I've been thinking about this one a while. Generally, when running a uniflow tank the practice is to either

1) position the uniflow vent into the airstream where it's basically a pitot tube getting dynamic pressure, or

2) connect it to the muffler.

So for option (1) it would follow that when the model slows the dynamic pressure drops (with the square of the velocity) and the net positive suction head available at the fuel pickup drops, causing the mixture to lean, and conversely.

Muffler pressure seems to be proportional to RPM, (based on observations of the vent line on R/C engines) so a similar situation exists in that as the prop loads and RPM drops one would expect a drop in suction head.

Of course, both system previously described seem to work well and have won enough national and world titles you really can't argue with them.

But I still wonder, has anyone flown a uniflow tank with the uni-vent connected to a static port? In theory, that would give a consistent suction head at the fuel pickup regardless of model speed or engine RPM and would remain so until the uni-vent was uncovered.

Just thinking.

Chuck

I've been thinking about this one a while. Generally, when running a uniflow tank the practice is to either

1) position the uniflow vent into the airstream where it's basically a pitot tube getting dynamic pressure, or

2) connect it to the muffler.

So for option (1) it would follow that when the model slows the dynamic pressure drops (with the square of the velocity) and the net positive suction head available at the fuel pickup drops, causing the mixture to lean, and conversely.

Muffler pressure seems to be proportional to RPM, (based on observations of the vent line on R/C engines) so a similar situation exists in that as the prop loads and RPM drops one would expect a drop in suction head.

Of course, both system previously described seem to work well and have won enough national and world titles you really can't argue with them.

But I still wonder, has anyone flown a uniflow tank with the uni-vent connected to a static port? In theory, that would give a consistent suction head at the fuel pickup regardless of model speed or engine RPM and would remain so until the uni-vent was uncovered.

Just thinking.

Chuck

With uniflow  its better  to run the vent open to air, whether hidden in a stable area , or out in the slip stream, I personally run them out in the slipstream with a 90 degree very light plastic , or aluminum "L" shape vent on the end of the tube ,  pointing either up ,down ,or toward the inboard wingtip, this  technique stops the  charging of the tank when flying in winds, the rich/lean situation when you fly   with/against  the winds.
Muffler pressure makes the engines  "dumb" if you are cycling them, it pressure feeds the venturie and slows the cycling making it come on  too late when going uphill , and  off  too late when going downhill.  plus many times it stretches out the  break time until you get almost back to level flight, other times it kills off the  break all together.
Other problems are the engine sucks up much sand, dirt ,dust, insects , grass  pollen..anything on the ground in its path the prop stirs up and the venturie can suck the debris in the motor. Whatever goes thru  the engine, can wind up in the muffler and can then go into the fuel tank  via pressure line.
 Also the  burned oil coats the inside of the muffler and pressure fitting, this coating can, and does, fleck off from vibration and  can ,and does go thru the line into the fuel tank, or worse clogs up the pressure fitting.
In winter time castor gets very thick in the fitting and effectively makes it act as if it is blocked then can open up with running, making for an inconsistent run.
Pressure can help. if  your venturie is too large

Randy

Randy, can you elaborate on the "L" shaped item on the end of the uniflow vent? I don't think I have seen anything that looks like what you are describing. Is the idea to not ram bugs, dirt etc into the inlet of the uniflow vent?

Foremost products makes them from plastic, plus fuel end caps  etc, I make the from 3/32 aluminum tubing very short  "L" shaped tubes about 3/4 inch long, just enough to use a 1/2 inch piece of fuel tube to join them, and point it   out  of the   airflow, either  towards the tip  or up  or  down.  many dozens of flyers  use my techniques on this that I have seen.  When you fly into the  strong winds you get a  rich half of the circle and a lean half, this happens because the excess airspeed charges the fuel tank  and makes it run richer when you go into the wind... it will get leaner on the other side of the circle when you fly with the wind.

The  "L" s  are ultra light  , so they do not  add any stress or vibration to the tube coming out  of the tank

Randy

http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXG844&P=FR&gclid=Cj0KEQiA39_BBRD0w-_rmOrc__8BEiQA-ETxXRjNTrTlBPNvaLYt7poPe8Q6FCT2Dm9JywM2p4XyQIkaAsql8P8HAQ

For the last couple of seasons, I've had my uniflow tube hooked to a piece of fuel tube that is permanently fixed into the cavity of the backplate on my Randy Aero OS .46VF AAC. I started with open uniflow, briefly tried pipe pressure (because the pipe came from Norm Whittle with the fitting installed), and then this.

