Author Topic: Need Fox expert (Read 10961 times)

Chris Fretz · « **on:** April 30, 2016, 02:31:49 PM »

Ok fellas, I need help again. I have a Fox 35 that ran pretty good but the prop shaft is sloppy an it vibrates. Sooooooooo I put a different Fox 35 that has a nice tight prop shaft, but of course you do anything that is not level flight an this thing burps like crazy. Runs smooth, I raised lowered tank an nothing! How does one Fox run great an another not? Any suggestions? Is this a candidate for that burp valve I see guys taking about? How do you know if these Foxes are just worn out or what?

Tim Wescott · « **Reply #1 on:** April 30, 2016, 02:39:45 PM »

Is it side mounted? Folks who talk about the Burp say that it's much more likely on a profile. And from what I've read, yes, some engines do it and some don't.

If you just don't want to mess with it contact Scott Riese in Portland -- he does a lot of engine work, and always seems to have the best running Fox 35's in Classic.

Chris Fretz · « **Reply #2 on:** April 30, 2016, 04:54:06 PM »

Quote from: Tim Wescott on April 30, 2016, 02:39:45 PM

Is it side mounted? Folks who talk about the Burp say that it's much more likely on a profile. And from what I've read, yes, some engines do it and some don't.

If you just don't want to mess with it contact Scott Riese in Portland -- he does a lot of engine work, and always seems to have the best running Fox 35's in Classic.

Yes it is side mounted profile. Just drives me nuts, makes me want to hurl the Fox. I got one on a Smoothie mounted upside down an the motor looks like it was beat with a bat an painted black but it runs great, probably my best Fox.

Brett Buck · « **Reply #3 on:** April 30, 2016, 04:55:37 PM »

Quote from: #Liner on April 30, 2016, 04:54:06 PM

Yes it is side mounted profile. Just drives me nuts, makes me want to hurl the Fox. I got one on a Smoothie mounted upside down an the motor looks like it was beat with a bat an painted black but it runs great, probably my best Fox.

This is a long-solved problem. Just do the stuffer bypass fix and be done with it.

Brett

Jim Kraft · « **Reply #4 on:** April 30, 2016, 05:48:05 PM »

I think it was Bobby G that said Fox 35's run best when they are almost worn out. There may be some truth in that as my flying partner buys his on ebay that have a fair amount of running on them, and they run great on his Ringmaster. They run much better than the ones I have only a few hours on. I have one on my Sterling Mustang that runs really well. It is an old 60's motor with lots of hours on it. The later ones do better with the stick in the bypass and a hot plug like the Thunderbolt long R/C.

Chris Fretz · « **Reply #5 on:** April 30, 2016, 06:58:49 PM »

Quote from: Brett Buck on April 30, 2016, 04:55:37 PM

This is a long-solved problem. Just do the stuffer bypass fix and be done with it.

Brett

Ok how do you do that? Are there any videos or pictures on how to do it?

Chris Fretz · « **Reply #6 on:** April 30, 2016, 07:01:59 PM »

Quote from: Jim Kraft on April 30, 2016, 05:48:05 PM

I think it was Bobby G that said Fox 35's run best when they are almost worn out. There may be some truth in that as my flying partner buys his on ebay that have a fair amount of running on them, and they run great on his Ringmaster. They run much better than the ones I have only a few hours on. I have one on my Sterling Mustang that runs really well. It is an old 60's motor with lots of hours on it. The later ones do better with the stick in the bypass and a hot plug like the Thunderbolt long R/C.

I have a Testors McCoy series 21 on my Sterling Mustang. Wish Sterling was still around.

Tim Wescott · « **Reply #7 on:** April 30, 2016, 07:25:40 PM »

There's a thread right here on Stunt Hanger.

The theory is that the bypass passage has too much area on some motors (there's manufacturing variations), and you fix it by stuffing something in there to reduce the area. The reduced area makes for higher velocity flow in the bypass, which does whatever magic (probably keeping fuel droplets in suspension) that's needed to prevent the burp -- and Bob's yer uncle.

Note that I haven't done this -- early in my stunt career some guy named Brett convinced me not to mess around too much with the Fox 35.

