The Main Wing Spar

Q: The center section of the main wing spar calls for 3.5mm plywood, a size which is appeares to be unavailable. It seems my options are: 4mm on the aft face and 3mm on the forward face, or laminate 2 + 1.5mm to make 3.5 (messy) or to pieces of 3mm plywood (which actually measure 3.3 to 3.2mm minimum). What should I do?

A: Just use two pieces of 4mm plywood. That's what we do here.

From "Construction Notes" Falco Builders Letter, December 1994

George Richards has just finished laminating the main spar caps and is now ready to taper them on the inside, but he found that the taper provided is not a straight line. George asked about this cut, whether we did a straight cut or a curved cut, and if curved then how?

What we do here is that we make a straight cut to taper the spar. We also follow the dimensions on the drawing, and this gives us a curved cut! How does a straight cut produce a curved cut? Easy. We bend the spar while we are making the cut.

Because the original dimensions do not exactly lend themselves to this, we adjust the thicknesses slightly to make a smooth curved cut. On the upper lamination, at Sta. 8, 28 becomes 28.2, at Sta. 10, 22 becomes 22.3 and at Sta. 11, 20 becomes 20.3. On the lower lamination, at Sta. 6, 25 becomes 25.3 and at Sta. 9, 19 becomes 19.3.

As you can see these changes are extremely minor and all result in a spar that is slightly stronger. We taper the spar laminations on our 'Gonzales' machine and hold it in place on a precision vacuum jig that bows the lamination slightly. Over the length of the spar, the bending is only about 20mm and can be done with finger pressure. When you have a $35,000 custom-made Falco spar milling machine, it's very easy, and only takes a few minutes. I have no idea how scratch builders manage this sort of thing and I don't even like to think of the work involved.

I am commencing work on the main spar and the following questions rise:

1. I have received the spar wood from Jean Peters, and it is marked to identify each layer in the booms. The outer two layers of the top boom have been supplied as a 4440 cm board and a 3760 cm board for each layer. If I flip over the boards so that the joints are on alternate sides of the centre line this gives me a gap of approx 60 cm from scarf tip to scarf tip. While this seems to be in line with the 10xthickness(10mm)=10cm calculation I am a little uneasy as the 30 cm distance either side of the centre line puts the joints into the bend area of the spar boom. Please advise.

2: I am aware of the scarf principal as it applies to grain direction as the build manual. My inspector has expressed a desire that the grain lines when viewed from the end of the spar board 10mm*100mm in my case should run in alternate directions (exactly as you hatch on the construction drawings. Is this required or necessary. I believe it is his view that this will reduce the effects of grain on differential expansion.


1. Generally, I suppose it's best to keep the scarf out of the area that's bent, but I don't know of any specific publication that would prohibit this. If you like, you can cut a piece off one end and scarf it on the othe to makes things work out better.

2. I've never heard of such a thing and I don't want to argue with your inspector or demean his views, but I don't see the need to even consider this. When I hatch a drawing, it is hatched in alternate directions simply to make the drawing clear, and sometimes people interprete such standard drafting techniques to mean something. It is a drafting technique and nothing else.

Alfred Scott

From "Construction Notes" Falco Builders Letter, March 1991

Finally, I should mention our method of drilling bolt holes in the main wing spar and how we use dowel pegs. We drill the bolt holes for the landing gear fittings in the spruce structure before we glue on the plywood. Before we glue on the plywood for the aft face, we tap short pegs into the bolt holes. These pegs are nothing more than half-inch pieces of 5/16"Ø wooden dowels. The purpose is to keep glue from running down into the bolt holes and making a mess of things.

We ship the spars with the pegs still in place, so what you are supposed to do is drill out through the pegs and the plywood. Then before you glue on the skin for the forward face, tap in the pegs we supply and then repeat the process of drilling from the other side. This method makes a nice clean hole without lots of complications with the glue-but if you cover up those holes before drilling out through one face, you are going to be in a world of hurt.

From "Construction Notes" Falco Builders Letter, December 1990

When we ship the main wing spar, we have already drilled all the holes for the landing gear fittings through the spruce but not through the plywood. Before we glue the plywood on, we tap short dowels into the bolt holes to keep the glue from running down into the holes. This means that you must ream the holes out, and then when you glue the remaining plywood on the spar you should repeat the process. Thus, as part of the kit, we enclose a supply of dowels.

This business of drilling the main wing spar for the landing gear fittings is proving to be a popular thing with our builders. Our jig to do this came in at about $2500, but it allows us to drill the holes in 5 minutes, and the holes are a perfect fit with the landing gear. This relieves Falco builders from one of the most dreaded tasks in building the airplane, and as that task passes into history it's worth remembering that some years ago Tony Bingelis-not trusting the dimensions on the drawing-actually waited until the wing was completed up to the point of skinning before he drilled these holes. I've never figured out how he did that.