How would you infuse this part?

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#1

I’m building new inner landing gear doors for an experimental aircraft (see picture by clicking the following link):

http://i1095.photobucket.com/albums/i477/jbocek/Glasair%20project/Legacy_SB060.jpg

I’d like to infuse the part so I’m looking for some advice from those with more experience. First off, I plan on building a female mold to infuse the part so my question is more related to the specific process and stack configuration. The part needs to be super stiff as it will close over (or under in this case) the main landing gear on an aircraft that is capable of 350 mph so I dont want the doors twisting and bending open at speed. I was thinking of using the following stack: CF - CF - core - CF - CF (the CF is 9oz 2.2 twill CF and the core will be 3/8" thick). If desirable I could add another layer of CF on each side or possibly a layer of 9oz 7780 eGlass on each side. The parts (two of them) will be painted so I’m not worried about covering up the CF. (the thougth behind the possible extra layer of eGlass is just to give me somethig to sand for final fitting of the doors)

As you can see by the pic there are areas on the part that are coreless (hinge attach point - upper right side of “upside down L” shaped area, door bracket that attach area - upper left side of “upside down L” shaped area, and the circular area in the middle. The middle area on my part will actually bulge out approx 1" to provide clearance for the brake caliper. The part is fairly flat in one axis but slightly curved to match the underside of the wing airfoil in the other axis. Additioally, the stiffener is another core layer on top of the main core layer. Oh, and I will be using infusion epoxy…

The door when closed fits a recess in the lower surface of the wing so nothing is left hanging out in the breeze.

So… back to my questions:

  1. How would you infuse this part? Outer layers - cure - then infuse core and inner layers or?
  2. How would you orientate the fibers of each layer? I dont want the to warp when it cures.
  3. Is the 9oz 2.2 twill CF a good choice… if not, what would you use?
  4. What core would you use? Is 3/8" going to be stiff enough? Remember the core must be LIGHT and strong AND becuase of the bulge for the brake caliper some core might need to conform to a slightly curved surface.

Thanks in advance for the help!

James

#2

I would infuse at one shot. As core material I would choose a foam like Airex C70-55, that should be stiff enough, I also think 1/4" thickness will be enough.
Make sure that the core has holes all 1.5" of about 0.01in.
I would also take a plain weave as first in 0°/90°, a heavier layer of ±45° multiaxial fabric and a light layer of 0°/90° multiaxial fabric, than core and backwards.
The plain and the thin multiaxial should have the same weight together than the ±45° fabric.
Degas your resin, work temparature should be about 77°F and the vacuum should be 0.6inHg absolut.
Place flow media over the whole part and let it end about 1 in before the part ends. Place the evacuation hose 3 in away from the part lip and use peelply as brake zone, or place MTI hose direct to the part lip.
When the flow media is completely filled place resin pot 1yd deper than the part and wait until the part is filled. Than clamp the resin inlet.

Than it will work and you should get a perfect part.

PS: has the MTI hose arrived? You should use it.

#3

Thanks for the input DDC! BTW I have not yet received the sample of MTI hose you sent… Re your response I have a few more points/questions:

  1. The size of the part is (approx) 16" x 14"
  2. The foam core has two layers. One layer for the bulk of the part and a second layer for the stiffener… see pic)

Questions:

  1. Does the fact that the stiffener is actually a 2nd layer of core change your recommendation of infusing the part all in one shot? If no, then I assume the Airex core you recommend is made for infusion and will have no problem getting the resin under the core and to the first layers of reinforcement… is this correct?
  2. Re the fabric… I have some plain weave CF 10oz and 2.2 twill 10oz CF on hand… will this work for the two different layers on each side of the core? If no, what weight/style fabric would you recommend?
  3. Since the part will be painted white, should I first spray/paint in a layer of PE gel coat into the mold or no? I’m thinking the gel coat might give me a bit of material to sand and shape to get the gear doors to fit just right… am I way off base here?
  4. For my last infusion I used some compoflex SB RF 150 as the flow media (on top of the red stripe peel ply) and it worked great… Should I be using real flow media instead?

Thanks in advance for the advice/assistance…!

#4

Where the stiffer is bond a second core to the first one and drill new holes in it because the old ones will be closed if you glue two core materials together.
You can still make it in one shot. The Airex can be bought blank, drilled, slotted or combined.
If you can get it I would choose the drilled and slotted for infusion, but it also works if only drilled. The most important thing is the vac level, make sure you have the vacuum I told.
I usually take a wooven fabric as first layer for crack stop and the multiaxial non crimp fabric for better stiffness (and it is much cheaper). You can use other fabrics as well, that was just what I would use.
Painting a gelcoat will be a good idea if it will be painted, it is easier to paint and you wont see any small air bubbles that can be enclosed in the outer layer because infusing in one shot.
I think the compoflex will work, I don’t know why not.