Brett Buck · « **Reply #8 on:** April 30, 2016, 07:30:59 PM »

Quote from: #Liner on April 30, 2016, 06:58:49 PM

Ok how do you do that? Are there any videos or pictures on how to do it?

http://stunthanger.com/smf/engine-set-up-tips/the-fox35-'fix'/

Brett

RandySmith · « **Reply #9 on:** April 30, 2016, 07:34:39 PM »

Chris
If you do the bypass fix, when you put the FOX back on a profile, they most always run with the center-line of the fuel tank, raised about 3/16 to 1/4 inch above the center of motor

Randy

Brett Buck · « **Reply #10 on:** April 30, 2016, 07:35:06 PM »

Quote from: Tim Wescott on April 30, 2016, 07:25:40 PM

The theory is that the bypass passage has too much area on some motors (there's manufacturing variations),

  Almost all of them, in fact. Many, many people do not believe it ever existed at all, and we are all lying about it. GMA once famously told me it was non-existent, never happened, then that it was just doing what it was supposed to do (even when it quit), then gave several methods to "fix" it, none of which was the bypass stuffer. All in the same post.

   We all have our blind spots, even the greats.

   Brett

Chris Fretz · « **Reply #11 on:** April 30, 2016, 09:27:39 PM »

Quote from: Tim Wescott on April 30, 2016, 07:25:40 PM

There's a thread right here on Stunt Hanger.

The theory is that the bypass passage has too much area on some motors (there's manufacturing variations), and you fix it by stuffing something in there to reduce the area. The reduced area makes for higher velocity flow in the bypass, which does whatever magic (probably keeping fuel droplets in suspension) that's needed to prevent the burp -- and Bob's yer uncle.

Note that I haven't done this -- early in my stunt career some guy named Brett convinced me not to mess around too much with the Fox 35.

Bob's my uncle???

Chris Fretz · « **Reply #12 on:** April 30, 2016, 09:34:58 PM »

Ok ok I can definitely give that a try. I might have some new gasket kits laying around. If not do you think I can reuse the head gasket?

If stuffing it don't work I'm going to hurl it into the woods!

Tim Wescott · « **Reply #13 on:** April 30, 2016, 09:40:44 PM »

Quote from: #Liner on April 30, 2016, 09:27:39 PM

Bob's my uncle???

Your homework assignment is to watch "101 Dalmatians", or serve an apprenticeship in England. https://en.wikipedia.org/wiki/Bob's_your_uncle

Quote from: #Liner on April 30, 2016, 09:34:58 PM

Ok ok I can definitely give that a try. I might have some new gasket kits laying around. If not do you think I can reuse the head gasket?

If stuffing it don't work I'm going to hurl it into the woods!

The head gasket should be metal, and won't have a problem with you taking the head off and putting it back on. The crankcase gasket is almost certainly really thin fiber, and may or may not be salvageable -- a gasket kit should solve that problem.

RandySmith · « **Reply #14 on:** April 30, 2016, 10:22:37 PM »

The stock gasket is cork, and is almost never reusable unless just recently put on. and oiled before hand

Randy

Brett Buck · « **Reply #15 on:** April 30, 2016, 10:23:45 PM »

Quote from: #Liner on April 30, 2016, 09:34:58 PM

Ok ok I can definitely give that a try. I might have some new gasket kits laying around. If not do you think I can reuse the head gasket?

The head gasket it aluminum and you will likely never see it as a separate piece, it will be fine. I have never damaged the *fiber* backplate gasket and used them over and over. I have damaged the old cork type. If you do, it is very easy to make your own out of water pump gasket material.

Quote

If stuffing it don't work I'm going to hurl it into the woods!

That's always a good idea, but again, only Fox 15 Slantplugs can be used for recording official records.

Brett

Dennis Moritz · « **Reply #16 on:** April 30, 2016, 10:53:27 PM »

Ahh. The Fox 35 thread. Visited again. Must be a new month or season. Today at Nehaminy Park I saw 3 Fox35s running powerfully and reliably. Usually there's 7 or more. Did I say running powerfully and reliably. Usually many Ringers some CG profiles Magicians. Firecat is a new one. BiSlobs. A front cover Stunt News Galloping Commedian that has finished 3rd (maybe 2nd) at NATs in Classic. That bird a flapless OTimer. Ringers that have won Classic at Huntersille. And and. All stock no popcycle sticks jammed somewhere. All running year after year on Powermaster 22percent 50/50. 5 or 10 percent nitro. No extra castor. One high zoot crank motor. Let's see none of the advice I've read on this thread so far matches what we do.

