a unifow puzzlement

[Ara Dedekian]

       I pulled the tank out of an old Barnstormer after it continually quit with half a tank of fuel left. I found a rectangular uniflow tank with an angled back end and the pickup and vent tube attached half way up and at the rear of the outboard wall.

       Replumbed it and attached the pickup at the outboard bottom rear with the uniflow vent 3/4" ahead of it on the outboard bottom rear. There's no overflow tube. It has to be fueled Combat style with the plane positioned so the pick up tube acts as the overflow. Pressure test was OK when reassembled.

       The resulting flights with the old Fox 35 were perfect. Except the engine would quit with no warning when the fuel ran out. Running one moment and stopped the next. It was so consistent I had to keep close track of the air time to know when to stop maneuvering. A second old Fox 35 did the same.

       The third engine I tried, a Fox 35 I put together from parts, finally gave me the usual three lap lean run warning. The piston/liner fits were all good but the 'parts' engine had a really good fit of the crank in the sleeve bearing. The other two engines had fuel coming out the front of the bearing while running. Would that have caused the crankcase pressure to drop and somehow cause the problem?

Ara

[Motorman]

 Some Foxs will just quit if they get too lean some just sag off, don't know why. I do know a wet nosed engine will use allot more fuel.

MM

[Dan McEntee]

  An engine should run right up to the last drop and quit on a properly constructed uniflow tank. You should always be keeping time even if it does give you a burp or two in level flight as a warning, because in the wrong attitude it will quit instead of burp and you could lose a model. If all the engines in the airplane perform the same with the only minor difference in how they shut down at the end, I would say you have a good tank. As far as the fit of the crank in the front bushing, it may affect a needle setting a bit, but with proper amounts of oil, the engine should run just fine. I have seen and own several long time Fox.35s with front bushing wear and they run just as they should. In fact there are some noted excellent stunt fliers out there that think that in this condition is where they run best and put out peak power for a good 4-2-4 stunt run.
  Type at you later,
   Dan McEntee

[Ara Dedekian]

      Dan

          Thanks for the reply. It helped a lot. I've got a few Fox 35 engines that I put big black "X"s on because the cranks, when held by the threads and forced up and down, had a noticeable wobble. I'll have to resurrect them in the Barnstormer next season with my 27% lube fuel and give them another chance.

     Ara

[Tim Wescott]

I'm probably strange, but I prefer a tank setup that just cuts off cleanly.  Usually this means either a plastic regular-vented tank or a metal uniflow tank with a clunk.  Consider, though, that all I'm interested in is competitive pattern flying -- I just start with a fuel load that'll give me a pretty reliable 10-12 laps after the clover, and I remember that I have about a six-lap budget for extra laps between maneuvers, and I go fly.  As long as I don't succumb to the temptation to noodle after the clover, I'm fine.

[Steve Helmick]

That angle across the rear end of the tank in the Barnstormer likely does no good at all. The "no wedge" outboard wall will work, but not very well. Best plan is to make a "square wedge" tank like the Veco's, except make the "V" angle more acute. A bit of taper from front to rear (wider in back, narrower in front) on the outboard edge is also good. Make the tank 1.5" deep if you must...no harm. I'd be in favor of replacing the tank, except that it will then require more fiddling to get it working correctly...maybe more than a 30 year old Barnstormer wants?

Unlike Tim, I'd rather have my engine pick up some revs prior to shut-down...just like an electron burner with one of Tim's Ingenious Timers installed.  Steve

[Ara Dedekian]

     

      Steve

          I have a number of 'square wedge' tanks (Perfect?) with uniflow plumbing that are drop in replacements for the homemade Barnstormer tank in the photo. I'll try those first for comparasons sake when the flying season returns. Al Rabe described an angled tank he designed for a shut off on command with a loop at the end of the pattern. I wasn't quite sure where the angle was and thought maybe the angled rear wall of my tank was what he described. Was it instead the angle he created by the outside of the tank being wider at the rear than the front?

     Ara

[Tim Wescott]

     

      Steve

          I have a number of 'square wedge' tanks (Perfect?) with uniflow plumbing that are drop in replacements for the homemade Barnstormer tank in the photo. I'll try those first for comparasons sake when the flying season returns. Al Rabe described an angled tank he designed for a shut off on command with a loop at the end of the pattern. I wasn't quite sure where the angle was and thought maybe the angled rear wall of my tank was what he described. Was it instead the angle he created by the outside of the tank being wider at the rear than the front?

     Ara

It's the angle of the outside wall of the tank.  With a wall that's parallel to the nose of the plane the fuel tends to collect in the front of the tank, because the centrifugal force in front of the center of gravity tends to point a bit forward as well as out.  In full-blown form, when you run low on fuel the pickup uncovers, then the engine briefly goes lean.  The resulting acceleration on the airframe flings the fuel to the back of the tank, and (usually) gets to the venturi before the engine cuts out.  Then the process repeats.  The result is that classic "brrrrt ... brrrrt ... brrrrt" run at the end of a flight.

With a pre-made tank that doesn't have the sloped back wall, kick the whole tank out so that the back is about 1/4" more out than the front.  If you're building a tank, taper the whole tank.  It makes a nice puzzle for cutting out the sheet metal before you bend it.  If you look at any of my profile planes you'll see a plastic clunk tank with the rear of he tank kicked outward.

To get a few "warning laps" of lean running before the engine cuts, locate the uniflow vent a bit inward of the pickup tube.  When it uncovers, the engine will start going lean.  I can't tell you exactly how much to do this, because it's not an effect that I seek.

I have never managed to get the cutoff loop thing to work, so I'm not even going to try to tell you how to do that.

[Tim Wescott]

Here's a picture of a typical tapered tank, from a full-bodied stunter.

[Steve Helmick]

     

      Steve

          I have a number of 'square wedge' tanks (Perfect?) with uniflow plumbing that are drop in replacements for the homemade Barnstormer tank in the photo. I'll try those first for comparasons sake when the flying season returns. Al Rabe described an angled tank he designed for a shut off on command with a loop at the end of the pattern. I wasn't quite sure where the angle was and thought maybe the angled rear wall of my tank was what he described. Was it instead the angle he created by the outside of the tank being wider at the rear than the front?

     Ara

I don't believe "Perfect" made any "square wedge" tanks, so probably Veco, but then I think somebody else took over their line of tanks. But no matter. Old tanks are always suspect, what with corroded brass tubing, internal rust from soldering flux, etc. I'd pop the rear off the tank, and replace the brass tubes with copper.

From what I can tell of Tim's tank photo, it looks like there's no wedge, just taper. Taper and wedge is good. You can make the tank thicker and offset the wedge and pickup/uniflow tubes. Make patterns from paper before getting out the tin snips! Calculating the capacity can be done, but it's not very easy...  Steve

[Tim Wescott]

From what I can tell of Tim's tank photo, it looks like there's no wedge, just taper. Taper and wedge is good. You can make the tank thicker and offset the wedge and pickup/uniflow tubes. Make patterns from paper before getting out the tin snips! Calculating the capacity can be done, but it's not very easy...  Steve

It was an experiment.  It's a clunk tank.  Works great, with a clean positive no-warning cutoff.

[Steve Helmick]

It was an experiment.  It's a clunk tank.  Works great, with a clean positive no-warning cutoff.

The "no warning" part will eventually reach out and gitcha, sometime when you don't have quite enough fuel for that cold day with high barometric pressure. IMHO.  Steve