Sunday, December 30, 2012

Ch 7: The Fuselage Exterior - NACA Scoop

Now that the sides and bottom are glassed and assembled it's time to shape and glass the exterior.  At the completion of Ch 7, I'll have a rigid fuselage, or a least most of one, that I can actually sit in and make airplane noises.  It starts with building the NACA scoop.

The Scoop on the NACA Scoop
The NACA Scoop is on the bottom of the fuselage and its job is to pull in air during flight to cool the engine.  There are many archive emails on the Cozy Builders Forum discussing the plans NACA scoop and ways to ensure it works well.  You'll also find many opinions about the plans updraft cooling (air coming up from the bottom) vs down draft cooling (air coming down from the top) and everything surrounding it.  My research indicates that the plans updraft cooling works just fine provided the scoop is built correctly and you make sure your engine baffling has a good seal to direct all the available cooling air across the cylinders and out the the cowling.

A review of the Cozy archives and a couple of Central State Association newsletters (July '90 & Apr '11, reveals a few key items to keep in mind when building the scoop.  Also be sure to download the Cooling Forum Slides from Oshkosh 07 for some good info on cooling.  Key points I noted are...
  1. The entry point (throat) of the scoop ramp should not be a should be approximately 1" or so wide for our Cozy. 
  2. The throat should provide a smooth entry with no discernible joggle or dip...a joggle will create air flow separation and disrupt the smooth flow into the scoop. When you're done contouring the scoop, use a straight edge and rock it over the throat entrance. If you hear the ruler click against the fuselage rather than smoothly rock back and forth, then there is a step or joggle that needs to be eliminated. 
  3. The intersection of the scoop wall and the bottom of the fuselage should be a sharp 90 degree angle. 
  4. The intersection of the scoop wall and the bottom of the scoop should be a large rounded radius. I've seen comments that the corner radius should be approx. 1/2 the height of the scoop wall at any given point.
  5. The slope for the ramp of a NACA scoop should be 5-11 degrees with optimal being 7 degrees. The Cozy scoop starts out more shallow and then curves to a steep angle (~10 degrees) between the landing gear bulkheads and the firewall as the scoop gets wider. There's really not much you can do here to change it because there is not much room between the scoop and the landing gear.
  6. The aspect ratio of the scoop along its path should be between 3 and 5. That is, the width should be 3-5 times the height. As the scoop runs back through the LG bulkheads and to the exit (into the cowl), the Cozy scoop has a ratio of around 4.8.  If you wanted, you could make it better later by extending the opening to the cowling a little below the scoop.
Building the NACA Scoop
The scoop is actually formed by building up foam around where the scoop will be.  The building starts by cutting out the template.  After I cut it out, I modified it slightly to allow for an approx. 1" throat as mentioned in #1 above.  I simply widened the tip to 1 1/8" and redrew straight lines down to where the curve starts.  You can see in the pic where the original template lines are vs the slightly widened version.

Front part of NACA Template with extra width
Close up showing 1 1/8" wide throat vs plans point

Now the template is fitted to the fuselage center-line and cutouts in the LG bulkheads.  In my case, I had to trim a bit of the firewall and aft LG bulkhead width to match the template.  The FAQs say this is likely the case and most people trim the bulkheads.  Also, based on the FAQs, I extended the foam about 7.5", up to the edge of the landing brake, to ensure the foam made it up far enough to sand straight and level back to the bulkheads.

Scoop foam cured and ready for sanding
Then, foam is cut to match the template and micro'd in place.  (TIP) I did not use the urethane foam called for in the plans.  I asked the forum about experiences with this foam and I got many folks who told me they wouldn't use it again because its too brittle.  Several of them had to fix delaminations in this area and they haven't finished the plane yet.  I decided to use 4.5 lb Last-a-foam.  It's heavier, but it sands easily and isn't nearly as brittle as the urethane.  I'll probably use this in place of all urethane foam from here out.

Back and forth, back and forth, etc., etc....

Once the foam is cured, the sanding can begin.  I built a straight 4ft sanding tool with 36" of 80 grit belt sandpaper glued in the center on the bottom.  If I did it again I would have used 40 or 60 grit to go a little faster.  It didn't take too long but you do need patience so you don't get in hurry and screw it up.  The goal is a nice flat surface the extends from just behind the landing brake to the fwd bulkhead.  A little while sanding back and forth and the scoop was level.

The NACA Scoop formed and ready for glassing.  Note this pic was taken after the shaping of the bottom corners...that's coming up.
NACA Scoop Glassed

Onward, cab