uniflow tank

It works fine, but has one flaw. The fuel in the tank develops a "pressure head" from centrifugal force, which tends to force the fuel up the line to the engine due to the weight of its being thrown to the outside of the circle. (This is usually about 3 g's, which means if you have a 2 inch wide tank, you have the equivilent of about a 6 inch high head of fuel.) As the fuel is consumed, this "pressure head" decreases, which means there is less pressure forcing the fuel to the engine. Thus the engine begins to lean out more and more towards the end of the flight. In some cases it will go dead lean and just take off screaming. Now, enter the uniflow system.

In a uniflow tank there is only one vent tube feeding the tank which is located internally towards the outside of the tank and towards the back. When set up properly, this fuel vent is located about 1/4 to 3/8 of an inch ahead of the pick up tube, and right next to it. An overflow tube may be added for fueling, but this tube is capped off before flight. Thus the only vent tube is the uniflow tube which is covered by fuel at all times until, perhaps, 2 laps or so before the end. This has an added advantage because it gives a burp or other sign letting you know you are about out of fuel. Otherwise, during the flight, with fuel in the tank, it takes more pressure to force the air through the fuel head in order for the tank to vent. This causes a slight vacuum in the inside of the tank, which counters, to some extent, the added pressure of the "pressure head." The greater the "pressure head", the greater the vacuum. As the fuel is consumed, there is less pressure required to vent air into the tank, hence less vacuum inside the tank, and, in a properly set up system, the pressure of the fuel will keep flowing uniformly throughout the flight. thus the name "uniflow".

If you want to do an experiment to see how this works, take a 2 liter pop bottle (empty), and punch a SMALL hole in the side of the bottle at the bottom. Fill the bottle with water while you hold your finger over the hole, and leave it uncapped. This bottle length will simulate a "normal" 2 inch wide vented tank with about 3 g's of pressure (the equivilent of 6 inches total height). Now release your finger allowing the water to squirt out and run into the sink. With the top open, there is no vacuum here--just steady air pressure and decreasing "fuel" pressure. You will notice that at first you will have a long, steady stream, but as the bottle empties, the stream will grow shorter and shorter until it is barely dribbling out.

Now drill or punch a hole in the cap so you can get a tight fit for regular fuel tubing to pass through, but not let air in around it. Push or pull a tube through this cap that is long enough to reach all the way to the bottom of the bottle when the cap is tightened on the bottle. Fill the bottle again as you hold your finger over the hole in the side, and then insert the tubing into the bottle and tighten the cap. At this point the tubing should reach all the way to the bottom of the bottle, and there should be no way for air to enter the bottle except through this tubing. Now release your finger once again and watch that stream of water. It won't flow nearly as hard at first, but the stream will remain relatively constant throughout the entire emptying of the bottle.

On a uniflow set up you have to open the needle a bit more because of the lower flow of fuel, but that's what needles are for. And, oh, yes, on this bottle experiment, if you blow into the tube, you will notice that there is an increased flow of fuel, but as long as you keep that pressure constant, the fuel flow will still remain constant. And here is where muffler pressure begins to enter the picture. If the engine leans out going up hill or whatever, there is more muffler pressure developed. This causes more pressure to the fuel, richening the mixture, and slowing the engine down. If the engine richens up, the muffler pressure goes down. Less pressure in the tank means less fuel going to the engine, and it leans out. So we have two factors working together to get that uniflow. The tank itself, and the positive effect of muffler pressure. On our "normal" engines we usually use both.

Try it. Experiment with and without muffler pressure. You will like it.

And the two older posts in this thread are at least a half day day off ..

We will get it right one of these days.

On the Armor-All question, I see a lot of people using far too much, thinking that "if a little is good, then more is better" (not!). Shake your jug of fuel. If it foams like a glass of Budweiser, add one DROP (not a "squirt". I truly mean one drop). Then shake the jug again. If it still foams, add another drop. Usually two or three drops is all that is needed for any brew of fuel. Above all, don't add too much. We don't need to add any unnecessary contaminants to our fuel.

Carlton:
Others who are much more experienced than I am will probably give you more and better advice than I can.
My advice, however, is that you change your fuel ASAP. Since the OS 40 FP is a bushed, not a BB, engine, you need significantly more oil to give it proper lubrication. My guess is that you should have at least 22% oil (with at least 50% of it castor). Others might differ with me, but 25% all castor would be even better. This will reduce the operating temperature and significantly increase the life of your engine. The reduced operating temperature might also help reduce the leaning out that you are experiencing in flight.
Good luck!
Peter Hess

>... I am using a OS Max FP .40 . It starts right
>up and pulls good The fuel is Powermaster 15%
>Nitro-18%Synthethic/Caster.

