My own tailwheel design.

[muleyman78]

Here's my design.

[Chris In Milwaukee]

Ooo, that looks like fun. Let us know how well it works out!

[Battson]

Very popular idea these days, glad to see one of the Bearhawk crowd is trialling it.

How did you decide on the tube sizes to use?
I can visualise the hinge points taking a lot of punishment in an off-airport situation.

[muleyman78]

I figured a landing force over 4g would do far more damage than just the tail wheel. The average resting tail weight is 120 lbs from what others have told me. So that would be a down force of 480 lbs on a 4g landing. I then doubled that for good measure and the geometry said that a 4 ft piece of 1.25 chromoly .125 wall will bend at the center point of 2 feet when 1245 lbs are applied to the end. I know that's much stronger than the rest of the frame. I bought a shock that is progressive from 150 lbs to 600 lbs and a crush of 1300 lbs. I built a front mounting plate of 1/8 plate steel that is welded to the frame in 3 sides and I also added a small 4inch x 4ich plate for the top shock mount as well as two more pieces of tube to reinforce the back end. I'll be running the Tundra tailwheel that is lockable and unlocked from the cockpit. I weighed everything i added and it weighs just over one pound more than the stock.

[svyolo]

I love the concept. Spreading the tailwheel load farther forward should increase strength. But I think you this should allow you to reduce weight instead of vice versa. Worst case you should be able to increase the stroke, make the tailwheel stronger, and keep the weight the same.

[marcusofcotton]

I figured a landing force over 4g would do far more damage than just the tail wheel. The average resting tail weight is 120 lbs from what others have told me. So that would be a down force of 480 lbs on a 4g landing. I then doubled that for good measure and the geometry said that a 4 ft piece of 1.25 chromoly .125 wall will bend at the center point of 2 feet when 1245 lbs are applied to the end. I know that's much stronger than the rest of the frame. I bought a shock that is progressive from 150 lbs to 600 lbs and a crush of 1300 lbs. I built a front mounting plate of 1/8 plate steel that is welded to the frame in 3 sides and I also added a small 4inch x 4ich plate for the top shock mount as well as two more pieces of tube to reinforce the back end. I'll be running the Tundra tailwheel that is lockable and unlocked from the cockpit. I weighed everything i added and it weighs just over one pound more than the stock.

Not speaking from experience, and haven't even done any rough math, but I'm guessing the tail weight at full gross/aft cg will be much more than 120 lbs. I very much like the concept though. I've looked at numerous variations along those lines.

[whee]

Pretty cool Muley.

Marcus has a point. On some BHs 120lbs is close to the weight on the tail when empty. I can’t remember the exact number but when loaded up the TW weight is in the 400lb neighborhood.

[Battson]

Pretty cool Muley.

Marcus has a point. On some BHs 120lbs is close to the weight on the tail when empty. I can’t remember the exact number but when loaded up the TW weight is in the 400lb neighborhood.

That's my thinking too. Mine isn't a heavy machine, but has 120lbs empty. A rough calculation with some bad assumptions suggests 299lbs loaded to gross at aft-most CG location.

One might be tempted to calculate ~300lbs x 4g = 1,200lbs with a factor of safety of 2x, your ultimate load for the tailwheel system would be 2,400lbs.

However, the fatigue life will be what kills a well designed tailwheel every time. This is especially true in an off-airport environment. The repeated stress cycles cause microscope cracks in the steel to grow, until they reach a critical size and the steel rod breaks in two. The stress required to grow a crack is only a fraction of the stress required to exceed the ultimate strength of the steel.

Fatigue usually starts at the surface so that the quality of the surface finish is very important. The surface finish becomes even more important as the strength of the material increases, like in 4130N. From landing on gravel riverbeds, my tailwheel is peppered with dents and chips - compared to the minimum flaw size to start fatigue cracking, these dents are colossal, cracking is just a matter of time.

Long story short, inspecting for cracks is very important in any tailwheel spring.

[muleyman78]

Good info as always Battson! I guess only time will tell but be sure I will make that inspection at every preflight!

[S Lathrop]

I don't have that much experience with airplane structures but I do know a bit about race cars.

I like what you have come up with. If there is going to be a problem, my bet is it will be where the clevis for the shock ends on the tube going to the tail wheel. I would suggest that you think about extending the clevis along that tube and along the tubes going to the fuselage. Just a thought. Something that reduces the stress concentration at the ends of the clevis.

[svyolo]

The more Iook at it the more I like it. I think I would calculate some loads on the upper longeron shock attachment. I believe that would have me weld 2 vertical ( or 4 angled 30-45 degrees) members tying the upper longeron to the lower, spreading the load between the two. But I do like the simplicity.

[muleyman78]

The more Iook at it the more I like it. I think I would calculate some loads on the upper longeron shock attachment. I believe that would have me weld 2 vertical ( or 4 angled 30-45 degrees) members tying the upper longeron to the lower, spreading the load between the two. But I do like the simplicity.

I actually did add vertical tubes to disperse the load.

Lathrop, I also come from racing so thats where I pulled this plan from. Not sure if you can see it in the pics but I added a piece of plate that is welded in 3 sides and the clevises are mounted to those. I could add much much more support but then the weight goes up. I also added a bump stop shock tonight in addition to the one I have in the picture. Many of you made a good point about loaded weight on the tail. I had a small bump stop shock thats adjustable and will tripple my crush numbers for an added 6 oz.

[alaskabearhawk]

Just a thought....I'm in Alaska and if I break down in the field the ability to use readily-available and common aircraft parts is a must. The humble tailwheel spring is in use with thousands of conventional gear aircraft and the potential to find a temporary replacement in the bush is very good. This setup look great, but from my perspective and in my narrow mission environment it is unusable.

[EBPaul]

Some more pics would be great
Paul Roy

[svyolo]

It is functioning more like a motorcycle rear swing arm for suspension. Still using springs with the NWS?