Building the NACA Scoop

I apologize to my loyal readers, it would seem that I’ve been slipping again in updating the site.  The only thing I can say in defense is that I’ve been building like mad!  Now if I could just get the site caught up while I’m waiting for epoxy to cure…  if you were to look at the date stamp on the pictures, you’d see that this is being posted over a month after the work was actually done.

I was quite excited to tackle the step of building the NACA scoop!  Something about the challenge of sanding a huge block of foam down to a small triangular shape that fits just right scratches my itch for sculpting things.  This is the beginning of chapter 7, and the whole chapter involves a lot of adding pleasing shapes to things by hand.  Loads of fun!

The NACA scoop is built up from the bottom of the fuselage as it was left in chapter 6.  Up meaning up with the fuselage upside down on the table. To accomplish this, we put a 2″ thick block of urethane up against the front side of the bottom of the forward landing gear bulkhead, then put a 1″ urethane block in front of it.  There are actually 4 blocks in total, 2 on each side of the center line.  The template from the plans for the NACA scoop is used to trim the inside edges of the blocks, and what you wind up with looks like what you see below:

Foam laid out with NACA cut out

Then you take the blocks off, butter them up with micro, and put them back in place, with plenty of weight to make sure they stay flush with the bottom.

Weighed down

The bulk of the construction of the NACA scoop is then sanding the foam blocks down to be flush with the fuselage near the landing brake, and flush with the bottom of the forward landing gear bulkhead (LGB).  As many builders have found, I needed another foam block added on to the front side of the NACA scoop buildup, in order to make the foam flush with the rest of the bottom.  I added another 1″ block of foam extending up to the rear edge of the landing brake.

Flush with forward LGB

As is hinted at in the picture above, we also installed the plywood pieces called out in the plans between the landing gear bulkheads, and between the …. firewall?!  We hadn’t installed the firewall yet!  After a review of the plans, I found we were supposed to flox in the bottom half of the firewall at the end of chapter 6.  That was easy enough, after cutting the holes in the firewall for the lower longerons and LWYs, chapter 6 was officially in the bag!

Unfortunately, we don’t have too many pictures of the plywood pieces. Here’s a shot of Heather cutting them out of birch plywood:


Heather cutting wood

Here’s a top down shot of the area in question, you can see that additional urethane foam is used to fill the gap between the firewall and the rear LGB.  More blue PVC foam is used to form the channel for the NACA scoop in that area as well.

You may also notice in the above picture that the port side foam block doesn’t have a seam in it.  What gives? Well, I got a little too excited in my sanding, and wound up with a sizable depression in the foam.  I microed on another 1″ sheet of urethane from a pretty much trashed sheet, and sanded it level.  I can’t recommend this approach, as the edge of the sheet wound up being in the section of the fuselage to be rounded, but that’s a story for the next post.  Just take my word for it for now.  If you’ve gotta add foam, you can do it, but it might be easier to just use dry micro.

Also in the above picture, you may notice some stirring sticks wrapped in packing tape against the sides of the scoop.  Those are there because of more nicks in the urethane (man this stuff is a pain), and were used pretty successfully as dams for micro to fill in the nicks without messing up the bottom of the scoop. This worked a lot better than I thought it would, I apologize for the lack of after pictures.


Tongue depressor micro dams.

See the nicks?!

As an editorial note, it really bugs me when I visit other cozy build sites and can’t find much evidence of mistakes being made.  I like to think I’m doing a pretty good job putting this thing together, but I think that sharing the MANY mistakes I make along the way is much more interesting, and MUCH more helpful than writing a blog about how perfect my construction is.  Yes, it’s an airplane.  Yes, precision counts.  No, you’re not perfect.  For those out there pursuing a perfect airplane, good luck, but I intend to fly my cozy someday! *gets off soapbox*

Up next: Contouring the bottom!

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