It appears that this would meet the definition of a "Static Port". I read that Brett has his planes rigged to do this "sometimes", but it's not clear to me why it's not used all the time. I'm using it all the time, anyway. I have seen guys restrict the uniflow inlet, have seen the 90 degree elbows too. I'd rather avoid stuff that could get misplaced, lost, or come off in flight.   Steve

After about 30 years  of using the   L  on the  uniflow tube, i have never had one come off, or  ever seen one come off of anyone's tank. I use a very short piece of the  red Prather tubing and it simply does  not move.
It is also much easier to fill the tank with the  uniflow tube right on the side instead of having to plug and unplug the hidden  vent tube for filling

Randy

Well, I doubt anybody would see something so small as the elbow fitting come off in flight, but I also haven't seen any Prather tubing in a decade or more. If it did come off in flight, I'd expect some effect on the engine run, which probably wouldn't be a good thing.

The tube from my .46VF's backplate cavity comes out through the side of the cowl in a notch on the split (it was already there) and quite simply unhooks from the usual brass tube uniflow inlet for fueling and sticks straight out, right there, reminding me to hook it back up after fueling or at least before starting. I can also pinch it off if I need to, to lean the engine if it's obviously overly wet, just like slipping a fingertip on/off the uniflow tube, or clamp it off with forceps, to prevent potential siphoning. The siphoning is one thing that does concern me, but I haven't had a problem with it yet. I fuel in the pits...like I'm supposed to...

I think this detail is the bee's knees, cures the upwind/downwind rich/lean nonsense, and with nothing to come off or potentially lose. I see no downside. I made a little sheet aluminum fitting that holds the tube fixed in the backplate; mounts on one BP screw. Nothing fancy or difficult about any of it.  Steve

It appears that this would meet the definition of a "Static Port". I read that Brett has his planes rigged to do this "sometimes", but it's not clear to me why it's not used all the time.

  The main reason is that in most conditions, I have no issue with the ram air with the RO-Jett, and I can use it to control how much boost and brake I am getting in the wind. Part of the ram air effect - actually, most of it when it is windy - happens in the maneuvers, when it would otherwise whip up. With the Jett (or the VF) it's a pretty small effect. My "static port" is actually inside the fuselage, since it's another tube through the fuselage that used to be used for the pressure line. I just disconnected the pressure line internally. The tubes are about 3/8" apart so I can use fuel tubing connect one to the other, so it sucks air from inside the engine compartment. The Jett doesn't like pressure very much and certainly doesn't need it.

    This was all developed when I ran the PA 61, because it is extremely sensitive to the ram air effects. It was great to have that in the maneuvers, but it killed me in level flight.  I assume it is because of the  fuel restriction on the inlet side of the spraybar, and one of the things that the bored-out spraybar seemed to reduce was the sensitivity to fuel pressure. If I was doing it now, I would counterbore the spraybar out to whatever diameter David uses (don't bore all the way through because you won't have a needle seat any more!), or go back to running pressure or the internal static port.

    As soon as I took off at the 2003 NATs (with the PA using the RO-Jett tank and ram air pressure), I knew I was screwed, because as soon as I got out of the ground effects, it went dead rich due to ram air going into the wind. That year, I had to switch back to the PA the day before because the rear bearing failed in the Jett, and it worked fine in reasonable air.  And in fact, had I had it on pressure, I may have made it around the lap better, but pulled the wings off in the maneuvers *without* the ram air effect on whip-up. Who knows?

   I used to have a plug with a cross-drilled #60 air hole all the way from one side to another, and a blind hole to connect it to the tank. That's less good than the internal air inlet, but works fine and doesn't require an extra tube through the fuselage side.

    I would never recommend a normal-sized vent with the axis perpendicular to the airstream. It works fine most of the time, but every once in a while, air will come in one side of the tube and fuel will run up the other side and siphon raw fuel all over the airplane. And it's even more likely if you use some "anti-foam" agents that reduce the surface tension. If you do it, I recommend the restriction to make the opening as small as you can tolerate (most use a #60 drill but you can go smaller if you can get the hole drilled without breaking the bit) and still get sufficient air. The air flow rate into the tank is very small, only 7-ish ounces in 6 minutes. That's the same as the fuel flow rate, of course.

     Brett