Thorp Air Command - T18.net

Hi all
It's been suggested at some point that consideration be given to beefing up the tail spring attachment points.
The 2 *4130 steel brackets on frames 576 and 575.
I recall reading some accounts of modifications being being done in this area but not a detailed account. Attached is a drawing of the 583 fitting with one possible option for the bracket which would extend across the full width of the lower skin so it can be riveted onto the 580-4 longerons instead of just the frame.
Any thoughts on the attached drawing, does it look like a reasonable option, striking the right balance between added weight and added strength?
Stuart

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Stug

Stuart,

That is exactly what I ended up doing after in-service cracking occurred in the fitting. So far, about 1500 hrs later, no cracking.

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Bob Highley
N711SH
SN 835
KLAL

Mine broke at 300 hours, so I copied Bob, no problems in 1000 hours. As for the weight, it helps in the S-18 with W&B.

I would think that the extra weight would help the T-18 more than the S-18. The reason the S-18(T-18CW) is limited to 0-320's and up is due to the extra length moving the CG aft.

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Lee Walton
Houston, TX
N51863,N118LW
KEFD

I have the 0-360 with CS prop , needed all the help I could get for the forward CG. I have the Dayton motor for trim and aft battery.

I did a tailwheel attachment mod 1.5 years ago when installing a new tailwheel - attached are a couple of photos - the airframe had around 1700 hours on it at the time - I cannot remember the thickness of the piece I used, but am thinking it was .080 at least - had to find a good brake to bend it. Rather than remove and build another bulkhead, I decided to span the bottom longerons to help with load distribution - made sure and put a spacer (riveted to the new bracket) between the bottom skin and the horizontal flange to keep things tight to the tailwheel bracket - Instead of riveting the bracket to the longerons, I drilled 2 holes (at former rivet locations) each side and installed 1/8 or 3/16 inch bolts (cannot remember now). Only about 80 hours on the fix, but no fatigue evident as of last may (conditional) and it seems to be a pretty stiff setup.

OK it seems like this mod is worthwhile and the additional time invested now will save some later
Next question, is the heaver gauge steel recommended ? I assume that just distributing the stress loads to the longerons with the current gauge material will improve the resistance to fatigue cracking in the frame but if the steel fitting has a history of cracking then it might need beefing up as well?
I like Fraser's approach, using bolts through the longerons into the fitting instead of rivets, I would have thought that has merit in this scenario.
Stuart

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Stug

I think it's definitely worth it, Stuart. Many of us have done additional reinforcing in that area. Just another little thing to keep in mind is the necessity to cleanly de-bur all holes and file smooth, or even polish the edges of the steel to eliminate stress risers. I've seen several pictures of cracked fittings in this area which obviously had poor/rough edge finish. I'm sure you're aware of that, but it doesn't hurt to mention.

I'm also pretty sure that I increased the material thickness of the 583 fitting to .093" (same as the rear fitting), and did other mods as well to carry the loads over to the side skins and flanges of the frame. I also made a heavier bottom skin/plate in that area, again to distribute the loads more widely to the sides. I actually caught the tailwheel on something hard enough to deflect it up into the rudder once, but that's another story. 22 years - no signs of distress.

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John Evens
Arvada, Colorado
T-18 N71JE (sold)
Kitfox 7 SS N27JE