Fuel tank design

[Matthew Brown]

I’ve been actively flying the past year and a half and engine runs have been consistently poor. I know some of it has been me as virtually everything else I’ve flown in the last 20 years has been RC and tune for max rpm at wide open. So tuning for well under max rpm is a feel I’m learning gradually. However, most of my issues aren’t tuning. They are consistency throughout the flight. Frequently so rich at launch the plane is barely able to get airborne and by end of flight so lean the engine is getting way too hot.
I have tried quite a number of tanks on my various planes, from regular vented tanks to uniflow, with and without muffler pressure.
Currently all my planes are profiles. I had an Oriental ARF that I went through three tanks before finding one that ran decent.
So, what makes a tank work good? Does shape have a lot of bearing on it? The three tanks on my Oriental were all standard stunt tanks. Two I got from Brodak and one from RSM. They all looked the same externally and all were uniflow. The only plane I have that runs decent has one of my old GRS chicken hopper slow combat tanks on it. Even it loses 1/4-1/2 second/lap from full to empty.
I flew my new Ringmaster ARF yesterday afternoon and while it flew really well for a Ringmaster the tank sucked. I had used an old header tank from one of my old helis as it was all I had that would fit. It needs about 2.5-3oz tank and I don’t know what to get. I’m tempted to solder up one myself.
Ideas?

Thanks, Matt

[Perry Rose]

How about a picture of the tank, engine, prop installation?

[Matthew Brown]

How about a picture of the tank, engine, prop installation?

Possibly later this evening. Working now. No useable pics on my phone.

Matt

[Paul Smith]

In general, profiles are bad because the fuel mass is too far outboard.  Full body works better because the tank is behind the engine.

So the fix is to cut a hole in the body & get the tank onto the centerline.  Or at least, get the fuel as far inboard as your design allows.

[frank williams]

Matt ..... make sure you run a filter between tank and engine.  I would suggest that you take the offending tanks off and fill them about 1/2 full of acetone, shake vigorously, and then pour out the contents slowly onto a white paper towel.  I think you will probably find that there are little flakes of rust coming out of the tanks.  Flush tank with fuel afterwards to get acetone out.  Also while ou have the tank off pressure test it for leaks in the sink under water.

[Matthew Brown]

Matt ..... make sure you run a filter between tank and engine.  I would suggest that you take the offending tanks off and fill them about 1/2 full of acetone, shake vigorously, and then pour out the contents slowly onto a white paper towel.  I think you will probably find that there are little flakes of rust coming out of the tanks.  Flush tank with fuel afterwards to get acetone out.  Also while ou have the tank off pressure test it for leaks in the sink under water.

That is basically what I do with any tank before mounting although I’ve never used acetone. Always used rubbing alcohol. Probably worth it to go buy a can of acetone though.
Always a filter.
Recessing the tank to the inboard doubler may be worth a shot as Paul mentioned. Especially if I solder up a tank myself. I can put fill and vent lines in ideal positions that way plus I can make it fit perfectly between the hardwood mounting rails.

Matt

[Steve Scott]

I had a new 2.5oz profile wedge uni tank mounted on a Ringmaster and couldn't get it to run.  Inline filter but when I disassembled the filter, it was full of rust particles.  Flushed the tank repeatedly and got out more quantities of rust.  When I got home I unsoldered the rear plate and the entire tank was rusty.  This was a Taffinder tank.

The plastic RC klunk tanks are very difficult to fit on a profile.

[Garf]

I turn old style steel free flight rectanks into clunk tanks. They work well. I fitted one 5 1/4 oz rectank into the Noblarf and it worked beautifully. I did use muffler pressure on it.

Refer to this link for more details on the steel clunk rectank. I'll look for more pics.

http://hangarflying.proboards.com/thread/2225/fuel-tanks

[Tim Wescott]

In general, profiles are bad because the fuel mass is too far outboard.  Full body works better because the tank is behind the engine.

So the fix is to cut a hole in the body & get the tank onto the centerline.  Or at least, get the fuel as far inboard as your design allows.

The plastic RC klunk tanks are very difficult to fit on a profile.

This is very easy, if you don't mind staring at a fuel tank as you fly.  Picture comes from here.

[Matthew Brown]

I’ve wondered what kind of issues there would be with a tank mounted on the inboard side. Seems like the centripetal force would make it go rich.

Matt

[Bob Heywood]

Easy fix. Just put the engine on the inboard side and all of your problems will go away...

[Dane Martin]

I’ve wondered what kind of issues there would be with a tank mounted on the inboard side. Seems like the centripetal force would make it go rich.

