7, 1998 Design News
Engineers Drive a Light Aircraft Revival
Mark Allan Gottschalk, Western Technical
and Anna Allen, Staff Editor
Picture yourself flying off to the office in the morning.
You're clutching a mug of coffee in your right hand
and steering a light aircraft with your left along a
virtual road. This scenario isn't just some science
fiction writer's vision of the future, but rather the
expected results of design advancements in the general
aviation industry. Like a phoenix rising from the ashes,
general aviation is back. Nearly exterminated in the
1980's, the industry has recently seen a jump in research,
development, and production activity perhaps unmatched
in history. Around the world, engineers are on a mission
to launch light aircraft design into the 21st century.
The life signs are appearing not a day too soon. Over
the past 20 years, production of general aviation (GA)
aircraft plunged 94%, from 17,811 in 1978 to just 1,132
in 1996. During this same time frame the cost of a single-engine
aircraft as compared to the mean family income went
from a factor of 2:1 to 4:1. The average light aircraft
is 28 years old and incorporates antiquated flight deck
layouts and piston propulsion technology from the 1950s
Yet, light aircraft account for a surprising 62% of
the total flight hours, 37% of the miles, and 78% of
the departures in the U.S.--figures that advocates hope
to significantly increase by moving the popular air
travel metaphor away from that of an air bus and closer
to an air car.
To that end, NASA has made the revival of the general
aviation industry a significant portion of one of the
administration's "Three Pillars of Success."
It has allocated more than $115 million to two government/industry
collaborative efforts focused on the task. The largest
of these is the Advanced General Aviation Technology
Experiment (AGATE). Begun in 1994, the five-year program
matches more than $60 million with 70 industry partners
in an extraordinarily broad push to develop new technologies
in several areas, including:
The program's goals are highly ambitious. "We
can create a small aircraft transportation system for
the 21st century that delivers the ability to move about
in the air at four or five times the speed of highways
at the level of affordability we've come to expect from
automobiles," says Bruce Holmes, NASA's AGATE program
manager. "By 2001 we will have finished the essential
technology ingredients for the industry to produce a
first-generation AGATE plane; this should re-energize
AGATE is complemented by the General Aviation Propulsion
(GAP) program, a four-year, $55-million effort launched
in 1997 that targets the heart of every airplane: the
engine. Two powerplants will emerge from GAP. At the
high end is a low-cost turbofan being engineered by
Williams Int'l (Walled Lake, MI). For entry-level GA
aircraft, Teledyne Continental Motors (TCM) of Mobile,
AL, is designing a two-stroke, compression-ignition
piston engine that runs on Jet A/JP-8 or diesel fuel.
Both programs depict a future for the past 30 to 40
years pundits have insisted was just around the corner.
The difference this time? Engineering experts say we
are finally at the point where technology can deliver
Two factors are driving this potential sea change.
The first is the plummeting price and soaring abundance
of information, combined with ever-more-powerful and
cheaper computing systems to process it. "The convergence
right now of various information technologies will make
it possible to operate a vehicle in three dimensions
with much the same simplicity we've come to expect from
operating a vehicle in two dimensions," says NASA's
The second factor is the development of inexpensive,
quiet, reliable, efficient powerplants. "If you
look back at the aircraft industry, every major step
in aircraft development was due to a major step in propulsion,"
says Leo Burkart, GAP program manager at NASA Lewis.
"New engines alone won't reinvigorate the industry,"
he notes, "but a lack of new engines could prevent
Engines beget airplanes. At Williams
Int'l, engineers are busy designing the engine they
feel will bring jet propulsion to the masses. Called
the FJX-2, it's a 14-inch diameter by 41-inch-long,
barrel-shaped dynamo that weighs less than 100 lbs yet
pumps out 700 lbs of thrust. Its 4:1 bypass ratio helps
make it "the world's quietest jet engine by far,"
says Dr. Sam Williams, the company's chairman. But the
real secret is its low cost.
Williams is shooting for an order-of-magnitude reduction
in cost for a typical engine of this class. "We've
got to take these from hundreds of thousands of dollars
down to tens of thousands of dollars," says Burkart.
