Chris,
The more general rule I've found is the uniflow vent end inside the tank will probably wind up from 1/4" to 5/16" above the center of the venturii. It is that vent height that the engine "sees" as the top of the fuel surface with uniflow venting. That's the level of vent air, whatever the amount of fuel in the tank. Here's another thought:
Brett,
Thanks for bringing Frank's comments to a new audience! I'd only add that treating the symptoms isn't always bad, IMHO. I know you've seen my recommended pre-flight tank height test routine somewhere: it's been in here at least a few times. The idea is that - particularly for Fox 35s over the many years of production - not all of them are equal. (Wasn't that from Orwell's 1984? The boss pig told the other pigs, "Some pigs is more equal..." Not that I consider the Fox Stunt 35 a pig, far from it. Considering the advances in technology that it does not make use of, it ain't all bad.)
The test before a first flight requires that the tank height be adjustable, of course. The process is to start the engine, rotate the wings to vertical - outboard tip down. Set needle fairly rich, so RPM changes can be heard - or better, tached - easily.
Hold the fuselage centerline dead level at all times, and rotate the wings to about 45° "cockpit up" (keep fuselage centerline horizontal). Check for an RPM change. Then rotate to the other 45° "cockpit down" (keeping fuselage centerline level.) Check RPM in this attitude. The tank is relatively low in the rolled position where RPM increases, and the richer, lower RPM is when the tank is realtively high.
Repeat this a few times until you are sure you know which is the high (or low) side and shut off the engine. Safest way is nose straight down, outboard wing up. That puts the fuel pickup out of the fuel - always with a metal tank, and most often with a clunk tank. Adjust the tank to lower it from the high position; alternatively to raise it from the low position - it's the same intention.
Run another test as above. If there is no significant RPM difference at both banked positions, whatever the mysterious conditions inside the engine may be, they'll act much the same upright and inverted; insides and outsides. What you're doing is doing it until it's done, not by some arbitrary number which may work with one engine, but won't work with yours. There are enough dimension differences inside Fox 35s - and surprisingly even in "reasonably priced" CNC engines, that they won't all run exactly the same. (The newer engines are generally more alike, tho...)
Why does this procedure work? The rolled angles put simple weight - force of gravity - at something like the angle of the combination of centrifugal and gravity forces in flight - only a MUCH bigger angle. We can't increase the load on the fuel from CF while we're standing still, but we can exaggerate the effect with the greater angle. That acts like raising or lowering the tank while the engine is running on a test bench, which is easy to understand will richen or lean the setting without touching the needle valve.
After this "wig-wag" test to set tank height, you are at least more sure that is close enough you won't likely flood or starve out in a first flight. Depending on the model's "rig" -whether it flies nose-up or -down, or with one wing high or low, you'll probably need to fine tinker the tank height a bit more...
Luck!
Topic: Fox .35 and tank location on a a profile plane (Read 2469 times)