Tank positioning - physics

I know that it most often helps to have the fuel pick-up a bit above the venturi on a profile. My question is why? Why does this work and why doesn't it totally mess up inverted flight? This is with a uniflow tank if that makes a difference.

I'm not sure you will find the answer you are looking for here but there is a great deal of discussion.

http://www.clstunt.com/htdocs/dcforum/DCForumID1/7110.html

Thanks. Missed that thread

>I know that it most often helps to have the fuel pick-up a
>bit above the venturi on a profile. My question is why?
>Why does this work and why doesn't it totally mess up
>inverted flight? This is with a uniflow tank if that makes
>a difference.

It probably has to do with the location of the by-pass port on the engine (opposite the exhaust.) In theory a rear exhaust engine would be symmetrical and not exhibit this "problem". I have never tried a rear exhaust engine on a profile, however, so have no evidence of this. A Schnurle engine has a boost port in the same location. I never really paid much attention to our blocked boost port engines on a profile, which would also be fairly symmetrical. We just located the tank for an even run but didn't look to see whether it was above or below the center of the engine.

Tom,
This is an odd thing that may have a general explanation. I haven't found one yet that covers all the variations I've flown. Examples:

Fox 35 Stunt, sidewinder mounted: Tank height center (or at least, pickup and uniflo inside vent end) 1/4" to 5/16" 'above' center of venturii with model upright. Tank AT needle valve height (1/2") head-up or head down. Actually, this may be off by about 1/64"...

Various other crossflow, radial ported diesel and schneurle engines: generally 0-0 with center of venturii sidewinder, but all over the place for an even run when head-up or head-down.
And, yes, the effects of incorrect tank height are much more noticeable with uniflow ventued tanks. Not a bad thing, the tanks just run so uniformly that any difference shows clearly. In another thread in here, I mentioned a ground-check method that comes close.

Start engine on model. Rotate fuselage so outboard wing is vertically down and fuse centerline is horizontal. Roll wing to about 45º 'cockpit-up' and note the engine RPM or sound. Keeping the fuse centerline horizontal, roll it to about 45º 'cockpit-down' and see if the RPM or sound change. The tank is too high on the side where the engine goeas a bit richer, or same-same: too low on the side where it goes leaner.

Shut off the engine by pointing the nose down with outboard wingtip up to starve it off. Make any indicated adjustment and check it again. When it sounds or tachs the same in both rolled positions, it should be close enough to survive a check flight. Only a flight can confirm the tank is at the right 'height'.