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RandySmith · « **Reply #17 on:** April 30, 2016, 11:18:42 PM »

All of the FOX 35 made gaskets are cork ,for the backplate, they are very fragile and will tear easy, if you have a fiber gasket, it is one that someone made, or one from Eric Rule, who cuts them on his laser cutter, Brett is correct in that the fiber gaskets are much tougher, and will be usable over and over sometimes.
It will be very easy to tell what type you have, as they most always stick out of the side edge of the backplate

Randy

RandySmith · « **Reply #18 on:** April 30, 2016, 11:20:55 PM »

Quote from: RandySmith on April 30, 2016, 07:34:39 PM

Chris
when you put the FOX back on a profile, they most always run with the center-line of the fuel tank, raised about 3/16 to 1/4 inch above the center of motor

Randy

Dennis I think you will find this works, and works on profiles even at your field

Randy

Dennis Moritz · « **Reply #19 on:** May 01, 2016, 12:43:10 AM »

First off, no one asked the gentleman which Fox35. Was it a 40th anniversary, or 50th or 60th. Or older. He also said it burps when not flying level. If the bypass was an issue it would be burping on outsides. Period. Probably isn't a burp. Tank, vibration, piston/cylinder fit. Prop out of balance, dirt in the filter, dirt in the needle. A miss-aligned needle. Leaking coverplate. What fuel? What prop? Frustrating to hear experienced folks recommending mods to an engine when the universal issues of IC motors on model airplanes aren't looked at first. How does the engine run on a test stand? Will it run at an even rpm? Does the motor lock up when you go to a fast 2-stroke? Fox 35s vibrate, create harmonics, each one an entity unto itself. Different model airplanes, even of the same design, coming out of an same brand kit box, will vibrate differently. Is this a bad vibe issue? Are the engine bolts cinched up tight? What muffler? Any muffler? On and on.

If you are having an irregular run, look first to the fuel system. Dirt in needle, filter, bad fuel tank, pin hole in fuel tubing.

Is there anyone around where you fly who runs Fox35s so they work reliably? Or for that matter, is there anyone around who gets model airplane engines of any brand to run reliably on U-Control planes? Consult them first. Most bad run IC issues are generic.

Fox 35s vary greatly. Part of this is variations in machining tolerances. All kinds of tolerances can be off. Often engines run well anyway. Sometimes they don't. Issues like that are not forum solvable.

First thing I would do is take engine off plane and run on test stand. If engine will keep a steady needle setting you're at 1st base. If it doesn't keep a steady needle setting look for dirt somewhere in fuel system. Reverse flush needle by removing needle putting finger over fuel inlet blowing fuel from threaded side of needle, if fuel flows through outlet, you're needle is probably clean. Better to take needle out, reverse flush, then examine in good light. Check tubing for pin holes.

If the engine runs reliably on test stand without locking up when leaned out to a fast (not screaming) 2-stroke engine is probably ok. Mount solidly to plane. Check tank. Many hard tanks need to have their backs popped and insides looked at. If you don't know how to do this, do not know what the tubes placements are suppose to be, find someone who does. Check tank for leaks. Pressure test tank underwater by capping all openings except one. Use syringe to blow air into tank. Push plunger hard. Any tell tale bubbles. Seal any leak with solder. A well set up clunk tank can also work. These are setup slightly differently for UC then they are setup for RC. One thing, pick up clunk tubing needs to curve towards outside of circle. On profiles tank needs to be mounted to some kind of cushioning material, velcro works well. Tank needs to be mounted solidly. Tank height needs to be adjustable. And. And. See if your engine works now. If not. Check for excessive vibration. If it's bad vibes. That's a tough one. Solutions are various. Bad vibes can effect full fues planes as well as profiles. Not possible to diagnose and solve those issues at a distance.

Fox35s from 40th anniversary forward were set up with tapered cylinders, pinch at the top. When engine is at operating temperature, combustion chamber heat straightens out the taper. Better seal. If. Many of the 40ths especially were set up (in our experience) with too much taper. Too much pinch. Pistons often requiring lapping. Breaking in does not do it. Lapping needs to be done by someone who knows how. This might be an issue with your engine. If that's the case your engine will lock up when leaned out on a test stand.

If you are ever near Philly PA, send a PM. We enjoy trouble shooting Fox35s and sharing yuks.

