Fox 35 4-2-4 run

Hi all, just getting back into cl,flying a Brodak Cardinal,find that if I set my Fox 35 at very rich 4-stroke,just enough to let me take off it still breaks into a 2 stroke after first loop and stays there for rest of flight. Is it tank height? any sugestions please.
proctor

I assume you are flying with a uniflow tank---otherwise you normally go rich after takeoff with "standard" tank venting.

The cause for both effects is simply "centifugal" force. If the uniflow tank is wide, with the uniflow vent further out than the spraybar on the engine, "centifugal" force causes a negative pressure head once the plane comes up to speed. The uniflow location establishes atmospheric pressure (more or less), so any extra negative head makes it harder for the venturi vacuum to pull fuel basically "uphil". This is exacerbated by really wide tanks that you had to use with old style profile planes where there isn't enough space between the back of the motor and leading edge of the wing for longer tanks. I thought there was enough space on a Cardinal for a longer, but less wide tank.On the normal vented tank, the atmospheric pressure point starts off on the inside wall. In this case "centrifugal force adds a positive head, richening the mixture. As the fuel is burned, the air inside the tank also moves outward, decreasing the head, and causing the engine to progressively lean out.

Finally, the Cardinal is a pretty big plane for a Fox 35 (I think). It may be that the load is so high, that the engine leans out just because it is heating up. This might add to the problem.

Solution to the tank is to use one where the outside edge (with the uniflow vent) is closer to the spraybar. Then you don't have to richen it up so much. If the problem is the engine load, then only a larger engine would fix that (or more un-obtanium nitro added to the mix).

>Hi all, just getting back into cl,flying a Brodak
>Cardinal,find that if I set my Fox 35 at very rich
>4-stroke,just enough to let me take off it still breaks into a
>2 stroke after first loop and stays there for rest of flight.
>Is it tank height? any sugestions please.
>proctor

Is this a kit built plane or an ARF? I am wondering if the nose might be vibrating (ARF) and causing the fuel to foam. Shake your fuel jug. If it foams up and the foam stays there when you quit shaking, add a couple of drops (drops only) of original Armor-All to the fuel. It may not fix the problem, but certainly can help.

Next is the tank mount. Is it solidly mounted to the fuselage of the plane? Is the tank right behind the engine? (It needs to be as close to the engine as possible). What kind of fuel are you running? You need 28% to 30% oil in these Foxes. If you have lower oil, and especially the RC fuel that is so prevalent in the hobby shops, this is a big no-no!

On the Fox 35 when everything is built right and solid, with the proper tank set up and fuel, I like to set the engine so it is rich when the plane is level, but breaks into a 2-cycle when you lift the nose about 45 degrees. If you have it set right it will return to the 4-cycle when you lower the nose again.

And then there is the needle problem with the Fox. The threads are so coarse that you usually only have three settings with a Fox 35.

1) Too rich
2) Too lean
3) Won't run.

What kind of fuel are you running?

What prop? Too much load on the poor ole Fox will make it go lean. How I know this is too long to type here.
Don

I'm thinking that any Cardinal is way too much aeroplane for a Fox 35...and that it is going lean because it's overloaded
See Ya

Peabody

www.gscb.us / www.richpeabody.com

I agree with the answers, but I have never seen a profile Cardinal with anything less than a good .40 on it. I have never used anything but St .46, OS .40FP, OS .46LA. The Fox is just not powerful enough, even wtih 25% nitro. Use a larger engine and save lots of touble.
FWIW [photo not recovered: sipcof.gif]

I agree with both Ty and Rich - - The Cardinal will be much happier with a Brodak .40, or something equivalent.

On the other hand, let me add that if the Fox will "get you through the first loop", but then not return to a 4-stroke, the ONLY cause of that (that I've ever seen) is that the engine is not properly broken in. To test, start the engine and set it into a solid 4-stroke. Then point the nose up and the engine should break into a solid 2-stroke. Hold it there for 4 seconds. Then level the plane and the Fox should return to that solid 4-stroke run. IF it doesn't, it needs more (or better) break in.

Back in the 60's people were successfuly using 35's on planes with 600+ square inches I've flown profiles with about 640 squares on a 35 without run-away, the Cardinal only has 579 squares. The run away is a sign of too much heat being retained in the engine.

Engine not fully broken in, use more oil, use smaller prop, use less restrictive muffler, switch to smaller plane, use larger/newer engine. Easiest is to install more cubes THERE IS NO SUBSTITUTE for more cubes.

Carl

Sounds like a pinhole in your uniflop tank. Seal the vents with tubing, and pressurize it with a 2-ounce "hypo" syringe under water. Bet you find bubbles.

Another possibility is that the uniflow line was placed too close to the fuel pickup. (In my experience, they should be at least 1/4" apart.)

Try a conventionally-vented tank.

The stock needle valve works fine for most of us, but if you really need more fuel draw, try a Fox 40-60 size needle valve assembly. You'll have to add a small grommet or 1-2 flat washers to get the spray bar's fuel-feed hole in the center of the venturi.

John,

The Fox 4/2 run does not work when the engine is set too rich on the ground, for one thing.

IF the uniflow tank height is right, the fuel has enough castor, the filter is clean, the prop is not too much load, the plug is hot enough and the engine has had a good break-in, the 4/2 run can be set with the engine either just into the lowest possible steady two-cycle, or warbling between 4- and 2-, mostly two, at takeoff.

When things are right, the ground setting sound will change to a rock solid 4-cycle within a lap after release, as the prop unloads. The engine will chirp into 2-cycle under maneuvering loads and return to the 4-cycle when maneuver loads ease off.

