Thorp Air Command - T18.net

I have heard that spar cap extrusion on the T-18 is no longer available. I have an old damaged wing that may need the extrusion replaced. Does anyone have a replacement for the spar material?

Brent Schultz

When I visited Richard Eklund several years ago he had several main spar caps.

Bill,

Do you know who I could call to see if I can purchase one of them?

Brent Schultz

Richard Ecklund

What dimensions and alloy / "T" treatment is it? (Sorry, I don't have plans.)

It should be possible to substitute a similar but slightly thicker 6061 section.

I imagine the lower cap is still available; it's just an angle.

First off ... Do not try to repair or build a wing without a set of plans please.

Second, the opposite, the u channel is available the angle is not. Last I heard Richard recommended cutting the upper spar extrusion to make the lower angle.

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Lee Walton
Houston, TX
N51863,N118LW
KEFD

Ha!, so much for my guess.

If the thickness is the same, that's what I'd do to make the angle.

I did buy some 2024-T3511 Aluminum Structural Angle 1" x 1-1/8" x 48" - (.100") aluminum angle for outer wing panel spars from an ebay ad. Unfortunately, one of them had a big flaw in the middle, like a transition to a new billet passing thru the die. That is supposed to be culled.
anyway, the original 1" x 1" 3/32" 2024 T4 material would be .093, so .100 is pretty close. I don't know how the T-3511 stacks up against T4. I hoped the extra 1/8" for the flange would provide insurance, but in this case, the quality of the aluminum was so poor I couldn't consider using it.
I did buy some 1" x1" .125" 2024 T3 from Aircraft Spruce, it seems to be the only acceptable alternative?
It seems the Center section U extrusion is no longer available. Someone will need to figure out an acceptable replacement.
I did get an old unfinished outer wing spar set from the parts stock of my friend Gary Jacobsen (a Northwet T-18 enthusiast)
Today, we sheared & braked up 2024 T3 .032 sheet for 10 rear spars.
My old rear spar must have been bent over the edge of a table. Dimensionally it wasn't bad, but it had lots of fluting on the flanges, which would indicate it was bent in small sections with a block and a hammer.
Now I have enough parts on hand to start clamping ribs between spars. Perhaps I can save the old ribs by back drilling their rivet holes into the new skin. If not, I'll make some ribs....

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Scott Emery
EAA Chapter 326
T-18 N18TE

Speaking of the lower centre-wing spar cap.... What if, hypothetically speaking of course, one removed the spar box cover and discovered that the spar cap flange had been drilled for nut plates in three places. Would we (the Thorp group) consider that to be a bad thing?

Completely normal .:. Most spars are drilled and tapped with 10+ screws. Your 3(9) are no different.

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Lee Walton
Houston, TX
N51863,N118LW
KEFD

I have five #8 holes drilled and tapped to hold the front of the wing gap cover on. Consider all the holes drilled to hold the skins on. Just make sure you polish the holes top and bottom for stress relief.

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Bob Highley
N711SH
SN 835
KLAL

"Completely normal .:. Most spars are drilled and tapped with 10+ screws. Your 3(9) are no different."

"I have five #8 holes drilled and tapped to hold the front of the wing gap cover on. Consider all the holes drilled to hold the skins on. Just make sure you polish the holes top and bottom for stress relief."


Hmmmmm. OK...... So, would the T18 wing still be considered rated to +6G with multiple holes in the spar cap?

There is no major flexing of the member in this area. Like Bob said, consider the large number of closely-spaced holes already in the spar caps for the web, mounting holes, fittings, etc. Minimum edge of hole clearances should be maintained, and chamfering/smoothing/polishing the entry and exit of the holes of course is good. I have tapped holes in mine for the cover also. Many years and many thousands of flight hours have shown that a reasonable number of holes in this area are safe and do not decrease the strength of the spar to any appreciable degree.

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

Have look at the original setup from John Thorp. I believe he called for a fixed gap cover riveted to the spar beam.

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Bob Highley
N711SH
SN 835
KLAL

Many years ago, we had an A&P use a hole saw to get at the main wing mount bolts. He put two nice large diameter holes through the bottom cap. JT designed a strap for the cap that was riveted from end to end to fix the error. Sort of like the strap fix on many Beech products.

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Bob Highley
N711SH
SN 835
KLAL