dspeers

My Feedback: (22)

Dave I like your example above but I think I have an answer unless I am missing something.

With a properly positioned tank, standard setup the pressure variance full to empty would be 2" as exhaust pressure is additive. albeit what % of the total pressure is unknown.
Comparing upright full to inverted near empty again yields a 2 inch difference, this would be somewhat less comparing inverted full vs. upright empty as the exhaust pressure would be canceled/reduced by the head pressure when inverted.

With the mis-positioned tank described above the pressure variance is ~2" in a uniflow setup regardless of how full the tank or the plane's orientation, but in a standard tank the pressure variance is 2 " upright to inverted and the head pressure variance full to empty is 2 " and this would be additive for a total pressure variance of 4"

I say ~ because the constant pressure in a uniflow setup is based on the assumption that exhaust pressure is at least = full tank head pressure. If not there would be a pressure variance full to empty described by head pressure (full tank) - max exhaust pressure, but only if that total was > 0. Even if this were the case the delta would still be

Sport_Pilot

Sorry I am doing a poor job of explaining this. Also I stated that the pressure of the fuel pickup is always changing , that is true but only because the exhaust pressure changes with engine speed and load.

Let's use all pressure as fuel head. That is because fuel density varies depending on nitro percentage and oil type and percentage, also because it is easier to visualize.

So for a full uniflow tank 3" tall and mounted with the bottom 1/2 inch below the spraybar, which of course is too high, so with a 10" exhaust pressure the pressure at the bottom of the tank is 10" and the pressure at the spraybar is 9.5" or the exhaust pressure minus the height of the spraybar from the bottom of the tank. We are of course ignoring suction pressure for this example, but assume it is the same in all cases. When inverted the pressure is still 10" at the top but now the bottom of the tank but the pressure at the spraybar is 7.5" or 2.5 inch's less. The difference in pressure iat the spraybar is 2" from upright to inverted.

Now consider for a standard tank. The top of the tank is now the bottom and bottom is the top. When upright the tank pressure is the same as the exhaust pressure or 10" and the pressure is 12.5" or the tank pressure plus the head above the spraybar. When inverted the tank pressure is now less than the exhaust pressure by 3" or 7" because the vent is now at the bottom of the tank. The pressure at the spray bar is another .5" (the distance of the top of the tank to the spraybar. or 7.5". That is a difference of 5" from upright to inverted. That is 3" more because with the uniflow tank the clunk for the vent falls to the side closest to the carb.

If you follow the same logic through you will find why I say that a standard tank should be mounted with the centerline below the carb and how much for the same height regular shaped tank.

gerryndennis

'because the vent is now at the bottom of the tank.'

Click. I've just got it. I hadn't considered that when a standard tank is inverted that it now acts like a uniflow tank because the vent is now at the bottom of the tank. I guess that's what you were trying to say all along?

So in my example of the 2" high tank, half full, mounted 1" high the difference in head upright to inverted is uniflow 2", standard 3". So the uniflow is better. Of course if the tank is low it's the other way round, uniflow 2" difference, standard 1". Standard is better.

And yes that explains why manufacturers recommend mounting the tank slightly low.

I'll change my posts a bit.

Dave H