft: Toe IN is stabilizing as the CG is ahead of the main landing gear. Think about it…..

As noted above, when transitioning from tail up to tail down on landing, the camber of the wheels will move the wheels towards toe in. For the typical10 degree nose up three-point attitude this will translate to approximately (10/90 x camber angle) or .11 degrees per degree of caster. This argues for even MORE toe out to retain relative stability in the 3-point attitude. At my next condition inspection, I'll increase the toe out to see if this can be documented.

As an aside, by changing the toe in I increased my tire life from 240 landings to 600 landings, and the wear is much more even (both sets of tires were Goodyear Flight Eagle III)
Click to expand...

What Mel and Goliath said!

When I first built my RV-8 I had a very tiny amount of toe-in , ~ 0.1 degrees on each side. I assumed such a tiny amount would be relatively insignificant. As I gained some time on the aircraft, I found it to be somewhat “nervous” on landing rollout. It wasn’t super scary, but it just didn’t feel stable. Another experienced tail dragger pilot also agreed that it was a bit nervous or twitchy on rollout. I decided to install some axle toe-shims purchased from vans to remove the toe-in. I ended up with a hair (~0.05 degrees) of toe-out. The difference in handling was night and day and the plane became very stable during landing rollout after the change.

A -6 owner at my home airport has a significant amount of toe-out on his gear, which of course he cannot adjust. He also reports very stable handling characteristics, although his tires tend to where on the inside edges.

Skylor
 
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gear1 said:
Toe out or toe in?

Hi, Moe,

Mel beat me to it:), but here is an expanded explanation:

You are entirely correct that in racing cars, toe out is desirable to make the turn in more aggressive. The reason for this is that at turn initiation, there is little weight transfer until the entire chassis rotates to give the rear tires the angle of attack (known as "slip angle" in tires), they need to create a lateral force. Toe out makes the inside front grab more aggressively on turn entry, speeding up the rotation of the chassis. However, the opposite is true with a taildragger aircraft: Mainwheel toe OUT creates a stabilizing force.

First, let me explain my background to give a bit more credibility: I am a Mechanical Engineer, with 35 years' experience in engineering and driving of open wheel (Formula 2000 and others) cars, reasonably successfully. Chassis setup was my specialty. Now, why toe out?

As we all know, the tailwheel aircraft is inherently unstable on the ground because the CG is aft of the main wheels. If both wheels have toe in, both wheels will be creating a force towards the centerline of the aircraft when rolling straight ahead. When the aircraft diverges from a straight path (swerves), even slightly, weight is transferred from the inside wheel to the outside wheel, thus the outside wheel will have a greater lateral thrust capability and the inside wheel will have less lateral thrust capability - the net is an increase in force in the direction of the swerve, increasing the swerve rate, and requiring a very authoritative rudder response from the pilot.

Conversely, if the wheels have toe out, both wheels will be creating a force outward, away from the centerline of the aircraft when rolling straight ahead. When the aircraft diverges from a straight path (swerves), weight is transferred from the inside wheel to the outside wheel. Again, the inside wheel loses some lateral force, while the outside gains some lateral force. But here's the key: weight and lateral force capability is still transferred, but the toe out of the outside wheel helps to "steer" the airplane straight.

Note that the airplane is still unstable, but less so with toe out. In my own case, my RV8 had an initial toe IN of 1.25 degrees. I found the airplane to be controllable, but "nervous." Using the VAN's supplied shims, I set toe OUT 0.25 degrees. The "nervous" feeling was greatly reduced.

Now, look at the same analysis for a tricycle aircraft: Toe IN is stabilizing as the CG is ahead of the main landing gear. Think about it…..

As noted above, when transitioning from tail up to tail down on landing, the camber of the wheels will move the wheels towards toe in. For the typical10 degree nose up three-point attitude this will translate to approximately (10/90 x camber angle) or .11 degrees per degree of caster. This argues for even MORE toe out to retain relative stability in the 3-point attitude. At my next condition inspection, I'll increase the toe out to see if this can be documented.

As an aside, by changing the toe in I increased my tire life from 240 landings to 600 landings, and the wear is much more even (both sets of tires were Goodyear Flight Eagle III)
Click to expand...
Great info! Question however. What happens to toe when the gear flexes outward when weight is applied on landing?? Should this be considered when making adjustments with shims?
 
arplnplt said:
Great info! Question however. What happens to toe when the gear flexes outward when weight is applied on landing?? Should this be considered when making adjustments with shims?
Click to expand...
In car racing we call this "bump steer" and yes it should be considered. I don't think I have seen this subject discussed, even by DanH!😉
 
Per the designer of the round and flat spring gear. The round gear on a taildragger should be toed in 1/16" in the length of the axle. As the airplane is loaded the toe in will be reduced. Toe out is BAD NEWS on the round gear. i have no specific information on the flat gear but zero works on the Pitts and Cassutt and probably many others.
 
jrs14855 said:
Per the designer of the round and flat spring gear. The round gear on a taildragger should be toed in 1/16" in the length of the axle. As the airplane is loaded the toe in will be reduced. Toe out is BAD NEWS on the round gear. i have no specific information on the flat gear but zero works on the Pitts and Cassutt and probably many others.
Click to expand...
See my post #8 above. Slight toe-out is better and is specified in the plans. Plans call for 0 toe with the weight off the gear. The RV gear toes out when weight is applied, by design.

ref: "Landing Gear Design for Light Aircraft" by Ladislao Pazmany
 
Just went though this with my -6, which I built with (it turns out) 2.76 deg of toe out (weight off wheels, level attitude). This is an early kit without pre-drilled holes. Always had serious inside tire wear, but ground handling was fine. Also measured toe with weight on wheels in level attitude (4.6 deg toe out) and tail low (2.41 toe out). So, the gear definitely goes more toe out with weight on wheels in level attitude, like you'd get with a wheel landing.

