stunthanger.com
Engine basics => Engine set up tips => Topic started by: FLOYD CARTER on March 21, 2009, 03:21:42 PM
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Suppose you use a 2" round clunk tank, and it sits on top of the usual maple rails, the center of the tank is well above the venturi. This would normally indicate a lean run while inverted- as the clunk will be about 1" BELOW the level of the spraybar.
Why does it work?
I have done this only once, with a Brodak Legacy/ST51. The tank actually sits a little between the maple bearers, but not by much. It works, but I'm cautious about doing it again.
Floyd in OR
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I've never calculated it myself but apparently a stunter pulls around 3G in level flight so the outward affect on fuel level (and clunk position) outweighs the affect of gravity to a large extent. If you stood the model on it's outboard wing tip then that would simulate fairly closely the fuel level in flight which would put it close to the level of the needle valve relative to the ground.
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Guys,
lets remember the way a uniflow tank works is where the position of the Uniflow tube is,, not where the fuel pickup is. so with a clunk tank the clunk can bounce around all over the place and its where the uniflow vent is that matters.
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Mark...yup, agree completely, except that Floyd didn't say he had it rigged for Uniflow. Is it? Maybe Floyd will provide more information. #^ Steve
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Steve, fair enough, I made an assumption there, in the case of a clunk feeding with conventional venting not sure how that affects the run, I have NEVER run one that way, though some do I imagine,, hmmm
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The tank I'm thinking of using is a Sullivan 2" round. I soldered the uni-flo brass tube to the pickup clunk, so uni-flo follows the clunk around. Now, the center of the tank is a bit less than 1" above the engine maple bearers, ( the wood is slightly bevelled to lower the tank) and the OS venturi is 1/2" above the bearers. Therefore, if centrifugal force makes the pickup ride at 1" above the bearers, it will be 1/2" above the venturi. So far, so good. But the needle is set on the ground, where the clunk is at the bottom of the tank, and BELOW the level of the venturi.
This SHOULD cause a change in RPM as the plane gathers momentum. But I know it seems to work, based on one other similar set-up I'm using.
I'm just wondering if I'll continue to have good luck with this type of set-up, or maybe my other plane is a fluke??
Floyd
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It makes perfect sense that the center of the tank should be aligned to the height of the NV Assy., but that's almost never the correct tank height. Much more so with Uniflow tanks. With "standard" venting, it's fairly close. It's generally different for every engine, and I have had a muffler change require a tank adjustment. Also changed from a ST .60 to a ST .60 Clone and had to adjust the tank height.
Several stunt noteables have experimented with adjusting the spraybar height instead of the tank (via longer venturis) and found that didn't do the job. Those noteables were Fancher and Rabe, in independent tests. I believe both mentioned their findings in their columns in AAM...or was it MA? Safe bet is to use Uniflow, and if using a plastic tank, make the uniflow vent fixed, but adjustable for height. Solder a brass nut onto the tube outside the stopper, so you can just apply an ignition wrench and give it a tweek without disassembly...maybe right in the plane. The uniflow vent outlet inside the tank is what the engine 'sees' as the center of the tank, but it will probably have to be higher than expected. If you use a round tank, figure a way to index the tank when you remove/reinstall. They do respond to a red hot branding iron.
FWIW, one out of the two Sullivan clunk tanks I've used leaked like a pig. I changed to the dreaded Hayes tank that Da Dirt recommends, and found joy. I had to adjust the tank height the old fashioned way, and I wish Joy had been blonde and about 5'8"/130 lbs. But it works amazingly well, for no reason I can determine. It's not uniflow. Muffler pressure goes in at the top center of the tank, so it's covered for half the flight, and uncovered for the rest of the flight. I guess I can learn to accept the facts and not worry about the why's......eventually. n~ Steve
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Floyd,
I'm curious if you ran suction or muffler pressure with the uniflo in your original setup. Also, is it possible to set up the new plane so if the round clunk tank doesn't work, you can just pull it and switch to some other type of tank without a lot of modification?
Kim Mortimore
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I've never had good luck using muffler pressure. The uniflo tube is open to slipstream. This one has the tank inserted through a 2" hole in firewall. Have to remove motor, but I can service the tank OK.
Floyd in OR
Oregon state motto: "Sunshine? We don' need no stinkin' sunshine!"