Fox 35 on Profiles

In a previous thread here, Brett Buck said:
>It's a good engine in a lot of ways, having won tons of
>contests in the 50's, 60's and 70's. It's a waste of
>time trying to run it on a profile, but in a full-fuse
>airplane it has a lot to offer.

And Kiwi said:
>But my personal experience with an "infamous" fault for
>that engine (an interesting burble in power delivery at
>high altitudes in outside turns) has been minimal...

I had lots of personal experience with it last summer.
It "burped" big time everytime I did a figure eight.
But more annoying was the vibration, which was causing
bubbles in the fuel, even though I had the tank mounted
on rubber pads, and added a drop of Armorall to the fuel.
The vibration also was slowly crushing my nose, even after
I added an aluminum backplate.

I WAS going to build a full-fuse plane for it (a Fancy Pants)
but a recent *incident* convinced me I'm still too crash-prone
to invest the effort into that yet, just to see it reduced to
a pile of splinters, so I sold the kit.

So, what I want to know is if anyone has had success with
using a Fox 35 on a profile, if it is possible to eliminate
the burp, and how to deal with the vibration problem.
Or if it IS really a waste of time.
Thanks in advance,

It tends to be a waste of time on stock motors. Some work, some don't, the percentage is too low for me to recommend it.

It can be fixed. The vibration is solved by the use of the ABC cylinder/piston assy, $65 from Fox. While you have it apart for this change, add a piece of 1/8x1/4 spruce to the case bypass. Make it long enough to go from top to bottom, and if necessary taper the end to make sure it can't hit the crank as it turns, but cannot slip out. This will solve the burp issue by taking up most of the bypass volume, thus speeding the gas flow and making it less prone to acceleration effects.

Filling the bypass with J-B Weld, forming to clear the liner, and then routing a couple of 3/16 wide by bypass depth grooves for gas passage is a slightly more high-tech appraoch. I've been told that the combination of aluminum and the steel in the epoxy causes electrolytic corrosion after a while.

Or, alternately, get a stock 20FP, 25FP, or TT 25, bolt in, bolt on 9-4 or 10-4 prop and don't worry about it. Save the Fox for a full-fuse airplane. These are all cheaper than the trick ABC liner alone, and they fly the airplane better to boot.

I discourage beginners and even the moderately experienced from getting involved in engine modifications until and unless it's absolutely necessary. There's little justification to do it as long as adequate stock motors exist, and it certainly doesn't usually end up saving money.

Ed,

Of course it can be done, Profiles have run for years with Fox 35s in their noses. I have an RSM profile P-40 that has given years of service and runs and flies just fine. Nothing fancy in my setup: Box stock 40th anniversary Fox 35, Big Art chip muffler, 4 ounce Brodak uniflo tank and zinger 10x6 wide prop. I have small aluminum pads 1/16th thick and not much wider or longer than the engine mounts. Fuel is 5% nitro/29% all castor with one drop of armorall/gallon added and the gloplug is one of the cheapie Thundervolt R/C longs.
The plane flew well enough to get first in PAMPA Intermediate at Brodaks 2000 and placed fourth in Profile stunt at this years KOI. The plane has suffered more from the other bane of profiles (wobbly tail) then from any engine run difficulties.
I might point out that my Fox on that plane never exhibited the "burp" many have complained about, to the point that some fellow fliers accused me of having modified the engine in some way (which is not the case, I have never even had the backplate off). So as far as fixing the burp, I am clueless.
Mr Buck's comments about engine choice represent good advice, but nor would I heave the Fox into the dustbin. My Fox took 50 flights to run well and more than one hundred flights to run great. Good luck with the Fox whatever plane you run it in.

Steve

Steve

If the pads are no bigger than the mounts, what good can it do?
Or do you rackon than just a little bit larger is adequate?

Thanks,
Preston

Had 2 other Fox 35's in the early and late '70's. Mounted on Banshee's, I had the same luck AFTER I solved the problem of the engine cutting out overhead. The only thing different since my Ringmasted days was the spinner. Once I removed it, I was back in business.
Can't explain this one, except that maybe the airflow around the venturi is somewhat altered with a spinner.

Now I have 9 Fox 35's! (Life is good!) I'm in the process of running them all on the same model / tank, etc. as a control. So far, very different experiences with the first 3 engines. I need to allow for break-in time so the process is slow. Very pleased with the 60's vintage engine and a 50th aniversary with many mods to include the ABC piston/sleeve. But another 50th Aniversary with Fox's Hemi seems to go excessively lean rather quick.

