Getting rid of the fuel line bubbles on profile planes

[Andrew Borgogna]

The majority of planes I build are profiles and they all suffer, to different degrees, from the problem of bubbles in the fuel line.  This is not a problem in my built up fuselage planes. 

I put Armor All in the fuel and this helps, I pad the tanks with various types of padding but I still cannot come up with a sure fire combination that eliminates bubbles.  I am now adding another later  of balsa wood on the fuselage on the opposite side from the motor to add  more rigidity to the front of the plane.   But even that does not seem to help much.  I have even experimented with different tank types with no real verifiable difference.

I would be very interested to hear what others have learned in the fight against bubbles in the fuel line. 
Thanks
Andy Borgogna

[Neville Legg]

Andrew,
are your props and spinners well balanced?

Cheers      Neville

[TDM]

This is a symptom of a weak nose construction. Try to use a doubler ˝ Balsa on the inboard side of the fuse. Then put a nice filet from the doubler to the wing this should transfer the vibration from the nose to the wing get rid of the bubbles or at least most of the bubbles and get you a better motor run.

[Phil Coopy]

I have also noticed that some fuel tanks have very thin material for the walls and can contribute greatly to bubbling.  A plastic clunk tank, if you can get one to work, will also minimize bubbling.  as Andy said, a few drops of ARMOR ALL , tho I have never tried it, is said to help with the bubbling problem. Also be sure that if you use rubber bands that they are holding the tank very securely.

Phil

[TDM]

This is a symptom of a weak nose construction. Try to use a doubler ˝ Balsa on the inboard side of the fuse. Then put a nice filet from the doubler to the wing this should transfer the vibration from the nose to the wing get rid of the bubbles or at least most of the bubbles and get you a better motor run.

[Scott Jenkins]

Put a fuel filter in line going to the engine with the right mesh screen it will diffuse
the bubbles and what the other guys said vibration is your worse enemy, especially
on a profile fuselage a half ounce of material strenght is worth it rather than have
the motor burp at the high point of a wing over or in a 4 leaf clover.

Scott   

[Wynn Robins]

plastic tank - sintered brass fuel pickup/clunk  - problem solved

[Paul Taylor]

plastic tank - sintered brass fuel pickup/clunk  - problem solved

Did-O

[Andrew Borgogna]

I am currently using a 1/2" doubler on the last plane I built and the one I am currently building.  Where do you get the sintered fuel pickup for the clunk tanks?   Is that the pickup that also doubles as a filter?  I think I have seen them but not recently.  Regarding props and spinners, I do balance my props, but I take the spinners as they come.  I have no idea how to balance a spinner.  Thanks for all the recommendations.
Andy

[Wynn Robins]

I get my sintered filters/clinks from Joe at www.sshobbies.com - they are dubro product number 161

If anyone is interested - I can post pics on how to do the ultimate nose and profile fuse.......

[LARRY RICE]

I put a thick piece of foam rubber under the tank, and that works.
Larry

[Brett Buck]

The majority of planes I build are profiles and they all suffer, to different degrees, from the problem of bubbles in the fuel line.  This is not a problem in my built up fuselage planes. 

I put Armor All in the fuel and this helps, I pad the tanks with various types of padding but I still cannot come up with a sure fire combination that eliminates bubbles.  I am now adding another later  of balsa wood on the fuselage on the opposite side from the motor to add  more rigidity to the front of the plane.   But even that does not seem to help much.  I have even experimented with different tank types with no real verifiable difference.

  I don't think there is one reliable solution to the problem. I am kind of surprised we ever get away with it, frankly. The factors that matter are the weight and weight distribution engine and tank, the stiffness of the fuselage, and the RPM. Think of the engine, tank, and front end like it was a spring with a mass on the end of it. Such a system will want to vibrate at a particular frequency. The heavier the mass or the softer the spring, the lower the frequency, and the stiffer the spring or the lighter the mass, the higher the frequency. If that frequency happens to line up with the frequency of the engine vibration (RPM), you really get it moving, and maybe get bubbles. This sort of reasoning applies to the entire front end, and/or the "system" of the tank and whatever mounting system and isolation you have. If the RPM is either higher or lower than the "tuned" frequency, it will probably not vibrate too much and you get away with it.

