Author Topic: Re: The fox35 "fix" (Read 1162 times)

Timothy Payne · « **on:** November 08, 2012, 08:20:45 AM »

An interesting one, but surely part of the charm and appeal of these older engines is the fact that they were designed differently, and (the fox 35 in particular) was designed to run with a misfire, or 4 cycle.

An alternative would be to fit an expansion chamber to the exhaust. - Happy flying!

Andrew Tinsley · « **Reply #1 on:** November 08, 2012, 10:26:30 AM »

Hello Timothy,
I fail to get the drift of your post, can you be more explicit?

Regards,

Andrew.

Brett Buck · « **Reply #2 on:** November 08, 2012, 11:33:31 AM »

Quote from: Timothy Payne on November 08, 2012, 08:20:45 AM

An interesting one, but surely part of the charm and appeal of these older engines is the fact that they were designed differently, and (the fox 35 in particular) was designed to run with a misfire, or 4 cycle.

The Fox wasn't designed to run with a misfire any more than an ST46 was. It was set up to do it at a favorable RPM and with a favorable power change (almost none) but aside from that it's not particularly magical. I can set up a PA61 to do just about the identical thing with just a few changes and 3x the "power".

There is very little design-wise that is different about the Fox than any other engine of the time.

The "burp fix" has little to do with 4 stroking, 4-2 breaks, etc, it keep the engine from quitting on hard outside corners and allows it to do it's normal 4-2 break. The character of it, aside from the burp, is completely unchanged and you get either the same or very slightly more "power".

Brett

Dennis Toth · « **Reply #3 on:** November 25, 2012, 09:19:36 AM »

When the Fox 35 was originally produced stunt was wild and woolly, flown pretty fast. They did not run them in a 2-4 break. I think if someone knows Don Still down in TX he would attest to that. They used standard vent tanks for the most part and the run would get leaner as the fuel load burned off. In a flat 2 cycle there is no "burp". Also originally they used fuel with lower oil content somewhere between 20 - 25% all castor. I don't know when the first guy flew with the now familiar 2-4 break but my guest is it was a mistake on the needle and was off just enough to pull the plane at about 3/4 speed and the guy pointed the nose up in the climb and the motor came alive. Then George A picked up on it and stunt changed to PA with slower smoother precession maneuvers. All most all the engines of the day could run in a 2-4 just depended on the fuel, glow plug and prop load to get a reliable run. Later Fox started blending his own fuel and came up with Super Fuel for stunt. This had 29% castor and 5% nitro. It allowed running a bigger prop on the 35 because the added oil would provide a better seal, cooled the motor under the higher load and increased the compression very slightly for a little extra power. With this also came the "burp" which for the longest time was something we lived with and worked around. The "stuffer" backplate pretty much cures this and the stick in the bypass finishes the job.

Best, DennisT

Chris Wilson · « **Reply #4 on:** November 25, 2012, 06:58:45 PM »

You may find that, historically previous to glow plug ignition engines, spark ignition engines could do a very respectable 4/2 break (and still can do in old time stunt events.)

So perhaps it was some free flight guy who launched a sparkie in a rich 4 stroke trying to limit the power and then the engine went lean as it entered a climb.

But who knows its origin?