Fox 35 strange behavior.

How come Fox 35 goes rich when inverted with fuel tank mounted on the motor centerline? Brodak 40 mounted in the same plane does not exibit this behavior at all. Fox 29 shows just hints of going rich while Fox 35 is just suffering while inverted.

What could be contributing to this kind of behavior? Engine design? Engine loading? Lack of fuel suction?

I have a profile Veco Tomahawk clone(Dare's Sporthawk) that lost its rudder last Wednesday due to this problem.

Steven Yampolsky

P.S. Fox 35 has Fox backplate, glow head,ABC piston/cylinder and one head shim. It spins 10x6 on SIG 5% all castor(29%) fuel.

That is one of the "mysteries of stunt" which make CL so interesting.

LAST EDITED ON Jul-07-03 AT 09:34 AM (CDT)
 
Double check the spray bar hole alignment in the venturi. If it is cantered a little forward or backward, it will change how the engine runs when inverted. If you are using a FOX double hole spray bar, the two holes are NOT drilled in the center. Install it so the thinner part of the spraybar is DOWN. You should not see either hole when looking down the venturi. If useing a single hole version or ST or OS, the hole should be aimed straight down the venturi taper.
Hope this helps! You can also shim the tank, if it's not glued in to achieve the same run rightside-up or upside-down. So, if your engine goes rich when inverted, on the profile, RAISE the tank from the engine centerline about 1/8". Try it, then adjust, (raise some more)till it runs the same in both directions.

I have modifed the tank mount so that it can be adjusted but I would rather fix the problem instead of just patching it. I'll look into the way the spraybar is mounted. I was pretty carefull mounting it the last time though...

I really like the motor and the model it's on. They make a perfect match.

Foxes do that. Just raise the tank about 1/4" for starters, fine tuning the height as required. That will cure what ails it.
Bare

>Foxes do that. Just raise the tank about 1/4" for starters, fine
>tuning the height as required. That will cure what ails it.
>Bare

Strange thing about this particular bad habit is that the modellers who used Perfect tanks had less trouble with it. I first encountered this with the Veco T-28 tanks, which I could mount straight in-line on 36X's, but had all kinds of trouble with using Fox 35's, including the first instances in my recollection of needing to offset a tank.

That was before I used Uniflow tanks much at all, and then later on with those tanks, I noticed that several engines of mine besides Foxes wanted to have offset tanks, whether mounted inverted, sideways, or upright.

.
Kiwi

Perfect tanks? Who makes them? Where can I get one?
I've tried several tank motor combinations(4 different tanks all but one were Brodak tanks) on two different models with same results

I think you can get a good approximation from one of Brodak's tanks. I think the point Kiwi was making was that these were non-uniflow setups.

In a non-uniflow setup, atmospheric pressure is on the inside of the fuel level--which in normal flight is almost vertical--slanting a little towards the outside of the circle from bottom to top (vice versa when inverrted). Since the difference in running typically comes from a "perceived" head difference between upright and inverted, you can see that there isn't much of a difference in the normal venting (except the slant is reversed). With uniflow the atmospheric point is nominally right where the air bubble comes out of the uniflow vent into the fuel.

>I think you can get a good approximation from one of
>Brodak's tanks... >Alan

I THINK Brodak bought the Perfect line, plus others.

>Perfect tanks? Who makes them? Where can I get one?
>I've tried several tank motor combinations(4 different tanks
>all but one were Brodak tanks) on two different models with
>same results
>
>Steven Yampolsky

Well, I wasn't a fan of Perfect, or Kap-Pak tanks (Perfect Clones). But as someone else noted, they have been in the Brodak catalog. They had no internal baffles, and a sharply shaped wedge. Bubbles in the fuel was a common Perfect problem, and they were really bad about a very long over-lean part of a run, after becoming about 2/3 emptied.

Someone who influenced me when I was starting was so deprecating toward the tanks made that way that I hardly ever used them, starting off when Veco tanks were still made by Froom, and at the time cost about 3 times what the Perfect tanks cost (69 cents versus $2.00). Both kinds were plain atmospheric-vented, but the extra-lean end run part was shorter on the square wedges, and the baffles helped smooth things out, etc.

As I recall, the straight square wedge T21's (atmospheric vents) with profile mounted Fox 35's were not afflicted to much of a degree by needing the offset -- possibly just a little, and we didn't realize it at the time. Brodak is now selling the T-28's that my Fox 35's seemed to really HATE when those were still a new tank design (and there's a GMA story about those, for some other time), and if you can get a Fox to actually run out a full tankfull from one of those, you'll see it needs a lot of offset with whatever venting it has.

Personally, I would just move the tank 1/8" upward on the Sporthawk & compare running speeds upright vs. inverted before launching, in the pits after starting. If it still slows down inverted, shut down and move it up another 1/16" -- repeat the test. Only after it sounds the same, then you can try comparing the difference in the air.

