Chapter 25: Contouring the Fuselage Top


I contoured the turtleback, canopy, and the nose at the same time to ensure that all surfaces and joggles were congruent.  As part of this job, I had microed and contouring partially down the sides of the fuselage so that I could also contour in the rounded corners on the top of the fuselage. 

 

Contouring the Canopy and Turtleback

In general, the canopy and turtleback aren't all that hard to contour.  You just have to be careful that you don't get too rambunctious and scratch the canopy bubble or the windows.  I applied lots of duct tape over the windows and canopy bubble to keep the sandpaper from scratching the plexiglass.  Logistically, I found it easier to contour the canopy and turtleback by sitting/kneeling on top of the strakes.  I covered each strake with a quilt and I set a large piece of 3/4-inch plywood on top of the quilt.  The quilt keeps the plywood from scratching up the primer.  The plywood keeps your knees from denting the strake tops.  I did most of the contouring with my flexible Durablock and 36 grit sand paper.  The sanding pattern is 45-degrees to the centerline.  The stroke was from just above the windows to as far as I could comfortably reach across the canopy.  I would switch strakes often to ensure I was sanding evenly from both sides.

 

I switched to a small Durablock for sanding the areas between the windows.  It's very tempting to sand up and down or side to side.  But you must avoid doing that and stay faithful to the 45-degree sanding pattern.  It's hard, but it can and will be done! :-)

 

Contouring the Strake Fairings

I next moved to contouring the cosmetic strake fairings.  I have to truthfully say that these fairings gave me fits, mostly because I have no patience.  The fairings are complex, compound curves.  I mostly used my 3/4-inch sanding stick and my small durablocks to do this contouring.

 

Top Longerons and Upper Sides

I filled and contoured the rounded edges along the top longerons and the upper sides while I was doing the canopy, forward deck, canard cover, and nose top.  I don't recall doing any pre-filling.  The Big Filling was a breeze.  The contouring turned out nice.  I did the upper fuselage together with the canopy, canard cover, and nose top solely to ensure nice, rounded, cosmetically appealing edges.

 

Contouring the Nose

Contouring the nose top is a breeze, but you must keep in mind that all surfaces are curved.  You end up working the nose, canard cover, removable deck, and feathering into the canopy as if they were all one piece.  You must continually and methodically work the sanding board fore to aft and left to right, being careful not to sand too long in one spot.  Else, you'll end up with a large, localized flat spot that will be hard to correct without refilling. 

 

What I did was work the swath along the centerline first, shifted to working the fuselage sides until I got them smooth and flat, and saved the rounded edges for last. 

 

I started with 36 grit on my flexible Durablock.  Using the centerline as a reference I stood on the port side of the plane and used the now-all-too-familiar 45-degree sanding motion.  I started near the nose and worked my way back to the canopy glass.  I started each stroke near the port edge of the nose hatch and stopped the stroke when I got near the starboard edge.  I "projected" these lines aft as I worked my way aft to the canopy.  Therefore the "swath" got wider and the stroke got longer as I progressed toward the canopy.  After two passes, I switched to the starboard side and made two passes from that side.  I alternated sides until the micro got thin enough (translucent enough) for the fiberglass to_just_start_showing through.  At this point you do not want to remove alot of micro and make it too thin.  You're just roughing it in right now.  You want to leave a "buffer zone" so you can make adjustments to the various contours later on.

 

Once I got the top swath roughed in, I worked the vertical sides of the fuselage and roughed them in.  This goes relatively fast since you have a nice, flat surface to work with.  I say "flat", because even though the fuselage sides are actually curved, you contour them as if the surface was flat.  I used my rigid Durablock and made the 45-degree patterns holding the sanding board parallel to the top longerons.   Every third pass, I'd switch to my flexible Durablock.  I kept doing this until both sides were roughed in.

 

I finally turned my attention to rounding the edges.  This is where the rubber really meets the road!  You must be careful to work the entire edge continuously and stop often to assess how it's going.  The way to check is to stand off to the side of the nose, about 3 feet in front of the canard wingtip.  Shift your vision's point of reference by squatting and/or getting on your tippy-toes such that you're seeing the rounded edges as the "horizon".  You want the curve to be continuous without any sharp bends or discontinuities.  This is hard to explain, so I hope the pictures are doing this justice.  The problem area is almost always the canard cover itself.  The most common discontinuity occurs where the canard cover meets the forward deck, although I've also seen some where the cover joins the nose top.  That's because we carve the nose in Chapter 13, the canopy in Chapter 18, then try and bridge the gap with the canard cover in Chapter 24.  This is why I'm a big proponent for carving the entire top of the airframe at one time like I did in my Chapter 18.  The time to have corrected for discontinuities is when the pieces were carved, shaped, and glassed.  If you have discontinuities, at this point you're basically screwed.  The micro can only minimize the optical effect on the eye.

 

   

 

 

 

After I had everything roughed in with 36 grit, I repeated this process with 36 grit until the obvious high spots started showing up clearly from under the micro.   For me, the high spots occurred on the rounded edge where the forward deck, canard cover, and sides come together, and over most of the flanges.  The other pesky high spots are the 4-UNI layups that strengthen the outside skins at the top longerons.

 


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