The International Cessna^® 170 Association

Congratulations Joshua. Welcome to the ownership ranks!

Wow, my old head is swimming over all this talk about a seemingly simple tail wheel spring...more food for thought, had a friend years ago had a 55 B model that had two main springs installed, one on top of the other....Sure made for a stiff tail wheel spring. Might have even eliminated the breakage problem. Believe the plane is in Alaska now.

Ed, you opened a can of worms.

When I bought my first 170 it to had two main springs. I could not understand what folks where talking about breaking the main spring and loosing their tail wheel. When I mentioned two main spring installation on the Yahoo forum way back 15 years ago, someone who will remain nameless, relentlessly debate with me about how dangerous it was to run such an arrangement. He did not persuade me to change my setup. Instead I vowed to run that setup until I saw some reason I should not. That reason never materialized in over 10 years of operation. When the aircraft left my possession, uneconomically repairable due to wind storm damage, it still had two main springs.

However successful my installation seemed to be I do not recommend running two main springs. And my current 170 only sports a single main spring. The reason is that while I saw no damage or malfunction to anything regarding the installation, I could not be sure that, over time, I wasn't putting more stress on very expensive and becoming more rare spring bracket parts.

It is just to easy to leave the installation as it came from the factory and replace the single main spring every so many operations or hours to help insure it doesn't fail. And by doing so avoid any question of legality of the installation.

In the twin main spring configuration; are both bolted to the tailwheel or just one?
If Both were bolted down, it would make for an incredibly strong (and stiff) spring (if it sprung at all).

bagarre wrote:In the twin main spring configuration; are both bolted to the tailwheel or just one?
If Both were bolted down, it would make for an incredibly strong (and stiff) spring (if it sprung at all).

Both of them where bolted to the tail wheel assembly. It was like that when he bought it and flew it 20+ years before he sold it. No problems noted.

edbooth wrote:

bagarre wrote:In the twin main spring configuration; are both bolted to the tailwheel or just one?
If Both were bolted down, it would make for an incredibly strong (and stiff) spring (if it sprung at all).

Both of them where bolted to the tail wheel assembly. It was like that when he bought it and flew it 20+ years before he sold it. No problems noted.

I'd call that a long enough test period.

bagarre wrote:

edbooth wrote:

bagarre wrote:In the twin main spring configuration; are both bolted to the tailwheel or just one?
If Both were bolted down, it would make for an incredibly strong (and stiff) spring (if it sprung at all).

Both of them where bolted to the tail wheel assembly. It was like that when he bought it and flew it 20+ years before he sold it. No problems noted.

I'd call that a long enough test period.

Assuming it was flown in the last 20 years.
I've always felt that spring gear is designed to absorb shock. What kind of force is being transferred to the tail section of the fuselage when installing these thick springs? My preference would be to use the stock spring and replace it more frequently.

Aryana wrote:Can you imagine the shear tension on that bolt from one leaf trying to push as the other pulls when the spring is deflected?

Enormous but it's a pretty hefty bolt.
I wouldn't advocate this set up as I think it'd be TOO stiff for any practical application. It's a testament to the strength of the tailcone if there were no cracks after 20 years of use.

Aryana wrote:Can you imagine the shear tension on that bolt from one leaf trying to push as the other pulls when the spring is deflected?

That was the caution which the "unnamed" person opined in disagreement with Bruce about the set-up. The stiffness of that setup may be what delayed any failure by the shearing-action since it likely prevented any flexing of the springs. (It's all conjecture, of course.) From a purist' standpoint, as well as from the FAA view, it's not a good idea.

gfeher wrote:Congratulations Joshua. Welcome to the ownership ranks!

Thanks Gene! I'm pretty stoked. I've been looking forward to this since I was 14 years old.

gahorn wrote:

Aryana wrote:Can you imagine the shear tension on that bolt from one leaf trying to push as the other pulls when the spring is deflected?

That was the caution which the "unnamed" person opined in disagreement with Bruce about the set-up. The stiffness of that setup may be what delayed any failure by the shearing-action since it likely prevented any flexing of the springs. (It's all conjecture, of course.) From a purist' standpoint, as well as from the FAA view, it's not a good idea.

Some Piper aircraft have a similar set up, both ends of two springs pinned by bolts through them, tying them together. So the set up is not unheard of. IN fact is has some brilliance in its design as it has built in redundancy as the springs go, and a single spring failure is more likely a non-event.

In my experience there was no evidence of shearing action on the bolt so that fear I feel is unfounded. All other thoughts on stiffness of such a spring package are still a concern though in the time I operated this setup I did not see a marked increase in wear or damage of other components over the rest of the fleet. I did not baby my installation as 95% of my landings are 3 point, and 99% of my landings I try to do in as short a distance as I can which results in many landings with the tailwheel hitting first.

I still don't recommend the setup and never really did while I was using it.

Now in an Ideal theoretical world, one better solution to the single main spring failure might be to use two main springs with half the spring tension each so that tied together they have the equivalent tension as our current main spring thus eliminating any issue with stiffness over the original design.

If you're REALLY worried about it, get field approval for this bad boy:
http://www.airframesalaska.com/T3-Super ... -51343.htm

bagarre wrote:If you're REALLY worried about it, get field approval for this bad boy:
http://www.airframesalaska.com/T3-Super ... -51343.htm

ABI-51343-2T.jpg

Or do that!

That does not appear to offer any redundancy to a single bolt failure.... it appears to add additional parts to contribute to rudder damage when that single bolt fails.

The idea of splitting the spring-tension between two leaf springs also strikes the "unnamed" person as a bad idea. Weaker springs in the same setup would only remove the stiffness of each individual spring, thereby increasing the "sliding" effect of the flexing springset. It would also overload the remaining spring when one spring breaks... while still adding unnecessary weight to the tail.

This tailwhell setup is, simply put, not a "fail-safe" design. It is a life-limited part which has never been properly so-designated by the mfr'r. IM[unnamed]O.

It would seem that if the unnamed person and I were in agreement on a main spring setup well, something in the world order would be amiss.

No one was addressing the single bolt failure. It still exists. The bolt fails at the shaft of the bolt at the thread.

The two weaker spring design I have in mind would not necessarily have springs designed nor expected to operate individually in a normal manner we are accustom. They would be designed to operate as a pair at the same tension as the main and second spring are today. The difference being that failure of one spring would not automatically lead to a complete failure of the second thus the wheel assembly would not be free to flail about beating the tail feathers into submission as the current design single main spring failure does. This design would not make the tailwheel assembly a redundant design. As noted, the single bolt or the tailwheel king pin does, and still could, fail. But I believe the bolt and king pin failures to be at a much lower rate than the main spring failures and at least the failure of the king pin does not lead to wheel assembly damage on the tail feathers. By making the springs redundant, at least to the point they don't release the tail wheel, would be cutting the occurrence of other damage in a failure significantly. But this is all theory, just fun to think about.