First taxi, can't straighten after starting a turn.

[haribole]

​

This is true for any tailwheel with the king bolt at the correct angle.

As you add weight into the plane, the king bolt angle will start approaching vertical, and reducing the forces required to re-centre. It's a self-correcting system.

Are you guys using strong enough tailwheel steering springs??
You need a spring with a barrel about 1" diameter, and the spring steel needs to be at least 3mm in diameter. 2.6mm spring steel will not be strong enough to command the tailwheel without using brakes / power to help.

Bearhawk tailwheels can shimmy on hard surfaces, if you put a lot of weight on the tail while moving too fast. On grass, shimmy is never a problem in my experience.

I have a T3 suspension with Jim Pekola's Tundra lite lockable tailwheel, which is full castering with no steering springs.

Making progress on fixing this issue. Put my tail wheel on a dolly to eliminate the tail wheel / T3.

Screenshot_20200610-094339.jpg

​
With this, I was able to steer easily to both sides using the brakes.

Next, I changed the tail wheel to the original leaf spring and Bob wheel. That helped and I am now able to steer to the right using the rudder, and straighten after a sharp left turn, though with a lot of throttle.

Like before left steering is a non issue, but right turning takes too much effort. Talked to Jared and he confirmed that there should not be too much difference between the two sides. Being a zero time pilot I have no prior experience to compare with, so please bear with me.

Started looking into the landing gear / main wheel alignment. It seems the problem lies here. Here is what I found:

Wheel to wheel distance is 72.5" on empty weight.

Left Toe-in: 1°47'
Right Toe-in: 2°30'

Here is how calculated this:

Screenshot_20200628-203920.jpg

Not sure why my two wheels are not equidistant from the center. To make sure my fuselage was aligned correctly with the engine crankshaft center I measured the rear landing gear attach point and that seems to well aligned. Could this be due to unequal shock strut lengths? Will measure this tomorrow.

Screenshot_20200628-204727.jpg

Found this explanation of the effect of toe-in and toe-out which seems to make sense with what I am experiencing:

When the wheel on one side of the car encounters a disturbance, that wheel is pulled rearward about its steering axis. This action also pulls the other wheel in the same steering direction. If it's a minor disturbance, the disturbed wheel will steer only a small amount, perhaps so that it's rolling straight ahead instead of toed-in slightly. But note that with this slight steering input, the rolling paths of the wheels still don't describe a turn. The wheels have absorbed the irregularity without significantly changing the direction of the vehicle. In this way, toe-in enhances straight-line stability.

If the car is set up with toe-out, however, the front wheels are aligned so that slight disturbances cause the wheel pair to assume rolling directions that do describe a turn. Any minute steering angle beyond the perfectly centered position will cause the inner wheel to steer in a tighter turn radius than the outer wheel. Thus, the car will always be trying to enter a turn, rather than maintaining a straight line of travel. So it's clear that toe-out encourages the initiation of a turn, while toe-in discourages it.

​​​​​​

[yateselden]

I have the compression type of spring. Wire is heavy.

[yateselden]

Thanks for the tip, I think my springs are smaller. My hanger mate and I were both surprised at how large the springs were for his C150

[Battson]

... when I turn the tailwheel 90 deg. It lowers the tail (demonstrated, turning by hand) and is difficult to get it back centered. (lots of power) Long fork leverage as Whee said. ...

This is true for any tailwheel with the king bolt at the correct angle.

As you add weight into the plane, the king bolt angle will start approaching vertical, and reducing the forces required to re-centre. It's a self-correcting system.

Are you guys using strong enough tailwheel steering springs??
You need a spring with a barrel about 1" diameter, and the spring steel needs to be at least 3mm in diameter. 2.6mm spring steel will not be strong enough to command the tailwheel without using brakes / power to help.

Bearhawk tailwheels can shimmy on hard surfaces, if you put a lot of weight on the tail while moving too fast. On grass, shimmy is never a problem in my experience.

[yateselden]

You have described it to a T. When the tailwheel is 180 deg. It actually works better when returning the plane to the hangar. I assume I will have shimmy, unless I change the king pin angle. All new to me

[svyolo]

To me that sounds like the tailwheel is statically unstable. As soon as it goes off center, it wants to go further off center due to gravity. And it takes a lot of force, fighting gravity, to get it back to center

[yateselden]

I have the same situation as Hari. It's a T3 w/ Eric tundra setup. Bill is exactly right, when I turn the tailwheel 90 deg. It lowers the tail (demonstrated, turning by hand) and is difficult to get it back centered. (lots of power) Long fork leverage as Whee said. Not sure there's an easy fix, but wouldn't forward stick and getting the weight off the tailwheel help this situation? Landing would be different that's my concern

[Battson]

With respect to the T-3 tailwheel installation. There is a
situation that is problematic and prone to early failure.
One of the Bearhawk’s on the North Dakota Passport
challenge is equipped with the T-3 Tailwheel. We like the tailwheel. It works well. The point of failure is the
attachment of the Tailwheel to the aft saddle support below the tail post. The aft support cracked from the attach holes outboard and in the corner where the gussets are bent forming the attach plane where the
eyebolts connect the T-3 to the bracket. The concentrated point load applied through the eyebolt causes the plate to flex and crack out.

