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

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)
 
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)

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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