The only reason the turbine engine is not the dominant
form of propulsion in the light aircraft market today
is that it costs too much, he explains. They've got
pretty much everything else--high performance, low weight,
and natural smoothness.
Engineers made cost reduction a primary design driver.
Parts count is always an enemy, and they attacked it
ruthlessly. "A small engine is not just a big engine
scaled down," says Dr. Williams. "If it were,
you'd have 10,000 parts." Though he won't reveal
the exact number of components in the FJX-2, a small
cruise missile engine produced by the company has about
Designers combined as many parts as possible into one.
An example: the fan, instead of being an assembly of
blades attached to a hub, is machined from a single
titanium forging. Housings serve multiple purposes,
combining several functions into one part.
Another target is the mechanical power takeoff. On
a typical jet engine, the starter and mechanical power
takeoff for things like the hydraulic system might account
for as much as 20% of the overall engine cost. Williams
Int'l is said to be developing an integral high-speed
starter/generator, though the company won't confirm
The engine is aimed at single- and twin-engine four-
to six-passenger aircraft capable of 350 to 400 knots.
At the Experimental Aircraft Assn. convention in Oshkosh
last year, the company flew an experimental twin-engine
jet aircraft powered by FJX-1 turbofans developed previously
by Williams Int'l. Called the V-JET II, the flight demonstrator
was designed and produced by Scaled Composites (Mojave,
CA), and the aircraft serves as a showcase for advanced
aircraft structure technology.
The V-JET II's fuselage consists of just five parts.
An example is the monolithic structure that begins at
the pressure bulkhead in front of the rudder pedals
and extends to the tail. It includes the windshield
frames, door frames, bulkheads, longerons, wing mounts,
and skin. It was formed in a single cure with no secondary
bonds and no fasteners.
It's come a long way, but more work is necessary. "This
only goes part of the way to what I would consider acceptable
for mass production," says Burt Rutan, president
of Scaled Composites. "We need a fuselage that
is built by machine in 20 to 30 min; it will most likely
be thermoplastic, not thermoset."
Piston progress. Regardless of Williams'
ambition to make the turbine engine the only engine,
the piston engine appears to have a healthy future.
Aimed more at the entry-level airplane buyer are two
designs that shun the leaded gasoline used by today's
light aircraft. One is being developed by TCM, and the
other by a small research and development firm called
D-Starr Engineering (Shelton, CT). Uniquely, both are
compression-ignition engines and, if successful, would
be the first widespread use of this earliest of internal
Engineers have numerous incentives to design for heavier
fuels such as JP-8, Jet-A, or diesel oil. Today, general
aviation is the only application of leaded gasoline.
Lead is an environmental hazard, and leaded fuel is
not as readily available worldwide as jet fuel. The
heavier fuels are also much less volatile and dangerous
in an accident.
After fuel, significant design drivers included reducing
noise, vibration, and harshness, and increasing durability
and performance. NASA also challenged designers to cut
cost by 50%.
TCM's design consists of a four-cylinder, two-stroke,
horizontally opposed, liquid-cooled configuration, with
turbocharged uniflo combustion, four exhaust valves
per cylinder, and a high-pressure direct injection fuel
system. A mono block design reduces part count substantially,
contributing towards reduced cost, increased reliability,
and projected time between overhaul of more than 3,000
hours. Output is expected to be about 200 hp.
Though TCM officials won't confirm the figures, industry
experts say the engine displaces 241 cubic inches and
weighs approximately 290 lbs. Two-stroke diesel engines
normally require a supercharger for scavenging. "However,
we believe that supercharging is not efficient at higher
power output and we have added a turbocharger,"
explains TCM president Bryan Lewis in a written statement
to Design News.
NASA officials say that the company has been working
with Turbodyne (Woodland Hills, CA), a manufacturer
of turbochargers augmented with electric motors that
keep the turbocharger spinning when exhaust flow alone
will not. If successful, this technology might allow
TCM to eliminate the supercharger and its related drive
Another interesting development, though not part of
the General Aviation Propulsion program, D-STAR Engineering's
motor is also a two-stroke compression ignition design.
The company plans to create a family of engines ranging
from two to six