Vought F-8A Crusader was selected by NASA as the test-bed aircraft to
install an experimental
supercritical wing in place of the conventional wing.
The unique design of the supercritical wing reduces the effect of
shock waves on the upper surface near Mach 1, which in turn reduces drag.
Sometimes, breakthroughs in technology
are not used in the way that their inventors intend. They may lead to
surprising applications. The supercritical airfoil, first developed by a
NASA aerodynamicist in the 1960s, provides an excellent example.
An airfoil is the shape of a wing's
cross-section. Slice a wing like you would slice salami and you can see
the shape of the airfoil. This shape defines how much lift the wing
generates at various speeds. By the 1950s, airfoil research in the United
States and most other countries had reached a standstill. Aerodynamicists
had become more interested in other research questions, such as how the
air flowed over an aircraft's skin when it travelled faster than the speed
of sound (Mach 1) and how much wings should be swept back (or angled back)
from the fuselage of the plane. Although designers still developed new
airfoils, they did so on a case-by-case basis for use on specific
airplanes. Few undertook basic research on airfoil shapes for a whole
class of aircraft.
That changed in the 1960s when NASA
scientist Richard T. Whitcomb developed the supercritical airfoil.
Whitcomb was already famous for developing the "Area Rule" of supersonic
flight which won him the Collier Trophy and which resulted in some
fighters of the mid 1950s having a pinch midway along their fuselage, like
an hourglass. Probably no one understood how air flowed over an aircraft
as it approached the speed of sound better than Whitcomb, and he applied
that knowledge to a new problem associated with the compressibility of air
over an aircraft wing as it approached the speed of sound.
At the time, commercial passenger jets
like the Boeing 707 cruised at speeds of around Mach 0.7 to Mach 0.8
("cruise speed" is the speed at which an airplane is most fuel efficient;
commercial airplanes operate at this speed in order to be economical and
not waste fuel). The people who ran the airlines wanted planes that could
travel even faster, at Mach 0.9 or 0.95, and still be fuel efficient.
But as an aircraft approaches the speed
of sound, it reaches a point where the air flowing over the wings reaches
supersonic speeds though the plane itself is still moving slower than Mach
1, causing a dramatic increase in drag. The airspeed at which this occurs
is called the critical Mach number for the wing. For example, if the air
flowing over a wing reaches Mach 1 when the wing is only moving at Mach
0.8, the wing's critical Mach number is 0.8. The spot where this happens
on the wing is usually about halfway between the leading edge and the
trailing edge of the wing.
Cross-sections of three supercritical airfoils.
These are considerably blunter and thicker than conventional transonic
Designers deal with this dramatic
increase in drag by angling the wings back from the fuselage, making them
thinner, and using other features designed to reduce drag. But all of
these solutions increase structural weight, decreasing range and fuel
economy, and making them unattractive for commercial use. In addition,
thinner wings cannot be used to store fuel, a common location for fuel
tanks on passenger planes.
In the early 1960s, Whitcomb sought to
develop a new airfoil shape that would allow the wing to reach a higher
speed before the airflow over it reached the speed of sound. He proposed a
new airfoil shape featuring a well-rounded leading edge, relatively
flatter upper surface (not as curved or cambered as other wings) that
pushed the critical Mach point farther back on the wings, and a sharply
down-curving trailing edge that increased lift. He called this the
"supercritical" airfoil. Whitcomb tested this wing in NASA's 8-foot
transonic pressure tunnel at Langley, Virginia. These tests suggested that
the supercritical wing might allow planes to travel up to 10 percent
faster. Alternatively, a plane with the new wing could fly more
efficiently at the same speed (for example, a plane that normally cruised
at Mach 0.7 could be equipped with a supercritical wing and achieve better
wing on the F-8 research airplane.
The wind tunnel tests, however, involved
small models with low Reynolds numbers making tests of the supercritical
wing unreliable. For full-scale tests, NASA engineers chose a Navy Vought
F-8U fighter as a test aircraft. The F-8U was normally a fighter aircraft
capable of supersonic flight, but NASA engineers wanted to use it to
determine if an aircraft could cruise just below the speed of sound, a
speed range known as the transonic region. NASA engineers equipped the
plane with a slender, graceful supercritical wing and tested it in 1971.
The F-8U flight tests proved that Whitcomb's wind tunnel results were
correct: the supercritical airfoil would allow planes to cruise at higher
speeds. Passenger jets could be equipped with wings that would allow them
to fly at Mach 0.9 or 0.95 instead of Mach 0.7 or Mach 0.8, and still be
relatively fuel efficient.
NASA presented the resulting wind tunnel
and flight test data at a conference in 1972. Industry designers were
intrigued by the data and started to evaluate it. They then came up with a
surprising conclusion: instead of increasing cruise speed to around Mach
0.9, they would keep the speed around Mach 0.8, but use the supercritical
shapes to increase fuel efficiency. A more efficient aircraft could travel
farther on the same amount of fuel. The commercial airlines had told the
airplane builders that this was what they wanted—planes that could fly
farther more economically rather than planes that could fly faster.
By the mid-1970s, supercritical wings
were being incorporated into a whole range of aircraft, from subsonic
transports to business jets. In addition to being more fuel efficient, the
blunt leading edge of a supercritical wing improved takeoff and landing
performance, as well as manoeuvrability. As a result, the most
enthusiastic users of supercritical airfoils are designers of cargo
transport planes. The Air Force C-17 has a supercritical wing that gives
it excellent performance for a plane of its massive size. The
supercritical wing may also eventually see use for its original purpose:
commercial aircraft designers have recently begun looking at the
possibility of designing large passenger jets that can cruise just below
the speed of sound.