inboard tank, outboard engine

[Bootlegger]

 I have tried this set up on a Ring Master, and cannot get a steady engine run.
 The engine is an OS LA 25, and the tank is a 4 ounce clunk tank.

 It will run well when I short tank it, but with 3.5 ounces it runs rich.

  What are some suggestions???

[Massimo Rimoldi]

Good morning,
A uniflow setup can help.
In any case it may be necessary to adjust the engine a little leaner on the ground and expect that the carburetion is enriched in flight.

I hope this helps.

Regards, Massimo.

[Lauri Malila]

The faster you fly, richer the engine will go. Maybe your tank is too much "in"? I have settled to end of pick-up tube 5-8mm (1/4" +/-1/16") outside the point where fuel sprays out inside venturi. You must find a balance between power in overhead maneuvres and breaking exfect in strong wind. L

[Massimo Rimoldi]

.... Maybe your tank is too much "in"? ...

Hello Lauri.
If I have not misunderstood , the tank is very "in".
I think that the situation is this:  http://stunthanger.com/smf/open-forum/inboard-tank's/msg145400/#msg145400

Regards, Massimo.

[John Rist]

I have tried this set up on a Ring Master, and cannot get a steady engine run.
 The engine is an OS LA 25, and the tank is a 4 ounce clunk tank.

 It will run well when I short tank it, but with 3.5 ounces it runs rich.

  What are some suggestions???

If you are running a standard OS muffler I have had good luck with OS Max muffler presser.  Also make sure the clunk line is very flexible. 

[Brett Buck]

I have tried this set up on a Ring Master, and cannot get a steady engine run.
 The engine is an OS LA 25, and the tank is a 4 ounce clunk tank.

 It will run well when I short tank it, but with 3.5 ounces it runs rich.

  What are some suggestions???

   Use uniflow.

      Brett

[Dennis Toth]

The question is why are you doing this? If you are trying to get a consistent engine run for stunt like Brett said use uniflow tank of normal dimensions. If you are looking to use it in as a boost for fuel flow in Carrier or racing you might look in that section as they have used this arrangement.

Best,    DennisT

[Lauri Malila]

Of course Uniflow is allways good, but in this case the problem is too high fuel head pressure difference between start and flight. You must needle the engine very lean before take-off and then it richens in flight as the centrifugal force comes in play. It is difficult to get a constant and repeatable setting when the starting point is lean. L

[Brett Buck]

Of course Uniflow is allways good, but in this case the problem is too high fuel head pressure difference between start and flight. You must needle the engine very lean before take-off and then it richens in flight as the centrifugal force comes in play. It is difficult to get a constant and repeatable setting when the starting point is lean. L

    With uniflow that will not be an issue, of course. On suction, yes, the head difference from beginning to end will be large, and perhaps excessive. An outboard tank is close to centered so the overall fuel pressure averages out to about atmospheric pressure. With an inboard tank, it's higher than atmospheric at the end and much higher at the beginning.

     It's clear in this case that the engine is OK with the end of run pressure, so with uniflow, the pressure will be the same as the end-of-flight pressure from a suction tank. Alternately, use a narrower inboard tank on suction or on suction with muffler pressure. With the latter, the needle will get touchy at some point, because muffler pressure+beginning of flight head pressure, will cause the absolute pressure to be much higher than normal, meaning the needle will have to be adjusted to a very small orfiice, so very little movement will be required to change the cross-section of the metering a lot, percentage-wise.
   
    Brett

[Pat Chewning]

On my Ringmaster with FP 25 (same size as LA 25), I fit a 3 oz wedge tank mounted on the same side as the engine.   I don't think you need a 4 oz tank.

I suggest a smaller tank, mounted on the same side as the engine, and maybe even move the engine forward in the mounting beams to allow this.

[Brett Buck]

On my Ringmaster with FP 25 (same size as LA 25), I fit a 3 oz wedge tank mounted on the same side as the engine.   I don't think you need a 4 oz tank.

I suggest a smaller tank, mounted on the same side as the engine, and maybe even move the engine forward in the mounting beams to allow this.

  It is very important to make the tank narrow. A standard 2" wide tank, mounted flat against the fuselage, is a bit too wide for reliable fuel suction with 10-4's (which would otherwise be easy for the engine to spin). An 1 3/4 width seems OK.   I use a Sullivan SS-4 clunk tank with the wide face (the "bottom") up against the fuselage, and that always works. IF you put it on edge, then it won't work as reliably.

    A narrow inboard tank is ideal and will permit the use of a very wide variety of props due to the higher overall fuel delivery pressure, with minimal pressure change. That's why Dirty Dan suggested it.

     Brett

[dirty dan]

That's Brett, always trying to get the attribution correct.

When first fiddling with inboard tanks on profiles we were flying a relatively worthless event, Slow Combat. There wasn't a good place to put any normal tanks, mainly as that pushed the engine way forward, making the model nose heavy. Even at the time I had a fair bit of experience with RC models, one night realizing that an SS-4 Sullivan nylon tank looked to be a perfect fit on the inboard (left) side of a profile fuselage. That also allowed the motor to be pushed way back, which was all I was really trying to do.

Mr. Rush got his panties in a wad over the whole deal, rules were changed, yadda-yadda, yadda-yadda...

Anyway, it worked and once getting involved in Stunt and yet more profile models, the inboard tank/outboard engine scheme seemed logical and has proven successful. Yep, use uni-flo. Use muffler pressure. Shim tank up or down to get upright/inverted speeds correct. Route all tubing in a commonsense manner. Be prepared to launch at higher revs.

Dirty Dan

[Russell Shaffer]

Dan, glad to see you still alive and (hopefully) well.