?? Am I wrong???

[Bootlegger]

  In thinking that it doesn't matter where the uniflo exits the tank it will not upset the engine run as long as it is correct where it should be in the tank?

   I try to set the end of the uniflo at 1/2" in front of  the feed line so I don't get "goofy" running due to air getting into the feed line ..

   Is this correct thinking??

 Thanks a lot folks...

[Don Hutchinson AMA5402]

Yes

[Paul Walker]

Agree that the exit doesn't matter.

However, the inside is critical, as you stated. Like you I USED to put the uniflow exit 1/2" in front of the fuel pickup (at the back of the tank). I had issues "neelding" the motor, it being inconsistent. I finally moved the uniflow exit to the mid point of the tank. If the tank was 6" long it would exit 3" in front of the fuel pickup. With that, very stable "needle" settings and there was also consistency tank to tank. Back in the day of the 40 VF I used to have 11 tanks in 0.1ounce increments, from 6 to 7 ounces. Flying FAI with a 7 minute limit required this so I wouldn't have an overrun or underrun because the size if the puddle on the ground was the wrong size! Moving the uniflow to the middle made all these tanks run exactly the same.

Give it a try!

[Doug Moon]

Agree that the exit doesn't matter.

However, the inside is critical, as you stated. Like you I USED to put the uniflow exit 1/2" in front of the fuel pickup (at the back of the tank). I had issues "neelding" the motor, it being inconsistent. I finally moved the uniflow exit to the mid point of the tank. If the tank was 6" long it would exit 3" in front of the fuel pickup. With that, very stable "needle" settings and there was also consistency tank to tank. Back in the day of the 40 VF I used to have 11 tanks in 0.1ounce increments, from 6 to 7 ounces. Flying FAI with a 7 minute limit required this so I wouldn't have an overrun or underrun because the size if the puddle on the ground was the wrong size! Moving the uniflow to the middle made all these tanks run exactly the same.

Give it a try!

Do you have any idea why changing where the tube exited the tank changed the consistency?  I am curious.

I run clunks but I still like to know how things work.

[James_Mynes]

I'm all confused. Is the inflow "exit" inside the tank, or outside. It seems that the term has been used for both ends in this discussion.

[Gordan Delaney]

Hi Doug,
Tried to call a couple of days ago. Left message no response. Is the number in the pampa correct?

Gordy

[dale gleason]

Gil, and Doug,
In order to understand how niflow tanks work, it pays to "be the tank". This way one need not concern oneself with terms such as "head pressure", "fuel head" and such. Or how air is entering the tank as fuel is leaving.

Simply put, "the motor thinks the position of the niflow pickup is the position of the tank". That's in the vertical, of course. Horizontally, forward and back doesn't make any difference, as Paul has proven.

Some folks have the niflow on the inside wall of the tank, these tanks are not unflows, and these folks are not "being the tank."

Hope this helps,
Dr. Tank

(And "no", I don't ever recall you re-plumbing my own tank between flights at a Dallas Contest because I was in a snit and had put it together wrong. Were that true, I wouldn't be known as Dr. Tank.)   

[Bootlegger]

 Dale I sent ya' a P/M..  Thanks a lot

[Bootlegger]

  Paul Walker, I sent ya' a P/M.  Thanks a lot

[proparc]

Agree that the exit doesn't matter.

However, the inside is critical, as you stated. Like you I USED to put the uniflow exit 1/2" in front of the fuel pickup (at the back of the tank). I had issues "neelding" the motor, it being inconsistent. I finally moved the uniflow exit to the mid point of the tank. If the tank was 6" long it would exit 3" in front of the fuel pickup. With that, very stable "needle" settings and there was also consistency tank to tank. Back in the day of the 40 VF I used to have 11 tanks in 0.1ounce increments, from 6 to 7 ounces. Flying FAI with a 7 minute limit required this so I wouldn't have an overrun or underrun because the size if the puddle on the ground was the wrong size! Moving the uniflow to the middle made all these tanks run exactly the same.

Give it a try!

Paul, this is some SERIOUSLY good info. Is it possible for you to draw and post a sketch of your setup. I have been doing very good with my Saito 72 setup but, if I decide to switch back to 2 stroke, this would be good to have.

