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Hopping Up a Stock Medallion .049 - Page 2 Babe_b10
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Cox Engine of The Month
September-2019
Levent Suberk's

CEF Magazine cover featuring the Ex Model Engines Harpy Eagle .049



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Hopping Up a Stock Medallion .049

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Hopping Up a Stock Medallion .049 - Page 2 Empty Re: Hopping Up a Stock Medallion .049

Post  1/2A Nut on Sat Aug 31, 2019 3:53 am

Put a turbo plug adapter on the engine instant boost in power.
OS RP7 plug.

https://kamtechnik.com/product/ktk-11/ Thumbs Up

Hopping Up a Stock Medallion .049 - Page 2 Ktk11_11

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Post  DaveyMo on Wed Sep 04, 2019 10:16 am

Ken:

What a beautiful ship of the air you've got there. Thanks again for these photos - neat wheels and midship tank mount, for sure! Lots of adaptations that I can incorporate into my next project.

Seeing your setup makes me want to go back and do a complete rebuild of my little Flite Steak, but that's just not possible! Your discussion prompts a few questions, though. The tank I'm using is a home made wedge with uniflow. Watching it run, I can see that bubbles are induced, probably by vibration. At this point, I'm tempted to remount it (it's strapped on tight, flat against the fuselage) with a cushion and standard rubber band retention. Worth a try?

That midship tank mount is intriguing. Did you anticipate that mount while building and adjust accordingly, or was it a retrofit?

Yes, I tolerate the trial and error approach reasonably well. Thanks for the encouragement. I fly at vacant ball diamonds, and people drop by to watch, and often ask why I like this sport. Lots of reasons, but solving problems elegantly has got to be close to the top of the list.

Much obliged.

Dave...
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Post  Ken Cook on Wed Sep 04, 2019 3:28 pm

Dave, my tank mounting was actually due to using a TD. I'm quite certain you've heard that a TD doesn't draw fuel on suction as well as a Medallion does. This is really due to the placement of the tank vs the function of the engine. I had all intentions of using my plane to fly the pattern. My initial flight inverted caused issues and I had to raise the tank which in turn had me hogging out a little material. I started with only a slight protrusion through the fuse knowing that the more I moved it outboard, the higher the tank would rise. Yes, I had to put a little thought into it and most of the time the tank needs to be raised, not all the time.

The wheels are quite simple, 3 discs of 1/32" ply with a o-ring wheels. I Monokoted the discs. As for the bubbles, unless they're causing engine issues I wouldn't be concerned. I put 2 spritzes of Original Armor All in my fuel. Despite what the naysayers will tell you, it helps and it doesn't cause any issues in the 20 years I have been doing it. If that still doesn't help, I use a inline Dubro filter. I have also seen bubble wrap around the tank prior to securing it. I use rubber bands most of the time, the integral mounted tank is just spotted into the fuse with epoxy.
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Post  DaveyMo on Sun Sep 15, 2019 7:35 pm

Greetings all:

Well, the modified Medallion and I spent some running time in the garage this eve, and we both sounded like sputtering fools! But before I launch into the unpleasant details, let me thank Ken for his post above, again offering some great design and setup ideas.

So here 'tis: maximum RPM is 16,700 but it's not a steady run. Unfortunately nothing much new here in comparison to prior runs. The 16,700 was on 5% nitro; upping the nitro in one jump to 25% gave me 11,200 RPM. Beautiful motor on a beautiful airplane. I'm confounded, but I do get one-flip starts!

So I did a quick calculation of the possible number of permutations given these variables: two different props, two different tanks, uniflow and non-uniflow setups, shims from zero up to three, and four nitro levels in the fuel (all 20% castor oil) from 5% to 25%, and came up with 196 combinations. I bet I've tried at least 60 of them and not a good run yet. Can't seem to establish much of a pattern in any of this, but then I didn't keep records.

I'm almost embarrassed to report this here as I suspect you all question either my sanity or my methods. But there you have it.

I don't have a high compression head as I didn't really want to go that route. Bringing back the standard one-port Medallion cylinder looks tempting.

Oh, it's still fun, but not as much when this sort of thing happens!

Dave Mo...
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Post  ticomareado on Mon Sep 16, 2019 4:21 pm

A stock Medallion ought to get at least 16K on 5% nitro on a 6-3 Cox black prop
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Post  aspeed on Mon Sep 16, 2019 5:07 pm

DaveyMo wrote:Greetings all:


So here 'tis: maximum RPM is 16,700 but it's not a steady run. Unfortunately nothing much new here in comparison to prior runs. The 16,700 was on 5% nitro; upping the nitro in one jump to 25% gave me 11,200 RPM. Beautiful motor on a beautiful airplane. I'm confounded, but I do get one-flip starts!



I don't have a high compression head as I didn't really want to go that route. Bringing back the standard one-port Medallion cylinder looks tempting.

Oh, it's still fun, but not as much when this sort of thing happens!

Dave Mo...
I don't see how 5% fuel gives you 16,700 and then adding nitro loses at 11,200. The 16,700 is really pretty good for the low compression head. I don't even use them. If your crankcase is hot, it may be a bit too tight and need lapping. However if it has too much slop it could be leaking air, giving an erratic run as you mentioned. My Medallion seemed pretty slow compared to every other .049 I have. Even the Wen Mac was faster.
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Post  ticomareado on Mon Sep 16, 2019 5:40 pm

I can't believe that someone would make such a confession on Medallion vs. Wen Mac. We're going to have do an INQUISITION on that.
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Post  Cribbs74 on Mon Sep 16, 2019 7:13 pm

You should have added gaskets when going from 5 to 25%. Rule of thumb is one per 10% of nitro.

That’s more than likely why you lost RPM.  Too much compression is just as bad as low compression.

A Cox engine requires a bare minimum of 15% for a steady run and easy needling. You are wasting time with 5%. They may run, but never very well.

Keep on with the 25% and use 2-3 head shims, you should be much happier.

Ron
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Post  DaveyMo on Tue Sep 17, 2019 1:28 pm

Thanks, gents, for the good feedback. Will follow Ron's advice on my next run. And knowing that I should expect RPM over an unmodified Medallion's 16,700 will be helpful. Onward.

Tico, go easy on Aspeed when it comes time for the auto da fe!

Dave ...
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Post  ticomareado on Tue Sep 17, 2019 1:39 pm

Not to worry, I'll be his abogado pro bono publico
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Post  DaveyMo on Fri Sep 20, 2019 11:34 am

Uncle, he cries! I got one good run with the latest advice last evening, but just can't get it to start today. Back to the original, single-port #2 cylinder setup; I'll just have to live with the possibility that I missed the one or two correct combinations in all of the 196. It's been fun, I've certainly enjoyed the dialogue here, and have picked up some great suggestions. Will certainly return with more Cox problems in projects in the pipeline.

Thanks to all!

Dave Mo...
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Post  Cribbs74 on Fri Sep 20, 2019 6:16 pm

DaveyMo,

This should have been an easy hop-up. Something must be wrong.

How old is your fuel? Still using 25%? Battery condition?

Even a good glowing glow plug can actually be bad.

Good fuel, good glow plug and good compression is all you need.

Have you had the carb/venturi assy off? Sometimes people will drill the case for pressure, it can wreak havoc when running conventionally.

Ron


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Post  DaveyMo on Sat Sep 21, 2019 10:13 am

Ron:

I've gotta think you're right. For now, the #2 cylinder will let me get it into the air on shorter lines to recharge my spirit. Swapping it back to the #1 when I'm ready to give it another go won't be hard. Thanks for the suggestions and encouragement.

Dave...
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