| The EA-6B has a
significantly higher design gross weight than the
A-6E; however, the EA-6B employs the same wing
to carry the increased load. This increased wing
load contributed to an alarming number of EA-6B
accidents in the early 1980’s. During that
period the EA-6B aircraft experienced accident
rates in fleet operations that were nearly three
times higher than all other Navy and Marine aircraft
combined. The majority of these mishaps were attributed
to out-of-control flight and resulted in the loss
of the aircraft after the pilots were unable to
recover the aircraft and were forced to eject.
These losses prompted many fleet squadrons to restrict
the EA-6B from intentional maneuvers at high-angle-of-attack
conditions. While the restrictions substantially
reduced the accident rates, they also imposed constraints
on evasive maneuvers while operating in high threat
environments.
In late 1984, the Navy approached Langley to
undertake a research program to improve the EA-6B,
with emphasis on increasing maximum usable lift,
maintaining lateral-directional stability near
stall, and maintaining lateral control near stall.
Langley agreed to lead this effort, under the cognizance
of the Navy. Grumman joined the effort and provided
additional technical support to Langley, as well
as participating in additional wind-tunnel tests
at NASA Ames Research Center for the high-lift
configuration. This program evaluated the EA-6B
in ten NASA and Grumman facilities.
The modified EA-6B model in flight
in the Full-Scale Tunnel.
Langley researcher David E. Hahne
with the EA-6B free-flight model.
Langley research activities on the high-angle-of-attack
stability and control characteristics of the EA-6B
were led by Joseph L. Johnson, Jr. and his staff
of the Low Speed Aerodynamics Division. The efforts
to improve maximum lift and maneuver performance
were led by Percy J. (Bud) Bobbitt and his staff
of the Transonic Aerodynamics Division.
Johnson’s team conducted extensive tests
in the Langley 12-Foot Low-Speed Tunnel and the
Langley 30- by 60-Foot (Full-Scale) Tunnel with
flow visualization and force and moment measurements.
The Langley leaders in this effort were Frank L.
Jordan, Jr. and David E. Hahne. The results of
these tests showed that the vertical tail was adversely
affected by flow emanating from the fuselage and
wing root areas for high-angle-of-attack conditions,
which resulted in a severe loss of directional
stability. The problem was resolved by extending
the vertical tail above the existing fin pod, adding
leading-edge droop to the inboard wing, and adding
a strake to the wing-fuselage intersection. Roll
control at high-angle-of-attack conditions was
augmented by using the existing wingtip speed brakes
as additional ailerons. These modifications were
tested with a 0.12-scale free-flight model in the
Full-Scale Tunnel. The results clearly demonstrated
the potential of these modifications to improve
the flight characteristics of the EA-6B at high
angles of attack.
Bobbitt’s team worked to improve maximum
lift. This task was a challenge as changes in wing
contour were limited to the leading-edge slat and
trailing-edge flap to keep retrofit costs low.
Several advanced airfoil designs were tested by
William G. Sewall, Robert J. McGhee, and James
C. Ferris in the Langley 6- by 28-Inch Transonic
Tunnel and in the Langley Low-Turbulence Pressure
Tunnel (LTPT). The results of the airfoil studies
were used to design advanced slats and flaps for
the EA-6B that produced a substantial increase
in lift, as well as decreased drag at cruise Mach
numbers. Edward G. Waggoner and Dennis O. Allison
led computational and experimental studies to design
the wing configurations to increase the low-speed
maximum lift capability of the EA-6B with minimal
degradation in high-speed performance. Their work
ranged from the application of low-speed and transonic
computational methods to experimental verification
tests in the Langley 7- by 10-Foot High-Speed Tunnel
and the National Transonic Facility (NTF) at Langley.
The integrated efforts of these two teams identified
new leading- and trailing-edge configurations that
dramatically improved the high-lift performance
of the EA-6B. The modifications defined by the
Johnson and Bobbitt teams were then tested on a
model at Ames to assess the impact on the high-lift
configuration. Spin tunnel tests were also conducted
to evaluate the impact of the changes on spin and
spin recovery characteristics. No degradation was
noted during the tests, and the existing spin characteristics
of the EA-6B were projected to remain unchanged.
The final tests included wing loads tests led by
Langley researcher Charles Mercer in the Langley
16-Foot Transonic Tunnel. Tests of a full-size,
EA-6B semispan wing in the Full-Scale Tunnel led
by Jordan and Hahne validated the effectiveness
of using the speed brakes as ailerons and provided
additional loads data. Because the A-6E had significant
problems with fatigue life, it was important to
assess the impact of the modifications on wing
loads and wing root bending moments to ensure that
the EA-6B wing fatigue life would not be degraded.
The impact of the Langley-led test program was
substantial. Test data indicated the potential
for a 25-percent increase in maximum usable lift
in the cruise configuration. Lateral and directional
stability could be maintained to angles of attack
well beyond stall. Lateral control could be maintained
beyond stall by using the speed brakes as ailerons.
A performance improvement due to decreased drag
could be realized at medium and high altitudes
where a majority of the EA-6B missions are flown.
Approach speeds could be substantially reduced
at existing landing gross weights, and growth capability
was provided for higher gross weights. Finally,
an extensive, unprecedented aerodynamic database
for the EA-6B had been produced and was ready for
incorporation into simulators for EA-6B pilot training.
EA-6B model being prepared for
tests in the National Transonic Facility at Langley.
EA-6B Vehicle Enhancement Program
(VEP) test aircraft during flight evaluation.
The results of this outstanding NASA, Navy, and
Grumman joint program were summarized in several
technical papers in 1987. The effort received such
positive national attention that in the 1987 Applied
Aerodynamics AIAA meeting, an entire session was
dedicated to the EA-6B effort with five papers
presented. Aircraft modifications defined by the
joint EA-6B program were directed at an advanced
version of the aircraft known as the EA-6B ADVCAP
(Advanced Capability), which included a myriad
of improvements such as new engines and electronic
upgrades, as well as the airframe modifications.
Flight tests of a modified EA-6B designated the
Vehicle Enhancement Program (VEP) test aircraft
were conducted to evaluate the effects of the airframe
modifications derived from the Langley-led joint
studies. Flights began on June 15, 1992, and the
final flight occurred on April 4, 1994. The flight
evaluation of the test aircraft validated all projections
of the research program, and the performance, stability,
and control characteristics of the modified aircraft
were judged to be far superior to those of the
basic EA-6B. The aircraft performed flawlessly.
Unfortunately, fiscal constraints and other programmatic
issues restrained the Navy from funding the upgrades.
EA-6B simulators, however, have been upgraded with
the data from Langley.
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