Use an APC 10.5 X 4.5 prop with a line length of 58/59 or 60 foot length and a release rpm of around 9800/9900.

Allen Brickhaus

     I was getting a great 4-2-4 cycle break but the burp was annoying!  Just for laughs, I turned the needle valve in about 4 clicks and the thing ran like an electric motor.

     Now I ain't very smart, but has anybody else had the same luck with avoiding the Fox Burp by doing this? I did continue to get a great4-2-4 cycle break. Oh yes, I was using an O.S. n.v.a.


                                                                                           Any thoughts about this?

 

      Running that lean will probably end up damaging something, but parts are cheap.

     Burps are a long-solved problem, close to 20 years now. I recommend just fixing the burp directly per Frank Williams article. This fixes the burp definitively, then you can run it however you want.

     Brett

     Hello Brett:

     How can my Fox be running too lean if it breaks back into a 2 cycle from a 4 cycle while doing a maneuver? In level flight, it is in a strong 4 cycle and breaks into 2 cycle when going into a maneuver.  Is this not what we all strive for with a Fox?

     Running a Fox lean is not a problem! Try to catch some Foxberg Racing and see Fox .35 engines that are run lean.  They hot start very well and seem to last forever.

                                                                                                Stay well,

                                                                                                Frank

   How can my Fox be running too lean if it breaks back into a 2 cycle from a 4 cycle while doing a maneuver? In level flight, it is in a strong 4 cycle and breaks into 2 cycle when going into a maneuver.  Is this not what we all strive for with a Fox?

     Running a Fox lean is not a problem! Try to catch some Foxberg Racing and see Fox .35 engines that are run lean.  They hot start very well and seem to last forever.

   OK, then run it that way. Geez! You were complaining about a burp. It's a simple fix that takes about 5 minutes and cures the issue completely. I think that is a better solution than running it leaner.

    And, for the record, the stuffer backplate was designed (by the inestimable Marvin Denny)  for people who were running Fox 35s in racing events and wearing out the conrods. But I am sure that won't happen to you...  

    BTW how many complete AMA stunt patterns have you got on Fox 35s? Same question for any Fox defender. Because I probably have you beat (unless you are Bob Gieseke).

    Brett

p.s. OK, I was irritated by this argument. But, in fact, something useful can be gleaned by understanding WHY running it at full crank helps the burp. The fundametal problem  causing the burp is that the gas flow velocity in the bypass is too low at regular 4-2 break stunt speeds. When you have it on a profile with the cylinder outboard and you do sharp outside corners, it "burps". What is apparently happening is the acceleration of maneuvering slings the charge up against the cool outer wall of the bypass and either condenses it, or the flow attaches, and the flow of charge is interrupted. When you do inside maneuvers, it is slung up against the nice hot iron liner and nothing bad happens. Filling up most of the bypass greatly increases the flow velocity, making it less sensitive to the acceleration of maneuvering. You might expect that it would also reduce the power, but at stunt speeds the ability of the bypass to pass the necessary flow is not the limiting factor, and in almost every case, the observation has been that the power is either unaffected, or very slightly increased.

   Run it faster, the flow rate is higher, so the flow velocity in the bypass is higher, creating the same situation as the bypass stuffer.

    The issue is that the rest of the engine, which is dead-nuts reliable and very durable chugging around at stunt speeds, is stressed at the higher power levels. One failure mode is a slight misalignment leading to the connecting rod being pushed backwards, off the crankpin a little until it hits the backplate. This limits the travel somewhat but the tilt+load wears out the connecting rod. Not much of a problem at stunt speeds, big problem at just a little bit higher speeds. The stuffer backplate was intended to cut the clearance between the crankpin and backplate so that when the rod gets pushed off the back, the angle is smaller. It's anodized to provide a good wear surface for the conrod when it rubs up against the backplate.

    The next likely part to go away is the crankshaft itself. At stunt speeds with 5% nitro and light props it's mostly OK, add nitro even at the same speed and at about 15%, the crank will start breaking at an accelerated rate. Running it faster than normal with full-size props also stresses the crank.

