Pros & Cons of fuel bladders

[Bud Morrison]

Next year I will be flying my first RC Combat plane with a fuel bladder. I was wondering how many guys are flying with bladders and what the pros and cons are on using bladders instead of fuel tanks other than having to frequently replace the bladder.

[web-pilot]

Bud,

What kind of "Fuel Bladder" are you speaking of? There is the Latex Tubing or balloon type of bladder which is a pressure bladder, or the bubbleless tank which has a bladder inside of some type of sealed container and uses muffler pressure to pressurize the bladder.

George

[Don Pruitt]

Bud,
The advantage for me is consistency. There are no air bubbles from vibration and the increased fuel pressure gives a more constant fuel pressure regardless of G forces. The fuel pressure may be reduced slightly as the tank nears empty but not by a significant amount. There is additional advantage that if you employ a normal throttle control along with a fuel pinch off, you can make slight adjustments in mixture during flight. A big help in case you get a lean run and start to overheat. You will still get a slight increase in RPM due to the engine unloading during flight but you will get that regardless of the fuel system employed.

I am going to anticipate that there will be questions about the statement on mixture adjustment during flight so here goes.

Being as the fuel flow is controlled more by tank pressure than carburetor vacuum and exhaust pressure, changes in throttle position can only control air flow and not fuel flow. As you close the throttle you shut down the air to the engine but that has no control on the fuel flow. Fuel flow is controlled by tank pressure and needle valve adjustment. So, as you reduce air and not fuel, the mixture becomes richer.

Regards,
Don

[pat_willcox]

Several things to make your bladder fuel system user friendly. Be sure have a good "clothes pin" type pincher. Be famliar with a typical fuel flow drip for the engine you are running. This way you can put plane nose down, release the fuel pinch off and get a pretty close N/V setting. It is a good idea to flush the needle at first start of day. I usually take needle all the way out and let fuel gush out. Put it back in and adjust for the fast drip prelim setting. Also after every flight when you have shut off fuel line with the pincher, turn plane so as to allow any raw fuel to be emptied. Rotate prop back and forth with muffler down or carb down if draining through intake. This procedure will prevent hydalic lock on your next start.

[LouMelancon]

Bud,
I started flying bladder tanks early this year. The initial learning curve included making and acquiring the right tools to fill them. The materials are easily acquired from McMaster Carr on their web site. I have used several types of tubing for the bladders but have standardized on 1/4" ID natural latex for the bladder itself and 1/16" ID natural latex from tank to engine.

My experience with them, on OS LA .15s, is that you simply connect them up to the needle valve but do it backwards, i.e. you connect the incoming fuel to the nipple that would normally go to the needle. This allows the needle to control pressure better. There is no need for a second needle valve or pressure reducer in this set up.

Shown below is a bladder encased in a plastic flourescent light tube. The tube is built into the wing of the model and contains the fuel if the bladder bursts. This photo is a four and a quarter inch tube filled with 8 ounces of fuel when I was bench testing the tank system for my MIG 5 twin.

There is an excellent article on bladders on www.texascombat.com and a link to it on www.georgiacombat.com

Good Luck.

[boiler]

The biggest pro for bladders is the very low cost. In combat, a dirt nap or mid air can mean the loss of a tank. If you weigh the cost of the tank against the cost of a bladder, you could save the price of a plane each season with bladders. To build a bladder takes much less time than putting a tank together. I've used both when I flew u control combat in the 60's. This was my first year flying rc combat and I stayed with the tanks. I plan to try some bladders for next year.

[Montague]

I'm not disagreeing that a bladder is cheaper than a tank.

But, in the combat I've flown, I've never lost a fuel tank. I've had to replace the brass tubing a couple of times though. I've found that the sullivan flex tanks can take quite a hit with out damage. I've had them get ejected from my skull bandits several times and seen them bounce off all kinds of terrain with out any problems. I also like Hayes tanks for the planes that have enclosed tanks.

[sheepy]

Using the channel HOR design, I put the tank right against the engine, and any hard nose-in dirtnap punctures the tank. I suppose I could attach the tank differently, but haven't figured out a way to do it yet. I also duct tape the batteries to the tank which might give the tank more energy to expend during the dirtnap. I bet I could put a flexible shim in front of the tank where the tank would slide up the shim instead of coming to a dead stop against the engine mounting lug.

[Montague]

Hmmm, on my Skull Bandits, I have the tank directly behind the engine as well. I do have a zip tie that goes around part of the tank to prevent it from going forwards in to the engine, that might be doing it, I don't know. Usually the zip ties all break and the tank ejects cleanly. I also tend to use thin zip ties that break before anything else does. It's not uncommon for me to have to re-attach the battery and fuel tank after a hard auger, but they aren't usually damaged. (never damaged a battery, and only bent the tubing as I mentioned). I don't know if that would be possible in you case.

Maybe put a hole in the Al channel and run the tie though that so the tank can't go forward so easily?

[LouMelancon]

I use a pressure bladder for performance and consistency. I could not get consistent fuel runs on my .15s with tanks. I had to set them off rich and let them lean in during the first minute of flight. I was often going lean toward the end of the match.

A bladder stays at the same rpm from launch to land. The high pressure has an added benefit beyond consistency. It gives the engine all the fuel it can take. You don't have to rely on suction or vacuum to draw fuel to the engine you are force feeding it under pressure.

The limiting factor with a bladder is how much air you can get into the engine. There is no limit on drawing fuel. I believe the engines run faster on bladders than tanks. Since going to pressure bladders I have to limit nitro content to 5% so I don't exceed the SSC limit of 17,500 on MA 8X3 props.

[web-pilot]

Lou,

I have not had your experience with pressure bladders. Granted I have not used them as much, and I am still learning what materials to use and how much space to put them in.

I had a LL 60" SSC Falcon. I started with balloons, which ran pretty well, but did not last very long. I went to pressure bladder with 1/4" ID Latex tubing. The hole in the wing was not big enough, so I was getting false pressure when the bladder expanded to meet the sides of the hole in the wing. If I did not fill it up very far, I still had trouble because the bladder did not give a consistent pressure as it deflated.

The Falcon is history, and not because of the pressure bladder either. When I built a new SSC plane, I again tried a pressure bladder, but did not have good results. I thought maybe the latex tubing was too old, so I ordered some new tubing. In the mean time, I put in a Bubble-less tank into the bladder hole in the wing. I get far better performance on the bubble-less tank than I ever got on the balloon or pressure bladder. Same OS LA .15 engine. The engine turns more RPM on the ground, and tuning does not change from start to dead stick!

I am not saying bladders are bad, but I have not found the secret to them yet. I will though! I will keep working with them until I find how to make them work like I know they can. Lee can make them work, so I know it can be done and I will figure it out.

George