.
Kiwi

Sounds like tank selection/contruction is requires Black Magic.

Me too. Raise the tank. Saves on rudders. Been there, done that.

LAST EDITED ON Jul-07-03 AT 12:30 PM (CDT)
 
Fox rc plug helps. Make sure tank is clean. Did you open the back? Is the pickup tube aligned correctly (centered in the wedge about 1/8th inch from the back). Fuel filters need to be cleaned regularly. Squeegees shed rubber. Do not use them. Those humongous syringes made of plastic that cost ten dollars allow you to see the nature of fuel pumped in. That can be good. Shimming the tank height is part of the process. Adjust till you like the run. Old tanks can be problematic. Corrosion forms in little black globs that create all kinds of weirdness. (New tanks also at times have been known to decompose. Reason for popping the back and taking a look.) Stay away from flat profile tanks, the ones high and thin. I had bad luck with them. Their bad baffling idea was explained to me. Use fuel tubing over the needle valve threads to seal the fox original model T magic screw. They work and are precise enough for this forgiving engine.

My Buster had all of the above problems. Now the thirty-five year old fox runs good upright and inverted. Motivation to solve the head gasket leak.

These observations from a member of the Fox 35 preservation society. Among those who believe that every FOX 35 has the inalienable right to live life true to mother nature's original plan.

Dennis,

None of the items listed explain why other motors on the same setup do not behave the same way. I can move the tank up no problem but I think that will just cover the problem, not fix it. I'm aftraid the problem may rear its ugly head somewhere else in the most inopportune moment and cost me a plane. Flying rich inverted on the other hand is survivable.

I will try to seal the threads with tubing as per your advice though.

LAST EDITED ON Jul-07-03 AT 01:14 PM (CDT)
 
Even engines mounted upright (and different brands) often require the same tank head tuning. The experienced modelers I know make a provision for this, even with PAs. Complicated rocket science explanations often do not work, cost money and ruin good parts. The more I get into this hobby, the more I appreciate practical pragmatic solutions (and I have learned to trust them.) When you think about it the fluid dynamics of the air/fuel mix going through the venture being sucked in by a hole in the crank whirling at 10,000 rpm is impossible to model accurately. This mix then gets slopped into a port and shot up on to a piston with a ridge pumping up and down and hot. Need a super computer? Then add in the varriable of humidity and atmospheric pressure on any give day. Fox 35s tuned in with various simple adjustments can be made to work reliably. Like other lucky classic configurations (small block chevy, for instance) the sum of performance surpasses its humble origin. I would try the simple suggestions offered then find a Fox person (in the flesh) who's stocker runs well.

It's all in the bypass.

I don't have any reasons, just seems likely that its all in there, its always in there. Just shim the tank unless you want to start adding material and cutting it out again.

LAST EDITED ON Jul-07-03 AT 02:17 PM (CDT)
 
Steve,

I guess I will provide my opinion on this much discussed subject, " Why does a Fox .35 run (funny, rich, lean, stumble, burp)" on my profile masterpiece.

The basic problem is that the design of the Fox makes it sensitive to changes of fuel air mixture and if you understand a bit about what variables affect the fuel air mixture presented to the glow plug during ignition you can adjust things to improve the run.

The basic design element that causes the problem in a profile mount is the size of the transfer port located on the top side of the engine whem mounted in a profile. The volume of internal space in the transfer port causes the center of internal volume to be offset upwards from the centerline of the engine. In a uniflow plumbed tank setup the centerline of the tank will have to be aligned with the center of the engine's center of internal volume (acutally center of pressure in the running engine which should be offset slightly toward top of engine); so the tank would have to be mounted higher than the centerline of the engine. (But will depend on how fast the engine runs because the faster engine run should raise the center of pressure somewhat. Its a moving target!) The effect is to make the engine run rich if the centerline of the tank is higher than the center of internal pressure.

Now the next problem. As the model changes attitude in inside and outside turns Newton's first law of motion comes into play to mess with things. (Objects in motion continue in motion in a straight line unless acted on by an outside force.) The fuel air mixture in the transfer port is becomes richer in outside turns (fuel is heavier than air) than inside turns, because the fuel is forced to the outside of turns and into the transfer port(Good ole Newton). The effect is fuel is dumped into the cylinder on outside turns causing "the dreaded Fox Outside Burp" if you're lucky and "the dreaded Fox Outside cutoff" if you're not.