       Hello Brett:

       You suggested that a stuffer backplate would be a good addition for a Fox .35 that is to be run hard as in racing.  I do not know where the stuffer backplate would be legal in a Fox event.  I think that they have to be stock!

       Broken cranshafts in a Fox racing event? I have only seen one and it was mine in a Fox that was "Stock" but would go much faster than any Fox that I had seen then or since.  I have no idea as to what was done to it as the guy who did the work would not tell me.  It puked the crank after winning the feature race.  The engine looked stock inside and out but it was very fast!

        I was not complaining about a burp  but mentioned the fact that mine went away when I I slightly leaned out the engine.  I think that it is easier to do that than to mess with the case! KISS

        In any case, try to get to see some Foxberg racing and perhaps build a plane and compete.  It is indeed a lot of fun! You may gain more respect for the Fox .35.

       Oh Yes, how long does a Fox .35 last if it is just flown for stunt!

                                                                                    Be well,

                                                                                    Frank                             

Jesus, Larry what makes you think that other folks may not have different experiences and knowledge than Brett does?

Phil Bare, playing with toy airplanes and engines for 60 years.

I don't even know how to respond to that.

Larry, Buttafucco Stunt Team

I bought my Fox stunt 35s, four of them, on sale at $12.95 in 1976.  I have flown several thousand stunt patterns with them.  I lapped the pistons and cylinders. Early on I went to ST NVAs, and started running 11 x 5 props, of which the Taipan seems the best. Last several hundred flights have been on Sig Champion 10% nitro, 20% oil, half castor, half synthetic.  For those flights I ran a stuffer backplate, but the hemihead was too powerful, so I replaced it with a stock head. I have usually run a crankshaft extention and a heavy hub. I never fixed the burp, because I had learned to live with it.

I burned up one engine trying to get it to run right on crankcase pressure.  It now has a new piston and liner, which I have lapped and broken in on the bench. I have been moving out of CL, and it is the only one of the original four I have left.

One of the things that is different today is the use of mufflers on the Fox 35. When I tested it there was a drop of about 400 ish rpms. If you are trying to turn a 10x6 on it you will need to run it harder and the 4-2-4 starts to go more to a 4-2-2-2-4 where it breaks going into the maneuvers and stays until you hit level flight again. If you drop the prop to 9 to 9.5 x6 it picks that back up and the traditional break comes back. This works with a light smallish size ship (say <36 oz and 48" span).

I just did the bypass reduction on the motor I am running in the El Diablo, I couldn't use the stick in the bypass because I had filled down the lip at the bottom of the bypass that would hold the stick in so I roughed it up, degreased and used JB Weld to fill up about half the depth, I assume this will work.

Best,       DennisT

I listen to experts thoughtfully, but if their opinions conflict with my experience, I tend to put more weight on my experience.

My Fox 35s have run to suit me with tongue mufflers providing tank pressure. However, the last Fox 35 I flew has an innovative muffler of my design.  It is the lightest and least effective muffler known.  It is a piece of tin, bolted into the tapped holes above and below the exhaust stack. It covers 1/4 of the exhaust stack outlet.  An ear is bent into the stack such that the cast in brace in the center of the stack and the ear form a chamber. This chamber has a pressure tap for tank pressure.

I showed it to Bob Gieskie at a Sequin, TX, contest some years ago.  He remarked that my Fox 35 ran very well with my set up.  No argument with that expert opinion.

I showed it to Bob Gieskie at a Sequin, TX, contest some years ago.  He remarked that my Fox 35 ran very well with my set up.  No argument with that expert opinion.

[/quote]

Like 30 years ago. 

Joseph, I am sorry that you misconstrued the points made in my post that you responded to.

Better luck next time.

Regards, Phil Bare, playing with toy airplanes and engines for 60 years.

Joseph, I am sorry that you misconstrued the points made in my post that you responded to.

Better luck next time.

Regards, Phil Bare, playing with toy airplanes and engines for 60 years.

You are not sorry, you are just arrogant.

You are not sorry, you are just arrogant.

I'd like to buy you a beer Joseph.

