Wide Wedge Uniflow Tank Problem?

[Kafin Noe’man]

I have read few threads or articles discussing about wide wedge uniflow tank design that’s causing engine to lean out towards the end of the flight.

I currently have 2 Brodak’s Wide Wedge Uniflow Tanks
1. 3oz BH-548 1" H x 2" W x 3" Long for my XEBEC (a profile fuselage plane) with LA-S 25 and the uniflow setup works perfectly fine.

2. 4.5oz BH-551 1" H x 2" W x 4.5" Long for my P40 with LA-S 46 and I have my engine leans out and pulls me hard at the end of the flight.

I’ve also checked for any leaks, but I found none. Since this is a commercial tank, I don’t know how the internal plumbing is done. The recommendations I’ve got so far are:
- Add shim to the back of the tank.
- Reroute the uniflow vent to the inboard side.
I might try to do the first advice first and I hope it will fix my problem.

So, this got me thinking, with everything about the same, why does the smaller fuel tank work fine?

[Dan McEntee]

    The larger tank may be working fine also, until the end of the tank. You also do not tell us how many laps that the engine runs in this condition. Just a few laps , or 15 to 20 laps?  it's kind of normal for most engines to lean out a bit as the tank gets near empty, lots of things affect it. If you have the smaller model working well, you might never get the other one exactly like it, but will run satisfactorily to get you through the pattern and then lean out a bit for the last few laps. Some guys have posted on the forum here complaining that they have an airplane that gives them no "warning" when it is about to run out of fuel!! If a tank and engine are working perfectly on uniflow, that is exactly what can happen. Some guys fight it for a long time and never really get to that point.  The smaller airplane may be more in good flight trim. You  just got the P-40 going. You also more or less answered your own question. The P-40 may be flying more yawed nose out when flying. That make the tank fly at the same angle, , with the front of the tank farther out than the rear. centrifugal force makes the fuel run away from the pick up when it get down to a certain level, so you need to shim the back of the tank out get it more in the attitude it needs to be. Moving your lead outs forward can also affect the same change but you may have to move them farther than you are comfortable with. Shim the back end of the tank out and fly the airplane and see what happens. then if you want you can move the fill/uniflow vent to the inboard side of the fuselage. You should just get into the habit of rigging your fuel systems this way every time when you build/assemble a model. Having the fill/vent and over flow tube high on the inboard side of the fuselage, profile model or full fuselage, just makes it more convenient and easy to fill the tank for a flight.
  Type at you later,
   Dan McEntee

[kevin king]

OS LA. 46 on a 35 sized profile is a lot of torque. The large engine could be to much for the airframe, causing excessive vibration/fuel foaming. Try adding a couple of squirts of Armour All to a gallon of the fuel. Nose heavy planes will pull alot more than tail heavy planes, so I would check the CG, then hang it up by the leadouts to see how far down the nose hangs. . I use 1 or 2 degrees on my planes. The stock muffler adds even more nose weight. Might consider using a tounge muffler instead 

[Leonard Bourel]

Hey Kafine Your engine and plane combo are fine That P40 has a 560 sq inch wing The LA 46 is a great motor for that plane . Depending how long the uniflow tube is in your tank and how much yaw is happening with the plane will determine how long it will run fast at the end of the flight as Dan said Try the shim behind the tank I have heard about the armour all in the fuel but have never tried it . If you still feel the plane pulls to hard move the lead outs forward just a little each flight Soon Len

[Dave Harmon]

OS LA. 46 on a 35 sized profile is a lot of torque. The large engine could be to much for the airframe, causing excessive vibration/fuel foaming. Try adding a couple of squirts of Armour All to a gallon of the fuel. Nose heavy planes will pull alot more than tail heavy planes, so I would check the CG, then hang it up by the leadouts to see how far down the nose hangs. . I use 1 or 2 degrees on my planes. The stock muffler adds even more nose weight. Might consider using a tounge muffler instead

Absolutely correct.

