Fuselage Construction: Firewall and Sternpost
This page was last updated Friday August 3, 2007
Click on any photo to enlarge it.
Continued from Fuselage Construction: Main Bulkheads.
2007-01-19:
The Volksplane's sternpost is made from a piece of 5¾ x 10¼ x
¾ inch quarter-sawn Douglas fir sandwiched between two layers of 1/8
inch plywood. Unfortunately, I didn't have a wide enough piece of fir, so I
laminated two pieces together. To keep the resulting board flat, I clamped it
to a piece of plywood (protected with wax paper) while the glue cured.
I also cut a piece of ¾ inch supposedly marine grade plywood for the
firewall, but the material I selected turned out to be no good. It consisted of
very thin (maybe 1/64 inch) okoume veneer, with five plys of a very soft,
light, and crumbly wood. There were a lot of voids, as you can see in the
photo. I'm not sure how this can be called "marine grade", but it was stamped
as such.
2007-01-31:
After determining that the first piece of firewall material was inadequate, I
purchased a sheet of ¾ inch exterior grade (one of two choices in the
plans) fir plywood for a second try. I once again carefully measured this and
cut it to size.
Next, I trimmed the sternpost fir piece to its pre-bevel size of 5¾ x
10¼ inches. I also cut a pair of 1-1/8 x ¾ x 15½ inch
firewall-to-fuselage blocks from a length of 1 x 2 inch Douglas fir I found at
a local wood specialty shop (Snider Plywood Specialties, which carries far more
than just plywood). I cut these blocks so that the end grain would "connect"
the firewall and fuselage sides.
2007-02-01:
Today I glued 1/8 inch marine plywood facing on the front and back sides
of the sternpost, using plenty of epoxy, and clamped the assembly between
two flat boards.
After setting that aside to cure, I prepared the firewall-to-fuselage blocks
for gluing. To do this, I first carefully clamped them in their precise
positions, marked the locations of the bolt holes, and predrilled the holes
using a 3/32 inch drill bit. I then tapped 3/32 inch thick smooth nails (which
were a snug but not tight fit) through three of the holes in each block.
I could then glue the blocks in place and not worry about them shifting around
as I tightened the clamps. After about twelve hours, when the epoxy was almost
completely cured, I pulled the nails out while still leaving the clamps for
another twelve hours or so.
2007-02-04:
My clamping job on the firewall blocking wasn't good enough. There were some
spots where a thin piece of paper could be slid between the firewall and block,
albeit not very far. I decided to try to force more epoxy into this gap, using
a heat gun to thin it so it would flow. In the back of my mind though, I was
already thinking that I'd probably have to start over.
On the productive side, I trimmed, bevelled, and notched the sternpost
assembly. To trim it, I used a router mounted in a router table, with a flush
trim bit. Bevelling the side and top edges was done on the bandsaw, with the
table tilted the appropriate number of degrees. Notching was done the
same way as notching the main bulkheads (see this
photo and description),
except with the table tilted to the appropriate angle.
I also drilled the 3/8 and 3/16 inch holes that are perpendicular to the
surface of the sternpost, using brad point bits to make clean cuts through the
plywood surfaces. The sharp points on these bits also make it easy to centre
the drill exactly on intersecting pencil lines.
2007-02-07:
There were still gaps in the firewall block joints, so I attempted to force
more glue in. I think this is the airplane building equivalent of "throwing
good money after bad".
2007-02-21:
Still stubbornly refusing to give up on the second firewall, I bevelled the
firewall sides. Like the sternpost, I used the bandsaw to do this, but I had
to clamp extensions onto the bandsaw table in order to supprt the plywood as
I fed it through the saw.
Next, I drilled the 3/16 inch holes for the attachment bolts and countersunk
them with a separate countersink bit. The problem with this approach is that
the countersink bit can wander, resulting in a countersink that isn't
concentric with the hole. If it's significantly off centre, it will unduly
stress the bolt head. I decided I'd purchase a proper combination drill-bit
and countersink cutter and rebore the countersinks later.
