Graeme,
What we need to keep in mind is the following:
When we run off the main tank, about 28-30 liters of fuel goes out per hour
at cruise power (912ULS). Abt 10 of these liters return to the reserve tank
(i.e. abt 1/3 of the total flow out of the tank), and flow back over the
saddle into the main tank. This ensures that the reserve tank is always
full, even if we should fly a little uncoordinated once in while or
experience some bumpy turbulence, and the net drain from the main tank is
18-20 liters (or whatever the cruise consumption may be).
What we should NOT do is switching to the reserve tank before we have
decided that "OK, this is it for the main tank, from now on it's only the
reserve tank that will take me home." Take the following case: 9 liters
in the reserve, 11 liters in the main. If this were the case during the
last part of a flight, we have 20 liters total, i.e. one hour's flying time.
If due to uncertainty about actual volume remaining in the main tank, we
switch to the reserve tank (9 liters), then we have a little under half an
hour before the engine tells you something. If we now switch back to the
main tank, we suddenly do not have 11 liters consumable fuel there anymore!
Why not? Because about 1/3 of that fuel is returned to the reserve tank,
but the returned volume does not any more overflow into the main tank
because the reserve tank first needs to be filled to its top. 10-15
minutes' flying time is thus lost, which could be critical (unless we are
conscious about this, and switches back to reserve again).
What it all boils down to is knowing how much you have left in the main
tank, so we can confidently run this tank almost empty and switch to reserve
just in time before the engine tells you to, and then stay on the reserve
tank.
I completely trust my fuel totalizer (feed and return flow senders) and the
sight tube. By the way, I have connected the two vent tubes (the one from
the top of the sight tube and the one to the top of the filler neck) by a
crossover tube at their highest point. If one inlet should be blocked by an
insect or whatever, the other will provide equal pressure on top of the tank
and on top of the fuel in the sight tube. It is amazing how erroneous the
sight tube reading gets even with a very small pressure difference in the
standard tubing arrangement - just try it by blowing gently into one of the
outlets.
/////////
About return flow: I mentioned early this spring that I would re-wire the
fuel flow system so that I could get temporary direct reading of the return
flow only. I have the relay deck and the push button, but just have not had
the time to wire it in ........ Will do - will report!
Regards,
Svein
LN-SKJ
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