Cyberwolf

My Feedback: (4)

What the term FOAM means to me is air bubbles in the line ,sometimes depending on what sort of flying a person is doing this may or may not effect the engine but under a heavy load the engine will lean out,sometimes to the point of dieing .

One way to stop the bubbles is to use Armorall in the fuel about a teaspoon per gallon,It changes the surface tension of the fuel so it won't make bubbles,but I also understand it can have a side effect on the glowplugs.

What I do is use a good fuel filter between the tank and carb this seems to break up the bubbles enough to get a good solid flow of fuel to the carb.

The whole purpose of a tank being level with the spray bar and muffler pressure or a pump or regualtor is to keep a steady on demand flow of fuel to the engine at all times under any condition.

shakeelsid

Fuel Foaming is characteristically different from a, say, bubble bath, so don't expect to see huge bubbles going through the fuel line.
Methanol is an extremely good solvent, it miscible with water and most gasses in the air. Oil in the fuel does not mix with either water or other gasses and at best would form an emulsion, and thus you see the color changes if you had an impure fuel.

Now fuel foaming is a complex phenomenon, its not only the engine vibration mechanically transmitted to the fuel tank such that it breaks up the surface layer where liquid and gas meet (fuel and the empty space in the tank), but also the shockwave transmitted through what you call your muffler pressure. Remember what the muffler is doing there in the first place, it is dampening the shockwave, slowing it down before releasing it down the hose. At 12,000 RPM, a column of air mixed with some exhaust gasses is pulsating in your 'pressure line' at 200 beats per second. Thats right, the air is not getting pumped in, it is just pulsating at a very high frequency. This pulse is like a little bubble at the end of the pressure line in your tank, inflating and deflating 200 times a second. Each time liquid enters this pulse or bubble (Each time you fly inverted or roll or dive etc etc.), it is vaporised by the energy within.
Now, there is no need to over imagine all these things - they are, after all, at a very small scale. The fuel in the tank is around say 200 cc, and the pulse is may be less than tenth of a cc for a usual 1/8 inch ID tubing. But then how much of the foaming do you really need?
A giant bubble in your bubble bath is a mixture of perhaps 500 parts gas one part liquid - a hand rinse soap will easily produce 10 to 1 bubble, but the 'bubbles' in your foamed fuel are perhaps 10 parts liquid, one part gas. it is essentially gas dissolved in methanol, and very hard to 'see'.
But they are enough to reduce your fuel flow by say 10% - now, at the end of flight, when the fuel is all foamed in, 10 % of what is going through your fuel line is gas. Your engine needs to be richened by 10% if you wanted the same power output - too lean, and it quits.
And dont be surprised, at the top end, a glow engine needs a very precise mixture to maintain glow in the glow plug. some engines would not tolerate even a very slight change of mixture.

So there you go, respect your engine, and no, fuel foaming is not a myth - it has been tested in the labs, under controlled conditions, and some people have earned their PhDs just on this.

cheerios
sid

MJD

My Feedback: (1)

The most common cause of a deadstick landing is the engine quitting. How come I don't see that on the poll form?

Sport_Pilot

I see more MUS (made up stuff). Its when they publish it in magazines that they become myth. I doubt any one here can support or provide a cite for the bubble density, etc.

shakeelsid

Oil companies routinely check fuel batches for dissolved gasses (which is refered to as foaming here). Magazines are popular commercial publications and not a place for scientific reporting. Not many writers in magazines are trained researchers. If you are a keen reader, look at petroleum industry journals at your local engineering university library- there are over a hundred (and no, none of them are freeliy availanle online). You would be amazed at the level and depth of research on the topic. I am a researcher myself, although not in this particular area, and I would never even think of publishing my work in any magazine.

You are entitled to your opinion anyway

sid

Sport_Pilot

Oh! (Hand slapping forhead) You are talking disolved gas! I thought you were talking about bubbles. Sure the slosshing and muffler pressure causes some disolved gas, I doubt it is as high as 10% though. People are talking about a phenom which will cause your engine to stop, not lean out a little. I have not made up my mind if it actually occurs, though a lot of sloshing around is not good.

Rudeboy

It's real. And it's foam... very "fine" foam, but still foam.

