Author Topic: Bladder tanks for stunt? (Read 936 times)

Kim Mortimore · « **on:** March 07, 2008, 06:46:51 PM »

Has anyone tried using a combat bladder tank for stunt? I have heard that if you include a short length of small-ID metal tubing between the tank and the venturi, it can reduce the needle oversensitivity that comes with high pressure, while still maintaining the advantage of positive fuel flow.

I have plane with a *very* solidly glued-in tank (built up fuselage) that is a dawug, woof woof cough cough. The fuselag is nicely finished, and I would like to avoid major surgery (major butchery might be a better term in my hands

). There is room to fit a bladder. Before I experiment, I thought I would ask for suggestions.

Thanks,
Kim Mortimore

Ray · « **Reply #1 on:** March 07, 2008, 10:19:23 PM »

Yes, there was a plane fueled that way, at the World F2(?) event. A B-17 with four FP 15's.

In the really far back history of stunt, circa 1948, pen bladder tanks were occasionally used, but some fliers flew the event in the 85 - 90 mph speed range, or supposedly even faster. Low pressure "Tetra" tanks are sometimes used, but the current type combat and speed bladders are REALLY very high pressure, and I feel certain the engine operation (for aerobatics) would be compromised by that type fuel system.

Manuel Cortes · « **Reply #2 on:** March 07, 2008, 10:37:17 PM »

I´ve used bladders in little stunt ships, up to .20 with success. I began using them because of a engine change in a fused ship that made the tank off line in the new set un for aproximately 1/8 inch. It worked perfectly, due to the high pressure in the fuel system, once I learned to start accurrately the engine.

Never used bladders in full size stunt ships as never needed for it (Keep it simple, you know)
Hope it helps.
Regards.

dave shirley jr · « **Reply #3 on:** March 07, 2008, 11:35:39 PM »

Hi KIm
if you saw my 1/2 a cyclon powered stunt model i flew a few times at contest it is set up with a bladder tank. the cyclon has to be fed that way. it runs great but some things to remember.
1 it has to run fairly leaned out unless you use some kind of exotic regulator. it will not run rich, it floods and quits
2 it is way more trouble to use(needs to be fueled right before flying,not in the pits before your turn)
3 hard to do in a cowled in situation (you will need a mechanical pinchoff accessable from outside the cowl to aid in starting) and a remote way to refuel without removing the cowl, like a one way valve.
in other words yeah it works but probably not the way to go.
i can show you what i did on the 1/2 a if you want. its basically a setup from a fully cowled F1C free flight model
Dave jr.

phil c · « **Reply #4 on:** March 08, 2008, 04:32:32 PM »

A backplate pressure regulator like PW used in the B-17 helps a lot. Most engines have a hard time holding a steady four stroke setting with a bladder. I had some good luck with an LA 40 running on a bladder, but it had to be with a 9/4 prop. More load made it impossible to set.

You also need a needle valve with a fine thread. The stock OS cl needle valve has a .35 mm thread and is just barely fine enough. It works better with a 2-3 degree taper on the needle instead of the stock 7 degrees.

Howard Rush · « **Reply #5 on:** March 08, 2008, 10:04:15 PM »

If you run a bladder in a tube on the same axis as the crankshaft, the circular vibration mode can cause the bladder to rotate, twisting off the fuel tubing.

RandySmith · « **Reply #6 on:** March 09, 2008, 11:49:39 AM »

Quote from: Howard Rush on March 08, 2008, 10:04:15 PM

If you run a bladder in a tube on the same axis as the crankshaft, the circular vibration mode can cause the bladder to rotate, twisting off the fuel tubing.

How does Howard know this?

??

dave shirley jr · « **Reply #7 on:** March 09, 2008, 05:54:21 PM »

Yeah Howard
i found that out running bladders inside carbon tube freeflight fuselages.

there is a trick to prevent it. either mount the bladder on a spigot inside the fuselage like i did on my 1/2A or on the engine backplate like on some freeflight models, or run the tubing into a 90 degree fiting so it comes out the side of the fuselage , that usually does it
dave jr.

Kim Mortimore · « **Reply #8 on:** March 10, 2008, 02:42:23 PM »

Quote from: dave shirley jr on March 07, 2008, 11:35:39 PM

Hi KIm
if you saw my 1/2 a cyclon powered stunt model i flew a few times at contest it is set up with a bladder tank. the cyclon has to be fed that way. it runs great but some things to remember.
1 it has to run fairly leaned out unless you use some kind of exotic regulator. it will not run rich, it floods and quits
2 it is way more trouble to use(needs to be fueled right before flying,not in the pits before your turn)
3 hard to do in a cowled in situation (you will need a mechanical pinchoff accessable from outside the cowl to aid in starting) and a remote way to refuel without removing the cowl, like a one way valve.
in other words yeah it works but probably not the way to go.
i can show you what i did on the 1/2 a if you want. its basically a setup from a fully cowled F1C free flight model
Dave jr.

Dave,
I would like to take a look at your setup the next time we fly 1/2A stunt. I see it isn't on the list of events for the April contest.

Quote from: phil c on March 08, 2008, 04:32:32 PM

A backplate pressure regulator like PW used in the B-17 helps a lot. Most engines have a hard time holding a steady four stroke setting with a bladder. I had some good luck with an LA 40 running on a bladder, but it had to be with a 9/4 prop. More load made it impossible to set.

You also need a needle valve with a fine thread. The stock OS cl needle valve has a .35 mm thread and is just barely fine enough. It works better with a 2-3 degree taper on the needle instead of the stock 7 degrees.

Phil,
I assume this is because even with the regulator, the overall pressure level is still much higher than muffler pressure?

Kim

Author Topic: Bladder tanks for stunt? (Read 936 times)

Kim Mortimore

Bladder tanks for stunt?

Ray

Re: Bladder tanks for stunt?

Manuel Cortes

Re: Bladder tanks for stunt?

dave shirley jr

Re: Bladder tanks for stunt?

phil c

Re: Bladder tanks for stunt?

Howard Rush

Re: Bladder tanks for stunt?

RandySmith

Re: Bladder tanks for stunt?

dave shirley jr

Re: Bladder tanks for stunt?

Kim Mortimore

Re: Bladder tanks for stunt?