Will run them all and see what I got. No attempts at conclusions yet.

But I do believe in careful break-in, proper oil content in the fuel, and straight suction tank setups.

Then I put a "Blue Magic Head" on one day and the outside burp disappeared. Sold by Tom's Machine Shop (Stunt News Advertiser).
Basically, it is a hemi head with an offset glow plug. When you use it with a hot glow plug, it seems to be immune to the increased fuel charge that comes during an outside turn. As an added bonus, the Fox seems to run stronger with this head and it's very pretty anodized blue.

About your abandoned plan to build a Fancy Pants Fox 35 combination, .... good decision! One I built served as an acceptable Beginner Stunt model; but can be scary to fly through the complete Stunt Pattern. With its small wing area and flapped design, it has a tendency to stall in loaded turns and square corners. The outside turn at the top of the second vertical eight comes to MIND #@!*&()!!!!!

Bruce Hunt
Salem, OR
"Sun? What Sun?"

I put a hemi head with angled plug on a 50th aniversary Fox and believe it's more powerful. Started out by building an ABC and running with the standard head. No difference: no burp on the outsides...with or without the special head!

Spoke with another fox lover. He too says the burp was gone with the ABC...not the head!

This is why I'm cutting the variables by using the exact same model & tank...and run several generations of Fox 35's on it. I've had real good runs on profiles...and some atrocities! Just don't know what the "magic" is!!

Yeah, most of them have the Dreaded Burp, but first, let me try to define it. Usually, it is not so much of a hesitation, but rather a noticeable lessening of rpm that extends for about a half second (or a tad more) at the beginning of an outside maneuver, like a lazy 8.

You know, after experiencing it countless times, it is almost transparent and unnoticed by me, like breathing, or blinking your eyes, or something. And you can live with it, especially as a sport flyer. But...yes, it comes at precisely the time when you would rather do without it.

It's not just Foxes. My Johnson S imitates its Ft Smith soulmate. And other revered brands don't always get a free pass, either, when going the sideways route.

One thing about the wood insert for the bypass. Haven't tried it yet, but please indeed use wood. If anything metal inside that engine lets go...Also: Look at the bypass size on Stunts, starting with the 4 bolt head/2 bolt backplate Sandcast from '49 right up to the 50th anniversary. They vary, as the case design changes. So want to get your Master's Degree in CL? Grab about a half dozen cases, keep moving the same piston/sleeve/crank assy around in them (just changing one variable), and see if the burping is correlated.

But: Foxes on profiles? Sure, go ahead. It's practically the definition of the hobby for many.

But understand the caveat that comes with it.

You got me started on Foxes. And I'll tell a story on myself. You know how people from LA or some smog infested city say that they can't trust any air that they actually can see? This Summer, I got a Schnuerle Fox 25 made into an ABC job by none other than GMA himself.

It was butter smooth. And it bothered the living Hell out of me. It just wasn't a Fox anymore. Something at the subconscious level. I'm supposed to be the psychologist, but this one is beyond me.

Indeed! They even have sunsets (every so often)

[photo not recovered: sunbeach.jpg]

This is a beach in Florence, OR but, speaking of Foxes...

I've noticed the burp on the outsides but, frankly, it doesn't bother me. It's supposed to be a Fox and Foxes are supposed to burp. So I let it burp away.

>So, what I want to know
>is if anyone has had
>success with
>using a Fox 35 on a
>profile, if it is possible
>to eliminate
>the burp, and how to deal
>with the vibration problem.
>Or if it IS really a
>waste of time.
>Thanks in advance,

Ed;

This is always a fun topic. Fun because of old loyalties, fun because of different approaches, and fun because of the variations between engines.

First, lets go over the easy problem - vibration. There are a number of ways to handle this. One is to stiffen the nose. This gets counterproductive after a while. Secondly is to install the ABC piston/sleeve. This is expensive. Next is to try to better balance the crank. This works to an extent, but you have to know what generation your Fox Stunt 35 is from. I had an older one (early six bolt) that I ground the web on the crank to remove some stock as per some recommendations from a very experienced flyer years ago. Did it at the same time I replaced a piston/sleeve assy. Vibration was signifigantly reduced. Put it in the air, and the engine literally self destructed midway through the first flight. Found out later from George Aldrich that my mod was only good with later Fox 35s that had a different heat treating regimen than the older ones. Ruined a very sweet engine.... Finally, you can try to pad the fuel tank to reduce foaming that vibrations sometimes creates.

