FlyingTgr6

So I'm now to the part of the build on the wing that tells me to install the upper spars, however when I fit them, there is a small gap in between the spars and the web shears. I only sanded enough of the shears to get them flat after cutting them out. A couple swipes was it...doesn't seem like that'd be enough. The gap is approx. 1/16th of an inch. Is this normal? Should I fill in those gaps?

SeamusG

The shear webs need to be flush with both top and bottom main spars.

I'm gonna guess that the rib bottom edges aren't fully seated on (and around) the bottom main spar adding 1/16" of additional space between the bottom and top spars. If that is the case then you'll have to sand off the exposed edge of the main spar before covering. Or, you can take a new X-acto #11 blade and cut the ribs from the spar, adjust the spar cutouts so that the spar now fits flush with the bottom edge of the rib and reglue. This should take care of the extra 1/16".

When you position the top spar into the rib spar cutouts is the top of the spar flush with the top edge of the ribs? If it is above the rib edge just sand the rib cutouts with a little sanding block using (fabricated from some scrap and sand paper) so that the spar will fit flush with the top of the ribs. This should take care of fitting the shear webs.

If it is flush, how about just adding a strip of 1/16" scrap to the top of the shear webs so that when the spar is put in place all is good?

HTH

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The image was scanned from the LT plans. The red boxes are offset just a tab high and low leaving a gap between the purple shear web and the spars.

karolh

Rather than place the shear webs in between the main spars, I glue them to the front, rear or both front and rear faces of the main spars.

Karol

Hydro Junkie

When I built my Kadet Jr, I also glued the webs to the back side of the spars and then sanded them flush. Was much simpler than trying to fit them between the spars, the only difference being that the leading edge sheeting was extended 1/16" further aft to cover and tie in the webs

SeamusG

The LT-40 shear webs are pre-cut. Some kits do front only. Some kits do both front and rear. Some shear webs mate with the ribs on both sides of the shear web. Some shear webs don't attach to either rib. Go figure.

OldScaleGuy

My Feedback: (2)

Personally i would cut new ones to fit the front or the rear of the spars, plus they do not have to fit exactly from rib to rib, some gap there is ok. Fitting in between is difficult and not as effective if there are any gaps.

karolh

+1

Karol

acdii

Surprised that there is a gap if the spars are fitting correctly, unless you have the webs out of order. They are not all the same size. There should be a gap between the ribs, but not between the spars.  As designed, the webs are very strong when correctly installed and glued, I know this well since I smacked a wing into the nacelle of an A-26 that is behind the barn and all the ribs that had shear webs snapped clean at the spar, the last two ribs sheared all the way back and were more difficult to repair.

If you are going to cut new shears, I suggest putting them all the way to the tip.  What you can also do is take some 1/16th balsa sheets and cut strips the width of the spar and glue it to either the top or bottom spar and then the stock webs should fit well. This will also strengthen the spar a bit.

FlyingTgr6

It looks like the upper spar itself isn't seating properly...so I'll have to take a little out of the spar rib notches I think.

SeamusG

Try making a small "custom fit" sanding block by taking a piece of scrap spar material and glue a piece of 120 grit to one side that fits the rib notch using a contact cement like 3M 77 Adhesive. A couple of pulls of the block thru the notch will clean 'em up nicely.

J.Paul Saure

I helped someone build an LT-40 many years ago.
The shear webs were also cut too small. You must NOT leave space between the spars and webs.
I would also recommend cutting bigger shear webs and gluing them to the aft side of the spars, much easier!

Beware with the grain of the balsa, it must be perpendicular to the spars.

Have fun!!

acdii

The shear webs are laser cut, the ribs are die cut. Chances are the ribs are off and not the webs.  Make sure though that the edges are sanded to remove the carbon from the laser or the joint will be weak.   The ones on mine fit perfect, and the die cut parts were clean, so that could be why.