Wednesday, April 30, 2014

Ch 8.1 - 8.5: Headrest - Part 2/2

Return to the Headrests
Ok...back to finish the first 5 steps in Ch 8.  Note it's been 9 mths since part 1 of this post.  Mostly because I wasn't working on the Cozy for about 6 months while involved in some other projects, retiring from the USAF, securing a new job, honey-dos, etc.

Since I'm not using the plans version of headrests, I can skip part of step 1 and all of step 4 but of course that work will be replaced with other work to install custom headrests.  At the beginning of Ch 8, I already cut the shoulder support.  I also ordered and received my Saab headrests and the adjustable headrest guides from a Saab dealer.  Now it's time to get it all together.

First I located where the birch hard points for the shoulder harness attachments would go.  The plans call for 8.5" apart.  This was a change made in Newsletter #50 which reduced the spacing from 11.5" based on a European safety study that gave 8.5" as the max safest distance for shoulder harnesses.  I ended up adding 1/2" and making them 9" apart.  At 8.5" they were just a hair too close to my neck...and I don't have a big neck.  Researching the archives, I found that many builders questioned the change since there was no accident evidence to really drive it.  Some chose to leave them at 11.5" and use an "H" cross piece to ensure the harness stays on the shoulders in an accident.

Shoulder Brace, inverted,
w/ birch hard points curing with 1 ply BID
With that decision made, I cut the birch hard points and glued them in with 5-min epoxy.  Then I glued the two halves of the shoulder brace together.  I decided to go ahead and glass the inside...but not leave an overhang since I'm not installing it on the seat back yet.  This allowed me to have a more solid structure to work with while I figured out how to support the headrest.  (Post build note:  as it turned out, waiting to do this after I designed my headrest supports would have been better and saved me a little weight.)

Deviation Time...
At this point I had to start doing things a tad different.  I couldn't install the shoulder brace onto the seat back until I designed the support structure for the headrests.

After some thought, I decided to install aluminum tubing for the headrest guides to slide into.  This will provide more support than just gluing the headrest guides in place.  My fear is that someone will grab the headrest and lean on it while getting into the plane so I want to make sure there's enough support in case that happens.

After ordering tubing with the wrong wall thickness twice, I finally got the right size to fit the headrest guides...3/4" OD w/ .049" wall.  I located the holes for the tubes in the shoulder brace and drilled and glassed receiving holes for the tubes into the seat back w/ 1 BID to provide a more solid hold at the bottom.  The holes aligned with the front inside edge of the shoulder brace.  Later in the build, I'll adjust the length of the tubes if needed based on where I want the center of the adjustment travel to be...then they will be floxed in place.

Pilot side holes for
headrest support tubes
Tubes used to form glass
 down into the holes

Next I cut some leftover 1/4" birch to provide support structure for the tubes on the inside of the shoulder brace.  These support the top of the tubes while the holes in the seat back support the bottoms.  The tubes go through the birch right up against the front of the shoulder brace.  The back is cut to match the angle of the seat back so it will flox to the seat back just like the rest of the brace.

Birch supports for top of headrest
guide tubes
Checking everything before glassing
in the birch supports in place w/1 BID

Next I prepared the nut plates for the harness bolts.  I followed the recommendation in the FAQs to flox/screw the aluminum squares with the nut plate permanently to the bottom of the harness hard points instead of the hokey way the plans directs you to do it.  Each nut plate is secured to the aluminum with rivets.  Holes were then drilled in the opposite corners from the rivets for #4 screws to secure them (with Flox) to the underside of the birch hard points installed earlier.
Inside of the shoulder brace.  Headrest supports glassed
in place, ready to secure the 4 nutplates with
flox & #4 screws before installing in the plane

The shoulder brace was then installed per plans and taped at the adjoining sides.  Next I sanded a the sharp back corner of the seat back round to allow the 3 ply UNI reinforcements to take the corner properly.  They were installed longer than plans at 17" instead of the plans 13".  Since these are "reinforcements", I felt I wanted a little more than a couple of inches overlapping the seat back and behind the seat.

And here is the result with headrests temporarily installed and showing different adjustment heights. What you can't see in the pic are the black plastic adjusters at the top of the tubes.  During the finishing process, I'll flox in the support tubes and hide them in a small contour of some sort on top to make it look nice.  All you'll see then is the adjusters on top.

 Another chapter down...on to chapter 9 and the main landing gear.