CriCri MC-15 in NZ

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MadRocketScientist

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Joined
May 16, 2009
Messages
1,579
Location
Canterbury, New Zealand, Earth, Milky Way Galaxy.
I started on the fairing molds today, I grabbed a couple of offcuts of mdf and laminated them together to make the slab 62mm thick.
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While waiting (impatiently) for the glue to dry I cut some templates from 4mm thick pine ply
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once the glue had set enough (almost) I cut the slab into some blocks,
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Then used the templates to mark the sides for cutting,
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and cut them with the bandsaw. Kinda scary using the woodshop bandsaw when I don't use it all that often. 150mm of really sharp bandsaw blade cutting through mdf like a hot knife through butter is somewhat scary.
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I screwed all the blanks onto a strip of plywood and started machining them to shape.
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When I was rough cutting the second one, the glue line split. I should have left the glue to dry longer. A couple of screws held the parts together and I left it to dry longer while I cut the others. It turned out alright in the end.
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Once I had them all cut with a reasonable coarse stepover finish pass, I coated them with epoxy. Heating things with the hair dryer to get a good soak into the mdf. I will give this a couple of days to set and then put a fine finish pass over them again. With a bit of luck the epoxy will seal the mdf well enough for molding. In other projects I have used wax as a sealer but it can sometimes allow the resin to bleed through to the mdf when molding parts.
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Speedboat100

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Nov 8, 2018
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1,011
Location
Europe
Taking little breaks to read Hoerner is not how normal people live their lives :)

How far from the fuselage did you kill off that upside down airfoil. Tricky one to guess at. Nice detail to mess with. Still kills me that you think like a composite guy but then use all the composite mold making skills to make hammer forms for aluminum using CNC. Amazing.

Horner is good reading. I owe this to him.

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MadRocketScientist

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Joined
May 16, 2009
Messages
1,579
Location
Canterbury, New Zealand, Earth, Milky Way Galaxy.
I have the molds ready to lay up. The idea of using epoxy to surface the MDF didn't really work out as planned. Everything moved around too much after removing them from the CNC. When I went to do cut the final pass, the blanks weren't quite lining up and I had to cut deeper than I wanted. I think if I thinned the epoxy with 10% acetone it may have penetrated deeper into the MDF, and then leaving the blanks clamped to the CNC bed while the epoxy set hard.

In the end I recut everything and coated the new surfaces with super glue before sanding. A good rub down with beeswax polish gives the molds a shine.
I also added a couple of small location pins to each pair of molds, made from some 1/8" aluminium tubing.

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MadRocketScientist

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Joined
May 16, 2009
Messages
1,579
Location
Canterbury, New Zealand, Earth, Milky Way Galaxy.
I would certainly do that if I knew where to send it!
These are the details I have for him

Michel Colomban
37 Bis Rue La Kanol
92500 Rueil Malmaison
France
Fax: 0033 1 47 51 88 76
(Current as of Jan 2006)

I have no idea if they are still current and I haven;t been in contact with him at all. You could try writing a letter to him to ask but the post might be really slow with the Coronavirus and all.
 

MadRocketScientist

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Joined
May 16, 2009
Messages
1,579
Location
Canterbury, New Zealand, Earth, Milky Way Galaxy.
Spent some time today laying up one of the gear leg fairings in polyester resin and glass cloth. I had to use the hair dryer to get it to set since it has been a bit cold and wet lately. It was a bit of an act to get the layers to overlap neatly on the leading edge as the mold was closed. I used a curved piece of wire and used that to smooth down the overlap in the glass cloth once the two mold halves were clamped together. I used a length of rovings in the trailing edge as reinforcement, but it shifted a little bit after the mold was closed, still overall a success!

Current thoughts about how to attach the fairings to the gear legs is to use a calking gun type adhesive for initial bonding and then fill the hollow spaces with lightweight spray foam. I can clamp the mold halves in place to stop it inflating like a balloon while the foam sets. I am also considering making the fairings with epoxy resin rather than polyester. Anyhow back to the pictures.

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After the resin set to a rubbery stage I trimmed the overhanging ends off with a knife. When it had hardened up some more I split the mold halves apart. The bees wax worked quite well and nothing stuck.
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Here you can see how the top side of the fairing follows the shape of the underside of the wing. Getting the fairing onto the end of the gear leg was a bit tight. There is a straight section on the end of the gear leg where the wheel bolts on and the fairing has to flex over it when I slide it on. However with a bit more force than expected it does go on!
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The lower cable exit to the caliper sticks forward quite a bit, so I need to design the transition from the leg fairing to the wheel pant to cover it up, The gear leg fairing itself is longer than it needs to be and I will trim it shorter later.
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I need to change the mold for this side slightly to allow the brake cable to fit where it exits through the clamp pieces. I think I will match the other side at the same time. The cable exits to the rear on the opposite side so it would be a cosmetic change only. The red line is where the cable doesn't quite fit.
 

MadRocketScientist

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Joined
May 16, 2009
Messages
1,579
Location
Canterbury, New Zealand, Earth, Milky Way Galaxy.
I adjusted the molds for the landing gear fairings. A round file and sandpaper does wonders on MDF. I did the molds from both sides so they match.
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I cut some of the leading edge of the fairing away, clamped it back in the mold and glassed in the reshaped area. Fits much better this time.
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I also eyeballed the lower hole location for the cable exit, got it pretty close.
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Yellowhammer

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Joined
Feb 21, 2020
Messages
102
These are the details I have for him

Michel Colomban
37 Bis Rue La Kanol
92500 Rueil Malmaison
France
Fax: 0033 1 47 51 88 76
(Current as of Jan 2006)

I have no idea if they are still current and I haven;t been in contact with him at all. You could try writing a letter to him to ask but the post might be really slow with the Coronavirus and all.
Thank you. You are a gentleman sir.
 

