Rick6a

Rick6a

Well Known Member
Overall, I'm quite pleased with the cowl fit.....to one another and the airframe. But as the photos show, a serious mismatch exists in a very noticeable location. I want to fix this right. For those practiced in the subtle ways of fiberglass, I ask....what is the best way to address this problem that will insure pleasing, lasting results?

 
I had a similar - probably a little less- underbite...but you asked how to fix it RIGHT, so I will let the fiberglass gurus answer and sit back and find out what I should have done.....;)

Truly, all I did was build up the top cowl with a mixture of West System's goops and stuff until everything matched!

Paul
 
Its the long and short story off fiberglass. Sand and fill

I have never seen 2 cowl halfs come together perfectly in the front.
This area of the frontal intake takes quite a beating. I recommend flox. West 2 part epoxy and milled glass fibers. Both avail from ACS. Cotton would probably work, but you might as well get the glass. Its strong stuff. Here is a short lesson that can help get you started.
http://www.torreypinesgulls.org/epoxy.htm
Best,
 
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Not to tough

Looks like you already have the hinge set up. My first recommendation is BEFORE you begin any cowl fit work to the fuselage fix the intake fit. Work on the overlaping flanges with your Dremel. With a few minutes of work you can greatly improve this fit.

Now, before you do any other work, scrub the cowl with soap and water and a stiff nylon brush. Rinse thorough. I then go a step further and wipe down the entire cowl with acetone or lacquer thinner.

Tape the edge of the cowl that won't get filler applied. Put the cowl together and tape to hold in place.

Mix a thick batch of Micro filler (WS 410) and epoxy. Before application, wipe the area with acetone. Apply with a squeegee and work on blending with the other cowl half.

If you do this carefully you'll minimize the sanding and additional fill needed.

Repeat as necessary until you get that perfect look. Small patches and minor fill can be completed with Rage auto body filler. This is a catalyzed filler that will not shrink and will bond great. This will save you time.

On my cowl I covered all the fill work with a layer of 1 oz glass cloth to lock everything together. As Kahuna noted, this area takes a beating. It probably would have been fine but I wanted to be sure.

The cloth I used is very thin. I used Rage to feather these edges.

The process is not to bad. Work in steps and you'll be done in no time.
 
?

Why dont you just sand the rear of the cowl untill they match in the front?
Best
Brian
ps.. I'm not an expert in fiberglass... so no flaming please...
Thanks
 
not only that

brianwallis said:
Why dont you just sand the rear of the cowl untill they match in the front?
Best
Brian
ps.. I'm not an expert in fiberglass... so no flaming please...
Thanks
Click to expand...

but the center meets up and the sides dont. so he needs to build up. maybe if the hinges were not on he could have pulled them some. but you dont want to pre load things or else you'll be seeing broken eyelets on your hinges.
 
cowl mismatch

Hi all...
I came up with the same problem, although not quite as much Overbite. My solution was to clean up the INSIDE radius of the cowl, and lay up additional layers of cloth, followed by resin and flox to thicken the area. then I ground away the offending overbite slightly beyond the needed amount. Then I laid up some bid cloth over the cowl on the OUTSIDE and sanded to contour. Final fairing with micro , and it really fit well. Nice and strong, and only took about 3 hours to do.
Just one solution, I am sure there are many more!!
Hope this helps....Chris
 
brianwallis said:
Why dont you just sand the rear of the cowl untill they match in the front?....
Click to expand...
Brian,

Fair question. The cowls were trimmed at the rear to achieve a butt gap with the firewall relative to the desired prop spinner gap, in this case I went with 1/8". After some preliminary trimming, the cowl halfs mated together fairly well and the final trim to the airframe and installing the hinges essentially completed the cowl installation and turned out well if I do say so myself. But the fact is, even though the cowl halfs mate close enough everywhere else including the centerline structure behind the spinner, the upper and lower cowls at the outboard front area do not match each other perfectly....and never would regardless of what the builder does. This is probably due to the molds...something we have no control over. Like Kahuna, I have never seen cowl halfs mate up perfectly right out of the crate. Still, the area in question takes an aerodynamic beating and mine will require a significant build up of material to achieve surface flushness with the bottom cowl. I just want to make certain that the measures I take now will insure I won't regret what I did a couple of hundred flying hours later.

