What's the deal with profile tanks.

[Ara Dedekian]

      I've read on here that profile tanks are not recommended. The tank below proves that out. The runs are unreliable, most annoyingly by quitting inverted. Same plane and engine with a wedge tank (and Spectra lines) is  a consistent flyer easily doing the Old Time pattern and AMA maneuvers. The plane is a stock Buster/Fox 35 at 32oz without fuel.

      I plumbed the tank as I do my wedgies and C/L RC tanks; placing the uniflow vent midway between the pickup and front of the tank. They work, this one doesn't.

      Is it a geometry thing with the profile tanks? I cant the pickup end to the outside of the circle. Is the baffle, or it's design the culprit?

      Ara

[Dave Harmon]

     Is the baffle, or it's design the culprit?

      Ara

In my opinion.....both.

Remove the baffle and existing uniflow tube.
Then install a new uniflow tube parallel with the pick up tube which should be about 1/8" from the rear of the tank.
The uniflow should be soldered to the pickup tube and extend to about 3/8" or so from the end of the pick up tube.

https://stunthanger.com/smf/open-forum/clink-yer-clunk/msg636708/#msg636708
Read this all the way through for more details.....BTW....baffled tanks never worked properly for me either.

[Steve Helmick]

Do you run a hose to a remote uniflow intake on the inboard side of the nose? I'd expect that getting consistent airflow to the inlet would be pretty important. Running it to the muffler pressure tap would be fine with some engines, but not with all of them. If you commonly have mufflers fall off or similar mechanical failures, then muffler pressure is not a good idea, because the engine will go dead lean.  Steve

[Dan McEntee]

   As I see it in the photos, you don't have any holes at the back of the baffle. Depending on other things happening, it's possible you could just not be picking up the whole fuel load. As I see it, if the nose is yawed out at all, you would have fuel that could not transfer. That's the way it looks to me.
   Type at you later,
   Dan McEntee

[John Park]

I know Bill Netzeband once said: "Ask ten different guys which tank they use, and they'll show you ten different designs - all of which work best!".  Be that as it may, the best tank I've found for profile aerobatic models is the chicken-hopper design that used to be (and maybe still is) the popular choice for slow combat.  The only trouble is, it's fiddly to make and I doubt that it's commercially available.

[Joe Gilbert]

Vibration is a problem with profiles. Lots of tanks will work. But chicken hoppers
Have worked well for me ,Brodaks had them in hobby shop in June. If the fuel foams in Syringe as you pull fuel in to it  ,it will foam with vibration and you can never get a good run. sometimes  it is just best to put anti foaming  agent in fuel to take one thing out of equation.

Good luck

Joe Gilbert

[Steve Dwyer]

Not big on the Chicken Hoppers, never used one but while we're on the subject of tanks I recently opened the old Ruffy kit a bought couple years back. There in the kit the builder planned for a tank. It was a Veco tank with a crankcase pressure line with a built in check valve that rattles when the tank was shaken.

The Veco instructions with the tank said this new design would eliminate having to tilt the model upward when starting to prevent flooding the crankcase. Pressurizing the tank before mufflers must have been a challenge, the instructions said I would need to buy the crankcase fitting for the back of the Veco engine.


I'll never use the tank but now I have another collectors item, anyone recall this tank?

Steve

[bill bischoff]

Those were used in combat and rat race, in conjunction with crankcase pressure. The check valve let pressure into the tank, but wouldn't allow pressure (or liquid) to re-enter the crankcase through the pressure line. They weren't rare, but really weren't suited to regular suction/muffler pressure use. We've learned a lot about tank physics since then.

[kevin king]

I have a chicken hopper tank on a 35 size Midwest P40. I cant wait to replace it. Always seems the tank likes to remain in a half full status  that wont draw fuel.  Its probably me thats the problem, not knowing or caring to learn how to use it.
. If someone ever wins a world championship using a chicken hopper tank, I may take the time to learn. Both of which we know will not ever happen.     

[Dave Hull]

Seeing the construction of this tank, and then assuming you are running with the overflow capped and no muffler pressure to the uniflow line, my thought is that when you take this inverted, you will get a slug of fuel trying to backflow from the “hopper volume” into the uniflow vent. The fuller the tank is when you go inverted, the further this slug will go.

This can happen because the viscosity and density of air is so much less than the fuel. You could get a chug-chug-chug of air going in thru the flooded out tube, and the resulting pressure pulse and flow restriction to the fuel can’t be doing good things to the output flow to the engine.

If you like to kick your tanks out at the rear in order to get a clean cutoff, the baffle holes shown would not be ideal. I would also try a notch in the baffle at the aft center point. From the picture, it looks like the baffle butts right up against the aft cap, so no joy there, either.

The plumbing arrangement is also a bit confusing. For a conventional gear plane, especially an “old school” model that sits with a pretty good deck angle, having the overflow vent at the rear gives up a fair bit of usable tank volume. It almost looks like someone reworked this tank and converted the overflow tank exit position to the uniflow.

Other than that, this tank looks like it could win any ProStunt contest in America up through Expert. I would upgrade it and start practicing.

