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CriCri MC-15 in NZ

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MadRocketScientist

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May 16, 2009
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Canterbury, New Zealand, Earth, Milky Way Galaxy.
It has been a while since I ran my engine......I put a timing lght on it and I think it was variable , or maybe it jumped to 30 and stayed there from TDC for starting. I thought the model ignitions were pretty similar. My trigger is just past TDC so it does not run backwards......Can you vary your ignition?
I can't reprogram the ignitions, but it would be great to be able to do so! The sensors are on the adjustable bracket so I can shift the whole curve around. The ignition modules have a curve of sorts programmed in. I have attached a file with the two versions of the Rcexl ignitions that I am using, I have verified the A-01 curve but not the A-02, no reason to suspect its not good.
 

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proppastie

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Feb 19, 2012
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4,714
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NJ
Mine probably like the first one.....Can not understand why the second is like it is. You talked about changing from 28 deg. How would you do that. ..looks like you cannot change much or you may not be able to start.
 

MadRocketScientist

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May 16, 2009
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Canterbury, New Zealand, Earth, Milky Way Galaxy.
I tested some hardened JB Weld and it didn't seem to conduct at all, so I used it to pot a sensor into a bracket. I used some packing tape and clamped a flat bit of metal over the top to give a flat surface and let it cure overnight. I will test that it works well before potting any more sensors. Ideally a non metal epoxy would be better but finding some locally with a high heat rating and in small amounts is difficult.
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And some pictures of the fit on the engines.
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MadRocketScientist

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May 16, 2009
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Canterbury, New Zealand, Earth, Milky Way Galaxy.
I ordered some silicon o-rings from Ebay and they arrived pretty fast for times like these. I am going to test them as a replacement for the rubber bands on the engine mounts and possibly also on the artificial feel on the controls. Why? Well a few reasons, the rubber bands don't age very well and tend to fall apart with any sunlight on them. They also lose tension quite quickly and become stretched.
DSC00219 (Medium).JPG
Silicon on the other hand doesn't seem to have much creep at all and will return to its original size after being held stretched for quite a long time, meaning the force won't diminish over time. It withstands sunlight much better than rubber and generally lasts longer. The silicon does tend to have the downsides of being susceptible to gasoline and can be sensitive to nicks and cuts.

I did some calculations before ordering the silicon o-rings on how many would be needed and what size to order. Each attachment on the engine mount needs 6-7 grams of rubber bands spread over three screws. The rubber bands need to be stretched to twice their original length when installed. I calculated that four 32mm diameter, 2.4mm width o-rings should give the right weight when stretched 100%. Turns out that after some testing that the silicon o-rings have a higher spring rate than normal rubber bands, they only need a 40% stretch to reach the same force/weight as the rubber bands. This means I can either use less o-rings to achieve the correct tension or buy some longer o-rings and have more redundancy. Given that the engine mount rubber bands do tend to break a lot, I am leaning towards some longer o-rings, I may be able to use up these ones on the controls.

I also came across another issue when installing the silicon o-rings on the engine mount (pictured below). The round cross section of the o-rings don't nest onto the screws as well as the rubber bands meaning I could only get six out the twelve o-rings installed. I will leave these ones in place as a stretch test and compare them with some new o-rings to test the level of creep.
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High tech stretch test setup :p , a board with a hole in it and some digital scales.
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MadRocketScientist

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Canterbury, New Zealand, Earth, Milky Way Galaxy.
The rubber mounts I ordered to test on the exhaust mount arrived. The smaller ones might be a bit on the small side but I will test them anyhow.
DSC00221 (Medium).JPG
I tested some rubber bands and silicon o-rings on my homemade stretch tester and loaded the values into a spreadsheet. The following graph give a good demonstration of the sharp increase in force needed to stretch the silicon o-rings. The rubber has a more linear force increase over its elastic range. The amount that rubber deteriorates over time is quite substantial. I haven't tested aged silicon but it only looses a small amount after the first stretching. I ordered some longer silicon o-rings to test on the engine mount.
O-rings and rubber bands.JPG
I had put some of the first o-rings on the flight controls to see how well they age, one broke from a notch but the rest seem to be holding up very well so far. If the silicon turns out to be too fragile, plan B is to use some thin braiding covered bungee rope.
 

