Steve,
Here's my 2ยข worth:
(We call the upper tank a "nose" tank or "header" tank. The tanks in
the wings are the "mains" or "wing tanks.")
I try to always know how much my nose tank was below full. If I've come
to the point in my flight where I'm using fuel from the nose tank, a
significant fraction of my attention is given to the fuel level.
________________
To know how much my nose tank is below full, I calibrated my nose tank
gauge. To do this, I got a good, clean gas can and put a clean funnel
and hose below the gascolator's drain. I drained the nose tank completely.
Filling the nose tank was done a half gallon at a time. On *MY* plane,
at THAT time, with THAT fuel gauge and cork, it took 3.5 gallons before
the wire gauge came up off the bottom. You *SHOULD* know how much fuel
is in the tank when *YOUR* nose tank float-wire gauge gets to the bottom.
Filling by half-gallons and marking on rain-proof glass, I calibrated
the gauge up to full. When near full, I waited quite a while between
adding any more so I could determine what the gauge looks like when it's
at the full and drained-down-to-stable level. (When flying, with the
fuel pump pumping, the level will be somewhat above this mark.)
________________
Now, if I've used fuel out of my nose tank in a flight, I know
approximately how much to add to the nose tank. I'll top it off then
wait several minutes so any excess can drain down to the mains.
Then I fill the mains with the up-hill tank first (if there's any slope
at all) followed quickly by the down-hill tank. Doing it backward could
cause cross-flow resulting in overflow from the down-hill tank. Leave a
couple of inches of airspace below the filler hole to prevent overflow
if your plane gets parked on a slight side-slope or siphoning in
flight. Due to the strong dihedral, that small amount of airspace
shouldn't cost you much fuel capacity.
If you don't have the inside gauge (down below your knee on the cockpit
side wall) then you should have a float gauge on one (or both) of your
wing tank caps.
Whichever gauge you have on the wing tanks should also be calibrated.
Ideally, I'd do this:
Get a friend to help. Calibrate two pitchers and mark them exactly at
the one-gallon point. (I'm not sure what I'd suggest as an accurate
reference but try to find one rather than just assume some level in a
store-bought product is a gallon.)
Go fly and empty the mains completely. (Well, you could drain nearly
empty tanks into your clean gas can using the quick drains.)
Now, fill both pitchers and each of you add that fuel to the main
tanks. Put on your gauge (or check the inside gauge) and record the
point at which the gauge comes off the empty mark. Due to the Coupe's
strong dihedral, there's usually a fair amount of fuel still in the
tanks when the float gauge gets to empty and even when a dipstick used
vertically at the filler cap shows empty. You want to *KNOW* this amount.
From when the gauge comes off empty, record what the gauge says after
you add each two gallons.
Over the years, the cork in a float gauge may get saturated with fuel if
the coating was not perfect. This leads to the float being a bit lower
an you'll think your fuel level is lower than it is. That's a fairly
safe condition - to have more fuel than you think you have. But still,
I'd re-calibrate every five years or if I had reason to suspect inaccuracy.
As a side note, my nose tank float gauge got so saturated that in the
middle of a flight it just sank, suddenly. Talk about pucker factor.
Fortunately, the engine didn't notice the gauge's report and kept
running just fine. After landing, taking the float gauge out of the
tank and letting it air in the sunshine for a couple of hours brought it
back to floating for the next leg of my flight. A couple of days of
sunshine and air let it float fine till I got a replacement.
I, like many other Coupers, kept the nose tank as a reserve, never
planning to use it on my trips. I think I never used the nose tank but
once on any cross country trip as the mains can hold more fuel than my
bladder can hold . . .
Six gallons of fuel in the nose tank gives you an hour and a hundred
mile range at max cruise power. With power reduced to maximum range
power (about 5 mph above best glide), your range improves to, maybe, 150
miles. Now that's comfort!
One other advantage to all this calibration: Knowing your fuel levels
lets you keep within your desired gross weight when carrying different
weights of passenger and baggage. By knowing the level of fuel in my
nose tank and having the wing tanks empty I was able to take an
acquaintance for a 20 minute local flight even though he weighed as much
as I did.
All the best,
Ed
