Cowling and Extension - Part II

With the mold made, next step was to extend the top cowl.  My plan (such as it was / is):

• Wax the inside of the aluminum
• Scarf the cowl
• Place a layer of peel ply against the aluminum (to get a nice finish of the end result)
• Build a jig to hold it all together upside down to allow for rolling pressure, etc.
• 4 layers of glass / resin, with another piece of peel ply on top (which is really the bottom)
• The gap to be filled is 11".  The first layer is 12", 2nd 12.5", 3rd 13", and final is 13.5.  That should give a good bond (I hope)

I cut the glass and peel ply on our pool table (because no one really uses, it, right?)

Here is the contraption all assembled and ready to go.

The error that I seem to have made was hoping that the peel ply on the aluminum would stay put.  It did not, and I got puckering in the corners.  No matter what I did, I could not get the glass to sit on the aluminum.  Further, I had significant resin bubbles that I could not get rid of.  I kept laying the glass down hoping it would work its way out, and it just kept getting worse.  Watching the clock, I finally got desparate after the final layer of peel ply and very quickly cut a big piece of bleeder material, layed it on top, and rolled the heck out of it.  Thankfully that resolved about 90% of the bubbles and left a reasonable finish.

(above pic is with the bleeder removed)

Next step will be to disassemble, remove the peel ply, and see what I have.

Turned out OK.  Going to be lots of finishing work to make it look seemless, but the profile is what I was hoping for.

Need to do a rough cut on the overhang, and start the mold for the bottoms.  I am going to do these one side at a time, and then figure out what to do about the air exit bulge.

Starting on the lower cowl sides and bottom - left side first.  I switched to .025 aluminum fro .032.  Much easier to bend, and the bottom radius is much tighter.

Same process.  Jig it up with the same aluminum angle braces and get it to sit right, then insert the aluminum and cleco it in.

Next is chamfering the edges and jigging it up to lay the glass.  The profile on the firewall side is good, but there are some gaps on the forward side the will need filling later.  I think this is unavoidable.

The upper and lower edges are not cut to the final profile yet.  Just trying to get the shape right.

Mold ready for glass

Layup complete

Upper and lower cowl on the airplane.  They line up OK.  Going to need some finessing on the transitions, but that is just filling and sanding.  Overall, mission achieved.

Right side jigging and layup

After removing the form and putting it back on the fuselage.

Next will be to reinforce the outside (where there is no overlap) to stiffen everything up.  Then will be the air exit ramp.

Started with the top.  Ended up using a 2" drum sander barrel by hand, with a separate sanding block to get rid of the gel coat (or whatever the surface is that came on the cowl).

2 layers of glass.  1" strip, followed by a 2" strip. A lot of this will be sanded off once cured.

The peel ply looks all blotchy as I added a 2nd layer to soak up the excess resin.

Overall I am happy with the progress and outcome, but this is going to take another month or so to make it look right.  I think I have a solid foundation, but there is going to be a lot of filling and sanding.

That is enough for this post... Part III to come.

Cowling and Extension - Part I

 For weight and balance purposes due to the mass of the UL engine, it has been pushed out 10.5".  Francois did a lot of calculations to come up with that number.  It should allow full loading of the baggage compartment.

But this means that the cowling needs to be extended.

After thinking about this for a long time, the plan will be to cut ~4" off the cowl on the aft end, bridge the gap with and aluminum mold, and then add 4 layers of cloth it to bridge the gap.

First step in that process is to center the cowling on the crankshaft.  The inner diameter of the cowling at the crank is 8", and the crank itself is just over 2.75".  Make a disk out of MDF, split it in half, and then temporarily attached it to the prop flange.

Because it is not supported at the back, there is no way to line it up properly, so I dont see any way around just cutting off the back and hoping for the best.

3.5/8" marked off to cut.

On the bottom, I left the scoop in tact with the hope of extending it.  So in the bottom, I only cut off the sides, plus about 4" off the bottom.

Initial test fit.  The back 3 5/8" of the cowl is clecoed to the fuselage, using the holes on the cowl flange intended for the firewall mounting.

Unfortunately it does not line up. the corners on the top half are pushing out and not following the correct line.

So I believe the solution is the overlap the seam between the top and bottom halfs (temporarily) to get the right profile on the edges.

I mounted all of my 22" long aluminum braces.  The theory behind 22" long...

• The gap to fill is ~10"
• I need 2" of scarf on each side (14")
• 2.5 additional " on the back side to pick up the fuselage to carry the line.  16.5"
• Another 6" forward to support the cowl to get an even transition.

Starting in the middle and working my way out on each side.  All of the braces are numbered, as is both the fuselage and cowl.

Using this method, I successfully followed the curve.  And my pushing the forward cowling piece down, the whole curve is lining up.

The trick will be redoing the goggle - but that is a problem for another day.  This shows the amount of overlap to get everything to line up.

The plan:  do the top cowling and fit it back on the airplane post fiberglass / scarf joint.  Then use the same method on the bottom (starting at the bottom and moving my way up.

For the mold, I am going to follow Ted's advice.  Aluminum.  I bought a 60" x 16" piece of .032.  Lined it up in the center, and took off the two center braces and drilled the first, and worked my way out again.  Had to trim the aluminum a bit.  17" wide would have been better in hindsight.

The only issue is the .032 does not want to bend, so its pulling out on the sides.  Plan will be to made a frame for it to sit in while I glass it.

Next is to take it apart and scarf the joints as close to 50:1 as I can get.  Going to put peel ply on the aluminum to try and get a decent finish.