MadRocketScientist

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Joined
May 16, 2009
Messages
1,579
Location
Canterbury, New Zealand, Earth, Milky Way Galaxy.
I have been making progress on the fairing over the landing gear bow on the underside of the fuselage. Quite some time ago I was looking at this picture of F-PCLF from France and the fairing looked to be missing. I realized that the lines are stiffeners of some sort that hold the fairing to a neat curve. These aren't shown on the plans but it seems like a good idea to me. It does make me wonder what happened to the fairing that he needed to fly without it in this picture?
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With this in mind I cut some 9mm plywood parts to shape on the CNC. Primarily to check the fit before I cut them in 20mm PVC foam. I am already on revision 2 of these due to drawing the rear sections wrong initially.
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I stuck them in place with some double sided tape. I fitted three on the front to test how many are needed to hold the fairing to shape. Three works well enough but if I use four on each side I can also epoxy some tubing into the final foam parts and feed the brake cables through. That will also solve how to stop the cables rattling around too much.
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I have been looking around for some thin rubber to tension the fairing. After hunting around for some sheet rubber, I first tried cutting up part of an old car tyre inner tube, but soon realized that the rubber was too thick. In the end I found a mountain bike tube, it was still in good condition with no punctures or splits but it was sacrificed for the cause. Another brand new one is only $6 NZD when we finally get out of our lockdown. I cut it along the length and held it flat with some masking tape.
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After some careful measuring of distances to get a 5mm stretch, I cut a pattern out of 3mm MDF and double sided taped it onto the rubber. I used a tube from an old broken RC transmitter antennae in the cordless drill to cut the holes and carefully cut the outside with a box cutter.
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I made the holes oversize so that they clear the dimples for the countersunk rivets. I will use dead soft A rivets to hold the fairings in place, so hopefully it doesn't pucker too much with the rubber being all squishy and that.
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I temporarily attached both ends of the rubber strip to the fairing parts with double sided tape and then clecoed everything onto the CriCri. Looks good to me. I may be able to shorten the overlap slightly because is seems to be that when the bow flexes, the fairing pushes downwards more than pulls forward. I need to test this before cutting anything.
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MadRocketScientist

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Joined
May 16, 2009
Messages
1,579
Location
Canterbury, New Zealand, Earth, Milky Way Galaxy.
After thinking things through I put the fairing back on the CNC and trimmed 10mm from the back edge. I cut a thin outline with the CNC and then doublesided taped the fairing down to cut it. This way its easy to relocate a part on the CNC accurately. The cutting left around 8mm of overlap and should allow ample movement in normal landings. I have no idea if things will catch in a really heavy landing, but that would be the least of my worries.
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I glued the rubber in place with superglue, being careful not to glue it where it needs to stretch. I also cut a 10mm wide 0.020" 2024 backing strip for riveting through.
Here it is with a couple of rivets in the ends after the strip was glued on top. I used A rivets for this as they are soft enough no to compress the rubber too much. I dimpled the outside and made oversize holes through the rubber sheet to allow the undersides of the dimples to come closer to the undimpled strip. This also limits how much the rivets squish the rubber.
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After a couple of revisions and adjustments of the plywood filler/spacer mockups, I cut some from H60 PVC foam.
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While the outside pieces are 20mm thick, I didn't think that the extra spacers inside needed to be quite so chunky. So I cut them down the middle with the foam stripper, making them 10mm wide.
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They still needed more shaping than I cut with the CNC. Sometimes its easier and faster to shape the trickier parts with sandpaper over trying to model everything to perfection, even more so on the first parts.
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I used the double sided tape to hold the foam parts in place. I will Hysol epoxy these in place eventually. I used a straw for the cable guide to stop the cable sawing through the foam from vibration. Thinking out loud here, using superglue may be better for holding the straw in place as epoxy doesn't stick to the plastic so well.
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They spacers fill up the gaps on the ends of the undercarriage fairing pretty well. There is still a slight gap at the rear that I fixed after taking this photo. I forgot that I had to cut a slight recess for the ends of the rubber strip and rivets to make it sit completely flat.
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The plans don't show this overlap of the fairing, and usually the rubber strip is visible. My reasoning is that for the small amount of extra weight, the overlap looks a lot better, and possibly a very slight drag reduction (clutches at straws) :p I am considering putting a thin strip of PTFE tape under the sliding part as aluminium can chew through itself very quickly from movement. The tape would also stop any nasty black oxidation smudging out the back.
 

MadRocketScientist

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Joined
May 16, 2009
Messages
1,579
Location
Canterbury, New Zealand, Earth, Milky Way Galaxy.
Started on another little job that needs completing. The fitting of the wing pin latches. I had started on the drawing for the location for these quite a while ago and after some time spent double checking everything I cut a drill pattern from PVC on the CNC.
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The latch clecoed in place on the inside.
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I flipped the pattern to drill the holes on the opposite side.
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I need the dimple the holes in the skin at the front (where the polished part is). Since I cant reach it with the dimpler or C-frame I will have to use a dimple die backed up with a bucking bar and make the dimple using a rivet.
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MadRocketScientist

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Joined
May 16, 2009
Messages
1,579
Location
Canterbury, New Zealand, Earth, Milky Way Galaxy.
Riveted the wing pin latches in place today. Getting the skin dimpled was a bit of a pain, After attempting to form the dimple with the dimple die in a bucking bar I gave up and made a set of pop riveter dimple dies. I should have made them ages ago.:rolleyes: They make it much easier to dimple around things.

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Next up is to glue the last of the stiffeners in place. Usually the air vents are between these two stiffeners but I haven't decided if I will put them in the plans location or in the sides of the fuselage.
 
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