The comment about Fox 35s lugging large models in days past is true, but there was a difference from what we have available these days. Large, heavy models slowed noticeably in maneuver loaded climbing moves, and consequently, sped up noticeably in the 'downhills.' Pull varied accordingly, too, from modest to very light...

With the newer engines, we can control lap-time (speed) by prop choice - letting them either fly in an easy 2-cycle without running away (largish diameter, flatter pitch) or in a 4/2 sounding mode (again, flatter pitch allows them to loaf along when loads do not force 2-cycle mode.) This is like setting a standard transmission car in a lower gear, and holding road speed with a steady RPM; it is less likely to sound more stressed going uphill, or around curves, or to unload more evidently downhill and on smooth straights.

The engines designed 50+ years after the Fox Stunt 35, of course, can produce more power. ...and at higher RPM. We CAN set those so that pull and apparent speed change less through a stunt pattern. The magic of a good Fox 35 is that we can still get solid, dependable use of it, the same way it has been the past almost 60 years! And the sound of a "proper" 4-2 run is a magic all its own.

A more progressive needle valve assembly helps a lot. The original Fox NVA, with a needle flatted on one side, tended to go richer-leaner then richer again, then leaner again as it was dialed towards leaner. My guess is that the flat, exposed directly to one spraybar jet then the other, caused response to be more stair-steps than steadily progressive. Fox's recent tapered, round needle is an improvement, particularly with a short piece of fuel tubing between the inner 'wheel' and the spraybar to seal air leaks. ENYA and OS NVA are closer to the stock Fox 0.125 spraybar diameter, and don't obstruct intake flow as much as the larger ST or other, custom, NVAs. Fox 35 power is marginal by today's standards, so blocking too much of the venturii costs more power than it improves fuel draw...

So, with a 10% nitro, 28% castor (or 25% castor + 3% synthetic) fuel, a clean balanced 10-5 prop, hot glow plug, tank height that will run the same upright and inverted, sealed tank, clean filter, and enough break-in time on the engine, it should at least give you all it has, dependably - and surprisingly well - for a long time.

Thanks guys,overwhelmed with the response,in a way you all
seem to be correct.
I'm using fuel generally recomended for cl on this side of
pond,
20% oil (castor plus synthetic)5% nitro. will up the oil
content to
28 - 30 per your and Mr Fox's recomendation.
Have tried various props around 10x6 best seems to be an old
wide bladed 10x5.
Engine is new old stock and again you guys are correct not
fully run in. Done what I normaly do with new RC
engines,couple of tanks though it and fly. These old plain
bearing engines need much more running in.
Am I asking too much in my Brodak Cardinal profile ARTF using
the
Fox, maybe. I'm flying on 58ft 15 thou lines and in anything
near a 2-stroke it's in the air in 10ft or so and in level
flight it flies with authority. Havn't attempted big loops or
wing overs and I suppose this is where lack of power would
show up.
Using a deep wedge tank as recomended for the model by
Brodak,sorry to say I don't know what a uniflow tank is or how
to use it.
Seems I got just about everything wrong, thanks again for
your patience and giving me lots to chew on. Proctor

Proctor,

Not to worry.

There's an excellent site nearer you than we are: the (UK) Barton Club's site. Excellent place! Members from all the currrent and former Commonwealths and Dominions, even a few of us ex-colonials, too.

http://www.controline.org.uk/phpBB2/index.php

Some may be in the North, closer to you. They are all a helpful bunch, and some may be within hailing distance by telephone.

I'd add further to my longish Fox Stunt 4/2 comments, that the layout of that engine does not allow continuing up the RPM ladder on a usable prop size - which is evident from the narrow RPM change from full-rumble-4 mode to tearing-silk-shrieking-2 mode. There's only a few hundred RPM gain, at best, but it sounds twice as fast because it is firing every time, not every other.

In contrast, more recent engines enter 2-cycling lower in their useful RPM range, thus keep winding up, producing more horsepower, all in 2-cycle mode, until torque falls so far that HP output ultimately falls off. Related to this: trying to make such engines run in the classic 4/2-break mode can put them so far down their torque curve as to be, effectively, useless or marginal.

It IS possible; a knowledgeable engine reworker can adjust various things to - in effect - build-in the narrow-band where the break occurs as a natural response to loading and unloading the prop into and out of maneuvers.

Alternatively, for certain modern engines, tuned resonant exhaust systems (tuned pipes) can impose such a response in a more controllable way. Resonant pulses in the exhaust tract can return a pressure wave stuffing unburnt fuel/air back into the cylinder to be fired next time through. A form of supercharging? The width of the resonant frequency band can be narrow enough to degrade power should the engine try to run away to higher RPM.

In fact, as I understand it - the engine is "tuned" to a steady cruise at RPM where that too-fast degradation response is already evident, limiting further RPM increase, hence the model ceases accelerating toward higher speed. Then, as maneuver loads drag RPM down, the resonance provides higher boost. Torque increases. RPM drops less than it otherwise would. Airspeed appears - but really isn't - more uniform. It MUST react to imposed loads... The range of speed changes is certainly less, but they are still present.

Is the model too big for a Fox 35? Possibly - more in the sense that newer engines have more power in hand, and won't frighten you by going to softer pull in difficult attitudes. Otherwise, if you accept the idea that the loads in maneuvers are quite large, and there must be some effect from that, very often a good Fox Stunt 35 CAN pull a 45-50 ounce, 550 sq in or larger, stunter around quite well. (AND, all Fox 35s that have not been cruelly abused ARE good, if you listen to them and use what they can do for you.) I flew a 52 ounce Oriental for several years, on an essentially stock Fox 35. Well, I did do a few of my personal mods to it... [photo not recovered: devil.gif]

Best of luck - and keep in mind the comment: "The harder I work at it, the luckier I get..."

(edit: spelling)