The one thing you will NOT get away from is the vibration caused by the mass of the iron piston

Next comes something more controversial - the outside burp. Some folks tell you it doesn't exist. Well, it may not exist for them, but for the rest of us, we live with it or at least have to be aware of it. The interesting quirk is that each Fox Stunt 35 is a law unto itself. What works well with one fails with another. Why? Who knows? Some folks have luck with tank design and tank placement, some don't. Some engines respond to the stuffer backplate and modified head, some don't. Some respond to the bypass (intake port) partial block, some don't. And, I have had some engines that I couldn't make burp no matter how hard I tried. Take that Stunt 35 off, put on another Stunt 35, and even THINKing about doing an outside maneuver made the engine burp.

I agree with Brett Buck on the Stunt 35. It is a fine mill for what it was designed to do. Put it in a ship that allows it to maximize its strengths and minimize its flaws, and you will have a lot of fun. When it comes to a profile, though, the vibration issue (which is handled much better with a full fuse ship) and the burp issue make it akin to hunting with a flintlock rifle - it works, but there are numerous things that can and will go wrong. Yes, there are those who like the challenge. But, if you are building profiles and flying them because you are getting back into the hobby, then it stands to reason that engine reliability is beneficial as it helps reduce the number of variables that can "bite you in the backside."

The TT GP25 or the OS LA 25 (which are available new), or an OS 25 or 20FP (which you can still find used) will do better for you in a profile if reliability is paramount.

The bottom line is that Fox Stunt 35s will run on their sides in profiles, but they are a lot of work and often take quite a while to troubleshoot because of the wide variety of variables and all of the solutions or rememdies that need to be tried. I think they were so common decades back because of the prolific production of the Stunt 35 and their availability and price, not because it was the best mill for the job. "Everyone" had one.

What this country needs is a good, simple to build, full fuse ship that builds about as fast as profile, has decent looks, and runs somewhere between 450 and 500 sqi. This would be the plane for all of the Stunt 35s that sit on shelves, in engine boxes, or desk drawers and haven't pulled a ship through the air in years.

Bob,

That plane exists as Tom Dixon's Time Machine 40. The only problem is the somewhat punitive cost of the complete kit and/or the foam wing cores. It was designed around a DS 40 which as mentioned on another thread in this board has about the same power as a Fox 35.

Steve

Bob--As we've talked about in a few discussions this would be simple enough to do but few seem to want to do it. This I guess is mostly a psychological hurdle. A simple slab sided box fuse is as easy to build as a profile and is far less of a compromise as regards weight and stiffness. But to most it just doesn't look right. Too plain. Not stunt-like or airplane-like enough. Not enough of the good ol' tradition of stunt for even beginners to be attracted to. So it goes.

>
>Bob--As we've talked about in a
>few discussions this would be
>simple enough to do but
>few seem to want to
>do it. This I
>guess is mostly a psychological
>hurdle. A simple slab
>sided box fuse is as
>easy to build as a
>profile and is far less
>of a compromise as regards
>weight and stiffness. But to
>most it just doesn't look
>right. Too plain. Not
>stunt-like or airplane-like enough.
>Not enough of the good
>ol' tradition of stunt for
>even beginners to be attracted
>to. So it goes.
>

You are most correct.

I have tried my hand in this a few times, and never could get anything to come out that looked good. This is directly attributable to my definite lack of artistic talent.

There is another rub as well. My experience with non flapped ships is that you need constant speed for them to fly well. Running a Fox in a 4-2 does not give you that as well as an OS FP25 running in a constant, slow two cycle. To get a good flying ship with the Fox, you need flaps. Anyone who is taking the effort to build with flaps will probably want something a little more capable. For example, if you can fit a far more powerful TT 36 where a Fox 35 goes, then why not go with the TT 36?.... Gets into a vicious cycle.

I know the original mill in the TM 40 was the Double Star 40, and that engine is very close in performance and size to the Fox 35 Stunt. Some are flying TM 40s or variants with the Fox 35. Their success can not be argued. My concern is how close to the "edge" are they on power. 526 sqi and a very fat airfoil is a lot of the Stunt 35 to pull. I need to talk to some of the folks who are flying the TM40 with the Stunt 35 to get their feeling about it. My experience in growing up literally on the coast of the Atlantic around Portland, Maine, was that on windy days (and we always had windy days), power was a good thing, as well as a fat airfoil to keep windup manageable. That may not be a contingency that I need to worry about here in sunny, temperate, Gawga, but it stays in the back of my mind all the time.

