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Need some help with this...

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Old 02-18-2012, 04:07 PM
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flyierjon
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Default Need some help with this...

Hi all, I know this isn't really a model airplane but its close enough, and figured you all could help with my problem.

Im trying to repair my cowl on my ultralight airplane and don't know how I would go about this. I started out and added a starter system to my engine and therefore had to open up the front end of my cowl a bit to make room for the flywheel and starter. I need to glass the sides left and right of the flywheel so the top and bottom half of the cowl connect to each other. can any one help and give me tips on the best way to go about this with fiberglassing and such.... pics below of the cowl as it sits now and the gap between the 2 halves.

Thanks All
Jon

p.s.- the first pic shows the gap I'm talking about the best.
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Old 02-18-2012, 04:46 PM
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Default RE: Need some help with this...

I would probably try lost foam method
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Old 02-18-2012, 05:37 PM
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Default RE: Need some help with this...

If it was a toy plane, I'd jump in but...........

What ever your do, note that you have heat that may play a roll in the type of resin and material used.

Steve
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Old 02-19-2012, 07:46 AM
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Walt Thyng
 
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Default RE: Need some help with this...

You might have better luck in an EAA forum.
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Old 02-19-2012, 09:46 AM
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Default RE: Need some help with this...

Well I am not a certified AC mechanic, but to repair (modify) a cowling, this is how I would approach the project. I would start off with the opening around the prop. Since the cowl has been spread vertically, you would need to begin with making a form (dense wood or a plastic material) the correct diameter of the front opening. I would remove the prop and mount the form in its place. You will need the form to recess beyond the cowl opening, so that you can form the new front opening to this form. This will give you a starting point for the other pieces to be shaped. This should be sealed (if you use wood) so that you can release it from the pieces you are going to create.

Owens corning pink or Dow blue foam would be my shaping medium for the new pieces. The entire cowl will need to be repainted once modified, so the only areas which need to be protected are the areas we need to pull the new cowling sections from. So I would use 2" wide clear packing tape to cover the outside painted surface around the new sections. Make sure to cover the area completely, but try not to have a bunch of overlaps in the tape. You could tape and mask the area beyond this with masking tape and paper, if you think you need to protect it from any spilled resin.

The idea now is to use the foam to create an over-sized block around the parts to sand and shape down to the final size. Protect your engine area with plastic sheeting and build up the blocks and securing them together with some 5 minute epoxy. You will need to do a lot of cutting to get the foam as close to the engine as possible, so that the foam will be thick enough for the shaping. once all the foam is in place, you can use rasps, hack saw blades, hand sanders, sanding blocks or whatever you think to shape the foam to the shape and size you want. I would create your fillets to the existing cowling using foam and a heavy drywall compound, and smooth it out to the shape you want.

Once all the pieces are shaped and formed, it is time to lay on the fiberglass cloth. I would think 3-10oz cloth, followed by 2-6oz and then 2-3 oz cloth would be adequate, but I really don't know for a cowl this large. A lot depends on the shape and size of the part(Curves are much more rigid than flat parts). Cut all the cloth and lay it on the foam (heaviest to lightest cloth) to make sure it all fits, and then take them all off and place them on something, so that when you do the lay up, the pieces are in the order you will need to place them down in. You will need to use a structural laminating epoxy resin system, like Aeropoxy 3032 with 3660 hardener or MGS 285 with the 285 hardener. Both of theses resins will withstand temperatures up to 185+ before they get soft(MGS is over 200). Both of these resins are VERY stiff when cured.

Mix you resin in batches that you can use in 10 - 15 minutes, and get the resin on the cloth as quickly as possible. I would paint resin on the foam, and begin laying the 10oz down first. As you add each layer, use a disposable 1" paintbrush and resin as needed to wet out each layer as you go. By the time you get to the last layers, the cloth should not even need additional resin, and you should just have to smooth it out with the brush. Do this to all the pieces and let them cure for a minimum of 24 hours in 70-85 degrees F. Epoxy takes 7 days to be fully cured, but you can work with it after 24-48 hours. I would remove each new section, remove the foam and clean off any rough edges at this time. Sand each part inside and outside to smooth any rough edges.

Now that you have each new section built, I would clean off the tape, place each part and cut the cowl so that you can join the new pieces to the existing cowl. You want to sand the edges to bond the pieces to, as well as the inside of the cowl a few inches so that you can add some cloth and resin to bond the pieces permanently. If the new parts are softer than you want, you can sand lightly on the inside and add additional layers to stiffen it up. You can now do any final sanding and body work needed to get it ready for paint.

Like I said, this will get you in the ball park, but you will want to do some research before you commit your time and resources.

Best wishes,
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Old 02-19-2012, 11:50 AM
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Default RE: Need some help with this...

The above suggestion is a good place to start. The only thing that I would add is that the epoxies suggested would not be able to handle those temps until they have undergone an elevated temperature cure. They can be quite pliable at 130*F if they have only been cured at room temperature. This requires you to cure the part in an oven. The part will probably undergo a heat cure naturally as the part is used but it could be susceptible to deforming the first few times until the resin gets post-cured from the engine heat.

Another option would be to use vinyl ester resin. Vinyl Ester can reach an HDT of 185*F with a room temp cure (unlike epoxies) and 230*F with an elevated temperature post-cure. The vinyl ester also has better corrosion resistance from oils and fuels than some epoxies. The downside is that vinyl ester resin will melt any extruded polystyrene foam that is used to shape the form (unless you seal it).
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Old 02-20-2012, 06:41 AM
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Default RE: Need some help with this...

thanks all for your help ill try your suggestions
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