Larry, Buttafucco Stunt Team

      Hello Brett:

       You suggested that a stuffer backplate would be a good addition for a Fox .35 that is to be run hard as in racing.  I do not know where the stuffer backplate would be legal in a Fox event.  I think that they have to be stock!

       Broken cranshafts in a Fox racing event? I have only seen one and it was mine in a Fox that was "Stock" but would go much faster than any Fox that I had seen then or since.  I have no idea as to what was done to it as the guy who did the work would not tell me.  It puked the crank after winning the feature race.  The engine looked stock inside and out but it was very fast!

        I was not complaining about a burp  but mentioned the fact that mine went away when I I slightly leaned out the engine.  I think that it is easier to do that than to mess with the case! KISS

        In any case, try to get to see some Foxberg racing and perhaps build a plane and compete.  It is indeed a lot of fun! You may gain more respect for the Fox .35.

       Oh Yes, how long does a Fox .35 last if it is just flown for stunt!

   Ask Marvin Denny about the racing events he was referring to for the stuffer backplate. Fox racing has been around for a very long time, before there were any agreed-to rules. That's definitely what it is for.

    The Fox will last a very long time just running it for stunt, on 5-10% nitro and the requisite large amounts of oil. The weak point is the crank, and it gets astronomically worse if you go above 15% nitro. As long as you don't get a lean run, the cylinder will last a very long time. I have something like 40 gallons of fuel through one of mine and it is fine. Thats A LOT of flights, as mentioned above. One lean run can wipe it out.

     At this point, there are enough aftermarket parts around that you can make it last indefinitely. Randy's High-Zoot crankshaft solves the crankshaft issue completely, and, very interesting, people tell me it reduces the vibration in most cases, even over stock cranks that have had altered counterbalances.

   The stock engine is dandy for stunt use IF you don't expect to have modern performance AND you use it on an upright or inverted mount. It was absolutely no accident that they won a bunch of stunt contests (although you might find it to be a lot less than purported if you actually add up the wins in big contests), they have some very good characteristics. Lots of power, good function on profiles,  and ease of use (in the year 2013) are not among them.

    I apologize for getting snippy above, I was getting frustrated (after having the same discussion 100 times over the last 20 years). But the burp is 100% a known quality, perfectly well understood, with a perfectly good, easy-to-implement, and cheap 100% fix. There's no reason I can see for not doing it, and once you do, then you can run it any way you want, including with less pitch and more RPM until something else starts to give up the ghost.

   As always, I have no real stake in this, and anyone can obviously do anything they want. Advice is just that, advice, no skin off my nose if you don't want to do it. But bear in mind, lots of us have lots and lots of experience and have seen people struggle with a variety of things that they really don't need to, and it's easy to get frustrated when the solution is at hand and people choose not to use it.

    Brett

p.s. http://stunthanger.com/smf/index.php?topic=4458.0 - why Marvin invented stuffer backplates.

To Mr. Bucks defense he is the guy who made a viable plug and play alternative known to the stunt community.  He doesn't have to do this.  He doesn't have to do research on a beginner set up that can be bought from Tower Hobbies, particlarly when he is regularly attacked for making statements which are true.  This is because Mr. Buck has made a contribution that does much to attract new people to the hobby.

  Thanks Joseph, I appreciate the defense, and that someone appreciates the contributions.

   But really, no one was going after me (except in the ratings... very amusing). It was a difference of opinion, and thats the way it's going to go sometimes. I am certainly not upset with anyone from this thread.

    Brett

Brett's accomplishments are notable, but no more so than many others that have chosen to repeat the pattern a zillion times.

Appreciate the notice, and aside from being in a fairly small fraternity of NATS winners, I agree that it may or may not be any more notable. You'll certainly never see me saying I am the greatest stunt flier who has ever lived!  Even though I was, amusingly, #1 in the all time scoring list when Dick Byron stopped keeping it. That has more to say about the validity of scoring lists than it does about me. Billy was something like 15th. OOPS!  I have a very realistic view of where I stand in the big scheme of things, and it ain't the head of the list!

     I would point out, however, that in fact, I didn't do it the way many other people have, I have a relatively small fraction of the "zillion flights" that used to be a standard way to win stunt contests. And it is relevant to this thread, to some extent.