MadRocketScientist

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Canterbury, New Zealand, Earth, Milky Way Galaxy.
Making slow progress towards getting the engine running again.
I installed the magnets in the prop hubs after alodining them. JB weld should stop them going anywhere. I also dipped the steel inserts in primer before pressing them back into place. Maybe I should have spent more time with the scotch-brite pad on the recesses in the hub as some areas didn't get scrubbed enough to take the alodine.
DSC00224 (Medium).JPGDSC00225 (Medium).JPG
I removed the mounting parts from the left engine, gave them a good working over with the scotch-brite pad , alodined them and then re-installed everything.
DSC00227 (Medium).JPGDSC00230 (Medium).JPG
I also made a puller for the prop hub, I should have done this a long time ago as it is far easier to remove and install the hub now. I used an M16x1 thread for the puller shaft. The hub is a press fit but comes off much easier after heating it with a heat gun, while being careful not to overheat the magnets.
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Setup for installing the hub...
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and removing it....
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MadRocketScientist

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May 16, 2009
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Location
Canterbury, New Zealand, Earth, Milky Way Galaxy.
The longer silicon o-rings arrived in the mail so I installed some on the engine mountings.

Firstly I used some superglue to stick on some new rubber strips onto the linking bars. I had removed the previously installed ones for alodining these. The rubber helps the next layer of rubber to grip the aluminium better when the o-rings are installed.
DSC00240 (Medium).JPGDSC00241 (Medium).JPG
I need to get some longer wood screws before I can fit the third o-ring in place on each screw but they do seem to hodl things in place well but still allowing some movement.
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I also headed down the local airfield and had a look at ZK-LBW. It has just passed its annual inspection and had the engine covers off. I had a good look at the setup and noticed that the music wires on the engine mounts looked suspiciously thicker than 1/8". I grabbed the vernier calipers out of my van and sure enough, it has 4mm (or possibly 5/32") music wire installed in place of the 1/8". I had a talk to the pilot and was informed that there is still minimal vibration from the engines with the thicker music wires in place. It has around 40 hours flight time so they seem to last much better than the 1/8" music wire as shown in the plans. Yet another thing to change on my engine setup!
 

Jay Kempf

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Warren, VT USA
Don't understand this part of the Cri Cri design. Why so many layers and safety critical stuff hanging on Orings? Vibration?
 

MadRocketScientist

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Canterbury, New Zealand, Earth, Milky Way Galaxy.
Don't understand this part of the Cri Cri design. Why so many layers and safety critical stuff hanging on Orings? Vibration?
The vibration from the engines tends to crack the tubing of the engine mount. Since there is a vibrating mass on the end of a pole, it tends to want to snap off very near where the engine moutn is bolted to frame 1. I haven't seen any documentation to support this, but my theory is that very early on in CriCri history, the engine mounts started cracking and this suspension design is what they came up with to reduce the fatigue to very low levels.

The music wire mounts allow the engines to move a small amount independently of the mounting tubes. The rubber bands / o-rings and plywood 'Y' bar, absorb the torque pulses when the engine fires. The elastic system reduces the resonant frequency of the mount to below idle speed very similar to the mounting system of a washing machine drum.
 

Jay Kempf

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I figured it was fatigue but that is rather elaborate. I think I would want bolts through holes with grommets so that there was no spontaneous disassembly path. I suppose that's a lot of rubber bands so a lot of redundancy. Is there a TBO list for tricky parts of the design? Or just pre-flight and replace upon condition inspection...
 

MadRocketScientist

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Canterbury, New Zealand, Earth, Milky Way Galaxy.
I figured it was fatigue but that is rather elaborate. I think I would want bolts through holes with grommets so that there was no spontaneous disassembly path. I suppose that's a lot of rubber bands so a lot of redundancy. Is there a TBO list for tricky parts of the design? Or just pre-flight and replace upon condition inspection...
The thrust forces (and some of the torsion) are all taken up by the 4 music wires clamped by bolts. If the rubber bands all break, the engines will stay put by the linking bars will flop around inside the fairings. The engines will still stay attached.

All the parts are replace on condition. The 3mm music wire mounts do break quite frequently. (I thought they were 1/8" from memory but looking at the plans last night they are listed as 3mm).

Most of the other builds that I have seen with boxer twin engines use the grommet and bolt style mountings. I am not sure if any of them have a huge amount of flight time but they do seem to work alright. I do know of one engine mount that snapped with the grommet style setup but that was because of a welding mistake and not so much the way the engine was mounted.
 

MadRocketScientist

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Yesterday I cut up a length of 5/32" music wire that I had laying around into 165mm lengths for the engine mounts. I think I can get away with cutting them down to 160mm but that can happen later. Since one length only makes 5 wires, I went to purchase some more from the model shop today but they were sold out. They say they will have more in on Tuesday.

