stunthanger.com
Engine basics => Engine set up tips => Topic started by: Kim Mortimore on March 07, 2008, 06:46:51 PM
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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 LL~). There is room to fit a bladder. Before I experiment, I thought I would ask for suggestions.
Thanks,
Kim Mortimore
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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.
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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.
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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.
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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.
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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.
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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?????? ;D ;D
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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.
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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.
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