The next problem is cooling effect of castor in our fuel. The more castor the more cooling. Not a problem unless the glow plug doesn't retain enough heat from fireing to fireing. If the engine is running 2-4-2 it will be firing every other time in level flight. Too much cooling and too cold a plug will result in cutoff in level flight. Most often this will happen in inverted flight because Murphy is at work when the tank is set close to the centerline of the engine. ALWAYS USE THE HOTTEST PLUG (The one that retains the most heat)you can get. I use the Thunderbolt RC Long plugs in all my engines for this reason. Replace your plugs regularly. Just because it worked last season doesn't mean it will continue to work. The idle bar on the RC plugs also provide help in reducing plug cooling when excess fuel is dumped into the chamber during outside maneuvers. Back to the cooling effect of the fuel, with Fox engines we are always using 28% castor fuel as opposed to the 22% 50/50 castor synthetic mixes in other engines. This makes the setup more sensitive to these problems. Running a bit more Nitro can help by making the fuel mixture more volitile (read as easier to ignite) 10% nitro will help maintain the run better than 5% nitro.

The next problem is the location of the plug in the head. The standard head places the plug in the head in the middle of the head where it receives the full effect of the fresh air fuel mixture into the cylinder. This magnifies the previous problem of fuel cooling the plug during outside maneuvers. There are after-market heads that move the position of the plug reducing the problem. I've found that Tom's Machine Shop "Blue Majic Head" is the best of these.

The last issue is "effective" venturi size. An engine can be thought of as an air pump. The air into the venturi is being moved through the engnine and out the exhaust. The flow of air for a given engine run can be restricted either at the exhaust or the venturi. Either restriction will result in less power and sensitivity to adjustment. The Fox venturi opening is for all practical purposes fixed by design with the only variation being the size of the needle valve assembly. The smaller the size of the needle valve assembly the better. The best setups I've seen use a reduced cross-section ST needle valve assembly. On the output side of the air pump, the muffler should be open enough to allow good power. Most mufflers on good running Foxes will have been opened up (more and or enlarged exhaust holes).

What this all means is that the Fox, as designed, runs best in an upright or inverted mount and is sensitive to many varriables in a profile mount. Your mileage will vary!

Wow, what an excellent explanation! It makes perfect sense. I'll give it one more try by shimmying up the tank and if it doesn't help, I'll retire the motor until I build an non-profile.

By the way, if I send it in for a re-work, would it help to fix sideways mounting issues. Does re-working a Fox35 improves upright/inverted configurations only?

If your problem is that the engine runs rich inverted vs lean upright, your problem the tank position. That is the problem to fix. You may be having these other things wrong as well and they may cause you some other problems. I will say that I was never able to get a Fox NVA ro run consistantly and swithced to an ST NVA early on. I have always run side mounted Foxes with the uniflow outlet in the tank pretty close to centered. I do see tanks out of alignment that run just fine, but mine have never worked that way. Put your tank where the engine runs right, wherever that may be.

Jim you're right, I do have other problems with the motor, I just want to figure them out one at a time. Who knows, maybe if I fix this one, other problems like sputtering on hourglass will go away.

Hi Steven,

Your running an ABC Fox? Use about 25% oil in your fuel. 29% is to much for an ABC set up. May help.

I will go to 10% nitro/22% oil(50/50) once I'm done with the gallon of all castor(5/29) to the break it in. I have about 1/3 left.

Steve,
The need for tank height offset is not unique to Fox 35s! Uniflow tanks run so dependably that they turn out to be MORE sensitive to a mismatch -- or at least you get to feel the effect more clearly. The over/under vented early tanks were doomed to run ever-leaner through a full tank -- fuel head (or "gravity" feed at force angles in flight) was greatest at launch and fell continuously until empty.

That's saying max fuel feed pressure/ least demand on the engine's fuel draw early... increasing load on fuel draw over the flight. Naturally they leaned.

In the day of the MAX 35S, I think the "reference" height for an inverted engine, built-up model was something like 7/16" below the mounts. I recently bench checked an ENYA 35 that settles at a different height. (Special bench lets me simulate the 'g' force directions met in flight.)

A way to come VERY close to an even run by tank adjustment? Without hooking up the lines (they'd just be in the way, for this), start the engine and hold the model with fuselage centerline level, and the inboard wing straight up. If you can, tach RPM at a steady high 4-cycle. Keeping the fuselage level, roll the wings about 45º canopy-up. Re-tach it there, or get a good ear for the sound. Roll it to 45º canopy down, and check RPM (or sound). RPM (sound) should be the same in each 45º rolled position. If not, raise the tank away from the leaner (higher RPM) position. (More "gravity" feed if you do, so it will tend to go a bit richer.)

After checking, if you see a need to shim it some, shut it off. Point the nose down about 45º and turn the outboard wing up vertical to put the fuel pickup out of the fuel and starve it off.

Make an estimated adjustment. Fire it up and re-check with the same wig-wag method.

There may (almost certainly WILL) still be some slight speed difference in upright and inverted. That is the final check. You've made adjustments in the Wig-wag process, so you have an idea what to do to NAIL it.

Sooo much more enjoyable when you don't need to sweat tank height effects!!

There's been no mention of what prop is being used. You might want to look at it also. Don't now what your swinging up front, but I know my fox's like APC's.