Paul Smith · « **Reply #20 on:** May 01, 2016, 05:13:04 AM »

I used to stuff engines with high temp epoxy. It worked fine, but was too much effort.

The recommendation was to stuff the bypass with a simple piece of spruce wood, not even painted. Strangely enough, it lasts long enough to do the job. Not all that scientific. Just take a scrap of 1/8" x 1/2" spruce and sand & grind it until it fits snuggly into the bypass. I guess the bypass was way too big because it smooths out the run with no loss of power. Maybe the decreased volume increases the suction as a byproduct.

Larry Renger · « **Reply #21 on:** May 01, 2016, 08:11:17 AM »

If you are putting the wood in the center of the bypass, do you now have "poor man's" Schneurle porting?

Dennis Moritz · « **Reply #22 on:** May 01, 2016, 08:25:23 AM »

We haven't established that the problem is a Fox 35 burp. It doesn't sound like a Fox 35 burp.

When run correctly, when setup correctly, when fitted correctly, the Fox 35 burp is a non-factor in the engine's performance. These observations are based on the experiences of many in my club. Folks who tune Fox 35s and other engines so they work well in stunt, sport, racing, BiSlobing, and elsewhere up down much of the Control Line spectrum. Club members who dig Fox35s. Folks who have shown time and again that Fox 35s can provide reliable power in many CL applications. Success that is documented by winning contests. There are big smiles on the faces of those in our club flying the 65 year old anachronism for the heck of it. Since all of CL and 95% of the folks flying CL are anachronisms, it is accurate to regard FOX35s as a paradigm of our sport. Love it and love what we do.

Chris Fretz · « **Reply #23 on:** May 01, 2016, 04:15:42 PM »

Older fox running on Brodak 5% 29%. The only anniversary Fox I have is a 40th an it is collecting dust in a box with a few others. If Philly wasn't so far away I would be over right now.

It ran fine on the bench, they always seem to run on the bench till you put them on a airplane.

Brett Buck · « **Reply #24 on:** May 01, 2016, 04:22:14 PM »

Quote from: #Liner on May 01, 2016, 04:15:42 PM

Older fox running on Brodak 5% 29%. The only anniversary Fox I have is a 40th an it is collecting dust in a box with a few others. If Philly wasn't so far away I would be over right now.

It ran fine on the bench, they always seem to run on the bench till you put them on a airplane.

Just do the fix and move on, and the various goofy half-assed "solutions" are moot.

Brett

Brett Buck · « **Reply #25 on:** May 01, 2016, 04:36:05 PM »

Quote from: Dennis Moritz on May 01, 2016, 08:25:23 AM

When run correctly, when setup correctly, when fitted correctly, the Fox 35 burp is a non-factor in the engine's performance.

I know it is confusing but this one is the shoe polish:

RandySmith · « **Reply #26 on:** May 01, 2016, 05:29:29 PM »

Quote from: #Liner on May 01, 2016, 04:15:42 PM

Older fox running on Brodak 5% 29%. The only anniversary Fox I have is a 40th an it is collecting dust in a box with a few others. If Philly wasn't so far away I would be over right now.

It ran fine on the bench, they always seem to run on the bench till you put them on a airplane.

Chris
That tank mount looks very suspect, I would try to mount it more solid, do whatever you need to give it a solid mount to lessen vibration, and start with the center line 3/16 above the engine center, ignore what is written about it does not matter, or does no good, Most EVERY FOX 35 side mounted engine I have seen needs this. If set dead level, they will burp and shutoff on outside manouvers
Use a good hot idlebar plug, make sure you have fresh fuel

Randy

Randy

Howard Rush · « **Reply #27 on:** May 01, 2016, 05:51:29 PM »

Quote from: Brett Buck on May 01, 2016, 04:36:05 PM

I know it is confusing but this one is the shoe polish:

I am late to this discussion. I spent the day digging a trench for Marilou's garden irrigation system (below). Boy, that was a pain in the ... um ...oh.

Dennis Moritz · « **Reply #28 on:** May 01, 2016, 06:07:45 PM »

Randy's right about the plug. Dunno about the tank height. Try the Powermaster fuel 22 percent 50/50. 10 percent. Brodak makes one similar. You're trying to burn beaucoup castor. Perhaps the shoe polish references is a political in-term meaning embarrassment and frustration with one's own.

Yep. Tank mount needs to be secure. Pad under it of some kind helps.