Matt

I run all of my profile planes with an inboard tank. I don't have any wierd issues

[Tim Wescott]

I fly Expert, but I've only been at this seriously for seven years, so I don't have the depth of knowledge of a lot of folks.  Here's what I know:

• You want well-regulated engine speed. To get this, you want constant fuel pressure, or even fuel pressure that helps you along.
• Muffler pressure can help, particularly if you don't have a uniflow tank.  For profiles, a plastic RC clunk tank and muffler pressure, no uniflow, often works Really Well
• Muffler pressure can really screw things up, particularly with a uniflow tank.  Isn't that nice to know, given what I just said about how it can be good?  The answer, if you're experimenting with combinations, is to try all four: uniflow & no uniflow, pressure & no pressure.  Keep going until something works
• Leaks will absolutely kill you.  A pinhole leak in a tank with muffler pressure won't leak appreciably when it's covered by fuel, but will leak like mad when uncovered.  This will lead to a run that's fine up to some point, and then horrid.  A pinhole leak along the inside edge of a uniflow tank will totally destroy the uniflow effect.  Pressure check your tanks, and double-check your tubing and connections

For a profile with a modern engine like an OS LA series or a Magnum XLS, probably the easiest and surest tank to use is an RC clunk tank on the inside (the side you see when you're flying), with muffler pressure and no uniflow.  There's a lot of other arrangements that all work great -- that's just the one that I like best right now.

As a final note, just to make you feel cheery and self-confident: it may not be the tank.  If you have a too-large venturi, or a too-large (diameter or pitch) prop, then you could have the world's best tank set up and the engine could still run away.  This has been treated over and over again, with the most recent iterations here and here.

[Brett Buck]

In general, profiles are bad because the fuel mass is too far outboard.  Full body works better because the tank is behind the engine.

So the fix is to cut a hole in the body & get the tank onto the centerline.  Or at least, get the fuel as far inboard as your design allows.

   The problem can certainly be that the tank is too wide, which result in having to pull the fuel too far "uphill" from the pickup. With a suction tank, it starts out OK and then slowly gets lower, with a uniflow tank, it stays at low pressure (same low pressure as at the end of the tank on a suction tank) for the whole run.

   The solution is either mount the tank inboard or make it narrower. For instance, a typical 2" side tank mounted flat on the outboard side of a profile is marginally too wide for a 20/25FP. 1.5" wide is fine. Looking at the same problem for a full-fuse airplane, the tank is generally centered so you only end up with an inch or so instead of 2" for a standard tank on a profile.  Adding muffler pressure improves the situation, but doesn't always solve it.

    You can sometimes recess the tank as shown and that will solve the fuel pressure issue, however, this cuts away so much of the front end that the vibration can sometimes be an issue, which just creates another problem. Even if you just cut away the doubler on the outboard and leave it on the inboard, we have seen issues (like with the original Doctor) with vibration.

    It boils down to the fact that a profile mount is easy, but not very good for engine runs, and this is one of the reasons.

    Brett

[Tim Wescott]

I’ve wondered what kind of issues there would be with a tank mounted on the inboard side. Seems like the centripetal force would make it go rich.

You compensate for that by what you adjust it to on the ground.  You need to do that for any plane anyway.  Just find the RPM that results in a sweet engine run in the air. 

One nice thing about it is that the centripetal acceleration works for you -- instead of making the engine run leaner the faster it goes, it makes the airplane run richer the faster it goes (and visa-versa).  So it adds to the speed regulation of the system, instead of helping the engine run away.  It's not a fix-all, but I've been very happy with my profiles with inboard tanks.

As an added bonus, there's always room for the tank -- there's a lot of short-nosed profiles out there that make tank mounting hard; with an inboard tank it's no worry.

[Matthew Brown]

This particular Ringmaster has an OS 25 FP on it with stock CL venturi. Stock NVA as well. I had a couple new 2oz plastic clunk tanks but they did not fit well between engine and LE. One will easily fit on inboard side. I’ll make that my first attempt rather than cutting into the nose.
Is it possible the stock venturi is too big? I was also running an APC 9-6 which I have read is a little too much. I plan to order a couple Xoar 9x5 but Tower is out of stock today.
If the venturi is too big, it would be a simple matter to make a new one if I know what size to make it.

Matt

[Tim Wescott]

This particular Ringmaster has an OS 25 FP on it with stock CL venturi. Stock NVA as well. I had a couple new 2oz plastic clunk tanks but they did not fit well between engine and LE. One will easily fit on inboard side. I’ll make that my first attempt rather than cutting into the nose.
Is it possible the stock venturi is too big? I was also running an APC 9-6 which I have read is a little too much. I plan to order a couple Xoar 9x5 but Tower is out of stock today.
If the venturi is too big, it would be a simple matter to make a new one if I know what size to make it.

Matt

Two ounces is a bit small for that engine in that plane, at least if you want to fly the whole AMA pattern -- 3 ounces is about right, if you can find one.  The old Hayes 3 ounce tank was perfect (and with work, fit behind the engine), but they're not made any more.  An import 90cc tank, on the inboard side, may work quite well.