The c/g mentioned in the Brodak P-40 ARF construction manual is much too nose heavy.
Move the c/g to balance on the wing spar with an empty tank.
Mine flys great here.
Then, hang the airplane by the leadouts and move the leadout slider for the nose about 2 degrees lower than the tail...while it is hanging.
You can shim the rear of the tank away from the fuselage about 2mm to start and all this will probably help.

As previously mentioned....the engine will lean out as it runs out of fuel....this is normal.
When the fuel level in the tank uncovers the uniflow tube inside the tank....the engine will go somewhat rich, then as the fuel is used up the engine will go lean until it uses all the fuel then it will suddenly quit .
This is what is supposed to happen.

You could add a few drops of armor-all in a pint of fuel as a test to determine of the fuel is foaming but I would not use it all the time because of the silicone deposit on the glow plug.
If the engine runs normally then leans out at the end of the tank....fuel foaming is probably not a problem.

[Colin McRae]

I have used many Brodak uniflow fuel tanks, both wide wedge and oval profile. Both with and without muffler pressure. All of my tanks have worked fine as expected. When the amount of fuel runs low towards the end of the flight, the uniflow vent exit (inside the tank) becomes uncovered with fuel. When this happens the 'uniflow' is lost and the engine goes lean until the fuel runs out. Totally normal. Depending on my model/engine combination, the lean-out maybe happens for 1-3 laps.

If running muffler pressure, the engine will also go lean if an in-flight tank/muffler system exhaust gas leak occurs. This has happened to me on a few occasions where the muffler bolts have loosened in flight. I always re-check my muffler bolts for tightness before each flight.

On an old Brodak 2.5 oz wide-wedge tank I had that was damaged, I was curious about the Brodak internal uniflow design. So, I cut the tank open to take a look. What I found was the uniflow exit was only about 1/2" away from the fuel pickup in the backside if the wedge. Pretty close. This is both good and bad. Good that the uniflow feature continues to function until the fuel is almost gone. Bad in that sometimes on the ground as I am adjusting the needle, the engine can sometimes be erratic as I believe air bubbles are getting into the fuel pickup. But once the model takes off and is in the air, the engine runs fine. I have only seen this in a few cases, not normal.

[Dave Harmon]

I think I just said most of that.

[bob whitney]

i love Brodaks to death BUT in the last two years i have had two Brodak tanks that didnt want to work right.after pulling the back cover off found that the tubes were no where near where they should have been to work properly .i called and told them what i had found.  Oh those must have been out of the bunch built in china.anyway i straightend them up and both worked fine,
  i fly with a couple of more or less beginners and have been trying to help them out .after working with one Brodak uni flo tank for ywo weeks he gave it to me and asked me to check it out.found that both feed and uni flo tubes were the same lengh at the back of thr tank and the feed tube was over half full of solder ,and neither tube was on the centerline of the tank.and to top it off the over flow tube was almost 1/4 in from the top of the tank.  i have pictures of all this but cant post them here as they are too big'
   what bothers me most is these beginners trying to get thier motors running with tanks like this,and there is not a lot we can do about it

actually this might be a good reason to go to an RC plastic clunk tank  RAD

[Dave Harmon]

I had the same problem with the Brodak tanks.
After going through that same problem, a couple of times I started buying the Brodak tank kits and building them properly. I don’t have any more tank trouble. I did a construction article sometime back on how I did it which is just a little different than what had been done before, but it works great.

[Motorman]

The problem is not big tank little tank it's big engine. The little engine draws less air into the tank so bubbles don't blanket the feed tube. The big engine draws more air throught the vent.

When the fuel gets low enough in the tank the air will blast through and break the vacuum which makes the engine go rich. You want this to happen right around the hourglass as the fuel head is getting low at that point. Several things can go wrong with this process.

In your case, where you go lean instead of rich, it sounds like there is too much turbulance near the feed line when the fuel gets low. I move the vent half way up the length of the tank on the outboard side right on top of the feed line. This prevents foam in the feed line.

If you have the vent on the inboard side, you'll do a couple of laps then be working with a standard suction system.

