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Posted on behalf of BILL HANNAHAN <wfhannahan@yahoo.com>:

Here is an article I wrote for my EAA Chapter
newsletter a few years ago, forward it to your
newsletter editor if you like.

  10/18/95

I am writing to describe an interesting
experience involving the use of  auto fuel in
hot weather I had while attending the Copper
State fly-in at Phoenix this year.

I was flying a Lancair 320 equipped with a
Lycoming O-360 A1A (carbureted 180hp) engine.
It has a 23gal fuel tank in each wing and a 10
gal. header tank.  It has a four-position fuel
selector valve, one for each tank plus off.
Each wing tank has its own electric boost pump
mounted on a rib a few inches from the outlet of
the tank finger strainer.  The outlet from the
selector valve is plumbed to a gascolator located
in the nose gear tunnel.  The gascolator outlet
is
routed to a flow transducer mounted in a
separate compartment just behind the firewall
which is vented to the nose gear tunnel. These
locations were selected to keep as much of the
fuel system as possible out of the engine and
passenger compartments.    The outlet from the
flow transducer penetrates the firewall and a
firesleeved hose conducts the fuel to the engine
driven fuel pump.  Another firesleeved hose
conducts the fuel to the carburetor.

For the last 120 hrs.  I have been
experimenting with the use of premium unleaded
auto fuel. This fuel is approved for use in this
engine through Peterson and EAA STC's.  I keep
100LL in the right wing tank and premium unleaded
in the other two tanks.  I always use the right
tank for takeoff and climb, then switch to the
unleaded.

  I flew down to Phoenix Friday and parked on the
ramp at Williams.  Temperatures were well above
normal with highs near 100F Friday and Saturday.
I took off Saturday afternoon about 2:30 after a
long taxi to runway 30 center.  Outside air
temperature was very hot and engine temps were
high but in the green.  Takeoff and climb were
normal.  About 50 miles west near Buckeye at 8000
ft.  with CHT about 190C and oil temp.  98C,  I
switched to the left tank containing auto fuel.

  After a minute fuel pressure began to drop while
indicated flow increased somewhat.  I have seen
this many times before, and believe it is due to
the resistance of the flowmeter causing bubbles
to form in the warm fuel passing through the flow
meter resulting in an artificially high flow
reading.  In my previous experience the engine
driven pump could usually maintain 1-2 psi,
sufficient for normal operation, but this time
pressure dropped essentially to zero and the
engine began to run rough.  I hit the left boost
pump resulting in an immediate surge on the flow
meter, the engine smoothed out, and after a few
seconds to fill the carburetor, fuel pressure and
flow returned to normal.  A typical example of
vapor lock up to this point, then the engine
began to roughen up again, which got my undivided
attention.  I scanned the instruments for signs
of a  problem, fuel pressure, fuel flow, oil
pressure, manifold pressure, mags on, everything
looked normal, the fire triangle was complete,
the engine should be running, but within a few
seconds it died completely, just windmilling.  I
eased the mixture out and the engine surged for a
second and then began to die, I pushed it in
slowly and the engine surged for a couple of
seconds then died.  I repeated this cycle several
times but could not find a mixture position where
the engine would run continuously.  After several
cycles survival instinct overcame engineering
curiosity and I switched back to the 100LL, after
a few more cycles the engine smoothed out and ran
normally.  An hour later with things cooled off
at 10,500' I switched back to the unleaded and it
ran normally.

  Last spring I was flying with an RV4.  We
stopped at a small airport to visit a friend.
After landing we taxied back the full length of
the runway and out to the friends home, found he
was not there, taxied back out and down the
length of the runway again to launch.  It was a
warm day and after several minutes on the ground
th engines were quite warm.  The RV4 launched
first, as he broke ground I started my run.  As I
lifted off and accelerated I  noticed my friend
low over the far end of the runway making puffs
of black smoke and I was gaining on him more
rapidly than normal.  He angled for a freshly
plowed field, landed and flipped in the soft
dirt.  He and his passenger were trapped in the
plane, fortunately there was no fire and they
escaped with substantial cuts and bruises, the
plane was totaled.  The investigation found
nothing wrong with the engine or fuel system.

