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Post  batjac Fri Jun 25, 2021 10:21 pm

I was trying to figure out how to reliably get a fine needle on a Medallion.  I know some guys take a 128TPI needle valve and JB Weld it into the Medallion body.  But I was wanting a less permanent way to do it.  I’ve never measured the I.D. of the Medallion needle valve opening, but I have two or three ideas for doing this sans JB Weld.  

1) Take a small nut that will fit in the hex well of the Medallion venturi body, drill it out for the 128TPI NVA, and then solder the NVA into the remainder of the nut as a stop for the NVA to slide in the venturi.  As I don’t have the proper equipment to mill a groove on the other side for the C-clip, I figured I’d try and find a push-on retaining ring to go on the other side and hold the NVA in place.
2) Next idea if there’s not enough meat on the nut after it’s drilled out.  Put a little solder ring around the NVA and then file the hex pattern into the solder to hold it in place, using the retaining ring on the fuel nipple side.
3) Just using two retaining rings, one on both sides.  Although I’m not sure yet how to keep the NVA from rotating in the venturi body.
4) Instead of using the retaining rings, use a piece of small fuel tubing to press against the side of the venturi.  Those 128TPI NVAs are pretty long. (Holy Crap!  I just had an even weirder idea as I was writing this!)
5) Just doing the JB Weld thing, as it’s only a Medallion body, and those are easier to find these days than a 128TPI NVA.


I did have another idea that I want to try, but it’s a little out there and I want to experiment with it first before I post anything, as well as the new idea that popped into my head when typing out idea #4.

The Mad Scientist Mark
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Post  Ken Cook Sat Jun 26, 2021 4:28 am

Or use a TD carb body with a KK needle or the spent glow plug method that you drill out the post and make your own size venturi with a through spraybar. You could press or solder the spraybar into  it. When I use a TD sprinkler setup for suction, I use the bushing from a Sig or Goldberg bellcrank and turn it upside down and put it into the TD venturi. This chokes the hole way down. I use a dot or two of CA to hold the bushing in. Obviously using bladder this isn't a issue.
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Post  944_Jim Sun Jun 27, 2021 10:34 am

@batjac wrote:
The Mad Scientist Mark

Another strange idea 8d40bc10
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Post  batjac Fri Jul 23, 2021 5:14 am

Well, my “Holy Cr*p!” idea turned more into of a “Meh…” idea.  It works, but not as good as I'd hoped.  It’s more of a field expedient solution compared to a permanent solution.  But while I was laid up this last week I had another epiphany when I was lying in bed trying to fall asleep.  Since I’m feeling better and can get around now, I decided to try and implement the new idea last night.

I found that a 5/32” brass tube is a tight press fit in the Medallion NVA holes.  The standard fine thread NVA from a postage stamp backplate is a loose fit in the tube.  The first thing I did was take my calipers and measure across the sides of the venturi where the stock NVA goes.  It’s 0.375” across.  I cut a piece of tubing that width and then measured the venturi.  The width of the throat is about 0.175”, so I marked 0.1” from each side of the tube.  I then used my Dremel to cut out the center of the tube, leaving a thin strip between the ends.  This made a nice carrier for the NVA barrel.  Hmmm… the cuts were not too even.  Next attempt I will take more time cutting the tube to ensure everything is square.

Another strange idea DSCN7940
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I then soldered the NVA barrel into the carrier, ensuring the brass strip was opposite the hole in the barrel.  Afterwards, I took my jeweler’s files and smoothed and shaped the brass backing strip so that it was narrower than the NVA diameter to allow full air flow through the venturi.

Another strange idea DSCN7969
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Before installing, I took some coarse sandpaper and ran it lengthwise along the carrier to knurl the tubing so it wouldn’t rotate in the plastic.  Then I just pressed it in with the hole pointing down.  The hole’s a little off-center, but since it’s bladder pressure that doesn’t matter.  

