Fuel Tank Follies

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Rich
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Fuel Tank Follies

Post by Rich »

In various contexts we've had some discussions around the operation and behavior of the fuel tank setup. So I thought I'd put together a bit of a model of the tanks with representations of the fuel therein at various levels. Reference the following diagrams:
DA40 Fuel Tanks - 40 Gallon.jpg
DA40 Fuel Tanks - 50 Gallon.jpg
Details:

1. The cells are accurately proportioned, to the best of my ability to measure them. There are some inherent inaccuracies, but the diagrams are qualitatively useful anyway.
2. The 5 degree angle of the cells assumes that they are aligned with the bottom surface of the wing, which exhibits a 5 degree dihedral, by both documentation and measurement. (NOTE: If you measure the upper surface of the wing you will get about 4.3 degrees.)
3. The green, blue, and yellow lines represent the fuel levels at which the levels just meet the lower outer face of each cell. They illustrate the level spanning cells as fuel is depleted.
4. The magenta lines represent an approximation of the fuel probes. This is really just a gross reference, as it's not clear what the calibration curves limits are or precisely the limits of the detection of fuel along the length of the probes.
5. I've made a stab at calculating volumes of fuel consumed from the various sections and thereby deduce the amounts remaining by adding the blue numbers together, showing the sums in red, with fuel remaining in parentheses at the various checkpoints. For example for the 50-gallon model, there is 3.48 gal removed from the outer cell and 1.94 gal from the center cell, meaning depletion of 5.42 gal, therefore 19.58 gal remaining. These fuel volumes, by the way, probably include "unusable" fuel, which is sometimes usable - partly - maybe. :D
6. Although shown with a high degree of precision, the volume calculations are made with some simplifying assumptions, so don't bet your life on these numbers.
7. I've depicted the cell interconnecting hoses just for reference, but have ignored their contribution to fuel totals. The picture gives a false impression of their size. In reality, the amount within each is negligible - of the order of a pint of fuel apiece. But they are still plenty large (~3 in. dia.) to allow the free flow of fuel between cells.
8. I don't show the vent lines at the upper levels of the tanks which are intended to provide for complete filling of the inner cells.

Some conclusions:

1. When the 50-gallon tank is at 20 gal per side, there appears to be less fuel in the outer cell than I would have guessed. That "flapper valve" deal might have some merit, but it would be a tricky device to construct such that it operated properly.
2. In the 50-gallon case there is a small window of fuel use (roughly between 19 and 16 gal remaining) where fuel is being drawn from all 3 cells. I conclude it would therefore be theoretically possible to eliminate that fuel gauge gap in the display. But uncertainty of the precision of the measurement when the outer cell is very low may make this unwise.
3. It occurs to me to wonder what design limitation led to implementation of the 50-gallon option by adding a 3rd cell, rather than making the outer cell longer. I presume it has to do with allowing for wing flexing.
2002 DA40-180: MT, PowerFlow, 530W/430W, KAP140, ext. baggage, 1090 ES out, 2646 MTOW, 40gal., Surefly, Flightstream 210, Orion 600 LED, XeVision, Aspen E5
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