How would you prepare the plug for this part?

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#1

I’m hoping some of you more experienced guys can help me with a part that I’m fabricating. I need to fabricate a plug for a set of landing gear doors for an experimental aircraft. In order for the finished product to match the contour of the lower surface of the wing I intend on hot gluing (or otherwise attaching) either a (a.) sheet of flexible plexiglass/lexan or (b.) 1/8” mdf board (I think I’m using the right terminology when I say “mdf board” as the stuff I’m thinking of is fairly flexible fiber board that I can buy at home depot with a relatively smooth/shiny surface on one side and a fairly textured surface on the other).

The process as I’m thinking of it (please correct me if I’m off base on any of the steps/materials):
[FONT=Calibri]1. [/FONT]Cut a piece of the plexi/lexan or mdf that is 5” larger than the gear doors (the doors will be 16” x 14” approx) and attach to the lower surface of the wing directly over the landing gear bays (the wing is currently in a cradle that allows me to roll it upside down and work on the lower surface of the wing).
[FONT=Calibri]2. [/FONT]Use expanding polyester pour foam to create an aerodynamic “bulge” near the center of the inner gear doors (this bulge is necessary because the brake calipers protrude slightly (1” or so) from the lower surface of the wing with the landing gear fully retracted. NOTE: The plexi/lexan or mdf will have a small hole to clear the caliper, this way the plexi/lexan or mdf will conform to the airfoil shape of the wing.
[FONT=Calibri]3. [/FONT]Once the hardened pour foam has been carved to a nice (but rough) aerodynamic shape I plan on filling and smoothing to the final shape with polyester body filler.
[FONT=Calibri]4. [/FONT]Next I will use a scratch awl to scribe the outline of the gear doors in the plexi/lexan or mdf (this will leave a nice part trimming line on the finished part)
[FONT=Calibri]5. [/FONT]Prime with Duratec surfacing primer (spraying it) – should I add the high gloss additive?
[FONT=Calibri]6. [/FONT]Wet sand/Polish/Buff the plug.
[FONT=Calibri]7. [/FONT]Wax with x coats to mold release wax.
[FONT=Calibri]8. [/FONT]Brush on orange tooling gel coat and allow to cure to the point that it’s still slightly tacky
[FONT=Calibri]9. [/FONT]Apply first layer of 2oz glass cloth/polyester tooling resin
[FONT=Calibri]10. [/FONT]Apply 6 layers of 2.5oz CSM two layers at a time… wait one day then apply the next two layers and another day to apply the final two layers.
[FONT=Calibri]11. [/FONT]Once the mold reinforcements have been completed I’ll glass two pieces of 1” pipe along the flange (on opposite sides) for reinforcement and to give the mold something to stand on when I layup the part stack.
[FONT=Calibri]12. [/FONT]Once completely cured I will de-mold/polish/wax and the mold will be ready to fabricate my parts via infusion.
[FONT=Calibri]13. [/FONT]My part stack will look something like the following eGlass, CF, CF, 3/8” closed cell foam core, CF, CF, eglass (eGlass is: 7781 and the CF is 9oz 2x2 twill)

Any input, corrections from you guys would be greatly appreciated!! NOTE: I like the plexi/lexan idea because of the smooth surface AND the scribe line might turn out better on plexi/lexan… I’m a bit concerned about wet sanding the mdf and having it deform from the water. Plus scribe lines might not be as clean on mdf…

I’m also open to better ideas on any of the above especially how to fabricate the “bulge” on the plug.

#2

One this right off the bat: a layer of 2 ounce glass and 6 layers of 2.5 ounce CSM is not enough mater to make a mold. Most of my open panel molds have over 150 ounces of glass or some kind of glass/core/glass construction. I like .17" thickness for smaller parts and much more thickness for larger panels.

I would probably shape the bulge out of XPS foam and laminate 12 ounces of glass on top of it and then blend it all together will filler. It’s hard to say how I would go about shaping the plug without being able to see everything. If you know the exact contour from a CAD file then I would extrapolate a set of templates to cut rails/ribs to make a sub structure to put the hardboard over to keep it all in place and stable.

#3

Correction: I just looked up my invoice from US Compoistes and I ordered 1.5oz per sq/ft matt for the bulking layers of my mold (13.5 oz per sq yard if I’m doing my math correctly). This mat (according to spec) is .045 thick per layer or .27" with all 6 layers of fg mat. Is this sufficient thickness for my mold?

#4

BTW: I see you’re from Cody wyowindworks… I’m originally from Sheridan, WY (just on the other side of the Bighorns)… small world!

#5

Ahhhhhh…per foot^2…forgot that about some CSM. My bad. I haven’t used CSM for eons as I always use epoxies. The stinky “ester” stuff doesn’t reside in my shop.

.27" will probably do it. Hopefully others with more experience with polyesters can chime in.

I would make the flanges rather thick to provide some more stability. A sandwich of some sort works well around the flanges.

I’ve lived in Cody for 6 years. It’s the composites capital of the world you know. :smiley:

#6

6 layers indeed is .27". Took me a while to figure out. I was raised metric… (at least we specify mat AND wovens per m2, to be easy)

Whether or not it is enough is hard to tell, size and shape matter, as well as backing structure.

#7

I will upload pics/diagrams/dimensions later today.

#8

If you use lexan or the like material, in the areas which aren’t getting bodywork and just lexan and you primer it, bear in mind the primer might pull from the plug and be on the surface of your freshy mint mould when you pull it from the plug. Scuff the lexan before you primer so you have a good bind to it.

Ask me how I know…