CriCri MC-15 in NZ

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JonMiller284

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Those spinners look awesome! I would still like to get 2 from you if possible… what size props are you using? The 34-14 that is recommended for the 210?
 

JonMiller284

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Although those engines mounted like on the one in that picture doesn’t look half bad…but I bet that changes the CG quite a bit and leads me to wonder what he did to counter that…
 

MadRocketScientist

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Although those engines mounted like on the one in that picture doesn’t look half bad…but I bet that changes the CG quite a bit and leads me to wonder what he did to counter that…
The CG would move but not in the direction that you might expect, the ZDZ engines are about half the weight of the JPX PUL212, so the CG needs to be moved further forward with the ZDZ engines.
 

JonMiller284

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The CG would move but not in the direction that you might expect, the ZDZ engines are about half the weight of the JPX PUL212, so the CG needs to be moved further forward with the ZDZ engines.
so then if I mounted them like m644 in the picture I might not have to change for the CG and it would be easier/ less work to mount the ZDZ210’s since I could use the rear mount holes! If I’m understanding you correctly Shannon

ohhh and that would make it possible to use the 34-14 props the ZDZ requires
 

MadRocketScientist

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so then if I mounted them like m644 in the picture I might not have to change for the CG and it would be easier/ less work to mount the ZDZ210’s since I could use the rear mount holes! If I’m understanding you correctly Shannon

ohhh and that would make it possible to use the 34-14 props the ZDZ requires
Yes that would work. The boxer twins that I have seen mounted from the rear use rubber bushings instead of the music wire setup of the JPX engines. Seems to work okay but I haven't heard of any high flight times so there isn't a huge amount of data on how the engine stalks hold up. VH-SZE in Australia seems to have the highest flight time that I know of with the engines mounted like this.

Here are a couple of pictures of ZK-CRI engine setup, this one was recently for sale.
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This one is of a build in the USA (I think) but not sure if ti has flown.
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And a couple of the engine mounts for the limbach engines that were on VH-SZE before it was converted to turbines.
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You might be able to get away with a bit more pitch than the 34x14 on the CriCri. A lot of the model engines are used on slower model aircraft that spend time hovering, hence the low prop pitch. The stall speed of a CriCri is a lot higher than the stall speeds that models can be flown at.
 

MadRocketScientist

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The exhaust bends arrived last week,
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And I have been working on a new canopy mold. There was another thread on this forum, I can't remember which one, that posted a link to a Russian Gazelle build. In particular the process used to blow the canopy.

This made me think about a way to put my canopy plastic in the oven and be able to quickly put it onto the mold for forming before it cools. Long story short I have decided to make a second canopy mold the will work better than the first one. There are also some parts of the first mold that I am not happy with and I would make it slightly differently the second time around. I have changed the 3D file quite a bit, most notably at each end there is a flat section that will allow the hot plastic to seal against. I have also tweaked the shape so that the canopy will fit on a raised or standard rear canopy frame. I am testing the shapes with a 36% canopy mold. This will be the right size to fit my RC CriCri and give it a more scale look. I carved the two halves on the CNC today and glued them together..
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MadRocketScientist

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Since I don't really have any clear acrylic or PETG thin enough to make a canopy from the 36% scale mold. I cut a piece of 1mm white PVC and made a quick heating box from cardboard and a hot air gun.
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I heated up the PVC until it was rubbery and draped it into the mold, mainly to get an idea of how large a piece of plastic will be needed. Since this one started at 41cm x 40cm and is slightly too small on the width, I think the full size canopy will need a piece 117cm square. Pretty much half of a 4x8' sheet.
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I am slowly thinking through the process needed for the oven and how to guide the plastic into the mold for forming. Keeping things repeatable will go a long way to fixing it when it doesn't go right at first.
 

proppastie

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will the model have a clear canopy ...does the PVC behave differently than Plexiglas ......will you scratch it when laying it into the mold? Will you have the finished mold in the heat box.....will it droop naturally into the mold that way?....Looking at the link from Russia he used a "male" mold.
 
