Center section spar overview

Chapter 14 is all about the center section spar - it is literally the backbone of the aircraft. Put simply, it's a box beam that gets attached to the fuselage just forward of the firewall - the wings later get attached to the center section spar.

To give you a better idea of where it is located, I highlighted it in orange below:

Hiding the wings, canard, canopy and cowl from view, you can see how the center section spar literally goes right through the fuselage.

Here is a front view. Notice how the bottom of the center section spar has a slight bit of dihedral to it. This not only gives the aircraft better handling characteristics, but also helps the fuel in the tanks flow towards the sumps where the fuel lines suck up the gas.

Here's an isometric view with the canard in place - center section spar is outlined in orange.

Top view... looks like an interesting insect!

Ok, so now that we know where it is located in the aircraft - how do we build it?

Chapter 14 starts off by explaining how to build a jig - which is then used to build the spar. It looks something like this:

The idea being that you build a perfectly straight and square jig - which in turn will give us a perfectly straight and square spar.

Very important to note: the jig is built such that the forward face of the spar faces up when mounted on the jig. All statements in the plans such as "top", "forward", etc. relate to how it is mounted in the completed aircraft - I will try to follow the same nomenclature here in the blog writeup.

After building the jig, you cut out various foam panels (as outlined on page 14-7 in the plans). I should note that I substituted ALL 1" urethane foam panels in this chapter to 1" H45 foam. Ever since I worked with urethane foam during the practice chapters, I am reluctant to use it anywhere in the aircraft that is structurally important. H45 is a little bit more expensive and a bit heavier - but at least is doesn't fall apart in my hand the way urethane does. Any who - moving forward... 

Notice the interior bulkheads. Also notice the spar tips get carved to allow for the wing attachment hard-point buildup.

The spar gets glassed on the inside and then the interior bulkheads get installed. Aluminum pucks are installed where the wing attachment points are and these areas get reinforced.

The lid of the box is then bonded on - shown below. After cure, the spar gets removed from the jig. The outside is contoured and carved to accept the spar caps. Metal inserts are installed at wing attach points, a bi-directional glass shear web is laid up, then the external bi-directional glass shear web is laid up. An oval-shaped panel is removed for access to the baggage area inside.

The spar then gets installed into the fuselage.

Wire-frame view shown in the orientation as it will get installed into the aircraft.

Simple, right? Let's begin!

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