[Paul Walker]

Doug,
Where the uni flow tube penetrates the side of the tank is not critical. It is where the end of the tube lets the air out INSIDE the tank that is critical.

Others,
The air exit point for the uni flow line is simply moved forward from its normal location to a point that is equidistant from the front wall to the back wall. The vertical location is where ever you like it. For me it was mid thickness of the tank. 

Nothing fancy.

[Brett Buck]

However, the inside is critical, as you stated. Like you I USED to put the uniflow exit 1/2" in front of the fuel pickup (at the back of the tank). I had issues "neelding" the motor, it being inconsistent. I finally moved the uniflow exit to the mid point of the tank. If the tank was 6" long it would exit 3" in front of the fuel pickup. With that, very stable "needle" settings and there was also consistency tank to tank. Back in the day of the 40 VF I used to have 11 tanks in 0.1ounce increments, from 6 to 7 ounces. Flying FAI with a 7 minute limit required this so I wouldn't have an overrun or underrun because the size if the puddle on the ground was the wrong size! Moving the uniflow to the middle made all these tanks run exactly the same.

      And you assigned Don McClave to carry around your box of fuel tanks, as I recall.

     We had talked about the uniflow position once (and unlike many others actually paid attention...) and I tried a similar experiment. Interestingly I ended up with utterly different results. My tanks are tapered in plan view , which may make a difference. Moving the air inlet forward and aft had the expected results; for the most part, the difference was when it started to lean out, since I couldn't move it strictly fore and aft, it also moved inboard and outboard.

    I ended up with the pickup as far back into the corner as possible. The air inlet ended up about 1/8"-3/16" away, about a tube width, in front of it. The pickup has to have a bend in it to go around the air inlet, it's so close. But it's bulletproof - it needles fine, it runs exactly the same from beginning to end (no richening or leaning out) and if I fly around level it runs dead steady, revs up for a fraction of a second, and stops, and leaves virtually no fuel.

    Brett

[dale gleason]

Suppose you are in your car, windows rolled up and gala Birthday helium balloons are in there with you. Maybe a clown in a clown suit is with you. You suddenly stop...which way do the balloons go? Now, inside a fuel tank, in flight, some air bubbles are being introduced inside the tank via the uniflow......  which way do the air bubbles go? (hint, it's the same physics as the balloons in the car, assuming the clown didn't impede their progress)
For this reason, I don't think air bubbles can get into the fuel feed line, they're being forcefully sent to the inside tank wall.

"Be the tank..."
Dr. Tank

[BillLee]

Dale, that's an analogy that is so hard to visualize as to be worthless. 

[frank williams]

Now I'm gonna have to go buy some birthday balloons to try this.    Generally when I step on the brake my car sounds like a pinball machine with all the golf balls rolling around.

Also, the exit of the uniflo tube can be moved up or down in the tank and it's just like moving the tank up or down.  I have some tanks with two of three uniflo tubes protuding from the tank.

[Randy Cuberly]

Now I'm gonna have to go buy some birthday balloons to try this.    Generally when I step on the brake my car sounds like a pinball machine with all the golf balls rolling around.

Also, the exit of the uniflo tube can be moved up or down in the tank and it's just like moving the tank up or down.  I have some tanks with two of three uniflo tubes protuding from the tank.

Gee Frank...do you have two or three needle valves on the engine too?? 

Nevermind!  Choice is always best...except of course when it comes to babies!

Randy Cuberly

[Howard Rush]

No clown suits here,
This is serious stunt.
We got some white outfits,
We'll be dressed like Bob Hunt.

[john e. holliday]

Suppose you are in your car, windows rolled up and gala Birthday helium balloons are in there with you. Maybe a clown in a clown suit is with you. You suddenly stop...which way do the balloons go? Now, inside a fuel tank, in flight, some air bubbles are being introduced inside the tank via the uniflow......  which way do the air bubbles go? (hint, it's the same physics as the balloons in the car, assuming the clown didn't impede their progress)
For this reason, I don't think air bubbles can get into the fuel feed line, they're being forcefully sent to the inside tank wall.

