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Scott Crossfield
Edition: World’s Most Reliable
Piston Engine?
Employing cryogenic technology,
the Scott Crossfield Edition engine
may be the most reliable in
general aviation.
By Bill Cox
Those pilots privileged to know Scott
Crossfield regarded him as one of
aviation’s most durable good guys.
In a career that spanned some six decades,
Scotty flew a little of everything, from the
Wright Flyer to the X-15, and he somehow
managed to survive them all.
Sadly,we lost Scott Crossfield in April
2006 to an inflight breakup in a Georgia
thunderstorm, but not before he initiated
development of the Crossfield Edition
engine. Scotty lived every day of his 84
years, and much of that time was spent
flying the world’s most exotic rocket
planes. At one point in the late 1950s,
Crossfield hadmore rocket time than any
other test pilot on Earth.
Scotty was more than just a talented test pilot, however. In his own words, he was first
and foremost an engineer,
and he regarded his flight
test talents as secondary.
Crossfield flew the X-1, X-4, X-5, XF-92, D-558-I and
D-558-II before transitioning to the ultimate rocket research plane, the X-15.
Having flown at ridiculous speeds and
altitudes (Scotty was the first man to exceed
Mach 2.0 and the first to survive flying faster thanMach 3.0),Crossfield knew that everything from guitar strings, golf clubs and shotgun barrels to NASA test
aircraft and spacecraft have been treated
cryogenically to harden them for the rigors
of their respective disciplines.
In the case of spacecraft, cryogenic
treatment helps guard against high speeds,
low pressure, high/low temperatures and
other rigors of high altitude flight. Why
couldn’t piston aircraft engines benefit
from the same process?
EnterVictor Sloan ofVictor Engines in
PaloAlto, California.Victor has been providing
some of the world’s most highly
regarded, balanced, blueprinted piston mills for more than 30 years, with some
customers even having their airplanes
flown in from Europe for installation.
Crossfield had flown behind a Victor
Black Edition engine in his Cessna 210A
for several years. The engineer had great
respect forVictor and felt a collaboration
could produce a truly durable powerplant.
Of course, the first question is why
cryogenics? If Crossfield was an expert on the subject,Victor wasn’t unfamiliar with the concept. An avid motorcycle
racer in his younger days, Sloan was conversant
with most of the go-fast techniques
in bike and auto racing, and he
knew that NASCAR and NHRA teams
had long been experimenting with cryogenic
treatment to toughen brake and
suspension systems, two of the most
abused components in racing.
OK, so what exactly is cryogenic tempering?
Cryogenic temperatures are typ- ically those below -244 degrees F, well
below mere “cold” treatment that demands
only -125 degrees F. A typical
cryogenic cycle includes placing the parts
in an airtight cryonic chamber, gradually
lowering the temperature using nitrogen,
holding at -300 degrees F and then
slowly returning the parts to room temperature.
In some cases, a short tempering
cycle completes the process. Cryogenic
treatment typically requires 48 hours, 12
hours to bring the metal parts down to
the proper temperature, 24 hours to cure
and another 12 hours to bring them back
to roomtemperature. It can demand up to
70 hours, depending upon the composition
and amount ofmaterial to be treated.
The result of the treatment is a remarkable
transformation of the molecular
structure of the parts. In addition to
relieving residual stresses within the
metal and hardening the material exponentially,
it’s suspected of actually refining
and realigning the crystalline structure of
the metal.
 The bottom line is a level of hardness otherwise uncommon in the engine overhaul
business. An engine consists of a
number of components, so toughermetal
components alone are no guarantee
against problems, but the cryogenic
process definitely does harden the target.
Accordingly,Victor selected the most
complex and powerful flat engine in general
aviation, a Lycoming IO-720-A1A, as
his test article for the Crossfield Edition
cryogenic application. In general aviation
terms, the 400 hp Lycoming is the peak of
the pyramid with a flat engine. (There are
a number of round engines with seven or
more cylinders that produce more horsepower,
but none remain in production. In
themid-‘60s,Lycoming introduced the IO-720 as the ultimate piston mill, and even
today, it’s the most powerful horizontally opposed
powerplant available.)
For better or worse, the only production
airplane to use the King Kong Lycoming
was the PA-24 Comanche,
perhaps the most hard core Piper ever to
wear an N-number. There are a number
of conversions – Aero Commander, Beech Queen Air – but Ted Smith
Aerostar was the only other manufacture
to experiment with a production model
800 that would have featured twin IO-720s. Smith dropped the project.
Ultimate speed and performance
were the goals for the Comanche 400.
The original Comanches of the mid-‘60s
were wide, comfortable machines, blessed
with pleasant handling, a docile stall
and reasonable economy. Despite what
youmay have heard about the invincible
V-tail Bonanzas, the 250 and 260 hp Comanches
weren’t that far behind the
Beech or the Bellanca 260, and they were
about even with the four-seat Cessna
210. In the early 60s, all four airplanes
used 260 hp engines to cruise in the 160-170 knot range.