By oblonging the gear rod retention bolt holes in the engine mount by ~.020", I was able to get to zero toe (level attitude, weight off wheels), which is Van's factory spec. Landing characteristics are unchanged, but it definitely rolls better now.
 
Ladislao Pazmany has been referenced a few times above. Here is the conversation that I had with Mr. Pazmany about 20 years ago:

https://vincesrocket.com/landing gear and related stuff.htm

Gear alignment: Toe-in versus toe-out

I have done more than just a little research and been in more than just a few "discussions" with other builders over whether toe-in or toe-out is the proper set up for our planes. I finally sent an email to Ladislao Pazmany, designer of many successful lightplanes and the author of Landing Gear Design For Light Aircraft. If anyone can answer this question it would be Paz. Here's the unedited correspondence:

Dear Mr. Pazmany:
For a conventional gear lightplane with tapered rod spring main gear, should the main wheels have any toe-in or toe-out?
Vince

Dear Vince:

Unfortunately your question doesn't have a straight answer. There is a big controversy on the subject. Please review the attached document in order to understand it better. (the attached document was scanned from my book Landing Gear Design For Light Aircraft )

Regards Paz

toe-in.jpg
https://vincesrocket.com/toe-in.jpg
https://vincesrocket.com/toe-in.jpg

 
A few more comments... worth what your paying for them. ;)

  • I haven't heard anyone talk about brake application. Putting the brakes on typically will cause the system to toe-out a bit.
  • Adding weight to the aircraft may, or may not, cause the wheels to toe-out. For tapered rod gear (RV-4, 6, 7, or the Rockets) more weight tends to toe out the wheels. For others.... maybe, maybe not. So, understand what gear you're working with before making that assumption.
  • Aligning the wheels with the aircraft off of the ground is a no-no. At least for the Rockets it is. Although it is appropriate to align the gear fairings and wheel pants that way.
  • My earlier post, #7, is the best technique for aligning your Rocket gear, and I'll wager that it is appropriate for most other conventional gear aircraft. (Yes, I expect exceptions to this statement.)​
For those who can't get enough of my sheer genius (pun intended), here's some light reading from 20+ years ago when I adjusted the camber and toe on my completed Rocket. It's a task that isn't for the faint of heart.
https://vincesrocket.com/Additions after 10-27-04.htm Scroll down to the section titled: Main Gear Leg Shortening and Alignment Changes

Enjoy! YMMV. Think twice, cut carefully, and always check behind what you're drilling into!
 
Mel said:
See my post #8 above. Slight toe-out is better and is specified in the plans. Plans call for 0 toe with the weight off the gear. The RV gear toes out when weight is applied, by design.

ref: "Landing Gear Design for Light Aircraft" by Ladislao Pazmany
Click to expand...
All the plans for older models specified a small amount of toe in with the wheels unloaded, to compensate for the increase in to out that occurs when the legs flex under load.
 
rvbuilder2002 said:
All the plans for older models specified a small amount of toe in with the wheels unloaded, to compensate for the increase in to out that occurs when the legs flex under load.
Click to expand...
Maybe they have been revised, but my -6 plans (circa 1989) specify "0" toe with the gear unloaded.
 
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The late Mr. Pazmany is absolutely correct and is in agreement with Steve Wittman who designed and patented the round and flat spring gears. When landing on one wheel in a crosswind the upwind wheel with toe out is working against the rudder input and in favor of a ground loop.
 
Another datapoint. My Rv8 was toe-in 0.13 deg weight off the wheels (as per Vans) measured wheel to wheel. It landed on the mains straight and tracked straight until the tail started coming down around then got wiggly. I put a grove shim marked 1/4 deg ( it actually measured closer to 0.2 deg) which resulted in a toe-out of 0.08 deg. I estimate the uncertainty in these measurements is +/- 0.04 degrees BTW.

I them lowered the wheels down onto a soap lubricated concrete floor in 3-point and pushed the plane back and forward. I remeasured the toe and found it to be toe-in 0.08 deg. Both wheels had a positive camber of 1.0 degree.

Landing before after in similar conditions qualitatively felt better. Semi quantitatively, I noted the lateral acceleration in the 1 second data from my EFIS on roll out. Toe-out reduced the lateral acceleration by roughly 50 percent.
 
When I built my RV-8 15 years ago, I set it as close as I could to zero toe angle in 3 pt. It was docile. About two years ago, I did a somewhat unintentional experiment where I ended up with 1/4 degree of toe-out. The airplane was treacherous. I kept wondering if it was me, just rusty because I hadn't flown in several months. Even after a lot of practice, it was much more squirrelly than it used to be. I had a partial ground-loop with a back-seat passenger (which makes it even more squirrelly) in calm conditions. What is wrong with me! Then I checked the toe and remedied the toe-out. Now back to its docile self.

Most simple hand-wave stability explanations get it wrong, because they ignore the weight transfer to the outside wheel in the initiation of a turn due to yaw. A slight amount of toe-in is more stable than a slight amount of toe-out, regardless of all the hand-waving arguments that say otherwise.
 
Maybe my impression is placebo effect. It’s tedious to make that change and do those measurements accurately.

More likely it’s the effect of wheel camber on the toe-in/out.
my plane has positive camber In three point The top of the wheels are out further than the bottom by 1 degree. As my tail goes down the wheels toe inward. I have no idea what it’s supposed to be but a heavier plane on the same landing gear legs will have less camber. If the camber is negative the wheels will toe out as the tail goes down.
 
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