Sent from my iPhone using Tapatalk

Dennis Moritz · « **Reply #29 on:** May 01, 2016, 06:35:26 PM »

It's running too cold and flaming out. Hotter plug. Fuel closer to Powemaster mix. Those tanks exaggerate effects of foaming. I'd use a chicken hopper. A pad under it. Do not let it touch the leading edge. Run engine leaner.

Hold onto the 40th anniversary. Might be the most powerful Fox 35. Piston might need lapping. They are worth the trouble.

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kenneth cook · « **Reply #30 on:** May 01, 2016, 06:55:27 PM »

   I see a Fox plug on that engine. Yes, they work and run the engine, but they're horribly reliable and they can cause more problems than what they're worth. A good plug makes a difference as Randy mentions. If you can find a Thunderbolt r/c long I would try that. A Mccoy MC-59, K&B 1L is another good option. Anything but the Fox. I would venture to say the instant you remove your plug clip provided your engine is warmed up, the rpm's fall off due to the Fox plug. It appears you have a Brodak tank. I would pressure test that tank underwater as if your trying to pop it like a balloon using a 5 oz syringe. Make certain you have no air bubbles leaking from the seams, tubes, end caps. As mentioned, your tank mount is very suspicious and I would certainly make improvements on it. Your fuel tubing appears to be Dubro, it doesn't fit the spraybar tight enough on the stock Fox spraybar. Use a piece of wire either floral wire or copper wire used for line wrapping and wrap twice around your fuel tubing to spraybar connection twist to ensure tightness. I see you have angled pieces of tubing on the overflow pipes. I would plug the overflow off using a piece of tubing with a plug ( screw or stick tightly fit in it) and cap it off leaving the top fill angled into the wind. I would make certain your fuel filter media is totally clear of any insects, dirt, rust, etc. Make certain your filter is tight not finger tight. The Fox needle valve is pretty poor in terms of adjustment, fitment and overall quality. If your using the stock needle, I would highly suggest cutting a piece of tubing that slips over the needle and kisses the spraybar tightly and squarely. Using a pair of side cutters, wire cutters, X-acto doesn't result in a square cut when cutting tubing. Insert a dowel into a piece of tubing about 1" long and chuck the opposing end of the dowel into a drill press or cordless drill. Using a single edge razor blade with the tubing spinning cut yourself several donuts of tubing anywhere from 3/16" long to 5/16" long. Insert one onto your needle valve. Take note to where your setting currently is and insure you have the tubing properly installed on your needle allowing for some adjustment. You have various sizes to play with.

   I'm personally not a fan of stuffing the bypass. For one, it appears you have a older Fox .35 with the wide bypass. Some Fox .35's burp more than others and some may require the plug. The high oil content in your fuel can be responsible for the extreme burp. Raising the nitro content to 10% and running the engine a bit leaner can assist getting through the burp issue. The problem with even taking apart these older Fox's is that the case can distort and warp if your not familiar with them. In addition to taking the engine apart, the backplate must be removed which destroys the gasket and seal. If a Fox backplate develops a leak, it generally shuts the engine off as if someone turned off a switch. You will need a new gasket or gasket maker to reassemble. I personally lap the rear of the case flat due to the casting being very poorly machined for a mating surface. I sand that portion of the case on a piece of glass using a piece of 400 grit paper and a bit of oil to do so. The head screws if not properly torqued can twist the case which distorts the liner and this can cause a serious bind. Sometimes finding this bind can take a hour or more as to what is causing it. Loosening one screw alone can result in fixing it. Adding a bypass plug can do the same thing making it either bind on assembly or when running when the liner wants to expand. This is no different than having muffler screws too tight binding the piston to liner fit.

   A Fox .35 doesn't burp when mounted upright or inverted at least not like when it's mounted sidewinder. I would try the above before doing what has already been suggested due to the suggestions being very simple. If the tank is leaking it will need to be soldered correctly. If that is a Brodak tank, some of them have clear coat on them which needs to be scarified and removed as this will contaminate soldering and not seal the tank correctly. A wire wheel chucked into a drill press can make this an easy process followed by a cleaning of lacquer thinner or acetone.