No big surprise, then, that Piper
elected to up the ante on the Comanche
by employing weapons grade horsepower.
In 1964, Pipermounted themonster, eight-cylinder, Lycoming on the
nose of a Comanche to create the Comanche
400, producing what was, at the
time, the fastest normally-aspirated roduction
single in the world.
Though the king-sized engine is definitely
more than a pair of four-cylinder,
200 hp, IO-360s welded together, it does
offer a total of 720 cubic inches of displacement
and 400 hp. Trouble is, more
cylinders and horsepower are supposed
to be anathema to reliability. Increase the
number ofmoving parts inside an engine
by 33 percent and you’ll nearly always
lower TBO and increase the incidence of
premature failure.
Piper incorporated a number of
changes to handle the big engine on the
Comanche airframe. Most prominently,
the tail was modified to increase rudder
and elevator authority.There’s essentially
no commonality between the empennage
on the lesser-powered Comanches
and the 400.
 The additional power also dictates
that the 400’s rudder be aerodynamically
balanced in a manner similar to that of
theTwin Comanche.Also, the Comanche
400 doesn’t employ the lead external balance
weights of the lower-powered single-engine Comanches.
Conversely, all the Comanche’s fly
above the same 36 foot span airfoil. The
Comanche 400’s fuselage is a foot longer than the original Comanche 180s, but the
airplanes have a startling resemblance
parked side-by-side on the ramp.
Victor went looking for a Comanche
400 demonstrator for the Crossfield Edition
engine and found one in the Midwest.
A farmer in DesMoines, Iowa had
a reasonably nice 400 parked in a barn
adjacent to his private grass strip,Victor
purchased it as was, relicensed it and flew
it to California for a complete overhaul.
“The point of the Crossfield Edition
isn’t necessarily more power,” Victor
comments. “You can’t legally produce
more than rated power, anyway. But the
exhaust system and all the accessories
drain away some power, so we wanted
an engine that would deliver the maximum allowable
horsepower to the prop.”
At a gross weight of 3600 pounds and
with 400 horsepower on tap, the airplane
enjoys a power loading of 9.0 pounds per
horsepower, the lowest of any general aviation, production airplane outside the
aerobatic category. Power loading isn’t
the only measure of takeoff acceleration,
but it’s one of the most important.
Flying fromVictor Engine’s home base
of Palo Alto, California,with Sloan in the
right seat and Pilot Peggy in back, the
Comanche 400 left little doubt that it was
not your grandfather’s Comanche. To fly
the finished Crossfield Edition demonstrator
is to experience what seems a
gradual tsunami of thrust.
Push the left knob to the wall, and the
engine pulls like a team of rabid Malamutes.
It’s not exactly the same as dropping
the clutch on a ZR1, but among
four-place, cross country airplanes, it’s an
experience you’re not liable to match.
Perhaps contrary to what you might
expect, however, the Crossfield Comanche
doesn’t feel drunk with power.
Acceleration does seem to go on and on
as the Comanche seizes the bit in its teeth and inhales altitude with enthusiasm, but
the Comanche 400 is easily manageable
in the pattern.
In keeping with the airplane’s considerable
power, I used about 120 knots cruise climb and still scored an initial
1400 fpm uphill as we cleared the class B
airspace to the west of Palo Alto and
lofted out over the coastal hills into unrestricted
airspace.
I saw 10,500 feet on the altimeter in a
little under nine minutes.This is one airplane
that’s not dependent on a turbocharger
for reasonable climb.The book
spec for service ceiling suggests
19,500 feet, so there’s plenty left at a
typical cruise height.
The big question, of course, is cruise
performance. The book number for 75
percent is 185 knots at 8000 feet. In 1964,
that made the PA-24-400 the world’s
fastest normally-aspirated production
single. Victor’s Crossfield Edition Comanche
generated more like 190 knots
on the same power, though with only 40
hours on the tach, the new mill was just
getting broken in. Sloan is confident he’ll
see another two or three knots of speed
when the engine is running at optimum.
An obvious benefit is that the Crossfield
Comanche can generate the same horsepower
at higher altitude, so better speed
is almost a given in the thinner air.
Remember, however, that 75 percent
of 400 hp is still 300 hp. Specific fuel consumption
is fairly immutable, about .43
pounds/hp/hr. Do the math, and the result
is about 22 gph, so the top Comanche
isn’t liable to win favor with the
Sierra Club.Thatmeans the standard airplane’s
100 gallon tanks are worth about
3+30 plus reserve. Tip tanks can add 15
gallons a side for a total of 130, worth
about five hours endurance atmax cruise.
As this is written, the Crossfield Edition
Cryogenic Engine has seen less than
100 hours use in Victor Aviation’s Comanche
400. Standard TBO is listed as
1800 hours.With the benefit of cryogenic
tempering, logic suggests it should last
considerably longer than that, perhaps
well over 2000 hours.
If I’m still here when that time comes,
I’ll keep you informed. 
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