The first time I really noticed it was back when I was flying competition funfly... you know, those one wheeled pod and boom models for doing touch-and-go's and limbo's and stuff. The first ones I built had the tank strapped rather rigidly to the engine. One time when setting my engine it occured to me there was a very fine stream of bubbles in the fuel line to the carb. Those were those microscopic, invisible bubbles in the fuel sticking together so they became so big that they actually became visible. Kinda like rain forms in a cloud.
I mounted my tanks on foam after that, and that solved the issue.

The absolute evidence was delivered a couple of years ago when I just started out in helicopters. My first heli had the tank mounted ridgid to the frames. I could see the fuel vibrating around when I was hovering. So I decided to fit a header tank, although I wasn't really having mixture problems... better safe than sorry as they say. The header tank was soft mounted: it could not vibrate at high frequencies. Now after 5 minutes of just hovering around, there was maybe 1/3 of an ounce of air in the 2oz header tank. The clunk in the main tank couldn't have possibly sucked air: it was always at the bottom of the tank while hovering, and you do not fly heli tanks dry like airplanes
That air in the header could only have gotten there one way: it came along with the fuel.
Maybe not 10%, but still a significant amount of air IMO.

NM2K

My Feedback: (14)

We had fuel foaming long before mufflers were around. I used to fly control line stunt/combat. That means I flew profiles and flying wings, mostly. I did build a Nobler once with a full fuselage and a Super Flite Streak that also had a full fuselage. Didn't like either of them.

Anyway, we used to have problems with fuel foaming now and then. Our tanks were solidly held to or within the fuselages. Ignorance was bliss.

However, if we had a problem with fuel foaming, we would loosen the prop and rotate it a random amount, then tighten it back up and try again. We usually found a tolerable position after a few tries. Those were very small fuel tanks when compared to what we use in R/C. I doubt that the rotating the prop trick will work. Didn't have Armor All back then, darn it.


Ed Cregger

Zombie

Fuel foaming is not a myth. If one has not seen it, it merely means they have yet to experience it. Some people just have to doubt that which they have not seen for themselves.

Fuel foaming can definitely make it difficult to tune an engine properly - or keep it running if the foaming is significant. I've flown SPADs and profiles with 4-strokes. I had one SPAD profile setup that initially shook so much the entire tank looked to be filled with nothing but foam. Really funny situation as the guys couldn't understand how I kept the engine running with so much foam in the tank.

I've never used Armorall in my fuel. Even though many guys say it won't have a negative impact on an engine, I just don't like the thought of adding such to my fuel if I don't need too.

[link=http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXD741&P=7]This Dubro Tank Filter[/link] has worked great for me - to stop the bubbles/air from leaving the tank and getting to the engine. It worked on the profile mentioned above. The engine was 100% reliable - keeping it's tuning and never quiting, all while the tank was foaming like crazy. I use them in all my setups and foam in the tank is never an issue.

If one likes to contribute to O.S. via their high prices, they have a [link=http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXXF25&P=ML]Bubbleless Clunk[/link] as well.

Azcat59

Interesting topics here, and quite a few good observations. I don't think it is the main reason for dead sticks, but one of the reasons I see is that guys will shoot for a slow tick-over idle before taking off, then experience a sudden case of "quiet" when they cut the power to complete some maneuver and then try to throttle back up. If the plane doesn't roll on a hard surface runway, the idle is slow enough. I taught pattern for seven or eight years, and I always had my guys run the idle up slightly after takeoff.....you need some prop wash to do certain maneuvers anyway. Then they can run the trim back down to land if necessarry.

Fuel foaming? I flew U/C stunt back in the fifties and sixties, and can't recall ever having a problem with it, but may have seen it happen at times on profile planes or light combat wings where the structure tended to be perhaps "whippy", i.e. subject to engine vibration. In RC, I built a number of the fine Japanese MK pattern kits, all of which provided a good gas tank in the kit, and the kits were always engineered for a tight slide-in fit of the tank between the hardwood motor mounts, bulkheads, etc. We never had a case of foaming, even though we were turning those piped engines at 14000-15000 on the ground. As a result of these experiences, I now always build in my tanks snugly with lite ply and balsa so they can't move (except slide to the rear for removal if needed), and I have not had any foaming problems over a total of 120 RC planes built in the last thirty years.

However.....this is definitely not to contradict Ed or others who support the foaming idea, as I respect his engine advice here on RCU greatly. This is simply my experience and readers can take it for what it is worth.

Clair

n8rloves2fly

My Feedback: (5)

running out of gas thats my #1!!!!????