Saturday, January 12, 2013

Ch 7: Contouring the Bottom

Time to turn this box into something with smooth sexy curves !  After sanding down the NACA scoop and sanding the foam to shape around the rear bulkhead area, the next task is to shape the aft end around the firewall so it smoothly flows into the forward fuselage.

The first step is to remove the foam covering the longeron and LWY from 5.5" ahead of the FW aft.  This is where the main spar will fit and you will need this area to tie in the glass when it's installed.  You also need to bevel the edge of the remaining foam into the longeron and LWY so glass from the fuselage can smoothly tie into those parts.  Some builders beveled the entire length of the foam edge but indications are that this is only needed where the foam leads to the longeron or LWY.  The pics below show my interpretation of how this is supposed to be done.

Aft fuselage side before foam removal
Aft fuselage side after foam removal

Aft fuselage side after sanding the transition
from the fuselage side to the firewall
Next, per plans, I sanded a smooth transition from the FW to approximately 25" in front, and at the bottom of the fuselage (top in pictures since it's upside down).  This is kind of a nerve racking task because it's a little hard to know if you're sanding enough or too much to get to the shape you need.  The FAQs give a pretty good description of what you're looking for and I also consulted multiple other builder sites for pictures of their results.  You'll likely start to uncover the electrical channel...if you do it's time to stop.  Here is what I ended up with.

Corner after the initial 45 degree shaping cut w/ a saber saw
Now I worked on shaping the squared corners of the bottom from front to back.  As the plans recommend, I used a jig saw to cut a 45 degree cut along the edge but I didn't cut all the way down to the longeron.  Instead, I cut a little shallow and used a belt sander to get down closer (although I still didn't get to the 1/4" exposed longeron...but close).

Here you can see my shaping tool
with center marks and my corner line.
Very smooth and even results
(Tip)  Using the template from the plans, I built a sanding tool and began the back and forth until I got a nice rounded shape.  I marked the side of the tool so I could keep it centered on the longeron.  I also ran a line down the center of the corner with a sharpie.  That way, when the line started to disappear, I knew the corner was filling the template tool and I was almost there.  Be sure to make long consistent strokes.

After the front half is sanded to shape to about the middle of the landing brake, the area starts to change so that it blends nicely into the aft shape at the landing gear bulkheads.  This was the hardest part to get correct and make sure both sides match.  I over sanded a bit in some areas and had to go back and fill/sand with micro to get it correct.  Here's how it ended up.


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