MM

[Doug Moisuk]

I’m a convert to the Squishy tanks. I’m changing all my planes over, profile and full body.

[Dave Harmon]

I think this link is worth reading and explains the OPEN UNIFLOW tank.....no muffler pressure.
http://flyinglines.org/cognitive.uniflow.html

Note that it explains why the engine goes a bit rich when the uniflow tube in the tank is uncovered.
Of course, shortly after this happens....the engine goes lean....because the tank is empty and the engine is now sucking air along with the dregs of the fuel.

This is why a properly constructed metal tank will give a slight warning (rich) then a couple of laps after that the engine goes lean.
Then a lap or so after going lean....the engine quits clean...no farting, wheezing or partial power for 8 or 10 laps like a plastic tank.
I didn't invent all this of course.....I didn't know diddley about stunt tanks....and it took a while to understand exactly why my engines were not flyable.
Ask Joe.....he saw it all.

One time a very smart flyer told me....."The tank...the tank....IT'S ALWAYS THE TANK!!"
For me....this has proven very true!

[Paul Van Dort]

the fact that the engine produces a rich warning at the end of the tank has everything to do with the flow resistance of the fuel in the fuel line. The flow resistance depends on the diameter of the pipe, the length of the pipe and the viscosity of the liquid.
Once the tank is empty, the feed pipe will start filling with air and the relative length of the pipe will decrease and therefore also the flow resistance. As a result, the fuel will gradually flow faster to the venturi, which gives a richening effect, until the tube is completely empty and the engine runs out of fuel , leans out and stops.

[Dan McEntee]

I’m a convert to the Squishy tanks. I’m changing all my planes over, profile and full body.

     Move that over flow line up to the same level as the fill/vent tube. As you have it, it's possible for fuel to syphon out of the tank if your cap leaks at all. Up high at the same level as the vent/fill tube, and no chance of that.
  Type at you later,
  Dan McEntee

[BillLee]

the fact that the engine produces a rich warning at the end of the tank has everything to do with the flow resistance of the fuel in the fuel line. The flow resistance depends on the diameter of the pipe, the length of the pipe and the viscosity of the liquid.
Once the tank is empty, the feed pipe will start filling with air and the relative length of the pipe will decrease and therefore also the flow resistance. As a result, the fuel will gradually flow faster to the venturi, which gives a richening effect, until the tube is completely empty and the engine runs out of fuel , leans out and stops.

Interesting conjecture!

Another:

Given:
The engine "sees" the fuel head based on the point of regulation, i.e., the position of the uniflow tube outlet inside the tank.
When the uniflow tube outlet uncovers, the point of regulation is now the free surface of the fuel.

Important terms: fuel head, point of regulation, position of the uniflow tube, free surface of the fuel.

A thought process:
Assume the tank width exceeds the position of the needle valve.

Tank is draining, uniflow uncovers, fuel continuing to be drawn out,  leaning out seen.

Until the fuel is all gone from the tank, remainder now in fuel line.

As that fuel in the line is consumed, the free surface of the fuel now moves back inboard towards the needle valve.

Resulting in a richening of the setting.


Hmmmmmm.

[Paul Van Dort]

Interesting conjecture!

Another:

Given:
The engine "sees" the fuel head based on the point of regulation, i.e., the position of the uniflow tube outlet inside the tank.
When the uniflow tube outlet uncovers, the point of regulation is now the free surface of the fuel.

Important terms: fuel head, point of regulation, position of the uniflow tube, free surface of the fuel.

A thought process:
Assume the tank width exceeds the position of the needle valve.

Tank is draining, uniflow uncovers, fuel continuing to be drawn out,  leaning out seen.

Until the fuel is all gone from the tank, remainder now in fuel line.

As that fuel in the line is consumed, the free surface of the fuel now moves back inboard towards the needle valve.

Resulting in a richening of the setting.


Hmmmmmm.

I think the decreasing flow resistance is more dominant. But perhaps both effects are in synergy.