#5

Again, thx for the advice DDC! The saga continues as today I was down at a local composites supplier and he had on hand some 1/8" thick Divinycell on scrim cloth that was cut into 1/2" or so squares all held together with the scrim cloth for me to touch/feel. The shop also had a sandwich prepared with the stuff (one layer of glass reinforcement on each side) and I was surprised how heavy the sample was (MUCH heavier than some other samples of carbon - honeycomb - carbon sandwich samples he had on hand. So… my question is this: Is the Airex much lighter? I was told the weight was high becuase of all the cuts in the Divinycell that filled with resin as well as the scrim cloth that saturated with resin.

Since I’m trying to keep the parts as light as possible I’m curious if I can infuse with just the Airex core and no scores/holes (would the resin flow 16" or so under the core using only the reinforcment as the flow media? If no, which method (holes or scores in the core) would be lighter yet still ensure the bottom of my laminate is fully wetted out?

Lastly, how flexible is the 1/4" and 3/8" Airex C70 core material (I’m not set up to thermomold the stuff) and who sells the stuff in the states?

Thanks again!

#6

Rohcell foam is the best foam I’ve used so far. It’s pretty light as well. I would incorporate the stiffener into the shape of the core. If you are using two cores there is no guarantee you have proper vf between cores unless you destroy one. Be sure to incorporate a glass ply at attachment points and also copper for ground strap. For the perimeter of the part incorporate a glass buffer between carbon and wing/fuselage. Vibration and flutter can cause galvanic corrosion at these areas. Proper ground is pretty important.

#7

Don’t take a foam that is cut in squares, that will be to heavy of too much resin consumption.
Rohacell is good, but not really better than the Airex, but much more expansive. And it is also not lighter, that depend on the type of foam you buy. The Rohacell IG51 is similar to the Airex C70/55.
The Rohacell is a bit harder than the Airex and has smaller pores, wich means it will suck less resin than the Airex, but that is really not much.
Take a drilled and uncutted foam, you can form the foam by heating it with a hairdryer and fix it with spray adhesive.


To be sure that the bottom layer is wetted out your vac level and your flow speed is important.

#8

I too love Rohacell but have never used it for infusion. Dallas, what did you do to prep the Rohacell for infusion?

#9

You can also buy it drilled and/or slotted for infusion. You can also drill every 1.5 in a 0.01 in hole.

#10

Fabrication is done either with a mill or a giant band saw. You can order preformed if you want. Channeling is done with a table saw and slide. Hotwire works but not to well… As opposed to other foams. I like it the best.

#11

Thanks for all the advice guys! Very helpful!

A few more questions/comments to the above posts:

  1. Is Rohacell as strong as the Airex? I ask becuase the specs on the Airex say its “structural core”.
  2. I havent looked for a supplier for the Rohacell but I cant find a USA supplier for the Airex? Any suggestions for USA suppliers for both would be greatly apprecited.
  3. Dallas re your comment “Grounding is pretty important”: I hadnt thought of that… is that becuase the part is constructed of CF? I dont beleive the kit mfr calls for grounding the landing gear doors so I’ll have to think about that one for a bit.
  4. Dallas re your comment “For the perimeter of the part incorporate a glass buffer between carbon and wing/fuselage”: Would fabricating my stack something like the following do the trick? eGlass - CF - CF - core - CF - CF - eGlass /// NOTE: The doors will be primed/painted so no CF should physically make contact with anything… more paint to paint contact than anything
  5. Dallas re the comment about two cores: I’m planning on bonding the stiffener (L shaped layer of foam core) to the main foam core via epoxy and letting it cure before infusing… is this acceptable?

Finally, it sounds like buying predrilled core (or drilling with 1/16" holes every 1" or so) would be the best bet if trying for a light weight part… do you all agree?

Again, thanks for all the input/advice.!!

#12

You should contact rohcell directly… If not I think aircraft spruce has it. Glass the inside perimeter that Fay surface with the gear well. 1" taper is perfect. Ground strap is wise because static charge buildups occur on graphite structures. Typically you see more static wicks on composite surfaces. As for lightning most wing tips and aux flight controls have copper laminate inside the layup to make a lightning strike more survivable. The ground strap can be installed with a electrical bond at the aircraft hinge on fuselage and on the panel. It’s wise to have a copper electrical bond laminated in the door. Static charge will wick through the strap an out the plane.

Paint wears off fast… And gear doors like to flutter.

#13

Rohcell will also fab the core for you if you send them a drawing… FYI. Also make sure the hinge has glass under it to prevent corrosion. Brush coat ground strap fasteners with 8802a1/2.

#14

Copper laminate is heavier and currently in GA the preferred sacrificial material is aluminum. A very thin aluminum diamond wiremesh that is usually used as a sacrificial layer between two thin layers (40gm/sqm) of glass. Its that what is grounded.

N