I contunue to have bad luck with uniflow tanks...


I am using a homebuilt uniflow tank, and I get a stream of bubbles.  Adding a fuel filter kills off the most of them, but I would like to stop them at the source.  The runs are OK if a filter is used, and the airplane is probally flyable.

The tank is a 4.5 oz square wedge tank made up from a brodak tank kit.  It has only 2 lines, a feed line and a uniflow line.  The uniflow line is right next to the feel line and ends about 1/4- 1/2 inch forward of the feed line input.  To fill I stand the airplane up on its nose an fill through the uniflow line.

The bubbles do not seem to be vibration induced, this is a OS 40 with and I can grab the tank  and it does not change the bubble stream.

The tank had a pinhole, but does not leak after the repair.

The tank has a pad under it.

Muffler pressure seems to make the stream worse.

I can make a fox 35 run on a regualr tank, but this is giving me fits. 

Engine is an OS40FP
Airplane is Twister with longer hard birch engine mounts and aluminum pads. Solid front end.
Fuel is SIG 10% half and half

Is the uniflow line too close to the feed line?

Any ideas?

Dave Siegler

Check the tank again for holes by pressurizing the tank and putting it underwater to look for leaks. Then repair and check again one other item would be to reduce the size of the muffler pressure hole it does not take near as much pressure as some think to pressurize a tank. If all else fails switch to a suction uniflow tank.

Scott

Seems it's the time for tank problems. Mine are different, but I seem to be getting them lately

This is a metal tank, it passes pressure tests,  There is a stream of bubbles in the feed line.

The fuel filter makes it flyable.  I suspect foaming (now) when there is less than a full tank.  There is almost no bubbles when the tank is full to the top.  After thinking about it, I did not completely fill the tank for my ground tests.

I just moved the pickup further forward will test today.  Also switched out the remote needle for a front mount one, simplifies the fuel line routing.

I believe it's all about resonance.. Your uniflow vent isn't causing the problem and moving it forward won't fix it. Back in the 70's I built many STOCK Twisters most powered with an OS 35 S. When I came back in 99 or 2000 I built the Twistmaster which is basically a Twister. I used long motor mounts and all the stuff we read about to stiffen the nose. Guess what.. I couldn't get a 35 S to run on that airplane no matter what I did.. Fuel foaming city.. never had this problem in the 70's when I used the stock mounts and lite ply doublers.. Only thing I can figure is the stiffer nose resonated with the engine right at the RPM it needed to run.

I solved the problem by changing engines but anything that will change the resonance frequency of the nose might help. Changing to a plastic clunk tank, changing engines, adding a muffler etc.

Hi Bob,
I'm having trouble with a evo 36, air bubbles with different tanks. I put the engine on a test stand and still got air bubbles. The engine leaks a lot of fuel from the front bearing when running, just wondering if this could be the problem??? I've had several engines that leaked fuel from the front bearing and usually didn't have air bubbles in the fuel line, mostly just couldn't get the engine to hold a good setting. I've checked all items for leaks, tanks, fuel line, filter, etc. The only thing leaking is the front bearing. I guess either throw the engine in the trash or fix the front bearing (new engine it shouldn't be leaking from the front bearing). E-mail horizon and they told me to send it in if I wanted too??? They didn't mention warranty??? I'm sure they'll honor the warranty. Never bought anything from them before, so I'll find out. Tower is great to honor warranties and so is fox After I get the leak fixed, I guess I'll continue trying to get the engine to run correctly. Great little engine when she runs. Have a great day, Gary