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Old Jul 20, 2005, 03:17 PM
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JoeVen's Avatar
Venezuela, south america
Joined Aug 2004
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More room for the fuel tank in my Piper Cub...!!!

I can’t fly properly adjusted my new Piper Cub powered with a Supertigre GS-45 ABC engine, because the panel instruments doesn’t let me room to lift the fuel tank up to the right position. When I spin the prop to "choke" the engine the fuel flow well but once I release it the fuel goes back again leaving empty the tubing. Although the engine runs very well, this trick makes very hard to start it, even has spun clockwise throwing the air like a home fan!! Once flying neither the trust keep constant nor idle is sustained. Thank is bottle type, 12 oz.
Is not usual to see fuel tanks trough that big windows and a fuel pump wouldn’t be handy for a easy model like mine… any smart suggestion from the Piper lovers?

Crashed plane in "dope an silk" thread, but now was fixed and fliying again...
http://www.rcgroups.com/forums/showt...=347013&page=4
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Old Jul 20, 2005, 03:46 PM
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Banjul
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Try an 8-oz tank. That should be more than enough for the GS-45.

Viper
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Old Jul 22, 2005, 06:48 PM
Flying RC since 1974
Del Norte, Colorado
Joined Dec 2002
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Smaller Tank!!!!

12 OZ tank is huge for a 40 sized airplane.

Even six ounces ought to give you 15 minutes flying at full throttle and 20 or more if you throttle back to "scale" speeds.
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Old Jul 22, 2005, 08:18 PM
In Massive Debt
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I would say go with the 8 oz tank, i really doubt that a 6 ounce would give you 15 minutes of flight time at full throttle, as my magnum .52 fs, only gets 18 minutes of mixed flying off of an 8 oz tank.
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Old Jul 22, 2005, 08:26 PM
Deletedfor proving Nauga wrong
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a .40 class engine can burn 1 ounce a minute at full throttle... But... most of the Cubs don't need more than about 1/2 throttle most of the flight, which could give a 15 to 20 min flight on 8 oz.

***********

what I have found is that a lot of people like to side mount the engine and use a Pitts muffler in a Cub. There's a problem inherent in most Pitts mufflers... inadequate backpressure.

try plugging off one exhaust pipe on the pitts muffler if that is what you are using.

Inverted mounting of the engine in a Cub brings the carb centerline down lower and will improve the fuel flow situation with the 12 oz tank. However... that usually (with a 2-stroke) leaves the muffler hanging out (and usually appearance wins over practicality)
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Old Jul 22, 2005, 08:45 PM
Flying RC since 1974
Del Norte, Colorado
Joined Dec 2002
736 Posts
You're right Fred, I keep forgetting about altitude. At 8000 feet, I run MUCH leaner and my 40-46's do get 15 minutes on 6 ounces.

At putt-putt speeds I can keep the LT-25, with it's LA-40 in the air for close to 35 minutes.

But down there at sea level, you have your needle valves open at least another turn or two.

Be that as it may, a 12 ounce tank is a big sucker to be cramming under the dashboard of a 40 sized Cub and with the tank too low and too big, the fuel flow problems are not going to go away by just boosting muffler pressure.
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Old Jul 28, 2005, 02:52 PM
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Venezuela, south america
Joined Aug 2004
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Lot of good advices!

Oh yes , a 12 OZ tank feeds my supertigre GS-45 for almost 20 minutes, flying the piper at medium trottle. Additionaly I got more thrust and lower trottle turning a 11X7 prop instead of the 10X6 I was using from the begining in my first ugly and heavy crash-trainer plane.
Uhhh, plugge one exaust tube off sound interesting, I´ll try the trick.
I thougth an smaller fuel tank wouldn´t help me because it still should be lower than the intake carburator line, so I took a drastic solution: I made more room upright cuting the ply instrument pannel off...
It hurts, but the engine runs smother. It doesn´t stop at low RPM anymore, ...just in the approach for landing!!!
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Old Jul 29, 2005, 11:32 AM
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I forgot to mention... Nitro content has a large effect on fuel consumption.... More nitro means you burn more fuel per combustion cycle. (and you get more power.) This reduces the time the engie will run at the same throttle setting when you are using high nitro.

Note... I have a Cub... .40 HB engine side mounted with the Pitts muffler... and when it blows the plug out of the one side of the muffler (which it does occsionally) the engine dies at 1/2 tank every time. (and I can forget having it idle below 2/3 full tank with the plug out)

Maybe I should design a pitts muffler that really works and market it... All that I have seen on the market have inadequate expansion chamber size, no baffles and excess exhaust (to make up for the inadequate chamber) and they don't really do well at reducing exhaust noise.
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Old Jul 29, 2005, 01:45 PM
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What i do is jsut take some pliers, and pinch both pipes a little bit, this way there are no plugs to blow out .
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Old Jul 29, 2005, 09:37 PM
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Venezuela, south america
Joined Aug 2004
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What was blowed?

Hey guys, what does "it blows the plug out of the one side of the muffler " MEANS??? Are you talking about the pressure fitting outlet on the muffler? I never have had a silicone tubing out of the pressure fitting, but once I heard a friend´s engine to sput the glow plug out, he forgot to tight it of course, sounded like a .21 shot!!! Lukily we didn´t shot anybody around...
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Old Jul 30, 2005, 02:13 AM
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what I mean by blowing out the plug is... I have been plugging one of the two tubes comming out of the Pitts muffler to increase backpressure. Its something I have been ADDING to the muffler whch is being lost... not a part of the muffler as it is shipped.

I'll have to try the crimp method.... don't know why I never thought of that.
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Old Jul 30, 2005, 06:19 PM
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Also, try to fill it with RTV silicone. That stuff will bond directly to the metal of the muffler, and you can get it out easily should you decide to do so.

Also, try a check valve with your fuel filter (you are running a fuel filter, aren't you?) to hold the fuel at the engine as you start it.
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Old Jul 31, 2005, 05:12 AM
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check valve by the filter never helped me... and check valve in the muffler pressure line has allowed me to blow tabnks apart.
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Old Aug 02, 2005, 10:01 PM
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I didn't SAY put it on the pressure side! I've never had luck with it on that side either, but I just wasn't getting enough puressure through the check valve into the tank (I coulda swore I was putting it in with the cone leading to the tank! ). If you get the fuel past the check valve, the valve should hold it at the engine.
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Old Aug 05, 2005, 03:24 PM
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Venezuela, south america
Joined Aug 2004
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Check valve for the fuel inlet sound right for me, may be those used in bycicle tube tires suits well.
But you don´t figure what an italian modeler shown on a old RCM issue to solve his backpressure problems... just conected a second CARB to the muffler exhaust outlet and then he linked the two trottle arms so both trottle barrels opens and closes together. At low RPM the exhaust carb closes to create backpressure effect, just when the engine needs it...
A sort of exhaust valve!!! Very engenius, isn´t it?
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Last edited by JoeVen; Aug 05, 2005 at 03:29 PM.
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