I keep looking at a 10% reduction of Leonard's basic DUS. Ed Culver was kind enough to send me a very nice sketch (with dimensions) of his modified TM 40/60 called the Cirrus. The Cirrus has nice lines and could easilly be scaled to 500 sqi. And, I keep fiddling around with my CAD drawings. Who knows. Even a broken (analog) clock is right twice a day... I may come up with something that has some eye appeal, is easy to build, and allows us to use some the million or so Fox 35s that have been made over the past 50 plus years.

Isn't that what you are working on with your "stealth project"?

As to the engines you mentioned, the TT 25 is a MUCH better engine than the LA 25. Lighter, better quality, and cheaper, too. Not a bad combination. It is a true FP clone for all those who wish the FP were still available. You have to convert it to CL, but that is what people were doing with the LA before they finally saw the light and did it themselves.

Leonard Neumann

Nothing stealthy about it. Basic moments from the DUS. Wing area running between 450 and 500 sqi. Slightly thinner airfoil, as I still wonder about the old Stunt 35's ability to pull the big, fat airfoils through the air.

>As to the engines you mentioned,
>the TT 25 is a
>MUCH better engine than the
>LA 25. Lighter, better
>quality, and cheaper, too.
>Not a bad combination.
>It is a true FP
>clone for all those who
>wish the FP were still
>available. You have to
>convert it to CL, but
>that is what people were
>doing with the LA before
>they finally saw the light
>and did it themselves.

And with some of your work, would be even better. Perhaps you might consider advertising them on your site as an alternative to the Stunt 35. Yeah.... that's what you need - someone suggesting that you do one more thing... .

"....What this country needs is a good, simple to build, full fuse ship that builds about as fast as profile, has decent looks, and runs somewhere between 450 and 500 sqi. This would be the plane for all of the Stunt 35s that sit on shelves, in engine boxes, or desk drawers and haven't pulled a ship through the air in years."
========================
It's too bad that no one kits Al Rabe's Mustunt 1. It offers the best of both the profile (simple, speedy construction) and built-up fuselage (upright or inverted engine for better vibration control, ease of starting, and no burp for the Fox .35.) worlds.

I stole his idea for the nose treatment of my current Banshee, and I'm sold. It's a simple mod that really tames the Fox and makes the plane fly very well. I'll not hesitate to use it again.

OS

>kits Al Rabe's Mustunt 1.
> It offers the best
>of both the profile (simple,
>speedy construction) and built-up fuselage
>(upright or inverted engine for
>better vibration control, ease of
>starting, and no burp for
>the Fox .35.) worlds.
>
>I stole his idea for the
>nose treatment of my current
>Banshee, and I'm sold.
>It's a simple mod that
>really tames the Fox and
>makes the plane fly very
>well. I'll not hesitate
>to use it again.

I had forgotten about the Mustunt series. I build a Mustunt 1 back in the 70s, but used an OS Max-S 35 on it.

Al Rabe posts here from time to time. I am surprised he didn't chime in. I still rememember looking at that fat airfoil and wondering why the h*ll anyone used something that thick. Of course, look at what the standard is today.... Al was ahead of his time. That was the first ship I ever had that, on a windy day, after the third loop and going inverted I wasn't at the threshold of giving the class C speed boys a run for their money...

I believe at one time, somewhere, you posted a picture of your Banshee with the Mustunt type nose. I can't find it to save my life. Think we might talk you into reposting?

[photo not recovered: cfc5802c.jpg]

If I did the thing correctly, the OS and Banshee should appear above. If not, click the link that appears.

OS

Great post! I never thought about it, but a Fox on the bench was a one or two flip starter until a "stranger" came around. At which point it alternated between a runny nose and belching fire. The Fox seems to like us older men, doesn't like females, and somewhat tolerates younger men with no talent.

This is probably irrelevant to the discussion, but it reminded me of an old practice in combat back in the mid 50s to early 60s. No one wanted to fool with bladders (bladders were for the speed guys that flew with their wrists in U-shaped contraptions), and the hot setup was tapping the backplate and running a pressure line to a sealed metal tank.

A necessary patch for this system was to file three or four full length grooves the full length of the brass crankshaft bushing, about .020 deep with a needle file. I never thought about it, but perhaps we lucked out and altered max crankcase pressure to an extent that diminished the bypass flow. I don't know.

I do know they didn't burp. This was a short time, as Fox 35s were quickly surpassed by superior combat engines and bladder fuel systems.

I know that sometimes people have problems with Foxes on profiles. But it doesn't happen often, and it certainly doesn't happen every time.

Reading this thread would lead you to believe that Fox .35 stunts
will not run on profiles. This is not true.

De Hill