    For example, I have flown maybe 500 flights in the last 10 years, and in the year leading up to my win in 2006 I flew 25 flights (from my crash at the 2005 NATs to end of the Walker flyoff in 2006), and only 3 flights from the 2005 NATs to a week before 2006. The other 22 were starting from scratch with a brand new airplane and rebuilt engine, 5 flights at home before we left, and 17 at the contest, including all the practice and the official flights.  

    There are a lot of reasons for this, but the way I can do this and still be competitive is because I have less-than-average piloting skills (compared to the other top competitors) but better-than-average engineering skills. I make up for my weaknesses in one area by excelling in another. That's why this event, with all the variety of skills it takes, is so accessible to everyone. As a result I am flattered by the fact that other competitors seek my assistance in the area of trim and engine/prop setup. 3 of this year's top 5, for instance. I am proud of that, I have to admit.

    When these arguments come up, I have to say I do get mildly irritated/amused when I have people like David Fitzgerald and Bob Hunt seek out my advice, and pay attention (whether or not they actually use it), and other people who are maybe not as accomplished ignore it or tell me I don't know what I am talking about. Whether it irritating or amusing depends on when it happens and how, I have good and bad days like anyone else.

    I do my best to provide good advice no matter who it is and am always willing to provide help as needed. I didn't get to 3500 posts here, and ~10,000 on SSW (and a few thousand on the old RCO and a thousand or so on the old compuserve forum) by refusing to help people or by trying to keep my hard-won experience to myself. No one is special enough or good enough to be "above" talking to anyone else. Although there are plenty of people who DO think they are too good for it, they are wrong - and note the contributors here, and you can see that the guys who are genuinely top experts are more than willing to answer any question you have. Like all 5 of this years Walker Flyoff pilots, who have all posted in the last few weeks.

    I don't respond to threads that I don't know anything about, and I don't start a lot of threads, I answer other people's questions to the best of my ability.

     Sometimes people take offense when it doesn't meet their expectations, and sometime I get bent out of shape for whatever reason, usually from someone naysaying something that is demonstrably true. We are all human and we occasionally get into arguments over silly things, that's the human experience. We get over it and move on.

    Brett

Brett, the charge velocity through the transfer passage is, of course, what the burp is all about.
Reducing cross section helps keep the velocity up, just as turning the engine up does. No doubt about that.

I have always maintained that if the engine is leaned out and turned up, there is no burp.

Revs and flat pitch props work on all sorts of engines.

Regards, Phil

  I made my stunt career on that theory! 

     Brett

Works great, doesn't it?               

Last Seguin contest was maybe five or six years ago. 

Brett, that is an interesting post and covers lots of ground.
I fail to see how high revs-low pitch would not work with any engine that will turn 11- 13 thousand RPM or thereabout.

   Well, it doesn't. An ST46 will spin 13000 rpm - if you put a small enough prop on it. That's the problem, the prop required it so small that the low pitch doesn't compensate, and there is no *breathing room* , the torque curve is plummeting at that rpm.

    Even a 40FSR - which is not much of a stunt engine by todays standards - works FAR better in this respect, in fact, it's better than an ST60. The issue is, a 40VF, or PA40/51/61/65/75 or Jett 50/61/65, is far better still.

    You are approaching this like it was a hypothetical argument, but people have been experimenting with this for close to 40 years now, and there's no serious debate about what it going on or why.  We have tried just about every variation on the theme, and around 5" is the low end for the ST46/60 and the Fox.

   Can you stick a 12-4 on an ST46 and fly an airplane with it? Of course. It just doesn't work very well compared to 5.5" and a lower RPM. Either of those works far less well than a 40VF or PA61. Same story with a Fox35 and smaller airplanes. It works about as well now as it did in 1955 (although it's FAR harder to support and maintain due to lack of easy access to fuel). It's not nearly as good as other, far easier to deal with, alternatives.

It seems to me that any engine that  a novice can learn to operate and fly is adequate for the job and with the many choices available among the new stuff and the thousands of old engines floating around, the bases should be well covered.

