Lean inverted run

I'm sure this has been addressed many times in the past on this forum but I cant seem to find the answer through the search engine. On my Twister, I am running a standard vented tank (Acme U3) with muffler pressure through a "T" to both top and bottom vents. The tank has the feed line centered with the venturi on the OS 35 FPS. On the ground the engine will be running 2-stroke but after launching will run rich for several laps, with the corresponding lack of power. If I flip it over to inverted the engine immediately runs lean with good power and line tension. I also notice that I have leaner runs and more power on the outside loops as compared to the inside loops.
Would changing tank height help in this matter or is there some other "fix". It would be just fantastic is the engine could run the entire flight the way it runs inverted or on the outside loops. Thanks, Brian

Sounds like you have the answer-raise the tank an 1/8" or so and see if it improves. Very possible it will need to be up 3/16" or so. Failing that, try a true uniflow tank. I've had nothing but good luck even with my poorly hammered together uniflows. It will likely still need to be above the centerline a little.

From your description of the symptoms, I'd advise you to lower the tank (with the plane upright) a bit at a time until you get equal running characteristics and lap times upright and inverted.

Brian --

>I'm sure this has been addressed
>many times in the past
>on this forum but I
>cant seem to find the
>answer through the search engine.

Yes, it has been addressed. The smart guys reccomend starting out with the tank 1/8 to 3/16 above center, and making the mount adjustable.

>On my Twister, I am
>running a standard vented tank
>(Acme U3) with muffler pressure
>through a "T" to both
>top and bottom vents. The
>tank has the feed line
>centered with the venturi on
>the OS 35 FPS. . .
>If I
>flip it over to inverted
>the engine immediately runs lean
>with good power and line
>tension . . .

Us old guys just glued 'em and flew 'em. Thanks to this forum, I learned trimming is quite an art, and one of which I was unaware. Your problem doesn't quite make sense, as a tank on the CL should run a little leaner upright. At least that is what the folks here say, and matches my experience.
Kelvin had it upside down (I think), and Sourdough had it right (I think). Following Sourdough's advice, the tank will be below the CL of the needle valve after you make the change, which seems odd. Never heard of anyone having to make an adjustment that severe. However the data points to having the tank too high, so lower it.
On a separate note, reminds me that trimming in general, I learned from this forum. I now make everything adjustable -- leadouts, rudder, tank location, and wingtip weight box. I am not sure how to thank all those smart guys. These are things we never thought of in the old days.

>I'm sure this has been addressed many times in the past
>on this forum but I can't seem to find the answer through
>the search engine. On my Twister, I am running a standard
>vented tank (Acme U3) with muffler pressure through a "T" to
>both top and bottom vents. The tank has the feed line
>centered with the venturi on the OS 35 FPS. On the ground
>the engine will be running 2-stroke but after launching will
>run rich for several laps, with the corresponding lack of
>power.

What you haven't said is what happens when you hold the plane in an inverted orientation on the ground? That's an easy way to start finding the appropriate tank relationship to the engine.

Incidentally, I have never known anyone in Texas using the Acme tanks, so if they have unique properties, those will be totally unknown to me in both fact and in rumor. In the earliest days of building and flying model planes, lining up the engines and tanks on each others' centerlines seemed to work just fine.

As engine and tank technologies changed, folks began to find that
typical model engines were RICHER when inverted than when upright, which required an average of 1/8" of offset, upward, for Uniflow tanks on most slow combat, sport, and stunt models.

>If I flip it over to inverted the engine immediately runs
>lean (K: "lean-er") with good power and line tension.

That is backwards from the usual symptoms.

>I also notice that I have leaner runs and more power on
>the outside loops as compared to the inside loops.

That sounds in agreement with the first symptom.

>Would changing tank height help in this matter or is there
>some other "fix".

If the Acme tanks have some unusual internal plumbing, maybe the situation can be dealt with by LOWERING the tank in relation to the engine. Start with a small amount and retest. But start by testing ON THE GROUND.

>It would be just fantastic is the engine could run the entire
>flight the way it runs inverted or on the outside loops.

One of the very first goals in trimming aerobatic type planes is to equalize their upright and inverted speeds. In some cases, it takes shims under the engines (for an inverted engine).

The K.

