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sidsel.svein(at)oslo.onli
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 Posted: Sun Jun 23, 2013 1:29 pm Post subject: SV: Use of reserve tank |
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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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davidjoyce(at)doctors.org
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| Posted: Mon Jun 24, 2013 1:27 am Post subject: SV: Use of reserve tank |
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Svein, That is all sound advice, except there is nothing
to stop you switching back to the main after having run
the reserve dry, to tap into what you left there and what
will have spilled over from the reserve. Some of that will
of course then go via the return line to the reserve side
again and when your main runs dry you can yet again switch
to reserve and expect to find a little bit there. I am not
suggesting this should be your normal practice - clearly
it is preferable always to plan to land with your reserve
fuel unused, but if one of these days you find yourself
having to do a massive diversion for say unexpected
weather problems, it is worth remembering that either tank
running dry gives enough notice (with engine coughing) to
allow you to switch to the other tank and that you can get
a few extra miles by repeating the process.
Regards, David Joyce, G-XSDJ
On Sun, 23 Jun 2013 23:29:28 +0200
"Sidsel & Svein Johnsen" <sidsel.svein(at)oslo.online.no>
wrote:
| Quote: |
Johnsen" <sidsel.svein(at)oslo.online.no>
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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