Modified nose cowl
12-23-2006, 09:52 AM
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Modified nose cowl
I have just obtained a very nice short kit for a motorised 60 i/c glider. Unfortunately the cowl is made from thin plastic in two parts, split along the engine centerline. They are a little flimsy and I would like to stiffen these up with fiberglass and epoxy on the inside or use them as moulds to make new cowls. Although I have used fiberglass before, I have never carried out this type of modification. I have only one chance to do this properly, as I cannot experiment and do not want to screw this up.
Can anyone please indicate how this should be done and any pitfalls that I should avoid. I do not know what grade of plastic was used originally.
Thanking you, John.
Can anyone please indicate how this should be done and any pitfalls that I should avoid. I do not know what grade of plastic was used originally.
Thanking you, John.
12-24-2006, 03:16 AM
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RE: Modified nose cowl
What you're describing sounds like white ABS molded plastic parts. The two pieces may have a small overlap on the inside to ease fitting them together, or they may just butt to each other. In either case, it should not be hard to fit them together and hold them temporarily with short strips of masking tape. If there is an overlap, you can drop some plastic cement in the seam, which should set firmly in just a few minutes. If no overlap, tape the entire length of the seam from the outside, then drop in plastic cement on the inside.
With or without using plastic cement, I would lightly scuff the inside surfaces with 150-200 grade sandpaper, fit them together and hold with masking tape. Then brush on a coat of 30-minute or laminating epoxy over the entire inside surface. Cut short strips of fiberglass (I would probably use 2 oz grade) and stipple them into the epoxy with the point of a disposable brush, taking care to leave no air bubbles under the glass. Just let the ends of the strips hang out over the edges, and trim them with a sharp blade when the epoxy has partially cured--after maybe 8-12 hours. I'd probably go for two laminations in one go, crossing the strips more or less. The interior laminations will be fairly haphazard, but that's OK. It's not really necessary to have a double lamination over every part of the surface; the point is to provide some reinforcement to protect from rough handling.
The interior may be left to simply cure with no further work, or you may want to clean up any pimples with sandpaper when it's well-cured (24 hours or more).
Depending how beautiful you want the exterior to be, you can also scuff that and lay on some lightweight glass (3/4 oz grade) and epoxy that down as you did on the inside, again using narrow strips to conform closely to the curved surface. Whatever overlaps there may be can be easily sanded down smooth when everything has cured. Before painting I would sand everything smooth, maybe dry-sand with 200 then wet sand with 400. Make sure you use fresh, clean sandpaper at every step. Keep paper towels handy to wipe clean as you wet-sand. This should give you a surface to be proud of.
You'll have less unraveling of the glass cloth filaments if you cut your strips with the edges at 45 degrees to the weave. Also, give the glass cloth a light shot of hairspray before you cut it (using sharp scissors), as that will also keep the fibers from coming loose under your brush. I normally drape the cloth over a sheet of cardboard and prop it up vertically before I use the hairspray. You don't want so much hairspray that you clog the weave of the cloth, so keep it light. You want the cloth to absorb the wet epoxy through the weave.
Paint the cowl with whatever you have handy for modelers spray paint. Prepare the surface again by sanding with 200-400 grade paper. If you have little pits or cracks in the surface, you can fill them with glazing/spot putty, which comes in small tubes at your local auto parts store. This stuff goes on easy with spatula or fingertip, dries quickly, feathers out beautifully, sands nice and smooth.
If the cowl is going around an IC engine, I would bolster the mounting holes with a small patch of CA hinge material, epoxied to the interior surface and overlaid with another slightly larger patch of glass cloth. Let it cure, then drill your mounting holes through the center of the patch. This will give plenty of resistance to the tendency of the cowl to vibrate against the mounting screws and result in cracks radiating from the mounting holes. I also cut a tiny doughnut of silicone fuel tubing to put under the head of the cowl mounting screws to absorb any vibration.
