Cox Medallion issue!

[Larry Renger]

I inherited a Flying Clown from Barry Baxter, and am charmed with the design's potential. However, I have run into a problem!

After some bench testing of the originally installed, steel-fin Fox .15 versus a couple of Cox Medallion .15s, there was no comparison on the power difference (1000+ rpm!). That was the good news. The bad news is that the Medallion I chose has the pinned piston instead of the usual ball-joint. Wow does that sucker vibrate! It tore the nose clean off the model on the previous flight. On examination, the glue join of wing to nose was sorely lacking, so that has been carefully repaired. I figured it was a construction problem, Right? 

Today, however, after one flight, one of the engine mount screws backed out 1/8" despite double-nutting with a Nylock nut torqued over a T-nut! Never seen the like!  The engine run was also very unsteady, but that could have been due to the, then, sloppy mounting.

For the time being, that Fox is going back on the plane.  I may need shorter lines to compensate for the reduced power, but there seems to be adequate turning capability for the OT pattern. I suspect that going from an 8x4 prop to a 7x5, will restore the speed I need.

For the Medallion, I will find a nice fitting ball joint piston to fit, and see if that cures the problem. More updates as they happen! 

[Motorman]

have you tried running the Cox in a different rpm range?


MM

[Dennis Leonhardi]

No offense Larry, you're a good guy - but I'm laughing my butt off!  Will there be a Cox Medallion Hurl now!?!

Back in the '60s and '70s, I mentored quite a few kids who could handle the Fox .15 quite nicely, and won trophies in both .15 Combat and Goodyear Racing - beating such dead chunks of weight as the SuperTigre .15s of the day ...


Dennis
 

[Brett Buck]

For the Medallion, I will find a nice fitting ball joint piston to fit, and see if that cures the problem. More updates as they happen!  

  Are you sure you have a Medallion piston, or maybe a TD Special or Special II? I thought all the Medallions had ball/socket pistons and conrods. And I thought all the 15s tended to be shakers because there wasn't enough diameter for a decent counterbalance.

   Or, we could do a Hurl as a demonstration event, just submit your wind tunnel testing data so we can calculate the ballistic coefficient.

     Brett

[Larry Fernandez]

No offense Larry, you're a good guy - but I'm laughing my butt off!  Will there be a Cox Medallion Hurl now!?!

No offence Dennis, but Im laughing my butt off
As much fun as it might be to "Hurl" Medallion .15's, It would never come close to the fun and excitement that comes with "Hurling" a slant plug Fox .15 at a major contest on a warm summer afternoon in northern California.

Ask any participant. Its the highlight of the contest season.

Ask a former Nationals Champ, Brett Buck. Of all of his accomplishments, be it professionally or hobbies. He will tell you flat out that winning the "World Famous Fox .15 Hurl, has been and will be the highlight of his life.

I have won it three times and I know the euphoric feeling of a great "Hurl", coupled with a good bounce and a great roll.
I just cant describe the feeling.

Larry, Buttafucco Stunt Team 

[Carl Cisneros]

Larry
can you post a picture of the engine please?
I been scratching my head about that piston assembly yours has.

thanks

Carl

[Larry Renger]

The piston is, very likely, out of the infamous Tee Dee Mark II. That engine was reputed to vibrate so much that it could set a plane on fire from friction between the wing and fuselage. 

The piston looks like any other piston except for a huge, solid piston pin.  If that were drilled out it might help, but Just switching to the standard piston is the easy way out.

[Larry Renger]

I pulled the engine apart, and weighed that piston vs a ball joint one. WOW! 2 grams difference, starting with the ball joint one at 7.5 gm.

The bad news is that is the only ball joint piston I have and it only fits one cylinder! It locks up in every other cylinder I have. What does this matter? I'll tell you why it matters!

NO! DON'T SHOOT (LOUD REPORT, FOLLOWED BY A BODY HITTING THE FLOOR)

[Tim Wescott]

I think I have a Medallion 15 cylinder and piston.  It's extra-special, because they're both oval.  So when they're lined up the engine runs fine, but then as the piston rotates through 90 degrees (less, actually) it freezes up.

You're welcome to them...

[Larry Renger]

Umm. Thanks, but no thanks, I don't need problem children. Then again, it might be interesting to measure them out and see how far Cox could screw up. That might be really a rare set! I was shocked to find that the one ball joint piston I have would only fit one of several cylinders while the useless pinned units fit the rest just fine. OMG Cox had feet of clay! 

[Brett Buck]

Umm. Thanks, but no thanks, I don't need problem children. Then again, it might be interesting to measure them out and see how far Cox could screw up. That might be really a rare set! I was shocked to find that the one ball joint piston I have would only fit one of several cylinders while the useless pinned units fit the rest just fine. OMG Cox had feet of clay! 

   Expanding the idea that worked so well as an 049 to 15s seemed to be a bridge too far.

     Brett

[Serge_Krauss]

Should I expect similar vibration problems with my Olympic .15 with smaller venturi throat?

[Larry Renger]

I think that the pinned piston only originated with the 2nd version of the Tee Dee 15. That is the one with the reputation for vibration. It was supposed to be fixed in the third incarnation of the engine.

Most Medallions and the prior, reed .15s should not have that problem. Why I only have one ball-joint piston in my parts bag is a mystery to me! I would think that the pinned ones would be the rare item.

I wouldn't mind getting a few ball-joint pistons to go with the engines I have in parts.