FG Cowl Construction
 

Moved this over from Composites and Fabrication as noone seemed to be able to respond......Starting soon, so all thoughts are appreciated:

Fiberglass Cowl Construction (post # 1)

I am about to undertake a project to construct a fiberglass cowl using the "lost foam" method for a kit that comes without a cowl. While I have researched most of the threads that contain this subject, I did not reach any conclusions on weight of cloth, best type of foam for plug material or layers recommended. So, without beating a dead horse, I would like input on the following:

1. Should I concentrate on one weight cloth with multiple layers or vary to lighter cloth with the last layer or two? This will be a 40 size sport plane, so I want to keep the cowl light but sturdy.

2. What type of foam is the best plug material? I have heard of people using blue or pink foam found at alot of homecenters to guys who take an old ice chest or packing styrofoam and layer it into a big cube and start sanding to shape.

3. Resin is according to everyone's preference I'm sure, but what about micro balloons to fill the weave and help with the smooth finish?

4. Foam removal. Everything from gasoline to acetone has been suggested. My plan is to use dremel tool and pick to get as much as possible then clean it up with some sort of solvent. Thoughts?

5. Finally, just looking for thoughts on this process. I saw it somewhere....I plan to bolt my cube of plug material to the firewall. After trimming and sanding to desired shape, will remove the plug and begin the glassing process. My plan is to determine the depth by trial fitting the engine to find length from firewall to prop hub. That will give me the minimum critical length to the prop hub. Adding to that length will give me what I need for overlap onto the fuselage. Does all of that sound right?

Thoughts, comments and suggestions are welcomed from all.

John

FG Cowl Construction
 

I just did a cowl and canopy that way, so here's my two cents:


I used 3 layers of 3 oz cloth, then one of .6 to try to save some filling. It turned out that I sanded through most of the light stuff getting the imperfectons out.

I used epoxy, and then epoxy with lots of microbaloons as a filler.

Blue foam is very fine grained and a little sturdier than the white, so getting the shape right worked better with it.

I dissolved the foam out with lacquer thinner on the canopy but it took quite a bit of thinner and kinda softened the part (it firmed up later) so I gouged, picked, and ripped out most of it on the cowl and just used the thinner fo finish up.

Good luck

FG Cowl Construction
 

Strat,

Thanks for the suggestions. Can you tell me what size plane this was for? Also, what was the overall thickness of the cowl when you were finished?

John

FG Cowl Construction
 

The airplane is a .15 size sport plane.......looks like one of the big pattern planes w/ inverted engine and a bubble canopy.
The canopy is about 11" long and 2" wide and quite flexible til it was guled in place. I'd estimate the thickness as a little less that 1/32".
The cowl was a little thicker, I think I may have used four layers, and made it stronger because it's just held in place with three screws. In my experience, how things are mounted has a lot to do with how strong they need to be.

FG Cowl Construction
 

I like to use 6 oz when I find it. Other than that, 4 or so. I do 3 layers and end up sanding about 1 layer off. Use the aluminum sanding bars with aluminum oxide sand paper, 80 grit to rough it in. I like to tack glue with just 4 dots of elmers or tightbond then sand to shape. I found out after severl times that right after you've sanded to shape, brush on a layer of thinned elmers glue (with water) over the foam. Couple of coats is all thats needed. Then fill in with light weight spackling and finish sand. Then a couple more coats of thinned elmers again. Now you can glass the plug right on the plane. It makes it easier to see how much overlap you have. Usually 3/4" is all I need. Too much and its tough to break the plug off. I do all my sanding while glued to the plane. When happy, break it off, pour a little gas in and scrap out the goo, then rinse with water. You'll notice that the inside is nice and smooth. The thinned elmers glue does this for you. If you dont do it, you get a bunch of little spikes that need sanding down on the inside. I let whats left of the goo dry and use a dremil with round and crown wire brushes to finish off the inside. And you're done. Takes me about a lazy weekend to do a cowl. I generally use regular styrofoam. You'll be amazed at how light it is. Very durable. I normally laminate 1/32" ply where mounting screws will go or just add some more glass on the inside after its cleaned out.
Edwin

FG Cowl Construction
 

Edwin,

I never thought of doing the glassing on the plane. I was planning to just mount the plug to a dowel rod after final shape was achieved and put the dowel in a vise. How do you keep from messing up the fuse where the overlap occurs? Wax paper?

Nice info on the elmers. Have to give that a try. Thanks.

John

Plug release
 

If you use a couple of coats of simonize or bowling alley wax
on the plug the cowl will come off without much trouble (professional makers use the plug over & over) oops u got to be careful not to key it together a an opening or reverse curve .