I agree with the others that it's probably the tank height. However, your use of a T piece to apply muffler pressure to both vents of the tank. I've never come across this before, though all that really means is that there is yet another hole in the lacework of my experience. However, it's not unreasonable to suggest that the problem may lie here - even if it's only an air leak in the T piece.

So, two questions, particularly if your tank is going to be hard to shift.

First: why do you use the T piece?

Second: do the symptoms change when you use the more usual "block the overflow and put pressure on the uniflo vent"?

The reason for the "T" is that the ACME is not a Uniflow tank, it is a standard vented design. I was running pressure to only the overflow and capping the fill line but that caused the engine to lose power on the outside loops. Jim Thomerson reccommended using the T setup and it has solved the outside power loss, but now I have the inverted lean-er runs. I found that running pressure has helped with the runaway problem somewhat, such that if I dont use pressure the engine runs far too lean toward the end of the tank.
Other info inquired about- I am running a 11x5 Zinger with 15% Champion fuel. I like the performance of the 11x5 much better than the recommended 10x6 for the 35 FPS.
On a side note- does everybody use uniflow tank setups routinely on stunt ships these days? I get most of my kits through TCI and just told Andy that I needed a tank to go with the Twister and the Acme standard vent is what he sent. This is the same tank we used 25 years ago, I am sure tank technology has come far since then. I am thinking that maybe I should just replace the tank with a Uniflow design. Nils has a plastic RC conversion to uniflow that he said works well, maybe I will try that. Heck the Acme tanks are not cheap anyway, about $10 compared to about $4 for the plastic tanks. Any other suggestions on tanks? Does anyone out there use the Acme U standard vent tanks for stunt or am I an anachronism?

It appears that some engines require different tank/engine align- ments. Ted Fancher did some theorizing on why raising the NVA on an extended venturi did not produce the expected/desired results.
I don't recall what the name is of your semi-uniflow plumbing, but I would try disconnecting muffler pressure, and also try deleting the whole vent/overflow plumbing system before I started moving the tank. A fair number of older stunt fliers claim that uniflow plumbing is what makes the tank height adjustments required. If you leave the plumbing as-is (be sure there are no leaks), then you need to lower the tank relative to the engine, plane upright. What prop pitch and fuel are you using. That will make some difference in getting the steady runs you are looking for. Steve

A fiend of mine, getting back into it, uniflowed a tank just like I told him to. Didn't run well at all. He ended up with the T fitting as discussed here. Worked for him. I'm vague about the Ted Fancher words on NVA position. I have found it to be very important on upright/inverted engines. The uniflow ends up more or less in line with the NVA when you get it working right. I emphasize "more or less", as it may need to be off a little one way or the other.

The only Acme tank I have run was uniflowed, of course. The problem with any commercial metal tank is that you really don't know what's inside unless you pop the back off and take a look. I would never run a commercial metal tank without opening it up first.

I've not tried it myself, but it seems that going with a plastic clunk tank is a fine idea. Many of the top stunt flyers use them with great success, uniflowed or not.

Hi all!

Here is a link to the Busholator page that explains all these T-pieces - an excellent source of which is aquarium shops!

Tony

I've heard they work very well!

http://www.aeromaniacs.com/busholator.htm

I learned to make those plastic uniflows from talking to Bob Hunt on the phone & then watching his video on the subject. The way I do it is slightly simplified copy of how Bob shows them on his videos. Bob & Billy Werwage both use plastic uniflow tanks on PA airplanes. They've always worked well for me with one exception. The only thing I've found that you have to look out for is the stopper leaking if you don't use a remote uniflow & vent. If you don't run some fuel line to remote uniflow & vent tubes the stopper can start to leak from being wiggled by the constant fueling, capping & uncapping of the vent. I had that happen once after about 500 flights on my Oriental. Come to think of it, it was just about a year ago, just before last years COLD contest. I don't bother with remote plumbing on my Ukeys with the slant front tanks & I haven't had any trouble with the engine runs on them. but, contrary to what I've read on these boards sometimes, I've found Fox .35's to be pretty forgiving compared to higher output motors.
Nils

Hi Nils!

I have a plastic clunk uniflow about ready to go made from your instructions - I'm still waiting to be in the right place at the right to to get flying!

I also have a standard vent tank (that I'll try the busholator mentioned above on).

And a couple of metal profile uniflow tanks.

