Monday, April 20, 2015

Ch 9.3: Main Gear Installation (2 of 4)

Creating Attach Tabs

Layups in place and "gently" clamped
Now that the gear was jigged, it was time to fabricate the attach tabs which form the structural connections for connecting the main gear to the fuselage.  There are two layups.  First, a 25 ply UNI layup goes from one side, over the underside of the gear (remember the gear is upside down), and down the other.  The second layup is a 20 ply BID layup that just wraps over the underside of the gear on top of the 1st layup.  In all, the layups end up 45 plys thick except right in the middle of the underside of the gear where it will only be 25 ply.

Cured Attach Tabs ready for shaping
The thickness of each ply of UNI and BID should add together to make this layup approx .5" thick.  But mine, along with several others I found in the archives only came out only around .38" - .4" thick.  The archives indicate that as long as you are sure you have the correct number of plys, that's what counts.  However, there's also an email in the archives from Nat that states to just add plys until it's ~.5" thick.  I ended up adding 8 additional plys of BID, gently clamped, and got them to .5" or very close.  I may not have needed to do that, but if nothing else, I'll have a little extra strength where it counts.

Cutting out one of the Attach Tabs
After drilling the holes using a hole jig like described in the plans, I cut out the tabs to the ~3" wide shape with the 1.5" radius around the attachment hole.  I used my Fein tool to cut a shallow line around the shape, then a small chisel to take off a few layers at a time.  Wash, rinse, repeat until all the unneeded glass is removed.   When I got down to just a few layers left, I just blended the remaining into the gear leg with a sander so as not to accidentally cut into the torsional plys.

Two Steps Forward, One Back.
Fwd, Port Attach Tab ended
up being cockeyed ~10 degrees
One critical part of this is being sure to get the glass going down
the jig to be perpendicular to where it crosses the gear leg.  When the glass is wet with resin and heavy with all the plys, it can slide down the gear leg a bit. (Ask me how I know this :-) )  When this happens, the attach tabs will end up being angled over to where the hole is which will compromise strength a bit.  My fwd/port side tab ended up about 10 degrees.  This ends up compromising strength somewhere between 7-10%.  According to Marc Z, this isn't significant.  This, combined with the fact that it just didn't look right, led me to a do over on the port side attach tabs.

I only built the half of the jig I needed
 and supported the other side as needed.
Perfectly Aligned
So I spent an afternoon removing the entire port tab and another afternoon jigging the gear up again to re-lay the cockeyed attach tab.  I think I got the gear jigged even better the second time around.

Port Attach Tab - Before
Cockeyed to the left
Port Attach Tab - After
Re-built...this time it's perpendicular.
Again, I ended up adding 8 additional BID plys to get the thickness to ~.5".  This time I made sure to draw a bold center line where I wanted the plys to go as well as some edge guidelines to help me make sure things stayed centered.  The resulting attach tab was nearly perfect.

This was just the external plys, now it's time for the internal attach tab plys.  Onward, cab