:cool: :cool:

FG Cowl Construction
 

I use 2" wide clear mylar tape. Just remove it when done. In talking with others, they thought using vaseline should work just as well. Havent tried it yet. Makes sense, I use it to keep hinges free of epoxy. I find the sanding goes much faster on the plane, and you can use the TAR method of aligning and checking your finish.
Edwin

FG Cowl Construction
 

Thanks everybody for the information! Others please keep it coming and share your techniques. I've already found a few ways to adjust the method I was planning on.

Appreciate the help.

Regards,
John

FG Cowl Construction
 

I use blue OR pink foam (what ever I have on hand). Blue sands easier than pink.

After the plug is carved/sanded to shape, cover it with a LOW TEMP film (stops the resin from eating the foam). I then wax the plug. Depending on the plug shape, you may be able to save it for a second cowl.

For a 40 - 60 size I NORMALLY use 3 layers of 4oz cloth. I use finish resin.

I apply the cloth/resin as follows: LIGHTLY spray the cloth with 3M #77 adhesive. Then cut the strips of cloth and apply to the plug. I then mix the resin (about 30 - 45 minute work time) AND CUT IT about 25% with acetone (makes it VERY thin). This thin mixture I work into the cloth with a "flux" brush until the film color is clearly seen. Let dry (about 48 hrs). Then rough sand and apply a coat of regular mix resin and let dry.

Make sure any excess resin and cloth are clear of the bottom and cut a 3/4" - 1" hole in the top center of the glass. If you have done everything "right", the plug should pop out and be reuseable. If not, make an X slit in the back and pour about 2 - 4 oz of gasoline onto the foam. Come back in an hour and pull the film and "sludge" out.

The cowl is going to be VERY flexable at this point. Do any final shaping and let dry for an additional 72 hours. THEN you can sand and fill as needed (I use the red spot putty for filling any low spots).

I use automotive type sandable primer to prime the cowl and paint with your choice of color/paint. If the paint is fuel proof, clear coat it with a THIN coat of "Crystal Clear" Lustercote.

Hope this is what you wanted.

FG Cowl Construction
 

Campy,

Very detailed, Thanks. I was planning on bolting the plug to the firewall for shaping, then removing for application of the glass. Would you recommend this or should I leave it bolted to the plane? If I leave it bolted to the plane, should I put some wax paper or monokote on the fuse so the "overlap" will stick to that instead of the fuse?

Thanks everybody.

John

FG Cowl Construction
 

I just finished the cowl on this 40 size plane about a month ago. I took a chunk of white foam and tack glued it to the fire wall. Then I carved and sanded it to the shape and length that I wanted to match up with my spinner. After this I put a thin coat of Spackle on and finish sanded everything. I then searched my house high and low for something to protect the plane and cowl. After much searching my wife suggested to use something made out of latex, and it worked perfect. Protected my plane and cowl, and the cowl just popped right off when I was finished.

This was my first try and it worked very well for me.

Just my 2 cents.

FG Cowl Construction
 

Definitely remove it from the plane.

The application of the resin can get messy. In addition, I USUALLY make the cowl a little longer than what I need, then do the final trimming after the cowl is made.

Two other items -
1. For a cowl that is going to go OVER the fuselage, make your plug about 1/64" - 1/32" larger than the fuselage. This will give you room to slide it over any covering with little or no damge to the covering.
2. If you decide to do a "flush" mount of the cowl, you will need to make your plug about 3/32" smaller all the way around to allow for the thickness of the cloth and resin.

Ted

FG Cowl Construction
 

You can see a "flush" mount cowl I did with this model.

FG Cowl Construction
 

Campaholic,
Took me a few minutes, but I figured out the latex product you used. A man's gotta do what a man's gotta do....hehehe. Great looking cowl.

Campy,
Mine will be a slide over the fuse type. Here's a tip that a guy at my field suggested. After the cowl plug has been shaped, you can cut into the foam your hard points for mounting with the appropriate thickness of light ply. Tack glue them in and when you glass, they will become part of the cowl. He tells me that this is great for wear and tear where the mounting screws go, and is much stronger that adding a couple extra strips of cloth in those areas.

I think 1/32" should be enough, as any slop can be taken out on the underside where the pitts muffler will come out. As for spinner distance measurement, I am planning to mount the engine. Measure from the appropriate place on the fuse where I want the overlapout to the prop hub. Add 1/16" and viola, the needed spinner clearance. Any minor adjustments can be made on the back end prior final mounting. Sound right?

One more thing: Using your cloth recommendations, would this cowl be strong enough to hold a filler valve, or should I build a mount and just put it at the bottom of the firewall?

