I was out practicing today with my Open B Battle Axes trying to get my thumbs limbered up for this weekend and found that I am having some severe fuel foaming problems when the engine hits certain RPM's. It's definitely a harmonic frequency thing, not normal "vibration". Different props have different max RPM's and it doesn't foam too badly with the APC 9-4 but foams a lot with the MAC 8-6 Combat. It REALLY foams a lot with the MAC 8-4 Scimitar. The foaming starts at about 16,500 RPM. I guess I can always use the slower prop.[xx(]
I can't use bubble-less tanks on the Battle Axe B because there isn't enough room between the radio box/wing leading edge and the throttle servo mount. I've test fit several tanks and they are all too long. Unless perhaps I turn the tank sideways...[:(]
I'm not going to use pressure bladders because I want to be able to throttle down. These things are a lot faster than SSC's and I tend to get disoriented occasionally.
There is an old C/L stunt fliers trick to reduce foaming where you add a few drops of ArmorAll to the fuel. Apparently the silicone in the ArmorAll prevents bubble formation. It works, but it only reduces foaming, it doesn't prevent it. I tried it today and it helped a little bit, but didn't totally solve the problem.
I'm going to remount the tanks with some 1/2" black neoprene foam (like a skin diver's wet suit, except thicker) under the tank and see if that helps, right now they only have the non-slip shelf liner stuff that comes with the kit.
Anybody else have any suggestions that I haven't thought of yet?
Fuel Foaming
Moderator: hbartel
There's a difference between foaming and air bubbles. I shorten my clunk about 1/4" and that seems to help with the bubble problem.
Had to edit my post. Gilbert, I can't claim any rights to the use of Armorall. Eric showed it to me several years ago and have just passed it on. Yep, I use it and that's the main reason I fill from a quart jug. It has to be added every day.
Had to edit my post. Gilbert, I can't claim any rights to the use of Armorall. Eric showed it to me several years ago and have just passed it on. Yep, I use it and that's the main reason I fill from a quart jug. It has to be added every day.
my first combat engine was an OS 28F with iron piston. That thing just had too much vibration over 17k and I got fuel foaming, I had to run a 9x5 prop with that engine, and used 9x4 on my ABC/ringed engines. To this day I still have the habit of putting a big 4 on my props with paint marker because until I did that I would occasionally put the wrong prop on my iron engine during an emergency prop change, and it would immediately (after launch, of course) inform me of my mistake.
THe 28F ABC, ring, and 25FX engines I now use are so smooth that I have gotten spoiled not having to do much to isolate the tank. On the Bat Trick/Chokers, the tank was just crammed into the fuse, on the rest of our planes, they are velcroed and taped on with no foam isolation. On SSCs, though, I fought bad engine runs for a long time till i figured out it was fuel foaming. So on SSCs, I have the tank quite loosely attached, sitting on a bed of barely-compressed foam. That solves the foaming, but then the tank will shift around easily. AJ taught me a partial solution which is to put hook velcro on both the fuse and the tank, with the foam in between- the velcro acts like claws and helps keep the tank from shifting around.
Other possibility as you have already seen is to just pick the right prop that avoids the harmonic, similar to what i had to do with my iron engine. I also balance my props which is a time consuming PITA but it might help.
THe 28F ABC, ring, and 25FX engines I now use are so smooth that I have gotten spoiled not having to do much to isolate the tank. On the Bat Trick/Chokers, the tank was just crammed into the fuse, on the rest of our planes, they are velcroed and taped on with no foam isolation. On SSCs, though, I fought bad engine runs for a long time till i figured out it was fuel foaming. So on SSCs, I have the tank quite loosely attached, sitting on a bed of barely-compressed foam. That solves the foaming, but then the tank will shift around easily. AJ taught me a partial solution which is to put hook velcro on both the fuse and the tank, with the foam in between- the velcro acts like claws and helps keep the tank from shifting around.
Other possibility as you have already seen is to just pick the right prop that avoids the harmonic, similar to what i had to do with my iron engine. I also balance my props which is a time consuming PITA but it might help.
I had fuel foaming problems from squashed tank padding from taping the tank on to tight. I like to use the foam insulation they use on pipes. I use the same tape used to cover the plane to attach the tank. The tank doesn't shift much with this method. I used to use electrical tape but if you put it on just a little to tight you will have problems.
Thanks guys. I'm using balanced props, Armor All, etc. When my friend with the special foam gets back from his trip (AA Eagle pilot) I'll grab some of it and try to isolate the tank a little bit better. Hopefully that, in combination with the other things will solve the harmonic resonance problem. Other than that, I'll just use a prop that stays out of the harmonic range and accept the performance penalty. It's not like I can fly well enough for that to make any difference.