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Old Jun 26, 2006, 12:03 PM
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North Central TN USA
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Pressurized tank

I'm building a Dumas 33" airboat with a Tower Hobbies .46 engine with muffler and pressure tap and a 10X7 prop. The tank is mounted directly below the engine. I know nothing about pressure tanks. What else will I need and how do I hook it up? Thanks.
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Old Jun 26, 2006, 01:24 PM
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Joined Feb 2003
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420tee,

When you say the tank is directly below the engine do you mean vertically below.
If this is the case you may have some trouble starting the engine as the fuel will drain out of the carb back into the tank.

The tank is usually mounted behind the engine roughly on the same level. Pressure is usually applied to the tank from the pressure nipple on the exhaust, connected via a short length of silicone tube. All other vents and fillers should be blocked for running.

Andy.
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Old Jun 26, 2006, 01:25 PM
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Banjul
Joined Jan 2001
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A fuel line from tank to needle valve on engine . . .
A fuel line from tank to muffler tap . . .
A fuel line from tank stopper going to needle valve to rear/bottom of tank ending in a clunk . . .
Two small pieces of brass tubing for stopper . . . one from muffler line curved to top of tank . . . one from needle valve line to fuel line ending in clunk.
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Old Jun 26, 2006, 01:32 PM
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Yes Andy, the tank is vertically directly below the engine. That's the only place it can go. I have a check valve I will put in the line. Thanks.

Thanks Viper, That sounds easy enough. I read somewhere about using a pump and all that jazz that I didn't want to have to get involved with.
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Old Jun 26, 2006, 01:32 PM
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Banjul
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I recommend the Uni-flow system, though for trouble-free operations.
See attachment.
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Old Jun 26, 2006, 01:50 PM
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Quote:
Originally Posted by 420TEE
Yes Andy, the tank is vertically directly below the engine. That's the only place it can go. I have a check valve I will put in the line. Thanks.
.
I think a check valve will require so much pressure difference to open it your engine might not have enough suction. Pressure applied to the tank may help but I suspect this won't be enough.

Best of luck.

Andy.
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Old Jun 26, 2006, 03:48 PM
Flying RC since 1974
Del Norte, Colorado
Joined Dec 2002
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Quote:
Originally Posted by 420TEE
...the tank is vertically directly below the engine. That's the only place it can go...
Really? I used to have one of the Dumas airboats and I don't recall any problem setting up the fuel tank in the proper location. However, since an airboat always has the same pitch attitude, vertical location of the tank is much less of an issue in a boat. If you can get it started, it should run OK even if the fuel has to flow uphill a bit.

With airplanes, the issue is that you want tune to be roughly the same whether the airplane is pointed up or down, upright or inverted, so the tank needs to be on or near the center line of the carburetor.
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Old Jun 26, 2006, 04:00 PM
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Quote:
Originally Posted by ctdahle
With airplanes, the issue is that you want tune to be roughly the same whether the airplane is pointed up or down, upright or inverted, so the tank needs to be on or near the center line of the carburetor.
I can't help thinking it is a bit of a myth that the tank in a plane needs to be close to the centreline of the carb for anything other than starting, this being so the fuel doesn't run back into the tank or flood the engine whilst there isn't a cotinuous demand. My point is that the tank is rarely on the horizontal centre line whilst in aerobatic flight and yet most engines don't have any problem with this at all. Even more than the difference in height, some manouvers can exceed 20G causing the head of fuel change by the same factor and yet the engine stays going.

My 2 pence worth!

Andy.
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Old Jun 26, 2006, 04:20 PM
Flying RC since 1974
Del Norte, Colorado
Joined Dec 2002
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I started in control line stunt. Properly tuned, tank position makes a big difference, and in controline, conveniently so. Ukies are tuned so that when the plane begins a climbing maneuver, the motor breaks lean for more power in the climb and then breaks rich in a descending maneuver. This is known as a 2-4 break. Some pattern flyers strive for similar engine behavior, however less so now than in the "olden days".
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Old Jun 26, 2006, 04:51 PM
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ctdahle-Dumas has several airboats. This is the "Big Swamp Buggy." Directly under the engine on the "fuel tank shelf" is the proper location per Dumas. It would have to go behind the engine to be in line and the two rudders are there. As to the check valve, I've used one before with no problems. It is not spring loaded, closing by gravity to prevent backflow. Not exactly in the point in the build to test all these ideas though. Time will tell. Thanks all.
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Old Jun 26, 2006, 11:42 PM
Flying RC since 1974
Del Norte, Colorado
Joined Dec 2002
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I checked it out on the Dumas website and I see what you are talking about. Kind of an odd way to set it up. I'm sure ours was a "Big Swamp Buggy" also, but the motor was mounted sideways and the tank was alongside the motor. Maybe we modified it, or maybe my memory is bad. It was a good 25 years ago.

Regardless, I'm guessing that you won't have any particular problem even without the checkvalve. As I was suggesting above, tank position has more to do with maintaining consistent and predictable running characteristics in a variety of attitudes than with getting the engine to run at all.

For an operating airplane, I'm real careful about tank placement, using a header tank if I can't get the main tank where I want it, but I've been known to set up an engine on a test stand on my work bench and feed it from a gallon bottle sitting on the floor 30 inches lower. If you can get fuel to the carb, it's going to draw just fine, especially if you run a pressure line from the muffler tap to the tank vent line.

There is no reason to mess with a uniflow set up, a pump, a three line tank or any other fancy plumbing for an airboat. However, you may find that it starts best by putting your finger over the EXHAUST rather than the carburetor intake to "choke" as you flip the prop.
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