between pistons and turbine jets.” (Jet
fuel doesn’t contain lead.)
FOE suggested aviation grade ethanol (AGE85) fuel as a substitute.
Development of this fuel has been
ongoing for some time, but efforts
to establish an industry specification
have been slow. Not all aspects of the
fuel have been explored and shown
to be satisfactorily compatible with
the existing aircraft fleet.
“Ethanol drastically changes the
water separation of the fuel and
its vapor pressure,” warns EAA’s
Macnair. There’s also a corrosion
potential from regular use; fuels
with ethanol left sitting unused in
vented tanks tend to phase separate
causing corrosion throughout the
fuel system. Ethanol requires more
fuel flow, which leads up to a 40
percent reduction in the aircraft’s
range. Outside of the warm climate
and abundant ethanol infrastructure
of Brazil, Macnair says, AGE85 just
won’t be viable for aircraft.
Not only is ethanol harmful, but
the autogas STCs held by EAA and
Petersen Aviation forbid the use of
ethanol blends. EAA holds STCs for
Octane Needs Vs Compression Ratio
Octane needs for various compression ratios can be estimated for air-cooled aircraft engines as follows:
Low compression (7: 1 to 7.2: 1) type certified to 80 MON
Mid compression (8: 1 to 8.5: 1) type certified to 91 MON and STC’d
to 87 MON premium autogas
High compression ( 8.7: 1+) & turbocharged engines type certified
to 100+ MON
71 engines and more than 100 airframes from 17 manufacturers. As of
May 2008, 65,000 STCs have been
issued. (EAA first tested lead alternatives in 1964 and in 1982 opened
the door to STCs for automotive
fuel, or autogas/mogas.)
That experience will help EAA gain
new STCs for new fuels or modifications to the high-performance aircraft
fleet. “What we’re looking for is a way
to expedite the process to certify groups
of aircraft, or families of engines, to use
new and existing fuels. We are working
with the FAA to find new and creative
ways,” explains Macnair.
There’s a mountain of problems
to address besides the performance
loss that engines exhibit when using
unleaded fuel. Changes in fuel flow
or horsepower output will affect performance, and aircraft handbooks
will have to be rewritten. A fuel that
weighs more or less would throw off
an aircraft’s weight and balance, as
would a new and heavier engine.
EAA estimates at least 80 percent of
the piston aircraft fleet will function
without problems on unleaded avgas,
specifically the low- and mid-com-pression engines designed for octanes
up to 80 and 91 MON, respectively.