"Be the tank..."
Dr. Tank

Remember the video the Tulsa GlueDobbers had a few years ago showing the plastic tank and fuel position while the plane was flying?   The fuel was to the outside lower corner of the plastic tank.  Even inverted it was to the outside lower corner.   Notice I didn't say top or bottom of tank.   The wedge tanks usually keep the fuel pretty well centered between top and bottom.

[Paul Walker]

Yes, my tanks were rectangular in plan form. If not rectangular your results my vary. 

That's the way the bubbles bounce!

[RknRusty]

Why does the engine never suck air on the descending side of a wingover or hourglass?
Nose down must uncover the pickup long enough to pull some air in. It seems like by the time the plane reaches the bottom and levels off or begins another climb, that bubble would reach the spraybar and the engine would cough. It doesn't though, even on the last quarter of a tank.

Rusty

[Dave_Trible]

There is usually enough centrifugal force to keep the fuel out there.  In the 'good 'ol days' when we were trying to fly 40 mph or less I recall an occasional flame out in the overheads.  That's why the baffles in some old Veco (and other) tanks- to keep enough fuel on the pickup to get through the maneuver.

Dave

[RknRusty]

When I was doing my overheads in Huntersville, the wind up top was gusty and I went slack. At the same time it coughed and I aborted on the last outside loop. Maybe lack of centrifugal force had something to do with that stumble. Since I've switched from the Wedge to a square Dubro clunk, it's been nothing but perfect runs. I'm sold on that system. If it just wasn't so aesthetically displeasing.

Rusty

[Dave_Trible]

Rusty I would say you like your new setup more because of the clunk than the shape of the tank.  You could put a clunk in a wedge ( though no reason too) and you couldn't tell the difference.  I still prefer the rigid, hard plumbed tank but a clunk allows for some unusual aerial situations. 

Dave

[Steve Helmick]

I understood Dr. Tank's car/balloon description completely, tho I didn't see that the clown was doing anything worthwhile unless he was making balloon animals while riding.

To answer Rusty's question...if your lines aren't tight, your plane is flying (roughly) straight and not in a hemisphere, thus the fuel will go wherever it wants, and not where you want. Might consider shorter lines for rough conditions?   Steve

Edit: I have no white clothes
       Don't fly that well
       And far too much hair
       To look like Bob Hunt
       It don't rhyme,
       but I don't care
      

[Joseph Patterson]

    Dale I understand! Was Howard there?
      Doug 

[Brett Buck]

Why does the engine never suck air on the descending side of a wingover or hourglass?
Nose down must uncover the pickup long enough to pull some air in. It seems like by the time the plane reaches the bottom and levels off or begins another climb, that bubble would reach the spraybar and the engine would cough. It doesn't though, even on the last quarter of a tank.

Rusty

     There's still plenty of centrifugal force, so the fuel surface angle+ fuel depth determines whether or not it will suck air in. In the descending leg of the hourglass, assuming you are descending at a constant speed (which it isn't, but is conservative) the fuel surface angle is only about 21 degrees (atan of 1 g/2.5 gs). You have to be pretty low on fuel to have 21 degrees uncover the pickup. And, in fact, if you are low on fuel, the hourglass is usually the first place it becomes evident, by leaning out briefly right at the bottom, or shortly thereafter.

     In FAI contests, that's how I can tell whether or not I am close on fuel, if I get a brief speedup flying along the hourglass bottom, I know not to put in an additional establishing lap before the 4-leaf. My tank is tapered with about a 5 degree taper so the effect is only about 15 degrees fuel surface angle instead of the full 21.

    By the way, "nose down" is not the sole determining factor. I think that the longitudinal acceleration is around a G, or more at times, just because of the maneuvering drag. That can make the fuel pressure when it reaches the engine change in ways you can't explain by just looking at the local vertical gravity vector. That's how you can dismiss the 4-2 break asa pure mixture change, the fuel pressure doesn't follow the apparent "lean" or "rich" engine run sound at all, in fact it goes backwards in many places.

   Brett

[RknRusty]

...Might consider shorter lines for rough conditions?   Steve...