    That's why the problem is so hard to resolve. Sometime you need to make the tank mount or front end stiffer, and sometimes you need to make it softer, depending on what the rest of the front end is like. And for a tank, which changes weight from maybe 6 oz to an ounce and half, you get an entire range of frequencies that you need to avoid. Ted's original Doctor with a variety of 40's had an very apt example - it would start and run for about the first 3 minutes of the flight with no issues, ran great. Then about halfway through, it would suddenly go back and forth lean for a few laps, then stay lean for about a minute and a half.  Then, over the space of a lap or so, it went back to running fine. What eventually dawned on me was that it was starting out with a heavy full tank, and the natural frequency of the tank and suspension was lower than the engine frequency. As the fuel ran out, the tank got lighter, and the frequency went up, until it got close to the engine frequency, and then it vibrated the tank so much it started foaming the fuel. Of course that sped it up, detuning it until the tank got lighter still, then went solid lean. Eventually enough fuel ran out that the tank frequency got higher than the engine speed and it stopped foaming, and went back to the original setting.   That never got completely solved, but I did try sticking about an ounce of lead on the tank to test the theory. Sure enough, now it ran correctly until almost the end of the tank, then went lean and stayed that way. So adding the weight delayed the point in the fuel burn until less fuel was in the tank.

     Of course the less the engine vibrates in the first place, the smaller the total vibration of the parts. But if the front end is particularly stiff, its more like a tuning fork and  you can still get in trouble if the frequencies line up.

    None of that is a direct answer to the question you asked, but I don't think there *is* a general solution to the problems. I think the solution is to just fiddle with the stiffness of the foam underneath the tank and the tension on whatever holds it down until you detune the system for the parts you have.

    Note also that this is the same reason I think you have a lot better chance with a full-fuse airplane. Its very stiff in the direction that the engine vibrates the most (up and down along the cylinder axis). On a profile the same direction of vibration lines up with the weak direction of the fuselage.

     Brett

[Warren Leadbeatter]

There is a sure fire solution to this problem... install electric motor. 

[Larry Cunningham]

Use only aircraft birch plywood (never Lite Ply) for profile nose doublers and extend them to the high point of the wing. Use only hard maple (not basswood) engine mounting beams and extend them back to the front of the wing. This will lick most of the problems. (Also, don't drill holes in the aft engine mounts - weight savings is minimal and it can compromise strength.) A thick balsa sheet inboard tripler (?) can be helpful.

Use a modern engine, they tend to run smoother, handle easier, and have a softer 2/4 break. Try removing an inline fuel filter, as they can sometimes make the fuel line resonant. Use a uniflow tank, make sure there are no leaks in the fuel system. (I've also found recessing the fuel tank, so that its centroid is near the center of the profile fuselage, a la Imitation, or a la full fuselage to be very helpful, though not related to vibration problems.)

Use an engine mounting plate, and never use washers below the engine case mounts (e.g. to get engine offset). The idea is to have large contact areas between engine lugs, mounting plate, engine mounts, and fuselage nose area.

On a metal tank, use a thin (~ 1/8") sheet of rubber between it and the fuselage side. I find cheap flat rubber sink stoppers are a good source for such rubber. You can cut it with scissors.

Balance your prop. I personally like certain wooden ones (say 11/6 EW RevUps, no longer made), but APC molded composites tend to be good quality and low cost. Never use a crappy ancient molded black (10/6) plastic Master Airscrew prop from your uncle's ancient Flite Streak - in addition to being poor for stunt use, they tend to be poorly balanced, they're crude. (MA has some good modern props, I've heard.). The combination of a unbalanced prop and old Fox .35, no engine mounting plate, washers under engine lugs, Lite Ply/basswood nose construction, etc. can turn a profile ship nose into a 1" blurr at certain speeds.

I've seen plastic tank setups (say on Grady's Forerunners) work extremely well. I can see where the plastic could preclude some vibration on the fuel system, and a clunk seems to be a very good feature.

Brett is correct about the potential complexity and difficulty of profile nose vibration problems. However, once you settle into a workable setup, such problems will disappear, and a reliable stunt run results. What works well for others and for you are not necessarily identical. 

Electric power will definitely clear up your fuel line bubbling problems.

Good luck!

L.

