Stinger tail wheel spring

[Steve Fisher]

Has anyone come up with a technique to adjust the tail wheel pivot angle with a round "stinger" style tail wheel spring? The pivot center line on my plane is vertical - it should be angled slightly rearward (top of pivot center line towards rear). Without the rearward angle, the plane is subject to tail wheel shimmy.

Steve

[Bdflies]

My initial thoughts involve a two-pronged approach;
1st) get a set of 31" bushwheels on the mains. That'll reduce the caster.
2nd) stay off of asphalt and concrete. Never had shimmy on grass!

But, that might not be what you were looking for...., 😎

Bill

[jaredyates]

I've been dealing with shimmy problems and will say that while I've never felt the shimmy on grass, it certainly does shimmy on grass also. I didn't realize this until it was pointed out by an informed observer, and/or a gopro.

[Steve Fisher]

Bill,

That would help, but as you mentioned, not what I was looking for.

Steve

[JimParker256]

Steve, out of curiosity, what size wheels and tires are you using? Was your pivot angle setup correct before you started flying? And if so, about how many flights (or landings) did you make before you started to notice the problem? Inquiring minds...

[Steve Fisher]

Jim,

I have 8.50X6 tires and zero flights - still building. The angle of the spring mount on the fuselage and the angle of the Bearhawk tail wheel spring to pivot are fixed (welded).

​Steve

[JimParker256]

The reason I was asking is that I'm also still building, and using 8.50 x 6 tires, and my tail wheel pivot axis is also pretty vertical. But I have not yet adjusted the "spread" of the main gear at all - just threw them on to allow rolling the fuselage around the shop. I'm pretty sure the main wheels are even closer together than they should be, since the wheels angle outward at the top pretty significantly. If/when they are adjusted to the proper spread, it will likely lower the nose somewhat, which may well move the tailwheel pivot angle so it inclines forward at the top - exactly the opposite of what I understand is desirable...

Like you, I'm wondering what (if anything) needs to be done about it. Mark Goldberg has pretty much the same setup on his Patrol, and I didn't notice any shimmy on landing when I did the test flight at OSH. (And on a paved runway, at that.) Maybe the stinger-mounted tailwheel is less prone to shimmy than the leaf spring mounting on the 4-place?

[Glenn Patterson]

It should be possible to create an angled shim under the attachment to change the caster angle. Support the fuselage with the tail just touching and loosen the assembly and slide in a metal shim strips across the front until the caster gets to where it is best. Plus & Minus caster should be possible with a stinger through adjusting front or rear between the attachment & the fuselage. Tighten the attachment with the temporary shims and then remove the fuselage support & inspect the tailwheel under load to see if it is what you would like. Once the angle is determine with temporary shims then a full wedged shim could be installed.

If you do not want to do a metal shim then an angled shim of UHMW polyethylene or plastic wear plate could be done on a table saw or with a router. Once there is a length to height ratio then the angle for the shim is known and is the taper or angle to machine into the shim. UHMW is easy to come by & can be cut or machined with proper care.

We installed a piece of conveyor belt to cushion the fuselage attachment of the tail wheel as that is what is done on our flying clubs tail spring. The rear ski takes a real pounding a real pounding as the lake gets really rough in the spring from all the snow machine trails in front of the club. If we need to adjust the angle then it could be done where the rubber currently sits.

I have some interest in this as we mounted the flat spring tail wheel & did not like the set up. It was too big & heavy for starters so we changed to a stinger mount. We have not flown the set up yet but it looks good & is certainly a lot less weight & drag. Hopefully the stinger is durable enough. There has been enough headaches expressed with the flat spring.
Glenn
BH727

[Glenn Patterson]

The trial shims would have to go under the rear of the fuselage attachment to angle the stinger downward to move the tailwheel toward the positive caster in relation to the airplanes thrust line.
Glenn