Chris Fretz · « **Reply #31 on:** May 02, 2016, 06:47:50 AM »

Quote from: RandySmith on May 01, 2016, 05:29:29 PM

Chris
That tank mount looks very suspect, I would try to mount it more solid, do whatever you need to give it a solid mount to lessen vibration, and start with the center line 3/16 above the engine center, ignore what is written about it does not matter, or does no good, Most EVERY FOX 35 side mounted engine I have seen needs this. If set dead level, they will burp and shutoff on outside manouvers
Use a good hot idlebar plug, make sure you have fresh fuel

Randy

Randy

I tried to rig up a quick plate to adjust the tank height, guess back to drawing board.

So I'm guessing a clunk tank would never work?

RandySmith · « **Reply #32 on:** May 02, 2016, 08:04:31 AM »

Quote from: #Liner on May 02, 2016, 06:47:50 AM

I tried to rig up a quick plate to adjust the tank height, guess back to drawing board.

So I'm guessing a clunk tank would never work?

yes you can make a clunk tank work, I would use the 4 ounce clunk, close to the motor and plumbed uniflow, mount it solidly

Randy

Chris Fretz · « **Reply #33 on:** May 02, 2016, 08:17:41 AM »

Quote from: RandySmith on May 02, 2016, 08:04:31 AM

yes you can make a clunk tank work, I would use the 4 ounce clunk, close to the motor and plumbed uniflow, mount it solidly

Randy

Do you mount the clunk tank center line even with center of motor or do you raise it up like the metal ones?

Uniflow is just a line feeding the motor with one vent line correct? Not 2 vent lines?

Tim Wescott · « **Reply #34 on:** May 02, 2016, 10:04:44 AM »

Quote from: #Liner on May 02, 2016, 08:17:41 AM

Do you mount the clunk tank center line even with center of motor or do you raise it up like the metal ones?

Raise it up. But you may not be happy unless it's a uniflow.

Quote from: #Liner on May 02, 2016, 08:17:41 AM

Uniflow is just a line feeding the motor with one vent line correct? Not 2 vent lines?

I just looked for the n-th time, and couldn't find a discussion of a uniflow tank that I really like for your purposes. This one is technically very correct, but a beginner to the concept may find it overly technical, and it's aimed at the RC helicopter pilot, so for us there's added clutter that we don't need.

The basic idea of a uniflow is that in flight you have one operating vent that's located correctly, submerged in the fuel, during flight. By the magic of hydrostatics, the pressure at the submerged end of that uniflow tube is equal to the pressure at the tank end -- so it's either at atmospheric pressure or at muffler pressure. By putting the uniflow tube close to the pickup tube, you ensure that the pressure at the pickup tube is also close to atmospheric pressure.

You can't determine whether a tank is uniflow by counting vents: you can have a hundred vents in a tank; as long as 99 of them are blocked off and the last one open is in the right spot inside the tank then your 100-vent monstrosity will be a uniflow tank.

Note also that a uniflow tank doesn't have to be metal, and it doesn't have to have fixed feed tubes. You can make a uniflow clunk tank either out of metal or out of an RC clunk tank -- just put the vent in the right place.

Chris Fretz · « **Reply #35 on:** May 02, 2016, 12:33:39 PM »

Quote from: Tim Wescott on May 02, 2016, 10:04:44 AM

Raise it up. But you may not be happy unless it's a uniflow.

I just looked for the n-th time, and couldn't find a discussion of a uniflow tank that I really like for your purposes. This one is technically very correct, but a beginner to the concept may find it overly technical, and it's aimed at the RC helicopter pilot, so for us there's added clutter that we don't need.

The basic idea of a uniflow is that in flight you have one operating vent that's located correctly, submerged in the fuel, during flight. By the magic of hydrostatics, the pressure at the submerged end of that uniflow tube is equal to the pressure at the tank end -- so it's either at atmospheric pressure or at muffler pressure. By putting the uniflow tube close to the pickup tube, you ensure that the pressure at the pickup tube is also close to atmospheric pressure.

You can't determine whether a tank is uniflow by counting vents: you can have a hundred vents in a tank; as long as 99 of them are blocked off and the last one open is in the right spot inside the tank then your 100-vent monstrosity will be a uniflow tank.

Note also that a uniflow tank doesn't have to be metal, and it doesn't have to have fixed feed tubes. You can make a uniflow clunk tank either out of metal or out of an RC clunk tank -- just put the vent in the right place.

HaHaHa! That was amusing. Now its clear to me that I had no idea what uniflow was. Still a little cloudy on it. Nothing is simple with any of this stuff...

