10/31/2007. Messerschmitt-Bölkow-Blohm (MBB) developed this CCV
(control configured vehicle) aircraft concept for the
Bundesministerium der Verteidigung (German Federal Ministry of
Defense). The aim was to develop and test a quadruplex-redundant
electrically signalled flight-control system and to flight-test a
higly unstable combat aircraft. In other words, an aircraft with
"artificial" stability, i.e. stability produced by technical control
methods.
The F-104G has a high inherent longitudinal static stability, and to
move the overall center of pressure forward of the center of gravity
by the planned 20 per cent mean aerodynamic chord required both a
jettisonable aft ballast and a canard foreplane. Stability is restored
artificially by the a digital fly-by-wire flight control system.
Triplex-redundant, self-monitoring actuators controlled the hydraulic
actuators.
The Bundesministerium supplied a Fokker-built Lockheed F-104G
Starfighter, s/n 23+91 (later re-serialed 98+36), that was modified in
stages. The first test flights were made in basic, stable F-104G
configuration to calibrate the aerodynamic sensors. Next, 1,323 lb
(600 kg) of aft ballast was added to move the center of gravity aft
towards the aerodynamic center. The aft ballast was then removed and
replaced by 529 lb (240 kg) in the front fuselage, plus additional
fuel in the front fuselage cell. In 1980 the canard, a complete F-104
tailplane section, was added to move the aerodynamic center forward on
to the center of gravity, the bulges on the fuselage housing the
actuators. The aircraft could be flown manually in these conditions.
Unstable testing began by installing 882 lb (400 kg) of ballast in
the canard equipped aircraft, moving the center of gravity 10 per cent
of the mean aerodynamic chord behind the aerodynamic center. This
condition was re-stabilized by jettisoning the lead shot aft ballast.
The highly unstable configuration was reached by adding the full 1,455
lb (660 kg) aft ballast then rescheduling fuel usage. Fuel was kept in
the tip tanks to refill the forward and main cells for emergency
re-stabilization.
Between December 1977 and December 1981, MBB completed more than 120
flights. Tests covered the complete flight envelope. including
supersonic speeds, in neutral, unstable and highly unstable
configurations. MBB studies indicated that artificial longitudinal and
directional stability would reduce overall drag by 20 to 30 per cent
while increasing overall lift, resulting in a reduction in
combat-aircraft take-off weight of 15 per cent relative to a stable
design.
Other advantages of active flight controls include: direct lift and
side-force control to improve weapon delivery accuracy; flutter
suppression to eliminate flight-envelope restrictions; gust
alleviation for greater crew comfort; and gust and maneuver load
control to increase airframe life. The test results from this program
have been used in the conception and development of the European
Fighter Aircraft that became the Eurofighter Typhoon. (View also photo
7026)
