Hi Linda,
Excellent questions, as always.
The fuel system of the Ercoupe can be very accurate when properly
calibrated. NEVER, EVER believe what you think an uncalibrated fuel
gauge is telling you (unless a three hour leg is your personal
maximum duration of flight).
The main problem with learning from experience is that sometimes the
"final exam" comes before the knowledge ;<) To paraphrase Donald
Rumsfeld: There are things we know, and things we don't know. In
emergency we may know things we didn't know we knew, and things we
didn't know we didn't know.
On my long student cross country just after a "fresh annual", I
avoided a forced landing by pure dumb luck. My mechanic/instructor
had mis-timed my engine and thus increased my fuel consumption in
cruise a LOT. I perceived a problem shortly before my cork "hit
bottom" some 25 minutes from my planned fuel stop over desert terrain
with no near airports, and I had NO IDEA how how much fuel remained
in my fuselage tank. The plane took 23.9 gallons to fill! From then
on I resolved to be "in the know" as to available fuel aloft.
A "dip stick", Fuel Hawk (hollow plastic tube with incremental
markings, or the "Little Dipper" measure wing tank fuel at the filler
cap(s). as Ed said, because of the Ercoupe's considerable dihedral,
such devices cannot measure the fuel residing in that part of the
wing tank below the level of the tank bottom at that (high)
location. If your wing tanks took 10.8 gallons to fill (and they are
nine gallon tanks), the wing dihedral was hiding 3.6 gallons PER
SIDE! That's why ERCO gauges in the right terneplate wing tank of
Serials 113-2623 AND in the left aluminum tank from Serial 2624
onward were located so as to measure fuel at the most inboard
location possible (but they still don't measure it all).
There are three types of fuel capacity.
1. Total fuel system capacity (including lines, fuel pump,
carburetor, gascolator) - necessary only for accurate weight and balance
2. Usable fuel capacity (wing tanks, fuselage tank/line to
gascolator = total measurable) - necessary for "informed
operation" (safe flight?)
3. Indicated capacity (wing tanks gauge & fuselage tank gauge) -
necessary for "informed operation" (safe flight?)
Ercoupes from Serial No. 113 to 2623 left the factory with 23 gallons
of fuel capacity in terneplate tanks. There were two 9-gallon wing
tanks and a five gallon fuselage tank. Some of these have since
replaced the terneplate tanks with stainless steel ones of the same
respective capacities, 415-48083R & L, which accommodate main gear
legs mounted on the front of the main spar (thru Serial number 812)
but also fit Serials 813 through 2623. The six-gallon aluminum
fuselage tank does not fit earlier fuselages delivered with the five
gallon fuselage tank.
During or after August of 1946, Ercoupe Service Memorandum No.39 was
issued describing how to install the later nine gallon aluminum
production tanks, 415-48187R and 415-48147, in Serials No. 813 thru
2623. I don't have a copy of ERCO drawing 415-48197 to determine
when "replacement" aluminum tanks of this number, both L & R, may
have become available. These tanks were shorter (presumably to
eliminate the need for cutting holes in the fuselage skin) and of
only 7.7 gallon capacity, thus reducing fuel capacity to
approximately 20.4 gallons. NO production airframes came out of the
factory with these tanks, and some have since been installed in outer
wing panels.
The "bottom line" is that total fuel capacity of all tanks is not
easy to measure without removal and cleaning them before
calibration. I am much more concerned with knowing my usable fuel in
the (interconnected) wing tanks and in my fuselage tank. I strongly
urge one and all to carefully calibrate their gauges in the process
of measuring usable fuel. Anyone can make marks. It's better to
make MEANINGFUL marks!
My method is to fly the wings dry so that the fuselage tank gauge
begins to drop, land, and level the plane at the sills. Pour 2
gallons and one quart in each wing tank and allow them to "level"
out. Whatever your gauge reads (if you have nine gallon tanks),
that's 1/4 (wing tank fuel capacity remaining in flight). Repeat.
That's 1/2. Repeat, that's 3/4. Repeat and you have the visual
"picture of where in the filler neck "full" is.
If you also use a "dip stick", Fuel Hawk, or "Little Dipper",
calibrate it accurately too as you do the above. Since you did NOT
fill the fuel pump and related lines, ignore that minor unmeasured
(plus the fuel in the gascolator and the carb) as a small "fudge
factor" always in your favor.
Take the plane around the patch once so the transfer pump refills the
fuselage tank. Level the sills again. Drain 1-1/2 gallons from the
fuel line at the gascolator (and plug it with a bolt or something).
Mark the fuselage tank gauge (wire or tube). Repeat. If the gauge
cork is in solid contact with the bottom of the tank, add fuel an
ounce or two at a time until it just floats. You now know that your
gauge can't measure much below half of the fuselage tank contents.
You know when it's full and 1-1//2 gallons down. You know either
when it's three gallons down or you will have derived how much less
than 3 gallons down it can measure. That's enough information to
manage that fuel until the engine quits (if you manage it right, it
won't ;<) Drain and measure the rest. You have just measured total
usable fuel from the fuselage tank because you got it the same way
the engine does...by gravity.
"Unusable fuel" in the wing tanks is negligible (and not infrequently
water or crud ;<).
The longest non-stop leg my wife and I have flown (so far) was
eastward from Carlsbad, NM to Tucson, AZ. We departed shortly after
noon and flew high and lean, landing with three gallons "reserve" in
the nose tank (as expected) after 4 hrs. 32 min. in the air.
Regards,
William R. Bayne
._____|-(o)-|_____.
(Copyright 2010)
--
On Sep 8, 2010, at 00:14, Linda Abrams wrote:
My experience last Friday confirms what Ed wrote below. At a fuel
stop on the way to Watsonville, CA, I filled the wing tanks to my
usual 1-2" from the top (any more and it spills out the cap vent
holes when taxiing or climbing out). The flight to Watsonville from
there took almost exactly 2 hrs., and I was pushing full throttle
most of the way (due to concern about the marine layer of clouds and
fog moving into Watsonville), which I don't usually do. Before
fueling up at Watsonville, I dip-sticked the wing tanks: one barely
wet the tip of the stick, and the other was at ~3 gal. mark (on a
stick calibrated by another Ercouper who gifted it). However, when I
then fueled to the same 1-2" from the top, it only took 10.8 gal.
That dihedral "hid" ~2 gal. per side.
BTW, do the wing tanks feed *all* their fuel out to the fuel pump,
or, put another way, is some of that ~2 gal. per side unusable fuel?
Linda
N3437H (Sky Sprite)
L.A.
Ed B wrote:
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.