Actually another plane has solved my problem, as well as confirming that I really can fly round loops and nice intersections with my feet planted in my beginner routine. No more dancing, I love my new Ray Copeland modified Skyray. I'm about to start building one to the same specs. Many of which are the Brett Buck mods. It's been a really nice surprise for me.

But yeah, I have a set of 58 footers I could put on the Shoestring, but I think it's mainly over powered and I was trying too much to slow it with the needle. 9x4 prop and lots of head shims and it was still too fast for me. It'll be a fun flier from now on, at 4.3 seconds on 60'. And it's still pretty, so I can live with that.

And Brett, thanks for the thorough explanation of the fuel feed.

Rusty

[Brett Buck]

Actually another plane has solved my problem, as well as confirming that I really can fly round loops and nice intersections with my feet planted in my beginner routine. No more dancing, I love my new Ray Copeland modified Skyray. I'm about to start building one to the same specs. Many of which are the Brett Buck mods. It's been a really nice surprise for me.

But yeah, I have a set of 58 footers I could put on the Shoestring, but I think it's mainly over powered and I was trying too much to slow it with the needle. 9x4 prop and lots of head shims and it was still too fast for me. It'll be a fun flier from now on, at 4.3 seconds on 60'. And it's still pretty, so I can live with that.

And Brett, thanks for the thorough explanation of the fuel feed.

   You are quite welcome for both. The problem with the Shoestring is more with the basic design that it is the engine. Very thin pointy airfoil, low aspect ratio, and tiny tail volume. It has to fly like a bat out of hades just to ensure it keeps flying in the corners, where it slows dramatically. It's in the Ringmaster category - a design from very early on, when just being able to loop was a really big deal.

     The Skyray is just a whole lot better airplane (based on a slow combat plane, Mikey tells me). I think you will want to be removing those head gaskets at some point.  Full-tilt airplanes are even easier to fly, the only problem is they are difficult and time-consuming to build.

       Brett

[RC Storick]

A good flying profile that can be speed built is the S1 Ringmaster. Fly's very well.

[Bootlegger]

 Focus, focus,focus this is a tank question...

[dale gleason]

Hopefully, this will help our good friend, Bill Lee, get a better handle on this tank business...get it? "Handle"    My balloon analogy was rather abstract....I apologize.

Another thing to think of when pondering any tank problem, is a centrifuge. One with an arm about sixty feet long. It rotates roughly 360 degrees every five seconds. That is the environment in which your tank finds itself. Before I obtained my mail-order doctorate in tankage, I always thought of tanks in a static environment, which just isn't the way it is. The centrifugal force on a tank flying in a c/l plane is easy to calculate, but if your forte' isn't calcium or mathematics, suffice it to say it's a pretty formidable force which can be put to good use. The force is always with you.

A tank built with the outer wall angled outward (the opposite of what is normal practice) for example, would need the fuel pickup at the front of the tank.

Hope all this helps; hopefully, someone can avoid the pitfall I fell into, the dreaded "porky pine" stunt tank, uniflows  pointin' every which' way....little plastic vent covers lost in the grass....

Dr. "T"

[Jim Thomerson]

George Aldrich told me he did not like uniflow.  In the same conversation he told me the Bob Palmer tank was excellent.  At the time neither he nor I realized that the Palmer tank was uniflow (later confirmed by Bob Palmer). The Palmer tank has the uniflow at the front of the wedge, the pickup halfway back, and a baffle behind the pickup.  I've seen drawing of similar Russian tanks, but with the pickup at the back of the tank. 

I have installed uniflows with the in tank opening anywhere from front to back.  I am not expert enough to tell any difference in how they run. I agree that moving the uniflow in tank end up or down is the same as moving the tank up or down.  I put a one way valve on open air uniflows to stop siphoning and protect from changes in outside pressure with maneuvers.

[Brett Buck]

I have installed uniflows with the in tank opening anywhere from front to back.  I am not expert enough to tell any difference in how they run.

   I am not sure what difference Paul found, but I would expect almost all of the difference would be in ground adjustments. Dale's thought experiment illustrates the likely in-flight situation.

    Brett

[Paul Walker]

Yes, what Brett said. The difference was in more consistent needle settings. In flight, no difference!