Today I put a slight joggle in the music wire of around 4mm offset (not pictured).
The plans show 10mm offset for the 3.0mm music wire on the JPX engines and 8mm offset for the thicker 3.5mm music wire on the SOLO 210 engine mounting. The offset in the wires is really only for adjusting the positioning of the engine in the mount. It is adjusted so that when the engine is running the torque pushes it near centered but meaning that it will sit to one side when the engine is off. The adjustment also allows for holding the engine up against gravity. I think it does allow the engine bumpstop to sit against the engine mount when flying inverted but this doesn't seem to be a problem, at least I haven't come across anyone mentioning it.

A noticeably thicker wire....
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I drilled the clamps out to size using the offcut of music wire as a drill. When I made the clamps originally, drilling and reaming the holes before cutting the slots gives a really close fitting hole. Trying to drill a hole to a close tolerance when it has a slot intersecting right through it is a completely different animal.
After some thought, I clamped some aluminium off cuts in the slots, hopeful that by giving the drill bit 360° of aluminium to cut against the holes would drill neatly and not chatter. The holes didn't turn out anywhere near as good a fit as the original ones but the music wires do clamp up nice and tight so I think it will work. Running everything on the test stand should show up any issues.

No joggles in the wires in the photo below but the mounting is much firmer while still allowing the engine to move somewhat.
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MadRocketScientist

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Finally got back to the model shop today and picked up some more 5/32" music wire for the engine mount. I purchased a few extra lengths as spares or they might get used in other projects.
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I cut one of the 36" lengths into 5 shorter 160mm long pieces with an offcut left over.
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And filed the flats on the ends..
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And then bent the 'Z' bend into them using one of the previous ones as a template.
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Having made a set of these before I came up with an easier way to make all the bends at the same point. I folded a couple of right angles into this scrap of aluminium making sure that the bends were 40mm apart.
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This way I can drop the music wire into the vice down against the lower folded flange and the length is set. All I need to do is make sure that the two bends are on the plane and its good to go. I followed the instructions in the plans and overbent these before carefully bending them back to where the bent angle needs to be. Apparently it reduces any 'settling' of the suspension during first operation.
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And the wires installed on the left engine.
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TFF

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Apr 28, 2010
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Memphis, TN
The mount is very much like the Merle Hyde RC pattern mounts with a nose ring, in that the engine is cushioned but the thrust line is kept centered. I don’t know if the IMAC RC planes use nose rings; I don’t think they do even though they use the same engines. I think there has been less and less use of flexible mounts on them. Electric RC is making most of these fixes sealing wax.

I want to say on some of the high end aerobatic big planes, there has been attempts to restrain the front of the engine, but has not become standard.
 

vondeliusc

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Aug 13, 2009
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Kalispell, MT, USA
Shannon-
Nice adaptation for the boxer engines.
Looking at this, I would drill the rods at the aft end flush with the aft face of the mount for a safety pin;
also I would drill the clamp bolt heads to accept safety wire to prevent vibrational loosening so the mount rod
does not migrate fwds, just to be on the safe side.
 

MadRocketScientist

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Canterbury, New Zealand, Earth, Milky Way Galaxy.
Shannon-
Nice adaptation for the boxer engines.
Looking at this, I would drill the rods at the aft end flush with the aft face of the mount for a safety pin;
also I would drill the clamp bolt heads to accept safety wire to prevent vibrational loosening so the mount rod
does not migrate fwds, just to be on the safe side.
I would hesitate to drill the through the rods themselves, I think the rods need to be as smooth and stress riser free as possible, I may even polish mine, apart from where they fit in the clamps. I am not aware of any issues with the music wire rods slipping unless you have heard otherwise?

I remember reading somewhere that there were problems with the thinner music wires snapping during use. Someone said they had at least one wire break on each flight or so, that is possibly why the SOLO210 engine plans show the 3.5mm music wire sized up from the 3.0mm in the JPX engine plans. I have been considering doing a test to see how much thrust force the clamps hold. I could even do a graph for various clamp bolt torque settings. ;) My seat of the pants feeling is that they shouldn't have any issue with the modest amounts of thrust from the engines. but the test stand will show any issues.

I do intend to use some form of safety locking on all the engine bolts but I haven't decided to go with wire or if I will use locktite and marking paint. The locktite will be slightly lighter but will need to be inspected more carefully. Also some locations don't really lend themselves to safety wire unless a hole is drilled nearby for the other end of the lockwire.

You can see a snapped music wire in this photo of the Canadian CriCri, C-FPTJ.
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