illustration shows the reverse airflow on forward swept wings vs. the
airflow on the swept-back wings.
On the forward swept wing, the air tended to flow inward toward the root
wing rather than outward toward the wing tip as on the swept-back wing.
Sometimes technological developments to
overcome aeronautical challenges succeed but provide such a small
improvement in performance or operations that they are never widely
adopted. Such is the case for forward-swept wings, a design concept that
has never really caught on for any kind of aircraft.
French designers ( Landwerlin and Berreur) were doing research on forward
swept wings in the early 1920's at the Eiffel Institute. They built at
least one flying plane in 1922, a single seat racer with no horizontal
stabilizer. The elevators were incorporated near the wing root with the
ailerons at the wing tips. Although they were French a British patent was
issued for the design.
In 1936, a German aerodynamicist first
postulated developing an airplane with its wings swept forward, but nobody
built any actual models at the time. During World War II, the Germans
finally conducted tests of such an aircraft. The Messerschmitt company
built the tailless Me 163B to explore the design. The German firm Junkers
produced the jet-powered Ju 287 light bomber with forward-swept wings.
They did this not because the design had any inherent aerodynamic
advantages but rather to enable the wings to be mounted behind the bomb
bay. In 1944, the obscure airplane manufacturer American Cornelius built
one of the oddest aircraft ever to fly, the XFG-1 fuel transport glider,
which was an un-powered fuel tanker with forward-swept wings. Only two of
the ugly-looking craft were built.
After the war, the National Advisory
Committee for Aeronautics (NACA) in the United States conducted wind
tunnel tests of forward swept wings. NACA engineers even mounted a model
of the Bell X-1 with forward-swept wings in a wind tunnel. But they found
little inherent aerodynamic advantages to such a design. Even the Russians
conducted full-scale model flights of forward-swept-wing gliders but
abandoned the concept.
In 1964, the German airplane
manufacturer Hamburger Flugzeugbau built the HFB-320 business jet with
forward-swept wings. This design allowed the wings to be mounted behind
the passenger cabin along the sides of the fuselage. Only 50 of the
aircraft were manufactured and it remains the only aircraft with
forward-swept wings to enter actual production. For years the primary
purpose for developing forward-swept wings was structural—to allow the
wings to be mounted farther back on the fuselage so that their connecting
structure did not interfere with anything inside the fuselage (like bombs
or people). Wind tunnel tests made it clear that there were many problems
with forward-swept wings and few aerodynamic advantages. One major problem
was that the wingtips tended to bend upwards and cause the plane to
stall—inevitable for metal wings. But in the mid-1970s, a U.S. Air Force
officer noted that new composite materials then becoming available for
aviation could be incorporated into the wings of a modern jet and
eliminate the tendency of the wingtips to bend upward and cause the plane
to stall. At the same time, several U.S. aviation companies were exploring
ways to make planes that were highly manoeuvrable at transonic speeds
(i.e., near the speed of sound).
Aircraft with forward-swept wings are
highly manoeuvrable at transonic speeds because air flows over a
forward-swept wing and toward the fuselage, rather than away from it. By
the late 1970s, the Defence Advanced Research Projects Agency (DARPA)
sponsored a competition to build an experimental forward-swept-wing
airplane. Rockwell International proposed the Sabrebat fighter and General
Dynamics proposed modifying an F-16 Falcon jet fighter. But in 1981, DARPA
finally selected Grumman, which had proposed using parts from several
different aircraft to develop an experimental lightweight airplane soon
designated the X-29. The X-29 used the fuselage from the Northrop F-5A,
the main undercarriage and other equipment from the F-16, and an engine
from the F/A-18. Its wings were made of advanced composites and it was
equipped with small wings called canards mounted on the forward fuselage
rather than on the tail where horizontal stabilizers are usually located.
These helped increase the plane's manoeuvrability. The reverse airflow
inward from the wing tip toward the root of the wing did not allow the
wing tips and their ailerons to stall at high angles of attack.
X-29 Ship No. 2 technology demonstrator was flown by NASA Dryden Flight
in a joint NASA-Air Force program to investigate the unique design's high
angle of attack characteristics and its military utility.
The Grumman X-29 first flew in 1984. It
had a strange appearance, with the wings mounted well back on the
fuselage, and almost looked like it was flying backward. The aircraft
could only be flown with the help of an advanced computer control system.
In numerous tests over the next several years, the X-29 demonstrated that
the forward-swept wing design produced a 15 percent better ratio of lift
to drag in the transonic speed region. But Department of Defence officials
were not significantly impressed by this performance improvement to
approve any further experimental aircraft and the two X-29 aircraft were
soon retired to museums.
lower illustration shows how the canards on forward swept-wing X-29 with
lifting load and reduce drag as compared to a conventional aircraft in the
Forward-swept wings remained dead as a
concept until the surprising appearance in 1997 of the Russian Sukhoi S-37
Berkut ("Golden Eagle") with its forward-swept wings and canards. The S-37
uses the front fuselage of the popular Su-37K fighter, but is otherwise an
entirely new aircraft. It is significantly larger and heavier than the
X-29 and when it first appeared, Western experts speculated that it was a
prototype heavyweight naval fighter. But after several years of
laboriously slow flight tests, Sukhoi did not appear ready to begin
producing large numbers of forward-swept wing naval fighters and the S-37
remains a one-of-a-kind aircraft. Whether this is because the Russians
have been unimpressed with their forward-swept wing airplane's
performance, or simply because of lack of money is unknown. But it is
clear that the forward-swept wing remains a novel solution to a problem
that nobody feels the need to solve.