 
carguy614 said:
Hi all...
I came up with the same problem, although not quite as much Overbite. My solution was to clean up the INSIDE radius of the cowl, and lay up additional layers of cloth, followed by resin and flox to thicken the area. then I ground away the offending overbite slightly beyond the needed amount. Then I laid up some bid cloth over the cowl on the OUTSIDE and sanded to contour. Final fairing with micro , and it really fit well. Nice and strong, and only took about 3 hours to do.
Just one solution, I am sure there are many more!!
Hope this helps....Chris
Click to expand...

Mine was less severe, and only on one side, but I did the same as above. Very strong and easy to do. Holding up nicely.
 
My cowl had a similar problem, with one side lining up well and the other about 1/2" back (top left side short). I ended up slicing the top cowl from the center to the outboard edge, just forward of the honeycomb sandwich. Before cutting, I sanded the inside area of the cut thoroughly to roughen it up. I then aligned the piece by springing it into position (no real resistance here) and then placed a couple of aluminum tabs to hold it in position by drilling and clecoing with 3/32 clecos. Now that the correct position was fixed, I removed the top cowl, inverted it and layed up 3 layers of 3" wide 6 oz cloth. The next day I filled the gap with west structural filler and epoxy. It was very easy and I spent longer agonizing over different solutions that it took to complete. I'm very satisfied with the results and the structural integrity. I didn't feel comfortable just building up a large amount of filler.

The face of the cowling just behind the spinner ended up not quite flush also, probably about 1/8" on one edge. To fix this, I put the hinge pins on the two cowls to join them together, off of the plane, so it would stand on the floor upright. I then taped up where I didn't want filler to be, mixed up a good batch of west epoxy and filler ballons and spread this over the lower area. I then placed a glass plate (from my dining room table) on the top and weighted it down so it forced the filler out until a good flush surface existed. I had simply taped the plate with packing tape and put a little car wax on this to protect the glass. After this cured, the glass popped right off, the surface was flat and true, the glass was put back on the dining table and my wife never knew the difference:D

Hope this helps
 
Trib has a great solution above to the cowling misalignment - much easier than going through the buildup and sand down process I think. My cowling lined up pretting good around the inlets, but was not a good match behind the spinner - couldnt get a consistent width gap there. So, ala Trib's method, I made a cut to split it, wedged it open, then just some simple filling with flox and minor sanding. Works great.
 
Rick, this is how we ddi it at UA. I would star with a Half inch strip 120 weight cloth and apply it to the top cowling in the lowest postion.. Then apply additional strips with a 1/2 inch overlap. For example your first repair is .5 second is 1.5 third is 2.5 ect work untill the low area is brought up level with the bottom cowl. What your doing is feathering the area out with fiberglass in a uniform fassion. The final cloth is 181 weight which is finner the than the 120 which will help with the top coat and finsh. let it dry and see where you are.. If it is good I would use aero bond or simlular filler to bring it to shape..

ALso when you put screw thru honey comp you have to create a hard point or you will crush the honeycomb. Drill a hole thru one side into the honey comb, Stick a allan key ( small side ) inside the hole and attach it to a drill.. Destroy and and pulverize all of the honeycomb within range of the alan key, Inject with liguid shim and let it dry, Drill a hole all the way thru with a drill and set ( A 1100 )rivet like you would normally.
 
Rick,

Consider filling with flox the top cowl to the lip on the bottom cowl then lay up two plies of glass across both and after cure carefully cut the glass with a dremel along a line extended forward from the aft part of the cowl. The match would be perfect and the glass would provide some ding protecion. Peel ply across the lay up would minimize finish work.
 
I think the best approach is to fill in on the underside with resin, cloth, and flox and grind back the underbite to match the upper cowl. Your fit is really nice, otherwise, and this little bit of repair will do nicely. I would refrain from adding to the outside and building up the upper cowl. That's too much to add on and will be harder to match. My cowl was similar, just not quite as severe.

Roberta
 
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