Sunday, November 1, 2015

Ch 9.3: Main Gear Installation (3 of 4)

Alignment Time
It was time to see how accurate the alignment of the strut was when installed in the fuselage.  This is the ultimate test of how well I performed the previous steps.  I leveled and aligned the fuselage per the plans, marking F.S. 124.25 (firewall) and 108.25 (leading edge of the main gear strut) and ensuring everything was square and straight.  In fact I did it multiple times using my large squares and a laser.  This is not the step you want to screw up because some major holes get drilled based on this alignment that would be difficult to correct.

Starboard (Copilot) Side
Port (Pilot) Side

After leveling the fuselage, I centered the strut between the bulkheads and dropped plumbs from the strut leading edge.  I found both sides a little forward of where there were supposed to about .15" forward and the other about .25".  After a little adjusting of the bulkhead holes, I was able to bring both side to almost perfect.  You can see in the pics the plumb basically dead on the line...just hope my line is actually correct :-).

After getting the the alignment set, I rechecked the level of the gear...amazingly still level !

Rechecking strut level after everything is aligned
 (Truth In Lending Statement:  This pic is actually from a prior set up...
I re-did the entire thing because the saw horse legs would move to easy
 when bumped and would mess up the entire setup.)
Digital level showing strut
 still level after alignment

The last step is the installation of the mounting hardware to the bulkheads & the gear attachment tabs.
Onward, cab

Sunday, September 13, 2015

Ch 9.4: Main Gear Cover (1 of 2)

While I'm waiting for the main gear mounting parts to arrive from the Cozy Girrls, I decided to keep moving and work on the main gear access cover.

I did not follow the plans method for this.  Instead I followed Wayne Hicks method which I highly recommend.  Earlier when prepping the bottom for glassing, I did not make the joggles with the foam at the front main gear bulkhead because I new I was going to use this method.  With this method, you don't need those joggles, you don't need the aluminum slugs installed into the edges of the bulkheads, it's much easier, and will be form fitting when your done.

The first step was to stuff the area between the bulkheads with foam.  I found enough urethane foam scraps around so I didn't have to buy any.

Once the foam is stuffed, I sanded it down to the shape of the fuselage bottom and NACA scoop.  The picture to the left is in the middle of this process.

Once sanded to shape, I glassed the NACA scoop with 2 ply BID and then the rest of the bottom of the cover with 2 ply BID as well.

After the glass cured, but BEFORE I removed it, I attached two straight supports with Bondo across the NACA scoop.  These served to keep the cover in the proper shape while I work on the underside.  The picture at the left is just after I removed the entire foam plug with the top completely glassed.

After sanding the forward/aft edges of the cover to fit, I removed all but approximately 1/4" to 1/2" of foam all over the other side of the cover and all the foam around the perimeter of the cover about 1" wide.

There needs to be room for the gear so, as recommended by Wayne, I sanded some indentations under the NACA portion to ensure the cover would fit over the gear without rubbing.  Then I glassed the underside with 2 ply BID and shaped.

After that, I installed 2 ply tape along the forward and aft edges to reinforce the bare edges for the fasteners.

The only thing left to finish this up is to install the ledge and fasteners along the bulkheads that will hold the cover in place.  That will have to wait until I get the gear mounted because it would be harder to work on the installation of the gear after the cover mounting flanges are in place.


Ch 9.3: Main Gear Installation (2 of 4 cont'd)

With the outside LG tab layups now complete and straight, I can now accomplish the inside layups to finish off the LG tabs and be ready to start mounting the gear between the bulkheads.

Once the outside tabs are built and shaped, the inside tab layups are pretty easy.  This pretty much went according to plans however I did add layers of glass (~10 as I recall) to try to get the total thickness of each tab to approx 1" like the drawings seem to indicate.  You'll find multiple comments about this in the Cozy Group archives.  According to the archive comments, what's important is that the min # and type of plys are used regardless of how thick (or thin) it comes out to be in the end.

Glass ready for inside layups
Outside Main LG Tabs before
starting the inside layups
The one problem that might be created by added the extra plys is that there may be a bit of a stack up of glass where the glass makes the turn from vertical to horizontal over the gear bow...particularly on the thicker leading edge.  There might not be enough clearance for the MKMGA cross tube to be mounted without having to sand out a little glass in the middle first.  Since I added ~10 plys, I don't consider this much of a problem as I'll just be removing some of what I added.