Tim Wescott · « **Reply #36 on:** May 02, 2016, 12:59:06 PM »

Quote from: #Liner on May 02, 2016, 12:33:39 PM

HaHaHa! That was amusing. Now its clear to me that I had no idea what uniflow was. Still a little cloudy on it. Nothing is simple with any of this stuff...

It's all about keeping the engine happy by making sure it gets just the right amount of head.

When the tank is vented so that the surface of the liquid is at atmospheric pressure, the head (head = pressure, more or less) varies with the amount of fuel in the tank. Engines with a 2-4 break like the Fox are sensitive to getting just the right fuel/air mix, and that mix depends on the head pressure at the needle valve.

If you use a conventionally vented tank that's two inches wide (inside of the circle to outside), then as the airplane uses fuel the head at the needle valve will diminish by something like five or six inches of water at the needle valve (why more than two inches? -- because the centrifugal force is flinging the fuel against the side of the tank.). Many engines are sensitive to that, and for stunt we want a nice consistent run.

If you use a uniflow tank, then no matter how deep it is from inside to outside, the pressure of the fuel at the pickup will be essentially at atmospheric pressure (or muffler or pipe pressure, if you're using it). A uniflow tank will actually pull a vacuum at the inside wall of the tank (so even a pinhole leak can cause a problem).

This all means that, with a uniflow tank, the "right" needle setting stays the same from the beginning of the flight (as soon as any fuel is cleared out of the uniflow vent) until the end.

Chris Fretz · « **Reply #37 on:** May 02, 2016, 03:16:11 PM »

Quote from: Tim Wescott on May 02, 2016, 12:59:06 PM

It's all about keeping the engine happy by making sure it gets just the right amount of head.

When the tank is vented so that the surface of the liquid is at atmospheric pressure, the head (head = pressure, more or less) varies with the amount of fuel in the tank. Engines with a 2-4 break like the Fox are sensitive to getting just the right fuel/air mix, and that mix depends on the head pressure at the needle valve.

If you use a conventionally vented tank that's two inches wide (inside of the circle to outside), then as the airplane uses fuel the head at the needle valve will diminish by something like five or six inches of water at the needle valve (why more than two inches? -- because the centrifugal force is flinging the fuel against the side of the tank.). Many engines are sensitive to that, and for stunt we want a nice consistent run.

If you use a uniflow tank, then no matter how deep it is from inside to outside, the pressure of the fuel at the pickup will be essentially at atmospheric pressure (or muffler or pipe pressure, if you're using it). A uniflow tank will actually pull a vacuum at the inside wall of the tank (so even a pinhole leak can cause a problem).

This all means that, with a uniflow tank, the "right" needle setting stays the same from the beginning of the flight (as soon as any fuel is cleared out of the uniflow vent) until the end.

Geez Tim did you guys go to school for this stuff? How did you guys learn all of this fancy stuff?

Tim Wescott · « **Reply #38 on:** May 02, 2016, 03:18:47 PM »

Quote from: #Liner on May 02, 2016, 03:16:11 PM

Geez Tim did you guys go to school for this stuff? How did you guys learn all of this fancy stuff?

I can't answer for anyone else, but I stayed awake in high school physics and then just picked the rest up here and there. "Here", being, literally, here on Stunthanger.

You don't HAVE to know this stuff to make it work -- just listen to someone who does know, and follow their directions.

Chris Fretz · « **Reply #39 on:** May 02, 2016, 03:32:32 PM »

Quote from: Tim Wescott on May 02, 2016, 03:18:47 PM

I can't answer for anyone else, but I stayed awake in high school physics and then just picked the rest up here and there. "Here", being, literally, here on Stunthanger.

You don't HAVE to know this stuff to make it work -- just listen to someone who does know, and follow their directions.

I fell asleep in that class. So now that you have blown my mind... essentially what you want is the vent line (say a clunk tank) to go all the way to the back of the tank right next to where the pick up is? Yes no? An this is a uniflow? And my metal Brodak tank the way it sits right now will never be a uniflow?

Tim Wescott · « **Reply #40 on:** May 02, 2016, 03:39:34 PM »

Quote from: #Liner on May 02, 2016, 03:32:32 PM

I fell asleep in that class. So now that you have blown my mind... essentially what you want is the vent line (say a clunk tank) to go all the way to the back of the tank right next to where the pick up is? Yes no? An this is a uniflow? And my metal Brodak tank the way it sits right now will never be a uniflow?

