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Uniflow tank pressure

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Old 10-25-2010, 11:25 AM
  #1
bgerth
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Default Uniflow tank pressure

I have always had trouble with consistant runs out of my C/L planes. They start off rich and wind up too lean. I mounted a muffler onto a McCoy 29 and sent pressure to a plastic clunk tank. No matter how much I opened the needle valve the engine ran 2 cycle until it overheated. When it's running, as soon as I undo the pressure line the engine floods out. It runs fine when run as ususal w/o pressure ( About 3 rounds open). I checked, and the clunk is submerged in fuel as it should be, and no noticable signs of bubbles in the fuel line. What am I doing wrong here?
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Old 10-25-2010, 12:06 PM
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da Rock
 
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Default RE: Uniflow tank pressure

Sounds like your pressure isn't pressing much if at all.

The fact that opening the needle valve doesn't increase fuel into the engine tells you something is actually holding it back.

Check to see if your muffler pressure actually is muffler pressure. Put a length of tubing on the pressure fitting. Use another plastic tank with some fuel in it. Plug the pressure line to the tank and see what happens when you run the engine. You can hook to the delivery tube and see if you get strong bubbles. Or if it's a 2 line tank, hook to the overflow and see if your muffler pressure can push fuel out of the delivery tube.

Got a picture of your uniflow plumbing?

I used to use a 3 tube setup with muffler pressure. One was the delivery tube that ran to the needle valve. One was the uniflow tube that ran to the pressure fitting. One was an fill overflow that was plugged solid.

If your system isn't plugged solid (overflow isn't capped, or the tank leaks) the pressure won't do a thing other than go out the leaks.
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Old 10-25-2010, 08:23 PM
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Default RE: Uniflow tank pressure

Most likely the muffler pressure nipple is blocked so no air at all can get into the tank. Another way to check for muffler pressure is to just drop the end of the tubing (doesn't matter how long it is) into a glass of water. It should bubble until it gets maybe 6" under the surface.
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Old 10-25-2010, 10:57 PM
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Default RE: Uniflow tank pressure

Assuming your muffler tap is clear and there are no other massive leaks in the manifold then you may want to give some consideration to using a Tetra 'bubbless' tank and use only a two line setup.

There is no clunk or clunk line and tank positioning is not critical and finally the tank plug system with an external clamp/overskirt rather than a squished bung plug is far better.

You cannot get any better or more uniform fuel delivery system on suction/muffler pressure. The tanks are about three times the cost of a common clunk tank but worth every penny on a controlline airplane and certain problamatical RC airplanes as well as some types of competition RC airplanes.

John
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Old 10-26-2010, 02:39 AM
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Default RE: Uniflow tank pressure

BGERTH,
Make or buy a uniflow tank. I have a stunt ship with a McCoy .19/ uniflow tank and it runs perfectly from start to finish with no muffler pressure. on one flight, I forgot to plug the vent so the tank was operating like a regular stunt tank. The engine run was just like you described. I couldn't believe the difference and it made a firm believer out of me.
A while back, I got a plane from a friend and it had a Perfect stunt tank mounted. I was lazy and left the tank as is. The OS .15LAS that I used would start rich and then lean out through the flight although not as bad as the McCoy. I took the end off the tank and plumbed in a uniflow set-up and now the OS runs absolutely consistent throughout the flight. Another testament to the performance of the Uniflow principal and you don't need muffler pressure.

Orv.
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Old 10-26-2010, 06:02 AM
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Default RE: Uniflow tank pressure

I have made an attempt to convert an old tank into a uniflow following what i have read on this forum. I haven't tried the metal Perfect tank yet. I figured the plastic tank I'm using on the test stand is virtually the same thing. I'll try to upload pictures of my engine on the test stand and the Perfect tank.
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Old 10-26-2010, 06:05 AM
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Default RE: Uniflow tank pressure

I'll try to upload pictures of my engine/test stand and the Perfect tank.
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Old 10-26-2010, 06:08 AM
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Default RE: Uniflow tank pressure

One of the problems I may be having is that the muffler may not be building much pressure. It sure doesn't quiet the engine any.
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Old 10-26-2010, 08:06 AM
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Default RE: Uniflow tank pressure


Quote:
ORIGINAL: bgerth

One of the problems I may be having is that the muffler may not be building much pressure. It sure doesn't quiet the engine any.

That happens with a lot of mufflers. The usual, first thing to try is to plug one of the exhaust pipes. It often doesn't work because the muffler simply isn't a very good design and doesn't quiet. The ones that do usually produce backpressure.
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Old 10-26-2010, 08:16 AM
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Default RE: Uniflow tank pressure

Guys, it's dead simple to make a uniflow tank that costs exactly what your usual plastic tank costs. They almost all come with a 3 hole capable stopper. Use all three holes for lines. Put clunks on two of the lines. Make the third a normal solid overflow pipe. If you wish, make one of the two clunk lines a bit shorter than the other. I haven't found the need of that, but it might be good if your clunks are smallish. Smallish works great, btw.

The two clunks follow each other perfectly. The silicon fuel tubing we have today works perfectly with no tangles. The clunks don't have room enough to get into trouble.

The only problem I've ever had with any uniflow made that way is they are bad to siphon, so care is needed on routing the lines to stop that. I've also had one plane that was going to siphon no matter where I routed the lines. Got no idea why routing didn't work. So I'd shut the delivery line with a hemostat whenever there was fuel in the tank.

Uniflow tanks made out of everyday RC tanks cost you extra whatever an additional cluck costs.

You know, I've never even seen a special uniflow bottle tank. And actually never used a bottle on a control line plane. Every CL model tank was either made from scratch or a metal one that got modified. Funny, but I enjoy making those as much as doing everything else.
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Old 10-28-2010, 08:05 PM
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Default RE: Uniflow tank pressure

Thanks to everyone for their help. The engine runs great now. The pressue nipple was clogged. The fact that it was sticking straight up made me think that it couldn't be the problem. While I was filling the tank with a syringe I noticed pressure was released when I removed the the fueling line. This didn't make sense since the tank was vented to the muffler. That's when I found the blockage. The muffler was either installed on an inverted engine or the muffler was removed dirty and stored inverted. Anyway it runs great now, I'm ready to install it on my Twister now.
Thank you,
Bernie
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