And here are the tabs after the inside layups had cured and were trimmed. You can see how the plys stack up on the leading edge (left side of the left hand pic below).  That's where I might have to trim a bit out of the middle to make room for the MKMGA mounting tubes.
 Next up...Alignment and mounting in the fuselage.

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

Wednesday, March 4, 2015

Ch 9.3: Main Gear Installation (1 of 4)

Gett'in Jiggy With It.  

As bought, the strut doesn't have any way to attach it to the fuselage so you have to provide those by way of fabricating attachment tabs.

Note to future builders...Don't rush these steps.  Take your time, check and recheck measurements. The more accurate you are on each of these steps, the easier job you'll have installing the strut into the fuselage and having everything line up with minimal adjustment.  

The first step is to build the jig.  I built a perfectly vertical backboard and laid out the alignment marks that define the center line and location of the future attachment points.  A check of the FAQs indicated that it is likely that one attach point location would be slightly higher than the other just because the strut may not be perfect or you may have had some variation in the thickness of the previous 8-ply layups.  I had one side that was .1" higher.  The FAQs say to use the higher of the two and match them.  Now that the forward attach point located, I built the jig box per plans, bondo'd it to the strut, and transferred the points from the backboard to the jig..  This was pretty straight forward.   

Strut against backboard w/ Jig attached.
Profile of Jig and strut sweep angle.
Plumb bobs show the LE all lined
up on the table top.

With the jig built, I moved it to the work table and positioned it per the plans.  After some shimming and leveling, the strut is aligned such that the LE of the strut falls exactly 9.25" in front of the back edge of the jig.  This further aligns the strut and jig in such a way that all four attach points should be at the same distance above the table.  This then allows you to locate the rear attach points based on the measurements of the front points you already located.

Level Baby !

Complete setup, level both directions,
LE at the right spot, and braces to help  keep it there.

This all looked complicated at first but it's actually pretty simple and I think rather ingenious.  Now it's ready to build the attachment points.

Onward, cab

Sunday, March 1, 2015

Ch 9.2: Preparing the Strut...continued

With the reinforcements out of the way, I'm back to preparing the strut.

Glassing the Glass...
The next step is a 4 ply layup, ~35 degrees to the leading edge (LE), that provides the strut torsional strength that it doesn't have out of the mold.  The entire strut is made of S-Glass with all the fibers laid in the same longitudinal direction.  Without these layups over it, there's nothing to prevent twisting from landing loads.  You can see my ~35 degree alignment marks in the picture.  The glass runs from the LE up around the trailing edge (TE), and back down to the LE again...maintaining the 35 degree angle.  

This layup took me a lot more time than I thought it would.  What I estimated to be a 4 hr job took me 7 hrs just to get the first three ply's laid down.  At that point I was so tired I decided to peel ply and finish the last layer another day.

My difficulty was getting the long pieces of glass running the correct direction while at the same time ensuring there was enough on each side of the strut to cover it.  The first layer wasn't bad because I could get it in place on the strut and then raise it a little at a time to put epoxy under it.  After the first layer though the remaining epoxy was sticky and made it very difficult to get the glass laid correctly.  I came back in the next couple of days and finished the job with that last layer. 

A Place for Brake Lines...or Maybe Not.
Next the plans have you flip the strut over and install tubing for brake lines at the TE.  I skipped this task because I'll be installing gear leg fairings from Eureka CNC.  They provide a more efficient airfoil around the gear legs which reduces drag.  It is expected that you can pick up ~3 knots with these.  The prefabbed fairings have a cutout for the brake line conduit already built in.

More Torsional Layups...

Now four more ~35 degree layups...these wrap from the TE up over the LE and back down to the TE again.

(TIPS) I did three things different this time.  First, I used 15" wide strips of glass and I marked them with tape, cutting down the middle.  The tape keeps them from unraveling at the edge.  I cut the tape off once it was laid up on the strut so I could adjust groups of fibers individually.

Second, I raised the strut which put it at a more comfortable level to work on and allowed me more room to deal with the glass wrapping underneath.  I still couldn't get to the 1/8" gap the plans want you to try to achieve.  Frankly, I don't see how anyone can do that.  I did the best I could and then sanded the rough edges I had to a nice fade into the strut.

And third, I made sure to apply fresh epoxy right before each subsequent ply after the first..  This allowed the glass to be moved around easier to get it aligned correctly without sticking to the previous ply.

2nd set of 4ply layups complete, peel ply on.....Wheeew!!
 (Note the new wall sign I got for Christmas for encouragement)
Onward, cab