I would get a couple of APC 9x4 and APC 10x4 and try that.  If the Xoar pitch number is accurate, a 5" pitch is too much -- the FP engines like to spin up fast; if you try to slow them down by running them rich they fight back by running away in the air.  There's about a bazillion threads about using 20FP and 25LA engines on that sized airplane.  You may want to do some looking around to see what you can find.

Someone else is going to have to say what size venturi to use -- I can never remember.

[Matthew Brown]

I really miss Hayes tanks! They were the best for RC use. Never had an issue with one other than the interior clunk line rotting. I still have 4 or 5 NIB but all are too big for this.
Matt

[Brett Buck]

This particular Ringmaster has an OS 25 FP on it with stock CL venturi. Stock NVA as well. I had a couple new 2oz plastic clunk tanks but they did not fit well between engine and LE. One will easily fit on inboard side. I’ll make that my first attempt rather than cutting into the nose.
Is it possible the stock venturi is too big? I was also running an APC 9-6 which I have read is a little too much. I plan to order a couple Xoar 9x5 but Tower is out of stock today.
If the venturi is too big, it would be a simple matter to make a new one if I know what size to make it.

   It's not too big if you use the correct prop, which is an APC 9-4 or 10-4. The 9-6 is not "too much" prop, it's not enough prop and requires you to run at a much lower HP level, which greatly reduces the fuel suction - which makes the fuel draw poor just like the venturi being too big. It only has more load on the ground, in-flight it requires a much lower power level than a 9-4.

   There is no mystery to the 20/25FP setup, the baseline system is as well documented as anything in stunt. I do suggest a narrower tank, and 2 ounces is not enough for a full stunt pattern (although its plenty for OTS).

    Brett

[Matthew Brown]

Brett, when you say narrower, do you mean in top view?
Can’t get much narrower than the tank I’m using. It’s only about an inch wide, 1 1/2” tall and about 2 1/4” long.


Matt

[Brett Buck]

Brett, when you say narrower, do you mean in top view?
Can’t get much narrower than the tank I’m using. It’s only about an inch wide, 1 1/2” tall and about 2 1/4” long.

    That seems fine to me. But if you want to try to run a 9-6 (which it will do, no problem with running, big problem with flying), you will need a much smaller venturi, because you have to run it at much lower RPM, which also reduces the fuel draw dramatically. I would suggest instead that you use an APC 9-4. Set the engine by peaking it out lean in a 2-stroke, then backing off about 5-6 clicks until you get a *distinct* drop in RPM. It will probably be in the area of 11,500 rpm, but don't try to chase that setting, just peak it out and then back off 5-6 clicks. Fly it. You probably want something like 4.2-4.3 second laps assuming .015x60' lines. If it is too fast, or it seems to "sag" lean in maneuvers, particularly at the end of the flight, back it off by two clicks (you can observe the RPM but don't do anything based on the tach reading, just back off the needle and see what happens). If it ever drops into a 4-stroke, or it is too slow (more than 4.6 seconds/lap or so), then turn it in two clicks leaner, try again.

    The likely issue is that it is too fast, rather than too slow. That's because the 20FP is too large/powerful for the airplane. There are several possible solutions to that, but the easiest is to put a few layers of panty hose nylon over the venturi inlet. Cut it into about 1.5x1.5 squares, and hold it on with a tight-fitting o-ring over the outside of the venturi. Then repeat, the same things should happen, but at a few hundred less RPM.

   Note that this all presumes the engine is *completely stock* with all the original parts. I have flown and set up dozens of these engines and the setup and operation is remarkably consistent from engine to engine, if you follow the plan, it *will* work, or the problem is something outside the engine. If the engine is modified in any way, all bets are off, and my suggestion is to put it back to stock  and try again, if possible.

   Running a 9-6 loses a tremendous amount of performance even if you get it to work. This will make the flying much more difficult, and will expose the limitations of the airplane to a great degree. There's no reason to give up that kind of performance that I can see.

     Brett

[Dane Martin]

I’ve wondered what kind of issues there would be with a tank mounted on the inboard side. Seems like the centripetal force would make it go rich.

Matt

I've heard people say this before, and I've wondered about it. With a clunk tank, in the outboard position; isn't the fuel going to be forced against the outboard wall of the tank (and back of course) along with the clunk itself? Then the fuel will be drawn into the clunk line.
Now move it inboard, and it would all be pushing against the same wall of the tank, in the same fashion?

[Fredvon4]

Dane took me a while to understand until it occurred to me with tank on the out board side, the fuel must flow IN towards the venturi fighting the forces
With tank inboard, the fuel flows OUT to the Venturi not fighting to get there

[Dane Martin]

I guess I do them different? The intake is outboard of my fuel feed line (clunk line) even when I set them up as an outboard tank. So the fuel inlet of the engine itself has always been farther out than my fuel line. So in my situation, the fuel feed is always being helped by centripetal force.

[Dane Martin]

Here's another outboard and one inboard