Situation fixed by Welding a solid block going crosswise
to the saddle plate below the tail post. This block has a hole running through crosswise. Block is sized to fit just inside the T-3 mounting plates and a single bolt goes horizontally securing the tailwheel assy to the spring saddle mounting plate.

Once you see the failure there is an AH Hah moment.
Heavy tail wheel loads, when turning, huge torsion moment being reacted through thin un supported plate...........

Stock tailwheel spring, the bolt hole ears only hold the clamp and NOT point load weight of the tail.

Kevin D
Bearhawk #272

This is good information (above).

Having broken the tailwheel off a couple of times now, including breaking the spring, those bolts deal with a lot of stress. This shows that the T3 loads them up somewhat differently to the leaf spring. The leaf spring allows twisting and sliding which reduces the stresses on the airframe.

I don't understand the fix but it sounds concerning. If there's enough stress to break those attachment plates, strengthening the plate is only going to move the stress further into the airframe.

[Battson]

Baby out with the bathwater...

[AZBearhawk272]

True,

Been there, done that on a standard Bearhawk with leaf spring. The T-3 setup applies the loading in a non optimal manner. Devils in the details, got to keep an eye
on the hardware. The bigger the tail wheel the more vigilant one needs to be about ground handling.

Kevin D

[whee]

With respect to the T-3 tailwheel installation. There is a
situation that is problematic and prone to early failure.
One of the Bearhawk’s on the North Dakota Passport
challenge is equipped with the T-3 Tailwheel. We like the tailwheel. It works well. The point of failure is the
attachment of the Tailwheel to the aft saddle support below the tail post. The aft support cracked from the attach holes outboard and in the corner where the gussets are bent forming the attach plane where the
eyebolts connect the T-3 to the bracket. The concentrated point load applied through the eyebolt causes the plate to flex and crack out.

Situation fixed by Welding a solid block going crosswise
to the saddle plate below the tail post. This block has a hole running through crosswise. Block is sized to fit just inside the T-3 mounting plates and a single bolt goes horizontally securing the tailwheel assy to the spring saddle mounting plate.

Once you see the failure there is an AH Hah moment.
Heavy tail wheel loads, when turning, huge torsion moment being reacted through thin un supported plate...........

Stock tailwheel spring, the bolt hole ears only hold the clamp and NOT point load weight of the tail.

Kevin D
Bearhawk #272

Big tail wheels significantly increase the torsional force created by the tw when heavy and turning. Now that the tailwheel is solidly fixed to the saddle that torsional force is now a bending moment acting on the tail post. Watch it for cracking at the lower rudder hinge.

[AZBearhawk272]

With respect to the T-3 tailwheel installation. There is a
situation that is problematic and prone to early failure.
One of the Bearhawk’s on the North Dakota Passport
challenge is equipped with the T-3 Tailwheel. We like the tailwheel. It works well. The point of failure is the
attachment of the Tailwheel to the aft saddle support below the tail post. The aft support cracked from the attach holes outboard and in the corner where the gussets are bent forming the attach plane where the
eyebolts connect the T-3 to the bracket. The concentrated point load applied through the eyebolt causes the plate to flex and crack out.

Situation fixed by Welding a solid block going crosswise
to the saddle plate below the tail post. This block has a hole running through crosswise. Block is sized to fit just inside the T-3 mounting plates and a single bolt goes horizontally securing the tailwheel assy to the spring saddle mounting plate.

Once you see the failure there is an AH Hah moment.
Heavy tail wheel loads, when turning, huge torsion moment being reacted through thin un supported plate...........

Stock tailwheel spring, the bolt hole ears only hold the clamp and NOT point load weight of the tail.

Kevin D
Bearhawk #272

[zkelley2]

This is one of the many reasons I dont run a parking brake on tailwheels.

[John Bickham]

Never thought about it till this post, the brake break in would be a challenge for me with a free castering tailwheel. I'd need a wide runway. I know the brake won't behave or have a lot of stopping power till broken in correctly. May be a little part of the issue?

[svyolo]

Can you take another picture of it from the side, tailwheel centered, level with the tailwheel?