Your description accurately describes a normal plumbed tank. When you take off, you will build up an extra pressure head due the "centrifugal" force on the inner surface of the fuel. This will richen the run. Depending on the depth of the tank, this can be a big effect. Note that in level flight, the fuel surface will almost be vertical, and that after a moment or two, both vents will be uncovered, so I don't see a lot of need for the "Tee", but if it works for you, I'd leave it.

As the fuel is used up, the level moves further outward, reducing the pressure head, and leaning out the engine. This is different than uniflows, where the pressure stays constant.

I agree with everyone else that you should play with the tank level, (and readjust the needle to get the right mixture) to give equal upright and inverted flight times. One thing to consider is that the actual level of the pickup inside the tank may not exactly the same as what you can see outside. This might explain why you seem to need to lower the tank, instead of the "normal" raising the tank.

One
>thing to consider is that
>the actual level of the
>pickup inside the tank may
>not exactly the same as
>what you can see outside.
>This might explain why you
>seem to need to lower
>the tank, instead of the
>"normal" raising the tank.
>
>Alan

I dont think the level of the pickup, inside or out, makes any difference in the system. It is the level of the fuel surface relative to the spraybar outlet (venturi center) that determines the fuel pressure (head).

In a uniflow setup, it is the internal location of the uniflow vent-end relative to the venturi center.

Also, I agree that the head pressure will increase as the plane takes off--and this would tend to make the setting go richer than stationary setting. However, depending upon the engine and prop combo, the un-loading effect on the prop can make the engine run quite a bit FASTER after takeoff--making you think it is getting leaner.

This is why it is always so important to adjust the needle based on its in-flight condition.

With highly muffled engines, I have also found a tach to be an excellent aid to keeping track of changes.

If all else fails, I would suggest you get a plastic rectangular sullivan or pylon tank and Uniflow it. It really is very bullet-proof--and it will give you a nice consistent run from full to empty.

FWIW from a recent retread (200 flights)

Curt,
Now that I think about your point, I agree with you. What is interesting to me is that it appears that any sensitivity to the centerline of the tank must be coming from the fact that the fuel level while flying isn't exactly vertical, but about 10 degrees off --as I recall (gravity vs "centrifugal" force). Assuming a symmetric tank, you want it to be centered on the spraybar---I am ignoring the "real world" 1/8 inch positive offset---to get equal inverted and upright times.

I guess it is a little strange that if he is running muffler pressure, that the engine can still be sensitive to these effects.

>Curt,
>Now that I think about your
>point, I agree with
>you. What is interesting to
>me is that it appears
>that any sensitivity to the
>centerline of the tank must
>be coming from the fact
>that the fuel level while
>flying isn't exactly vertical, but
>about 10 degrees off --as
>I recall (gravity vs "centrifugal"
>force). Assuming a symmetric tank,
>you want it to be
>centered on the spraybar---I am
>ignoring the "real world" 1/8
>inch positive offset---to get equal
>inverted and upright times.

YEs--but I think that at our speeds, the angle is a little greater--3 g lateral and 1 g vertical so the arctan is about 19degrees. (off vertical)

>
>I guess it is a little
>strange that if he is
>running muffler pressure, that the
>engine can still be sensitive
>to these effects.
>
>Alan

WEll...muffler pressure will yield higher overall fuel pressures but it is the CHANGES in the pressure that mess with our runs. Muffler pressure or not, the head will change from full to empty by the same amount (the difference in fuel level measured on the resultant angle) Remember that the actual fuel feed pressure will be the SUM of the head pressure and the muffler pressure.
Reducing the head-pressure change is what Uniflow is all about. Since the uniflow vent will be vrey close to the pickup, the maximum change in head from full to empty will be on the order of a few 1/16" of an inch (instead of an inch or two).

Uniflow offers an effective steadying effect running with or without muffler pressure. Try it both ways. In some setups, the change in muffler pressure will also help stabilize the run. ie rpms drop, pressure drops, fuel feed pressure drops, mix goes leaner, rpms go back up. It is a stabilizing system. Sounds good on paper eh?. But many folks just vent the uniflow to the atmoshpere.

I spent ALOT of time searching all articles on Uniflow and fuel feeding last year, and am finally seeing consistent flight results--with predictible corrections for problems.

Hope this helps. I know many of the real experts are busy this week.

LAST EDITED ON Jul-09-01 AT 02:41 PM (EST)

whoops, too impatient!