That's not enough oil! You want the Powermaster 5/22 or 10/22 for that engine in a C-L application!

>plug was a new one given by a fellow flyer at the
>field-it has a idle bar and is black-don't know
>name but he will tell mw because he took it out of
>a blister pack.

In other words, you really don't know if it was a hot or a cold plug. For high speed running, cold is the right plug. For either a steady slightly-rich two-cycle that is right for an FP, or the breaking 4-2-4 run of a traditional engine, *HOT* is the order of the day.

You should be using an 11-4 or a 12-4 (perhaps shortened to 11 1/2) with that FP 40. With a 10-6, you have a racing plane.

>... the tank i made from the Nobler plans that
>Leonard sent me sometime ago. It had one side
>pointed and the engine tube goes into the coner
>and the pressure tube is soldered to the top
>in the rear.

That isn't any "Nobler Tank" I know about. George uses atmospheric venting, one up & one down, in his pinch- cornered tanks.

>As you know the tank is as high right behind engine
>and the needle sit almos 3/8 higher than the tube.

I have no idea what you mean the way you wrote this. The original Nobler plans from MAN included the pattern for a tank to use. It was one inch tall and 1 7/8" wide. The pickup tube was in the center of the pinch, so it was 1/2" from both the bottom and the top. The "standard" location for a spraybar in a C-L engine has been 1/2" above the mounts for 52 years. Thus, if the tank rested on the same bearers as the engine, the pickup was lined up with the spraybar.

When the vents are changed from atmospheric venting to either Uniflow or Muffler Pressure into a uniflow-arranged vent, there is a mismatch in the fuel flow from upright to inverted that calls for a wedge roughly 1/8" thick under either the tank or the engine (assuming it has an upright engine mount, the wedge would go _under_ such a uniflow tank in an All American Senior).

>So I always thought that they should be as close
>as poss but I would have to lower engine to do
>this. I could make another tank ands raise the
>tube as high as poss.

Attempting to read between the lines, you seem to have left the RC throttle in the engine, which raises the spraybar well above the 1/2" high position that would match an ordinary tank. You may need a *LOT* of wedge under the tank. It would be so very much easier to throw away the throttle than to start cutting open the nose, wouldn't it? Even a home-made balsa venturi stiifened with CYA would be better for you.

>

Yes. The shorts are used in small engines such as 10's & 15's.

>I built the pinched tank you described and ran just
>a vent as described in the first post-

George built them with two "standard" vents. One to fill, and a second as an overflow. The fill tube terminates at the inside bottom corner; the overflow ends at the inside upper corner. Sometimes the fill vent on a standard tank will siphon some, and sometimes is capped off to prevent that.

> ... yes I used the rc throttle because I know not what
>to do.

Isn't there a standard C-L type engine somewhere you can look at?

It's entirely possible to carve a balsa plug to slip into the intake stack in place of the throttle, make it fit nicely and tightly, then harden it by letting it wick in successive coats of CYA glue (no, not while it's stuck in the motor), then drill a venturi down through it and a spraybar hole across that.

But Tower still carries parts for FP's, including the C-L inserts.

>My plan was to use a .46 that Len is modifying but thought
>I would get the bugs out and learn to fly again...So I'm not
>bugging him. And I had this engine just begging to be put in
>a plane.

However long ago the FP series first arrived on the scene, I had one of each of the 35 and the 40. Without going into the larger expense of turning a GMA or a SSW loose on them, I got extra head gaskets, different intake restrictors, and a variety of props and fuels to try them on.

I was displeased overall with the 40. It didn't tame down well with the resources I used on it. The 35 was somewhat more of a civilized beastie. But before I got around to really getting too annoyed with them, someone else wanted them with the RC throttles back in them, for as much as I'd paid in the first place. Since I am perpetually budget-crunched for hobby stuff, I took the offer.

The 40 that I had was still trying to go too fast with 11-4's on it. I had it hanging on the front end of one of the larger old Slow Combat planes I had left over (550 squares or so), and it flew at about 60-65 in level flight and was winding up pretty strongly (maybe 70 mph) in loops, telling me I needed an 11-3, which at that time didn't exist.

Wrong. Same compression, greater coolant flow.

I decided to make my criticism of the dope at your hobby shop less public. The bulk of my further comment is in your eMail inbox.