Shot Peen Tail Spring

How to keep the Cessna 170 flying and airworthy.

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MoonlightVFR
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Shot Peen Tail Spring

Post by MoonlightVFR »

Why not? Main landing gear is shot peened. Big benefits for tail spring? More than a L19

1840s carriage makers used ball peen hammer forging springs. When inside curve peened . Less broken springs.

Shot peening is big in aircraft design. Jet engine manufacturers secretive about proprietary processes.

C170 Main landing gear is shot peened underside and edge per GA Horn.

Shot peening creates a subsurface comprehensive stress zone - theory prevents cracks. Strengthens the metal ensuring that the part will resist failure. Increased strength allows design of lighter weight part s that exhibit high wear and fatigue resistance. 600% increased fatigue strength.
Parts with 10,20 or 30 times service life.
Curtis Wright Surface Technologies even talks about protecting inter granular corrosion with shot peen.
gradyb, '54 B N2890C
bagarre
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Re: Shot Peen Tail Spring

Post by bagarre »

Aryana wrote: And now the mundane part...how would you log that kind of work and under what authority?
Step 1: Buy new tailwheel spring
Step 2: Send off to shot peen shop
Step 3: Paint spring
Step 4: Install spring on aircraft
Step 5: Log entry "Replaced Tailwheel spring"

Unless you stripped the paint and looked at the underside with a magnifying glass, you'd never know if it was peened or not.
bagarre
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Re: Shot Peen Tail Spring

Post by bagarre »

Aryana wrote:
bagarre wrote:
Aryana wrote: And now the mundane part...how would you log that kind of work and under what authority?
Step 1: Buy new tailwheel spring
Step 2: Send off to shot peen shop
Step 3: Paint spring
Step 4: Install spring on aircraft
Step 5: Log entry "Replaced Tailwheel spring"

Unless you stripped the paint and looked at the underside with a magnifying glass, you'd never know if it was peened or not.
My Aeronca Sedan had a previous owner with the same philosophy of "if you can't find or see it, then it never happened".

It took thousands of dollars to correct all of his "improvements" that we weren't able to discover until we tore the plane completely apart. Not cool.
There is a limit to everything but I wouldn't see shot peening a piece of spring steel as a big deal.
bagarre
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Re: Shot Peen Tail Spring

Post by bagarre »

Aryana, I do agree with you. I was just playing devil's advocate a little.
Even with 95D only having 1,400 TT, I've had to undo some very questionable work that even had log book entries and 337's to go with them.

Re- tail spring failure: Do they fail because the top is worn down causing a stress fracture or so they fail because the top spring place the entire bending load at a single point under the last spring?

The former is how we think about it but I don't know if a stress riser on the compression side of the spring (the top) would do that.
The bottom of the main spring is in serious tension and it's all focused at the point we see them break. In that case, shot peening could help extend the life of the spring.

It would take a lot of analysis and testing to determine which is the cause and how much peening would extend the life.
As you pointed out, there isnt much gain when the cost is $100 for a spring spread over a 5 year or 500 hour period. That's $0.20 an hour.
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gfeher
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Re: Shot Peen Tail Spring

Post by gfeher »

Aryana wrote: If just one new 170 owner in the future reads this thread and changes out their main spring to avoid disaster, I'm happy.
Well, I know I'm a beneficiary of advice like this. Because of it, one of the first things I did after buying my plane a few years ago was to change the main leaf spring. According to the log book, it had been on for well over 1200 hours. I figure that the savings from the bullet I dodged on that alone paid for my membership for the rest of my life.
Gene Feher
Argyle (1C3), NY
'52 170B N2315D s/n 20467 C-145-2
Experimental J3 Cub Copy N7GW O-200
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c170b53
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Re: Shot Peen Tail Spring

Post by c170b53 »

Ah...Gene for President :D
Nice to see a rational debate on this one. I'll just offer, the wear which occurs, the one we all see often results in deformation of metal to a depth greater than the subsurface protection provided by shot peening. Just my call.
Jim McIntosh..
1953 C170B S/N 25656
02 K1200RS
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GAHorn
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Re: Shot Peen Tail Spring

Post by GAHorn »

I posted a pic or two over the years where a tailwheel main spring broke...exactly where the trailing edge of the spring above had gouged a stress-riser. ( I'll try to relocate those pics. but that was several hard-drives ago.)

The thing I'll add to this discussion is that alteration of a landing gear falls under the definition of "Major Alteration". One of the few things that FAA Legal gets excited about is UN-recorded maintenance. Shot peening the underside of that spring will not likely extend it's life one-bit, and therefore there's little reason I can imagine to assume the risks of unapproved modifications in that regard.

<edit> Here's a pic of what a stress-riser looks like. The No. 3 spring on a B-model (some earlier assy's had 5 springs so the No 4 spring on those) has a sharp, lower trailing edge which erodes the main tailwheel leafspring. That sets up a "stress-riser" which can result in a concentration of stresses at that point which drastically reduces the spring's strength. I've seen 3 mainsprings which broke exactly at that stress-riser....which is enough evidence for me to change that spring before-hand. When we first started the Classic Cessna 170 Club at yahoo we ran a poll/survey which resulted in our recommendation to change the spring at 500 hours/cycles whichever comes first, based upon a conservative result of that survey. YMMV.
In the pic below, the wheel is mounted to the left (just outside the pic) and the right end of the pic mounts to the airframe.

Image
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons. ;)
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