   "Adequate" is probably right, if you are very patient. Buy why toss out the incredible advances of the last 30ish years just because someone is a beginner? Or, the way I would say it is why saddle a beginner with something that has less than the best available performance, is expensive (because to even make a go of it with a Fox you need some aftermarket parts, or just struggle with it like we used to - and don't say we didn't, I lived through it), uses unobtainium fuel, coats everything with oil, has terrible user-friendlieness, and shakes the airplane apart. When you can get a far-superior-performing and far easier to handle engine that uses any fuel you happen to find, uses props that cost $1.94 in bulk, has an effective muffler (that also improves the performance)  - oh, and if you wait around for a deal, costs about the same as the replacement crankshaft for the Fox.

    I did my first complete pattern with a Fox 35, bone stock, on a profile. I flew thousands of patterns with a Fox35/Nobler/TF 10-6/Fox Superfuel. I did thousands more flights with an ST46/Rev-Up 12-6 or similar, and was one of the very last holdouts (along with my mentor Ted Fancher). We knew what we were doing, and all the tricks there are. It's not like I am dreaming this stuff up out of whole cloth, I did it because that was just about the only way to do it at the time.

     Based on that experience, and the knowledge gained since then, it is my assessment that the very last thing a beginner needs is to be screwing around with any of these old engines, it took an amazing amount of knowledge to just keep them running ideally - and it had to be ideal because the performance is so marginal. I am an old fart just like most of us, but not the kind of old fart to tell people "well it was hard for me, and I turned out great, so it should be hard for everyone else". I want people to not have to struggle and fight with equipment to learn how to make it work, it accomplishes nothing.

    Brett

Brett, your opinion is noted, it just differs from many others and that's ok, just don't try to sell it as gospel.

<<snip>>

Watch a guy named Charlie Reeves fly his Big Job with an old long shaft Fox .59 and then tell him how much 'better' he would be if he only had a PA 61 in it.

( edited to add) BTW, Fox manufacturing still produces Fox S .35s and Super Fuel.

    Most of this is snipped because it bears absolutely no relationship to what I said. We were talking about low pitch/high RPM.

     I am not bashing Foxes, I am saying there are far better alternatives. Particularly for beginners.  Again, I would guess I have 5x the number of Fox flights that you do.

       You are absolutely correct about "Disinformation, old wives tales and just plain old BS does more harm to the hobby than anything.". Couldn't agree more.

   BTW, the "name dropping" isn't going to work, I know Charlie pretty well, and we have absolutely no conflicts, great guy. Please be sure to tell him, next time you talk to him, that you are straightening me out on how stunt engines work.

    I have been pretty polite here, I haven't baited you into anything, I have answered every question you asked (and everyone one else) patiently, and explained it in detail. Why are you going off on tangents about "Fox Bashing"?

    Brett

I wouldn't advise someone just getting started to get a Fox stunt 35.  There are other engines available which can be made to perform well by a person with a high amount of ignorance. I might make an exception if it was my Fox stunt 35, and I could work closely with the beginner to teach them the Fox Cabal Secret Handshake.   

As to low pitch/high rpm, the 1964 World Championships were won by Juri Sirotkin using a 4 pitch prop on an MVVS special edition engine.  It just didn't take hold on the stunt world at that time.

BIG Bear
RNMM/AMM

Leaning out a Fox S .35 and letting it turn does away with the burp. It gets the transfer port velocity up.
Lots of .19 to .25 engines both old and new that have and do fly .35 sized planes quite well.
My LA .25 powered S1 Ringmaster does a fine job but not a bit better than My Fox .35 powered Ringmaster.
The LA .20 and LA .25 are ok cheap engines but no better nore worse than a hundred others.

Oh, and BTW, lots of people over the past 50 - 60 years have said that a Fox S .35 is one of the most user friendly engines going.

Matter of fact, I would bet that more people learned to fly using a Fox S .35 than all other brands combined.

Do I think that it is 'the best engine going" nope but I do think that it is no worse than any others.

As I said before any engine that a novice can learn to run and will fly his airplane is as good as any other.

A leaned out Fox S .35 has no burp.

The OP indicates that fact.

Ed, you might have some thing there.