Somebody help the boy!!

Hi all, I haven't touched control line in about 25 years and I recently got the bug back. Either I've forgotten a lot or the advent of the muffler has me on the wrong track. Need a bit of help...

I bought an OS 40 and broke it in on a test stand. Engine started wonderfully and ran perfect. Having been encouraged, I built a Brodak .38 Special....always wanted to build a bi plane...

I installed a uniflow tank with centerline about 1/4" above the centerline of the engine after soldering tank mounts. Now, this tank has one tube coming out the top, which I connected to the muffler. The side tube went to the fuel intake, and I plugged the drain tube after fueling. So, the problem...The engine starts ok, but won't run smooth due to mega bubbles coming from the fuel tank. I move the plane around in various configurations and still the bubbles...lots of bubbles. Once when I was messing with it, I inadvertantly left the muffler hose off and it ran a bit better, but still with bubbles...I got disgusted with it and put it down before I threw it across the garage. Is this a simple matter of lowering or raising the tank or is there some fundamental thing I've forgotten.

By the way, I never got involved with heavy duty flying, just school yard stuff...I know the bipe will be a lot slower but I'd like to be able to do some basic slow stunts...is this plane gonna give me that?

I'll appreciate hearing any answers...thanks:-)

LAST EDITED ON Nov-21-04 AT 06:43 PM (CST)
 
Hi Dave,

The bubbles are probably coming from vibration you know. Try placing some stiff foam rubber between the tank and fuselage. A drop or two of regular Armor All is often used in a gallon of fuel to help keep the bubbles down. It only takes a couple drops.
Double check all the fuel tubing for no holes.

The 38 Special should do the entire pattern pretty well. All the ones I have seen were amazingly good pattern fliers for a profile biplane!

have fun

Bill <><

Armor All in the fuel......now I could have lived a long timne and not thought of THAT! Vibration might just be it.....my test stand was just a 3/4" piece of plywood in a vise...certainly no vibration there.....I'll try the armor all and if that doesn't correct it, I'll try the foam..thanks so much for your reply

If you're getting bubbles in the feed line then the vent tube is too close to the pick up tube inside the tank. You may have to pop the end of the tank off with a torch and move the vent tube forward.

As for the bipe, if it's straight, well balanced and you maintain velocity you should be able to do any stunt you want.

>I installed a uniflow tank with centerline about 1/4" above
>the centerline of the engine after soldering tank mounts.
>Now, this tank has one tube coming out the top, which I
>connected to the muffler. The side tube went to the fuel
>intake, and I plugged the drain tube after fueling. So, the
>problem...The engine starts ok, but won't run smooth due to
>mega bubbles coming from the fuel tank. I move the plane
>around in various configurations and still the
>bubbles...lots of bubbles. Once when I was messing with it,
>I inadvertantly left the muffler hose off and it ran a bit
>better, but still with bubbles...I got disgusted with it and
>put it down before I threw it across the garage. Is this a
>simple matter of lowering or raising the tank or is there
>some fundamental thing I've forgotten.

VIBRATION! It's sounds as if the tank is hard-mounted, and that transmits the vibration to the fuel, foaming it up. Most people soft-mount the tank to avoid this. A couple of workarounds - a couple of squirts of Armor-All in the fuel helps a lot. And be sure to carefully balance the prop. Going to a plastic "clunk" tank will also help - since the plastic is soft and doesn't transmit the vibration nearly as badly.

It still may end up being a problem - it's largely a matter of luck it doesn't happen more than it does. If the resonant frequency of something in the system (nose, tank, needle valve) happens to like up with the running frequency of the engine at some point, you are going to have problems.

It's not at all likely that the vent it too close to the pickup. It has to be nearly soldered end-to-end to make it suck up the intake air. It could be, but I would try al the above first.

The replies that are given above (or below) are all good. I notice that you did not say what part of the country (or world) you are in. If you give your location, you might find some good assistance for you at the drop of a hint. A lot of the people on this forum are ready and willing to assist anyone needing help.
Bigiron (in the Wichita KS area)

Dave, assuming that your OS .40 is a new one, such as an FP, LA, etc., it will probably run smoother at higher rpm. Try a 10.5 x 4.5 APC or 11 x 4 APC prop. You won't find the 10.5 x 4.5 at your LHS, but they can be ordered direct from APC very easily, at similar prices. Steve

Dave,
Others have mentioned a most common cause of bubbles - vibration. But just a reminder in case you haven't done this already, always install a fuel filter in the fuel line between the tank and the engine. Besides keeping any crud from fouling up a steady needle valve setting, it may also help dissipate some of the bubbles coming through the fuel line.
DennisV

Thanks so much for the ideas. Last night I installed some foam, put a little armor all in the fuel...haven't tried it yet. Figured I'd go light on the neighbors. I live in Battle Ground, WA, about 10 miles N of Portland, Or......this baby is ready to fly, if my bubble problem is cured, that is!! I guess I'm learning what variables can present themselves. Back in the 70s, I built many planes using my trusty Fox 35 and I don't remember ever having bubble problems so this was a first to me...thanks, again!!

One other simple trick, if the problem is the muffler pressure blowing bubbles into the fuel pickup tube. That can happen if the uniflow tube and the fuel pickup tube ends are too close together in the back of the tank.

Switch the pressure line to the overflow tube, and cap the uniflow tube for flight. The overflow tube's a lot farther from the end of the pickup tube.

The other tips in here should almost certainly solve things for you, but if not, this is another shot at it.

AH yeah....the 38 Special is NOT slow......a 40 will haul it along maybe too well.....I ran a 40LA on mine with an 11 x 3 APC and it was anything but slow.......the plane stunts well and you will not be dissapointed.