In the previous post I layed down the bottom main spar as well as the bottom rear spar. In the next steps I'm going to start laying out the ribs as well as the shear web. I can hear some of you right now asking, what is a shear web. Forgive me here if don't get the exact “word for word definition” of what shear webs are, but I'll sure do my best to get what they do correct. Shear webs basically give the wing a lot of strength. The “backbone” of the wing is the spar that runs the length of the wing, but it doesn't have any strength if the wing is flexed up or down. This is where the shear web comes into play. It will give the wing strength in the up and down direction. Shear webs are just plain wood, usually balsa, but it's very important that they be placed with the grain running perpendicular to the spars. In this case that means the grain will run “up and down”. See the diagram below for a little visual aid on this. Shear webs are attached between the 2 spars (top and bottom), and have the grain of the wood running perpendicular to the spars. When shear webs are glued in the don't necessarily need to be glued to the ribs on each side, but THEY MUST BE GLUED TO THE SPARS ON TOP AND BOTTOM. I know that sounds weird that they don't have to be glued on the sides, but remember that we are going for strength up and down. Now with that said, I do glue it to the ribs on each side. Every little bit helps, but as said above it won't hurt if you don't get it glued. See first diagram for an example of how shear webbing can be attached. It can be attached to the outside of the spars, and glued to the front or back of the spar. As I said above, it doesn't have to be glued on the sides to each rib, but IMHO it's better if you glue it to the ribs. The other way to attach shear webbing is to glue it in between the spars, forming an “I-Beam”. Both ways of building shear webbing will work. If asked for my opinion I'll tell you that gluing to the outside of the spars is definitely easier to build. On this LT-40 it calls for the shear webbing to be built in between the spars, as an “I-Beam”. Since the shear webbing is precut I don't see any reason to do it differently than the instructions call for.

I've learned a few things the hard way as I built kits in the past. One of those things that I learned the hard way is to test fit. I just started applying glue to parts and slapping them in place. If you can't already see the problem with that I'll tell you what it is. Not all parts fit correctly right out of the box. Sig makes wonderful kits that have great quality in their cut parts, but even those need to be test fit when you build. If you just start slapping parts on, you're going to end up with a mess of a plane. Trust me on this one, learn from my experiences. The biggest parts you going to need to test is the ribs, and how they sit on the spars. Since this is a flat bottom wing, and we are building on a flat building surface, we want to keep the bottom flat. And this can be our “point of reference”. You want all the ribs to sit flat on the floor when they are in place on spars and in the notch on the trailing edge. In the case of the W2 and W3 rib it will rest flat on the balsa sheeting built earlier in the build. As you can see in the second picture, there is a gap between the bottom of the rib and the sheeting. In order to close that gap we'll need to make the “notch” in the rib deeper, so that the rib will sit lower on the spar and the rib will be flush with the sheeting. Use your x-acto knife to trim material away from the top of the notch in the rib, see picture 4. When you are trimming away trim it A LITTLE BIT AT A TIME, and check often to see if you have it correct. Remember, you can always keep trimming but it's almost impossible to replace the wood. Once you it correct the rib will sit flush on the sheeting, or flat on the building board for ribs W4 and W5. If you have a gap between the top of the spar and the notch in the rib, see picture 6, you will need to sand a little bit off of the bottom of the rib. Use a sanding block to remove a little bit of material at a time. You want to use the sanding block so that you keep the bottom of the rib flat. Just like before, remove a little bit at time and check your progress often.

Ok, I'm going to wrap up this post now and for the next post I'll start putting the ribs in.

Pictures
1. Example of shear webbing, and where it adds strength
2. Difference in shear webbing mounting
3. Improper fit of W3 rib. Bottom of the rib not firmly against bottom sheeting.
4. Area needed to trim in order the adjust the fit of rib on the spars
5. Completed adjustment. Rib sitting firmly on the bottom sheeting.
6. Another improper fitting rib. Notch in rib does not touch the top of the spar.
7. Sand the bottom of the rib to adjust the fit of the rib on the spar.


Until next time

Ken