"I don't want everyone to like me; I should think less of myself if some people did." -Henry James

[Warren Leadbeatter]

 Regarding props and spinners, I do balance my props, but I take the spinners as they come.  I have no idea how to balance a spinner.  Thanks for all the recommendations.
Andy

On a serious note, I've had a couple of models where the bubbling stopped when I removed the spinner.  After checking the prop is balanced, and if you suspect no other issues, try flying the plane without a spinner. You might be surprised. Some of the plastic spinners I've had are way out of balance so rather than play around with them I just try another.

[Jim Kraft]

I seem to have better luck solid mounting the tanks than insulating them. Also found just changing engines ( sometimes of the same kind ) will stop the bubbles. Almost all of my McCoy Red Heads run better than my Foxes on profiles. The sterling S1 Ringmaster, and the P51 Mustang seem to run pretty well with Fox 35's as the front ends are shorter and deeper than most others. But again; Like Brett said, you just have to find what works on each one of them. The 10-5 APC prop has worked well on my Foxes on full fuse planes, but will not work at all on most of my profiles. But a 10-6 will work just fine, as it changes the "engine & front end tuning".

[afml]

Yet another solution to the problem is an "easy to build" plane with full fuse.
Got a "sneak preview" of this plane and it has all the "right stuff", upright motor,
and "fail-safe" construction techniques with no blocks to carve.
Very versatile with plenty of opportunity for "personal expression".

It's being developed by Blue Sky Models by Tom, and is called HOBO.

A winner fer sure and a GREAT step up from profiles!

"Tight Lines!"

Wes

[Ward Van Duzer]

Couple more thoughts...

Yes, I do the 1/2" tripler on the inboard side with an epoxy filet...
Double up on your fuel line filter. Most bubbles can't get through the screens...
Squeeze a thick layer of Silicone on the back of your tank for shock absorbsion. Trim to fit the tank...
Or use an old mouse pad. Both have worked for me.
Turn your prop 180 degrees on the engine. Works sometimes even with balanced props!
Try a 3 bladed prop. The odd number of blades reduces vibrations...
Change engines. A differant size, brand may solve your problem...
Use high RPM engines. 9,000 - 11,000 RPM and a low pitch prop. I luv my 11 X 4...

Your milage may vary!

Ward-O

[Jim Thomerson]

If you will switch to the black neoprene tubing, you won't be able to see the bubbles.

[Ward Van Duzer]

No,

You'll just fill up your filter with black krud 

W.

[Larry Cunningham]

Two filters may also move the resonant frequency of a vibrating fuel line and thereby solve a foaming problem..
Fuel line bubbles can drive a wooden Indian crazy!

L.

"It's all that the young can do for the old, to shock them and keep them up to date." -George Bernard Shaw

[Wayne Collier]

It would seem that a clunk tank, because of the abillity of the clunk to move around inside the tank, would help minimize the negative affects of viberation on the fuel delivery system.  The clunk can, to an extent, move with the fuel while a fixed pickup must move with the tank.  I realize that inertia and the flexibilty of the pick up line affect the movement of the clunk.  This should help offset viberation at the point of fuel pick up.  Unless every bit of fuel in the tank is foamed, it seems to me, that a clunk could have an advantage here.  I think I'll try a clunk tank.  Actually already have one just haven't used it yet.

[Chuck Feldman]

Use a Crap Trap fuel fillter. It may work for you.

[Andrew Borgogna]

After reading Brett and Larry's comments and the rest of you I am finally beginning to understand what I have been experiencing.  In particular why a solution that works on one plane has no effect on another.  The plane the prompted me to write this thread was an S-1 Ringmaster with a Fox .35 that has a reworked crank.  I finally gave up on it and moved on to another plane  a profile I am using to do an evaluation of the Avaistar .46.  I did solve the bubble problem by changing to a lighter tank and changing to a stiffer padding material for the tank.  I also changed from an APC 11x5 to an RSM (wood) 12x5.5.  Can's say which of the modification had the greatest impact but the end result was no bubbles, at least at launch time.  I would also add the engine runs steady as a rock during the entire flight.  I really want to thank all of you who responded, at least now I have an idea about the problem I am fighting.
Andy

[john e. holliday]

I guess it is just me but, I have never really worried about bubbles when there is a filter in the line.  Seems I read years and years ago that a filter helped with set ups that had bubbles in the fuel line.  Never really noticed the difference in engine run and I know for a fact that Armor All kills glow plugs.  My flying partner and I did experiments to see why we were losing plugs so often.  Fresh fuel, balanced props and solid mountings engine/fuel tank and plugs lasted mucho longer.