2007-03-01:
Today was the end of the line for this firewall. I noticed that it was
significantly bowed vertically, which was part of the reason it was so
difficult to clamp the Douglas fir blocks to it evenly. I decided to throw
it out and start over.
As far as real work goes, I drilled the angled holes in the sternpost,
including a row of ¼ inch holes that I then chiseled out to make the
landing gear spring slot. To achieve the proper angle, I constructed a simple
jig to place on top of my drill press table. This consisted of a 14 inch
long piece of particle board with a 1-1/8 inch wide block glued to it 10.7
inches from one end (since arctan 1.25/10.7 = 6°, the
desired angle).
The finished sternpost is a thing of beauty, if I do say so myself.
2007-03-24:
Another day, another firewall. Today I started on a brand new firewall, using a
piece of top-notch waterproof ¾ inch 13-ply Baltic birch plywood. This
is the same stuff used as flooring in some European horse trailers. I love the
material and the name has a nice ring to it as well. I also cut two new pieces
of Douglas fir firewall-to-fuselage blocking. In the photo, you can see it with
alignment nails in place (as described for the previous firewall attempt),
ready for gluing, which I proceeded to do.
2007-03-25:
After the glue had cured, I use a chisel and hand plane to remove the excess
glue out so that I'd have a flat edge to run along the bandsaw fence. I then
used the bandsaw to cut the 4½° bevel in each side and the
3½° bevel along the top edge. I also began pondering how to cut
the rounded corners so that they smoothly transitioned from 4½° to
3½°, and posed that question to the Volksplane discussion group on
Yahoo.
Notice in the photo that the end grain of the block is diagonal, forming lines
that will connect the firewall to the fuselage sides. If the grain went the
other way, the wood could easily split along the joint.
2007-03-28:
I purchased a set of combination bit-and-countersinks in order to obtain one
that could take a 3/16 inch bit and didn't counterbore the top part of
the hole (intended for the unthreaded part of a wood screw). It seems that
there's no standard for what size bit is used for a given screw size. In the
set I purchased, it's the #9 size that uses a 3/16" tapered bit. I replaced the
tapered bit with a straight bit and drilled the bolt holes, using the depth
stop on my drill press to ensure the bolt heads would sit flush with the
firewall surface. The result: perfectly concentric holes and countersinks.
Also today, I notched the firewall corners and sides, using the techniques
described above for the sternpost, and previously for the
main bulkheads.
2007-04-04:
I finally decided to cut the rounded corners of the firewall by cutting them at
a 4½° angle while following a line drawn on the forward face. This
would leave some excess material where the angle should be less than
4½°, which I'll sand off later when fitting the forward upper
surface.
Just for the sense of accomplishment, I set up all the bulkheads in their
approximate relative positions (except the sternpost, the top of which
should be in line with the top notches of all the other pieces). It took some
imagination to fill in the rest of the airplane.
Remaining tasks:
-
Drilling the engine mounting holes. I decided to wait until I
have the engine ready to install, since I have no idea if the
mounting hole spacing on the plans is still correct for a more
"modern" VW engine.
-
Cutting out the 8 inch diameter lightening hole. Again, I'll
wait to see how this will fit relative to the engine mounting
holes.
|
Continued at Fuselage Construction: Sides.
|
Stefan's General Aviation Web Site
|
E-mail Stefan
|
|
|
|
Disclaimer:
Although every effort has been made to ensure accuracy and
reliability, the information on this web page is presented without
warranty of any kind, and Stefan Vorkoetter assumes no liability for direct or
consequential damages caused by its use.
I am not a flight instructor, aircraft designer, or aircraft mechanic,
nor any other professional in this field. I am merely using this site
as a medium for sharing my experiences and the things I've learned.
It is up to you, the reader, to determine the suitability of, and
assume responsibility for, the use of this information.
Copyright:
All materials on this web site, including the text, images, and HTML
mark-up, are Copyright © 2008 by Stefan Vorkoetter unless
otherwise noted. All rights reserved. Unauthorized duplication
prohibited. You may link to this site or pages within it, but
you may not link directly to images on this site, and you may
not copy any material from this site to another web site or
other publication without express written permission. You may make
copies for your own personal use.
|
|