I am not inclined to get too uptight about the finish on my cowls, as they inevitably get abused from the normal stresses of flying and handling, not to mention the occasional nose-over or pranged landing. Just give it some color to match (or not) your airframe covering, fly and have fun. If you want a pretty model to look at, you'll have to get your advice from someone other than me.
This may sound like a lot of work, but it's not hard to do--just takes time to set up, cure the epoxy, sand, paint, etc. It's stuff like this that makes ARFs popular with people who are more interested in flying than messing around in the workshop.
It would be possible for a total klutz to screw this up, but the result would most likely be shoddy appearance rather than a dysfunctional cowl. I would not let that interfere with my flying pleasure. Guys with pretty airplanes that stay that way... well, good luck to them.
With or without using plastic cement, I would lightly scuff the inside surfaces with 150-200 grade sandpaper, fit them together and hold with masking tape. Then brush on a coat of 30-minute or laminating epoxy over the entire inside surface. Cut short strips of fiberglass (I would probably use 2 oz grade) and stipple them into the epoxy with the point of a disposable brush, taking care to leave no air bubbles under the glass. Just let the ends of the strips hang out over the edges, and trim them with a sharp blade when the epoxy has partially cured--after maybe 8-12 hours. I'd probably go for two laminations in one go, crossing the strips more or less. The interior laminations will be fairly haphazard, but that's OK. It's not really necessary to have a double lamination over every part of the surface; the point is to provide some reinforcement to protect from rough handling.
The interior may be left to simply cure with no further work, or you may want to clean up any pimples with sandpaper when it's well-cured (24 hours or more).
Depending how beautiful you want the exterior to be, you can also scuff that and lay on some lightweight glass (3/4 oz grade) and epoxy that down as you did on the inside, again using narrow strips to conform closely to the curved surface. Whatever overlaps there may be can be easily sanded down smooth when everything has cured. Before painting I would sand everything smooth, maybe dry-sand with 200 then wet sand with 400. Make sure you use fresh, clean sandpaper at every step. Keep paper towels handy to wipe clean as you wet-sand. This should give you a surface to be proud of.
You'll have less unraveling of the glass cloth filaments if you cut your strips with the edges at 45 degrees to the weave. Also, give the glass cloth a light shot of hairspray before you cut it (using sharp scissors), as that will also keep the fibers from coming loose under your brush. I normally drape the cloth over a sheet of cardboard and prop it up vertically before I use the hairspray. You don't want so much hairspray that you clog the weave of the cloth, so keep it light. You want the cloth to absorb the wet epoxy through the weave.
Paint the cowl with whatever you have handy for modelers spray paint. Prepare the surface again by sanding with 200-400 grade paper. If you have little pits or cracks in the surface, you can fill them with glazing/spot putty, which comes in small tubes at your local auto parts store. This stuff goes on easy with spatula or fingertip, dries quickly, feathers out beautifully, sands nice and smooth.
If the cowl is going around an IC engine, I would bolster the mounting holes with a small patch of CA hinge material, epoxied to the interior surface and overlaid with another slightly larger patch of glass cloth. Let it cure, then drill your mounting holes through the center of the patch. This will give plenty of resistance to the tendency of the cowl to vibrate against the mounting screws and result in cracks radiating from the mounting holes. I also cut a tiny doughnut of silicone fuel tubing to put under the head of the cowl mounting screws to absorb any vibration.
I am not inclined to get too uptight about the finish on my cowls, as they inevitably get abused from the normal stresses of flying and handling, not to mention the occasional nose-over or pranged landing. Just give it some color to match (or not) your airframe covering, fly and have fun. If you want a pretty model to look at, you'll have to get your advice from someone other than me.
This may sound like a lot of work, but it's not hard to do--just takes time to set up, cure the epoxy, sand, paint, etc. It's stuff like this that makes ARFs popular with people who are more interested in flying than messing around in the workshop.
It would be possible for a total klutz to screw this up, but the result would most likely be shoddy appearance rather than a dysfunctional cowl. I would not let that interfere with my flying pleasure. Guys with pretty airplanes that stay that way... well, good luck to them.