I suspect the problem in both cases was unleaded
auto gas boiling in the carburetor and surging
through the float bowl vent into the induction
system, flooding the engine.  The surging action
may be explained by the pressure changes induced
in the induction system as the engine RPM
changes.  My plane has a ram air induction system
with an air filter.  The filter uses up some of
the pressure generated by the inlet, about
0.5-1.0 in. HG depending on power setting.  I
suspect that when the mixture control is leaned
out and the engine surges, pressure at the float
bowl vent drops causing additional fuel to boil
sustaining the surge briefly.  As the fuel supply
diminishes the engine slows down causing an
increase in float bowl pressure, suppressing
boiling and cutting off the engine fuel supply
until the mixture control in pushed in.  When the
engine recovers on fuel flowing through the
normal path, pressure at the float bowl vent
drops again resulting in another surge of
boiling.  The same engine with another induction
system might have a different response
characteristic.  The same engine on a test stand
with no induction system may have a more benign
response.  There are some characteristics about
my airplane that might make this phenomenon more
likely.

1 It is tightly cowled and baffled.  Air inlets
are small, the baffling and oil cooler
installations use almost all the air going
through the cowl so that air temps below the
engine are higher than for average installations.

2 Due to the high power to weight ratio I tend to
use lower power settings than the engine
  would see in other applications resulting in
lower fuel flow rates through the fuel pump and
carburetor.  Cruise climbing out of Phoenix
settings were 1990RPM and 19.5" HG resulting in a
fuel flow around 6.5 GPH.
3 The aircraft's long range allows the transport
of high vapor pressure winter-gas to warm
climates.

There are also some characteristics that make
this problem less likely in my airplane.
1 The aircraft is painted white resulting in
minimum surface temperatures in direct sunlight.
2 The wing skins are made of two layers of
fiberglass separated by a 1/4 in.  Foam core.
This core substantially reduces the heat rate
through the skin.  A nonwhite metal airplane with
integral wing tanks could produce much  higher
fuel temperatures given the same circumstances.


I do not believe that people should be prevented
or discouraged from using auto fuel in their
aircraft.  There are several significant
advantages including cleaner engines, less plug
fouling, cleaner oil, lower cost and reduced lead
in the environment.  People do need to know that
this phenomenon exists and how to prevent it.  I
think that keeping a supply of undiluted 100LL on
board for takeoff and climb is one way. It also
provides  the valves and valve seats with a fresh
protective patina of lead at the beginning of
each flight.  There may be modifications that
would also help.

PS  11/00

I now keep 100LL in the header tank with mo gas
in both wings, and use the header for takeoff.

Should you switch back to 100LL for landing?
There are reasonable arguments for both sides.  I
used to switch back to the 100LL for each landing
but worried about a selector valve failure
between tanks or moisture condensation sloshing
down to the pick up point and flooding the carb
with water. Most landings follow a cruise decent
at low power, engine and fuel temps are low and
if there is plenty of fuel in the tank I now
leave the selector alone. If I am planning touch
and go's I switch to 100LL, I also switch after
an unexpected go around as temps will be higher
on the next approach.

After landing the fuel selector is turned off to
burn the mo gas out of the carburetor taxiing
back to the hangar.  For the next start select
the header tank to flush the residual mo gas into
the float bowl. After engine start the fuel
selector is switched off while taxiing to the
runup area burning the residual mo gas. The fuel
selector is switched to the header tank prior to
engine run-up and left there.

  Two years ago I insulated the fuel pump and
float bowl with 1/2 inch closed-cell black foam
(It came from a friend who did not know its
source, possibly soundproofing from Spruce pn
42725), and have not had any vapor-lock or
boiling problems since. The foam was tested by
placing a piece of solder on a sample and melting
it into a ball with a heat gun. The foam was not
affected. A propane torch will burn it but when
the torch was removed the fire extinguished. The
fuel system components are liquid cooled and
should be insulated, blast tubes are a waste of
cooling air on these, but make sense for heat
generating components such as mags vac pumps and
alternators.

Avoid methanol like the plague.

What would I do different?

1   Install the flowmeter downstream of the
engine driven fuel pump and insulate it.

2   Make the header location a baggage
compartment for more storage space and CG
adjustments.

3   Wet out the leading edge D-tubes. One D-tube
would be connected to its main wing tank to make
one large capacity tank. The other would be a
separate tank for AV-gas.

The engine and airframe have about 1400 hrs.
since new. The engine has burned a lot of mo gas
and seems to be in good shape. I clean the plugs
every 200 hrs, need it or not.


__________________________________________________



=====
BILL HANNAHAN
WFHANNAHAN@YAHOO.COM



__________________________________________________
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