Uphill water, Pascal's law, Uni-flow, Yada Yada Yada

[John Pavlick]

George,
 It's not as much work as a hopper tank. Just add an extra clunk pickup to
use as the pressure / vent and plug the overflow (3rd line) after filling.
Here's why I ask. I had to review a small 3D airplane where the motor was
inverted and the tank was a lot higher than the carb. To make matters worse
I wanted to use a motor with an air bleed carb (an O.S. .46 LA is a lot
lighter than an FX / AX). Recipe for problems. I knew what to expect but
since it was a review airplane I built it as designed / per instructions.
There was no way I could stop the siphoning and the motor wouldn't idle. I
had to start it on it's back, you name it. I ran into some uni-flow stuff on
the web. It looked like it might help my situation, so I gave it a try.
Complete success. The idle is low and smooth with good transition. Very
reliable. I was making low inverted passes for the camera with the motor
just above idle. No drippies from the carb. with a full tank waiting to
start the motor. Saved this plane's arse. I also run this setup in my Bravo
303 (the old style made out of Chinese Oak). This plane has a Perry pump
though. The nice part about the uni-flow setup is that the weight of the
fuel does not affect the fuel flow to the carb. as much as with a
conventional (vent / pressure at the top) system. The only bad points I see
are:
1. You must supply a good amount of steady pressure to a uni-flow tank
because that is the ONLY pressure in the tank.
2. It will still siphon if given the chance - not nearly as easily as a
conventional setup. A check valve in the pressure line helps avoid filling
the carb. with fuel when the motor isn't running. Just enough restriction to
keep air from getting in. (No air goes in vent = no fuel goes out pickup).
And it gives nice steady pressure when the motor is running.

Some guys think that a uni-flow setup increases the likelihood of air in the
fuel because the pressure line is always very close to the pickup. In my
experience, this doesn't happen because the pressure line is usually full of
fuel. The air enters very slowly as the fuel level drops - not like you
might think by the muffler pressure forcing it's way in. On the Bravo I have
a check valve in the vent, nom pressure to the tank because I'm using the
pump. It lets air in (from the atmosphere) as the fuel level decreases
during the run, and keeps fuel from running out otherwise (remember, the
vent is IN THE FUEL). The overflow line is plugged (as with any uni-flow
setup). This runs very consistently regardless of fuel level or airplane
attitude. A lot of this is because of the pump I'm sure but from what some
guys are saying about pump systems, maybe the tank setup is helping the
pump. The tank in the Bravo is in the original position where most people
have problems and end up cutting the nose of the plane to lower the tank
position. This was my uni-flow experiment and so far it convinced me. If
anyone else is using one I'd like to hear about it, especially if it didn't
work for them. So far I haven't found any reason NOT to use it.

John Pavlick
http://www.idseng.com


 -----Original Message-----
From: discussion-request at nsrca.org [mailto:discussion-request at nsrca.org]On
Behalf Of George Kennie
Sent: Thursday, December 02, 2004 9:44 PM
To: discussion at nsrca.org
Subject: Re: OS 160FX setup


John,
It was so long ago that I can't remember all of the specifics, but as I
recall, I had a particular engine that was giving me some kind of fits and
in desperation I tried a uniflow set-up. I can only tell you that since that
time I have never even thought about using a uniflow set-up again. I think
my conclusion at the time was that it just wasn't worth the extra effort as
I found no advantage to the system, although I have to admit, it sure
sounded good on paper.
Georgie

John Pavlick
http://www.idseng.com

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