The best place (AFAIK) for the uniflow vent is to have it come out at the outermost point of the tank, about 3/8" or so in front of the pickup. Get it too close and the bubbles coming out of the uniflow vent will go right into the fuel pickup, which will, in turn, mess you up.

A very important "and" to this is that you must seal the tank against all leaks -- if there's a leak, then air will come in, and (given a big enough leak) the tank will be at atmospheric pressure at that point instead of at the uniflow tube.

I don't know what Brodak tank you have now, but if it's not a uniflow you'd have to pop off an end and install a uniflow vent. Fortunately, Brodak does sell uniflow tanks, so you can just buy one (and pressure test it, to make sure you didn't get any pinholes free of charge).

Chris Fretz · « **Reply #41 on:** May 03, 2016, 07:35:08 AM »

Quote from: Tim Wescott on May 02, 2016, 03:39:34 PM

The best place (AFAIK) for the uniflow vent is to have it come out at the outermost point of the tank, about 3/8" or so in front of the pickup. Get it too close and the bubbles coming out of the uniflow vent will go right into the fuel pickup, which will, in turn, mess you up.

A very important "and" to this is that you must seal the tank against all leaks -- if there's a leak, then air will come in, and (given a big enough leak) the tank will be at atmospheric pressure at that point instead of at the uniflow tube.

I don't know what Brodak tank you have now, but if it's not a uniflow you'd have to pop off an end and install a uniflow vent. Fortunately, Brodak does sell uniflow tanks, so you can just buy one (and pressure test it, to make sure you didn't get any pinholes free of charge).

Do you have a picture of a clunk tank set up uniflow?

Chris Fretz · « **Reply #42 on:** May 03, 2016, 07:40:04 AM »

Quote from: kenneth cook on May 01, 2016, 06:55:27 PM

I see a Fox plug on that engine. Yes, they work and run the engine, but they're horribly reliable and they can cause more problems than what they're worth. A good plug makes a difference as Randy mentions. If you can find a Thunderbolt r/c long I would try that. A Mccoy MC-59, K&B 1L is another good option. Anything but the Fox.

The local hobby shop has some McCoy plugs so Ill check an see if they have that one. Yes the rpms do drop after igniter is removed.

bob whitney · « **Reply #43 on:** May 03, 2016, 08:18:12 AM »

one more thing that no one has mentioned. in the picture, not good to have the filter or fuel line in the way of the exhost

Tim Wescott · « **Reply #44 on:** May 03, 2016, 09:57:36 AM »

Quote from: #Liner on May 03, 2016, 07:35:08 AM

Do you have a picture of a clunk tank set up uniflow?

The picture below is courtesy of John Miller, from a thread on setting up uniflow clunk tanks.

The top picture is a view from top to bottom; the tube that curves over to the tank wall about halfway back is the uniflow vent.

Chris Fretz · « **Reply #45 on:** May 03, 2016, 10:05:07 AM »

Quote from: Tim Wescott on May 03, 2016, 09:57:36 AM

The picture below is courtesy of John Miller, from a thread on setting up uniflow clunk tanks.

The top picture is a view from top to bottom; the tube that curves over to the tank wall about halfway back is the uniflow vent.

Ahhhhh HA now we are getting somewhere!

Chris Fretz · « **Reply #46 on:** May 03, 2016, 10:06:33 AM »

Are there any O.S. plugs that will work in Fox 35?

Tim Wescott · « **Reply #47 on:** May 03, 2016, 10:08:15 AM »

Quote from: #Liner on May 03, 2016, 10:05:07 AM

Ahhhhh HA now we are getting somewhere!

Picture's worth a thousand words. Unfortunately, it takes me about as long to make a picture as to type 1000 words.

Quote from: #Liner on May 03, 2016, 10:06:33 AM

Are there any O.S. plugs that will work in Fox 35?

AFAIK there's long plugs, short plugs, and OS plugs, which are in between. I'm pretty sure that the OS plugs will fit -- but I don't know how they'll work.

Chris Fretz · « **Reply #48 on:** May 04, 2016, 05:42:48 PM »

Tank passed the leak test, filter clear, picked up McCoy 59 plugs, need to make some shims under the tank to clear landing gear.

Chris Fretz · « **Reply #49 on:** May 04, 2016, 06:44:23 PM »

Any concerns with this set up?