Originally Posted by
Cat 1
So... A bit of shop time today sorting out the fuel system on the Boxer test mule. The tank position (high) troubles me a bit and got thinking (which is always dangerous) so I spent a bit of time playing with a theory.
Some of you might know what a "Uniflow" set up is and its basic function.. I have used a Dual clunk uniflow on an RC plane before with an odd high mounted fuel tank and it worked well apart from an issue that plagues this setup for RC. When you Back off power the drop in muffler pressure causes a back flow down the "pickup level" vent and floods the muffler.. A check can be used but then the tank pressure stays high for quite a while and causes very rich low power mixtures.. Playing with how to incorporate this concept into an "air pump" system had the same issue - When the pressure would decay (pump slows down) it would flood the supply line and dump fuel out the air bleed in the air pump supply line. I thought a catch (and return) can might work (sort of like a crap trap but set to return the fuel when the pressure returns. Did some math and found that a throttle cut in a near empty 16 oz (500ml) could return over 1 oz (30ml) at 1psi decay.
I set up a test rig and confirmed this could be the case - Playing with plumbing I found a solution (I think) and it worked in the test rig - Also confirmed - using pressure differentials - the Uniflow would help with the high mounted tank.
One issue remains - And it is an issue with the crap trap setup - If you pull power when inverted - you can still flood the "fuel trap" (similar to a crap trap) but in this case the fuel can be returned to the tank.
I KNOW this is not a Simple solution as the plumbing is looking like a rocket motor
- But have a look and see what you think. The theory is (proven on my bench) that the Check valved vent at the top of the tank will dump the pressure on decay but keeps the Uniflow as the primary pressure supply in the system.
I think this system could also be used for a Muffler pressure setup - But the can would have to be set to eliminate return to the tank (as it does in the current config)
Just to bring things in perspective: a 500 ml tank is twice what you would need for the Boxer. 250 really is large enough for all purposes, even my BigLift can do 4 to 5 safe tows to 600 ft on that. So that reduces the amount of "regurgitation" by half. 15 ml of max reflux on a near empty tank (where you preferrably won't fly much) 7,5 ml on a half empty tank, and less than 4 ml on 75% tank capacity.
Any decent throttle management will make reflux a non-issue.
The only plane where for me reflux is a minor issue is the F3A plane, where sudden power cuts from full power to abrupt idle are regular and happen in negative G conditions (tank vent immersed).
I am a bit confused though why it is not possible to mount the tank lower or the engine a touch higher. Next to my BigLift, there is an OS twin in my club, admitted, the planes are high wing "floaters" but we do both not have real issues with tank placement. My Big Lift can be fuelled to the max without the fuel syphoning out, as it is a tail dragger, and the stance of the plane lowers the tank sufficiently. In flight the level attitude really is a non-issue, the engine consumes enough for it not to flood.
My suggestion: use a 250 ml tank, and test what the set-up does with needles as set for running, but engine stopped and the plane in its normal tail-down stance it would have on the ground. If it does not syphon with the engine off, you're good to go in the air.
Mind you: Even IF there is minor syphoning going on, it will only "flood" the headers, but not the cylinders, because fuel does not flow upwards. The excess will leak out of the carb.
Two or three vigourous reverse flips with the throttle full open will clear the headers.
IMHO, the uniflow style tankvent is not needed to get a good running engine.
But I absolutely HAVE to say: your system layout with the checkvalve for immediate pressure relief and a "pushback" line for any regurgitation is pretty clever, and it took me quite a bit of thinking before I got its workings. It can work with the airpump, but not with muffler pressure, because the craptrap is not supposed to return any backflush-fuel as it will be contaminated with muffler crap.
Then again, I still have to experience my first issue with a full craptrap. Even the F3A plane at best has about half of its fluid-capacity used after a flight.