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MadRocketScientist

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will the model have a clear canopy ...does the PVC behave differently than Plexiglas ......will you scratch it when laying it into the mold? Will you have the finished mold in the heat box.....will it droop naturally into the mold that way?....Looking at the link from Russia he used a "male" mold.
Yes hopefully I can source some clear plastic under 1mm for the model canopy. Failing that I can use 1mm thick as the model is quite underweight and could use some more mass for flying in any sort of wind.

The PVC is super soft and forms at a much lower temperature than any of the other three plastics. The areas that initially touch the mold are parts that get cut off before installation so the optical area wouldn't be touching the mold until the vacuum is pulled. I think I will use the model mold as it is and see what surface finish I end up with. I am using it more to get an idea of the process more than anything.

On the full size I intend to felt-line the mold to keep the optics as perfect as possible. I will have the mold outside the heatbox as the foam and glass materials it will be made from won't handle the 100°C temperatures. I am thinking of preheating the mold in its own oven to around 50-60°C to slow the cooling time of the freshly molded canopy. In the main oven I will use a frame similar to the Russian Gazelle canopy, but only to support the plastic as it is heated and to aid in placing the hot acrylic in the mold. Since the frame will need to face downwards to put the hot plastic into the mold, I do have concerns about the amount of stretching that the hot plastic will do and possibly pull away from the frame as it is placed. Another option is that I could place the mold over the frame and vacuum the canopy up into it, but that would also make checking for when the stop vacuuming a lot more difficult.

The Russian canopy was free blown for the most part, but the CriCri canopy is too skinny and tall for that to work.
 

MadRocketScientist

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I decided to start on the second canopy mold and so I cut out some extruded polystyrene sections to laminate up into a block
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Spray glue works wonders for sticking these together fast, I cannot remember what I used last time but it didn't really help the polystyrene stay stuck together.
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It took a bit of working out the slice shapes but once everything is stacked up its ready to rough cut.
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The roughing cut done and starting on the finish cut.
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Finish cut all done! This is the second side to be completed but just view all the pictures in the mirror to see how the opposite side was done. :p
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This form has a better shape than the first attempt with large flanges for sealing the hot plastic.
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Last job for the day was to cut some heavy woven glass cloth ready to epoxy in place. I didn't start the glassing today as it was pouring with rain and very cold. The weather is looking much better tomorrow with sunshine forecast.
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MadRocketScientist

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I started on surfacing the form halves with polyester filler today.
Firstly I cut some 6mm wide strips from an off cut of 3/16" thick MDF
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Then I took the worst of the gloss off the epoxy/glass surface with the multitool with a sander blade. Note that I have already surfaced the lower edge of the form with filler in this picture.
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I wrapped the strips with packing tape to limit how much the filler sticks to them. Then I tacked them in place in the form with hot glue, leaving them close enough that I can run the spatula along the tops to level the filler. I was also careful to keep the hot glue outside of the area to be filled as the filler doesn't stick the the hot glue. The filler also tends to pull and under fill the gap on the first filling and a second layer is needed to completely fill level to the top.
I have been weighing out the filler 100 grams at a time, and adding exactly 1.8 grams of hardener to each batch seems to give me enough working time. Hopefully this keeps the whole surface a consistent color and more importantly consistent hardness.
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Once the filler is set somewhat firm, the strips are removed and glued in the next locations to fill. The gaps between the previous solid filler are also filled in.
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Working my way across the form, I got close to half of the area done before stopping for the night.
 

MadRocketScientist

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Making steady progress on the filler this week. I completed the first half and have started on the second. The tin of filler I had ran out and I purchased another couple of 4 liter tins however the new filler is a lot more runny than the old tin I used up and tends to slump. I decided to add some of the filler shavings from when I CNC cut the first attempt so I ran the shavings through a sieve to take out any larger lumops. Adding 6-7 grams per 100 grams of filler thickens it up well and keep it in place.
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First side finished
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And after completing the first side I then started laying the strips onto the second half.
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And stopped here for the night.
 

drienkm

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Jul 29, 2021
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Wow. So many have said it, but I'll say it again. Spectacular craftsmanship!
Keep plugging away at that flight training. I recently became a pilot, fulfilling a lifelong dream. There were minor setbacks and pauses, it was expensive and it took some determination to finish, but it is so worthwhile! One of the common problems people face when they earn their wings is finding something they can afford to fly. You are ahead of the game there.
Best wishes!
 
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