Hope you have a nice tanksgiving,

Dave

[wwwarbird]

 I don't think it's "profile tanks" that's your problem, it's the Chicken Hopper design. I've never had a bit of luck with any Chicken Hopper, throw it away and replace with a properly mounted and plumbed Standard Wedge. Do that and you should be good to go, as far as tank issues anyway.

[Perry Rose]

Du-Bro 4 ounce plastic tank plumbed for uni flow on profiles.

[Dennis Moritz]

Engine vibration gets magnified on many profiles. My club has had a lot of luck with chicken hoppers when engine vibration is reduced much as possible. What do we do to dampen bad vibes? Lots. Solidly mount engine, keep mounting bolts tight, sometimes tightening bolts every other flight. Use an aluminum plate, usually 1/8", bolted to engine mount bolts on inside of circle. Increases rigidity of nose. Bubble wrap tank. Plumb to bypass uniflo. Plastic tanks also work when plumbed for UC,often work when left stock. Aluminum piston engines vibrate less than cast iron piston engines. Less mass up and down. Debris in tanks, all kinds of weirdness. Even a speck of dirt in a filter can cause erratic running. A speck of something or other in needlevalve will ruin your day. Pull out needle, reverse flush, check flow through fuel feed hole on spray bar. Block fuel intake on spray bar, squeegie through needlevalve opening. And. So on.

[Dan McEntee]

    I would still point out the top photo looking into the tank from the front. Why are there no holes in the baffle past half way from the front?? Lots of debris and such inside also. i would consider that less than clean. Again, I think this can cause issues that are magnified if the model is flying with the nose yawed outward. I make tanks similar to this from a drawing that I think Fred Kroeger designed, and I guess it's a sort of chicken hopper because it uses a small tank soldered outside the high point of the larger tank. These are made tall and narrow like this one to keep fuel load more in line with the crank case of the engine. They have always worked well for me and others here in the club. I don't like some commercial "chicken hopper" tanks because they are too wide and fuel draw has to work against centrifugal force when in the air.
  Type at you later,
  Dan McEntee

[Ara Dedekian]

    Why are there no holes in the baffle past half way from the front??


     Because of a world class brain fa( )t. I turned the tank 180 degrees to punch my own holes for the replumbing to uniflow. I completely missed, or forgot, that the baffle got reversed in the process and buried the fuel pickup. So the front end of the baffle, in the picture, is supposed to be in the rear by the pickup. I just realized that this morning after looking at the picture again.

     But thanks for all the suggestions. Dave Harmon's link to Aprils discussion was a great read. Swapping the end positions of the pickup and uniflow is what got my attention. A fix I'll definitely do.

     Positioning the overflow to the top rear of the tank is something I'd recommend. On a profile, the overflow tube is usually coming out the bottom. It's a pain to access when uncapping and capping and seeing the overflow stream. I have to pick up the plane to do and see all that and lose some fuel in the process. With it on top, it's in plain view, accessible, easy to cap off and away from landing gear, muffler lugs, mufflers etc.. There's no loss of internal volume since the overflow tube goes into the tank only as far as the hole flange.

     The first fix will be to get rid of the baffle. Again, Thanks for all the help.

    Ara

      PS - Steve H., If not using muffler pressure, the vent is routed to the inside of the fuse with tubing.

[Dave Harmon]

Ara.....
I would move the top overflow tube from the rear to the front bottom as done conventionally.
The front bottom overflow problem can be overcome by putting a copper tube crossways through the fuselage with the stopper on the inside of the circle.
Then add a piece of fuel line from the front overflow to the cross tube.
This will make it a heckova lot easier to refuel the tank....and....moving the overflow to the front will allow the tank to be filled all the way.....can't do it if the model is sitting tail down with a rear overflow....hahah

[Gary Dowler]

Looking at your tank, here is what I would do.   drawing is top view

Ditch the baffle.  Not needed and complicates getting the plumbing right.

 The uniflow vent tube end needs to be parallel to the pick up tube, and not terminate any closer than 1/2" from the end of the pickup tube. If I recall Pauls teaching correctly, it really should terminate closer to about mid way down the tank.

For consistent runs, you need consistent vent pressures.  Run a piece of brass tubing through the fuselage, then bend it 90 degrees so its facing directly into the airstream.  This location gives you the cleanest, most consistent, airflow.  Connect the end of this on the right (outboard) side to the vent tube in the tank with a piece of fuel tubing.

[Steve Helmick]

Studying the OP's photos further, it looks to me like it would be a MIRACLE if the aft end of the baffle would seal against the aft end of the tank well enough to keep fuel from finding its way into the ^ fuel feed area.

The rusty/crusty looking face of the tank makes me wonder if there's a leak? But maybe that's from contact cementing a foam pad to the tank?

Leaks don't work with uniflow. The overflow cap is also critical...some commonly used caps leak. Those cute little yellow caps can leak, as can a threaded screw in a bit of tubing. Solder flakes in the tank are bad news, along with small bugs and tiny seeds. It can be difficult to diagnose the exact cause of the problem. Is a fuel filter being used between tank and engine?

All that aside, I've never tried one of those profile tanks. My gut feeling is that the rather extreme height of the tank would potentially cause inconsistent fuel feed problems, but I could easily be wrong on that.  Steve