Another strange idea DSCN7977
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So, the question is, does it work?   Well, no.  Or, at least not the first few times I tried it.  Seems the two Cox glow heads I tried were pretty weak.  I put on a Merlin adapter and insert, and Bazinga! (©Chuck Lorre Productions, all rights reserved.)  






It seems a hair over-compressed, maybe.  I may try another glow head gasket.  But 18,650 or thereabouts seems pretty good.  It’s a Medallion, not a TeeDee.  Who knows what another gasket might gain or lose?  I’ll fly it when I’m able and see if I want to mess with it or just leave well enough alone.  Too bad Merlin seems to be out of the glow plug business.  When my current supply runs out, I guess I’ll just have to do turbo plug adapters only if I’m not going to use Cox heads.

Now that I’ve experimented and I know how I want to do this, I should be able to do the next one in about an hour.  I have six or seven of the fine NVA assemblies on hand for further experiments, if I want to do more of these.  I find that I am really liking the consistent RPMs of the bladder systems, and being able to do them with Medallions as well as TeeDees is a big plus.  The great thing is that this will also work for a Medallion .09 engine.  I don’t know about a Medallion .15, I’ll have to give that a try.

The Wizard Mark
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Post  getback Fri Jul 23, 2021 8:19 am

cheers cheers Good job Mark ! Sounds pretty good in my book 18.5 K is good with a 6x3 prop I think even if it did come out with a couple imperfections Beers You want see them in the Blur going by. Very Happy
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Post  Marleysky Fri Jul 23, 2021 9:05 am

Yup,that's some wizzardy connections for sure! Very Good Idea and well planned out and nicely done. I hope whatever has been keeping you down is going away and lets you get back in full action.
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Post  rsv1cox Sat Jul 24, 2021 6:08 am

Great picture tutorial Mark.

Gloved hand a necessity, those little APC? props can bite, sharp as a knife too.
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Post  batjac Thu Aug 05, 2021 2:33 am

Okay, I went to the flying field today.  I wasn’t up to flying, but I could start and toss for my son.  He’s been flying my Flip a lot, but I wanted to test out the Medallion.  Looking at the handle on the plane, I had 30 foot lines on it instead of the 35 foot lines I thought I had on there.  Fellow club member Jim Cameron first flew this on the existing 30 foot lines, and it flew slower than it did on the TD, which is what I wanted.  Then I put on 35 foot lines and it was just about right.  The engine is a little over-compressed, and it took a good bit of flipping to get it to start.  I'll add another head shim before the next time to the field to see how it goes.






Next, I have an idea for a handle I want to try to see if it will make flying smoother when I’m back on.


The Criminal Mark
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Post  getback Thu Aug 05, 2021 6:38 am

The Medallion was Screaming way to go !! I have never flew a Skyray but that one with the built up wing seems to fly Great ! I hope you can get your health issues under control so's to get behind the handle again . RC Plane
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Post  Yabby Sun Sep 05, 2021 4:22 am

Found on my 049 TD engines if I want them to work really well on suction, I get a venturi from Matt or Bernie, get a standard Cox 049 spraybar. Drill the Venturi to take the spray bar, block the sprinkler holes in the venturi and fix the spray bar in with an Aluminum base epoxy and then screw it into the carb body either using standard glow plug washers (not Cox type) or the spacers Matt sells for them. Now it takes the standard long NV that the Surestart Grey backplates use as its the same spray bar I put in. I seal it all up with anaerobic permatex and also mount a short piece of piano wire parallel to the needle valve. I fix the piano wire into the Aluminum base epoxy, and then I put some fuel tube over the piano wire and wrap a rubber band around the NV and it doesnt change setting with vibration and such. You can also adjust the venturi position such that the NV is pointing well back away from the Prop. This also works very well with a TD spring and a Cox round muffler fits perfectly. Getting 22K with this setup according to Android RPM gauge. I should probably post a photo of the Venturi setup. Works fantastic on suction or pressure and everything fits. Spring and Muffler! cheers

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