Regards,
John

FG Cowl Construction
 

Read this whole thread & saw no mention of the balloon method . Is this a lost art or haven't you troops ever heard of it. Really solves a lot of problems in making a small cowl . MAX H.

FG Cowl Construction
 

The cowl MAY be ok for a fueler valve. I would suggest a ply backing for it though.

If you cut the ply into the foam, the ply will stick out from the inside of the cowl. I would think you would want the ply flush with the inside of the cowl.

I would go with 1/8" instead of 1/16". Remember, it is easy to take the material off, but not so easy to "stretch " it if you are short.

One thing I have done for quite a while which works well for me, is to make a "washer" from a piece of fuel line. Then make the hole in the cowl SLIGHTLY smaller than the "washer". When you put the screw in, it expands the "washer" to form a snug fit and cushions the cowl preventing cracking. I think the latest issue of RCM has the same idea using servo grommets and the brass eyelets.

FG Cowl Construction
 

Big Max,

Not familiar with the balloon method. What gives?

FG Cowl Construction
 

Use your plug the same as they say ,one mounted on a stick. Put the stick through the bottom of a plastic bag & put stick in your vise (bag protects vise) . Brush on a coat of resin & place strips of glass cloth over the plug ,use long enough ones that start on one side & go over middle of plug to other side. Saturate with resin again . Take a child's round balloon blow it up & place the bottom of the balloon on the plug with the glass cloth & press down , as you press release air & let the plug go up inside the balloon until it is all the way on & the air is all out. The rubber puts equal pressure all around plug & makes a very smooth lay up , let harden cut off balloon & trim cowl to fit . May not be perfect the first time but does work quite well .If you use white foam for plug ,gasoline will eat it out. MAX H.

FG Cowl Construction
 

I have done a few cowls, the last one was for a .60 pattern. The usual method, Stick a foam cube, mark the exact thrust lines, stick a 3/32" lite ply mouth ring matching the spinner back plate taper lines. That and a few card templates made from the plans give me a great guide for carving the foam. Coat the plug with thinned white glue (sand finely between intermitent coats). This also gets the foam surface hard. Once sanded smooth, I overlap the fuse and cowl an inch either way with a clear cello-tape (which takes great care for the release from the fuse). Now I haven't done much experimenting, but so far a medium wt. Great PLanes glass cloth (single lay-up)is all that I have used. I run a thin coat of epoxy to soak the weave and after that dries, a thicker coat of epoxy. I heat the epoxy to be able to brush it on nice. After curing, I use balsa filler mixed with dope to keep it light and then sand it down.
Now, its a good time to mark the cowl openings. Usually I stick a syringe with thinner to burn the core near the fierwall till I am able to release the cowl off the fuse. Dig the foam from the inside. This is the point where I decide how much additional stiffening is required, depending on the flex and the openings. So patches of GF are applied. Next is to cut the openings.
To get a better surface finish , I cover the cowl with Monokote, works great with good heat and keeps the cowl light as well. It surprises me how well it goes over curves and minor dings don't show up at all. Its easier to do this with the cowl back on the fuse.
Thats my pennies worth, hopefully.
Good luck.

FG Cowl Construction
 

Thank you to everybody for the input. I started on the plane last night and should be ready to do the cowl in a week or so. I will be using many of your suggestions in my method.

I started a thread over in kit building about this plane, so keep an eye out for pics of the cowl when it is started. The thread is the one entitled 4*40 "Barbie" Building Thread. This is gonna be a fun project.

Thanks Again,
John

FG Cowl Construction
 

Plug came out pretty nice for my first attempt. Could not find any pink or blue foam, so I went to the craft store and got some of that foam they use to stick flower arrangements in. There were a lot of different kinds to pick from, so I got the kind that most closely replicated blue foam. I bought 4 blocks that were 8" x 4" at .99 each. Pretty cheap I thought.

This is not the stuff that dents when you touch it or disenigrates if you look at it wrong. It sanded nicely and consistently too. Tonight it will be coated with diluted elmers and hopefully Thursday or Friday it will get the first lay-up of glass.

When mounting to the fuse for shapping, here's what I did. First cut the cowl cheeks off with about 1/8" to spare. Sanded the "stubs" flat at the ends to mirror the 2 degrees of downthrust on the firewall. White glued a piece of 1/32" balsa to the back of the plug. The whole assembly was then placed on the "stubs" of the cheeks with thin CA. A dowel was run through the firewall in the center and on the thrust line all the way through the plug. Spinner back plate was then put on the dowel and the outline was shaped. When it came time to pop it off, I removed the dowel and gave it one little downward twist. Off it came! There was very little CA residue on the stubs and the 1/32" balsa just peeled off the foam. Nice and clean.

John