Background |
|
In the early 1950’s,
the Langley Research Center was recognized worldwide
as a leader in fundamental and applied research
on vertical takeoff and landing aircraft. Leaders
in the Langley research efforts included John P.
Campbell, Richard E. Kuhn, John P. (Jack) Reeder,
and Marion O. McKinney. The challenge of providing
efficient vertical flight with minimal penalties
and adequate payload produced a myriad of candidate
concepts, including aircraft-tilting (tail sitters),
thrust-tilting (tilt rotors), thrust-deflection
(deflected slipstream), and dual-propulsion (lift-cruise
engines) concepts. The Langley researchers had
accumulated in-depth experience with each concept
and had identified the limitations and complexities
that constrained the satisfactory growth of vertical
and short takeoff and landing (V/STOL) aircraft.
In view of this vast experience and their innovativeness
and visionary personalities, the researchers were
actively sought for assessments and opinions of
emerging V/STOL concepts..
In the United Kingdom, Hawker Aircraft Ltd. was
privately funding the development of a new V/STOL
tactical strike aircraft known as the P.1127. Initial
interest from the British government had been lukewarm,
and Hawker aggressively pursued potential funding
from the Mutual Weapons Development Program (MWDP)
of the North Atlantic Treaty Organization (NATO)
for development of the revolutionary P.1127 engine.
This engine utilized four swiveling nozzles to
redirect the engine thrust for vertical or forward
flight. The U.S. members of the MWDP were particularly
impressed with the P.1127 concept, and with their
outspoken leadership, critical development funds
were provided to Bristol Siddeley, the engine manufacturer,
in June 1958.
The support for the P.1127 project from the U.S.
military (particularly the Marine Corps) and NASA
has been a key element in the success of the Harrier,
which continues to the present day.
|
Contributions to
the P.1127 |
|
As Hawker proceeded
in the engineering development of the P.1127 from
1959 to 1960, numerous critical issues arose. These
critical issues included the design of the flight
control system; whether artificial stabilization
was required; the lifting capability of the aircraft
in ground effect; and the stability, control, and
performance of the P.1127 in conventional flight.
Perhaps the most daunting question was whether
the aircraft could satisfactorily perform the transition
from hovering flight (supported by the vertically
directed engine thrust) to conventional wing-borne
flight. Many skeptics—particularly in the
British government—believed that the transition
maneuver would be far too complex for the pilot
or that the P.1127 would not maintain adequate
lift to permit a safe conversion.
John Stack, then Assistant Director of Langley
and an active member of the MWDP, regarded the
P.1127 as the most significant advance since the
achievement of operational supersonic speeds in
fighters. Stack directed the Langley team to provide
full support to the emerging P.1127 technology
by conducting tests in the unique facilities at
Langley. Two model test programs were initiated.
One, free-flight tests of a 1/6-scale dynamically
scaled powered model in the Langley 30- by 60-Foot
(Full-Scale) Tunnel, was used to determine the
characteristics of the P.1127 in the transition
maneuver. The other program used a large-scale
powered model (with simultaneous simulation of
inlet and exhaust flows) for force tests in the
Langley 16-Foot Transonic Tunnel to determine the
complex propulsion and airframe interactions over
the operational flight envelope. The 1/6-scale
model was also used for tests on the Langley Control
Line Facility (a large rotating crane equipped
with control lines for testing powered models)
to determine characteristics during rapid transitions
to and from hovering flight. All tests were slated
for completion prior to the initial flights of
the prototype aircraft in 1961.
Shielded by a protective panel,
Langley researchersRobert O. Schade and Louis P.
Tosti
hover the 1/6-scale free-flight model in the airflow
return passage of the Full-Scale Tunnel.
Under the direction of Marion McKinney, the free-flight
model tests showed that the P.1127 model behaved
extremely well when compared with other V/STOL
designs tested by Langley. Transitions to and from
forward flight were easily performed, and thrust
management was relatively simple. Several problems
were identified, however, including the fact that
the model lacked sufficient lateral control power
for satisfactory behavior during the transition.
(The control power of the aircraft was increased
as a result of these tests.) A tendency to pitch
up due to longitudinal instability at high angles
of attack was anticipated based on Hawker wind-tunnel
tests and was readily apparent in the model flight
tests. (This problem was subsequently cured by
adding anhedral or droop to the horizontal-tail
surfaces of the P.1127 and subsequent variants.)
Despite these shortcomings, the P.1127 was judged
to be a superior performer by the Langley researchers.
The free-flight model tests were witnessed by
leaders of the Hawker design team, including William
Bedford, the P.1127 test pilot slated to make the
first conversion flights of the aircraft. While
at Langley, Bedford flew Langley’s variable
stability research helicopter, which had been programmed
to simulate the control powers and sensitivities
of the P.1127 design. The variable stability features
of the helicopter provided valuable information
that was used by Hawker in the control system design
of the P.1127.
Hovering flight tests of the P.1127 were conducted
in the United Kingdom on October 21, 1960, followed
by the first conventional flight on March 13, 1961.
Finally, on September 12, 1961, transition flights
both to and from wing-borne to jet-borne flight
were accomplished. The overall results of these
flight programs agreed remarkably well with the
Langley model tests and the helicopter in-flight
simulations of 1960. John Stack witnessed transition
flights in gusty conditions a week later and referred
to them as the smoothest transition of any of the
existing crop of V/STOL machines. Stack subsequently
noted that the precursor Langley tests had, in
fact, indicated that the P.1127 would have better
characteristics than any other concept previously
investigated.
Perhaps the most important compliment to the
Langley contributions prior to the first flights
came from Sir Sydney Camm, the Chief Designer of
Hawker (designer of the Hawker Hurricane fighter
of WW II), who said that the Langley wind-tunnel
tests were the most important tests for the P.1127
project prior to flight.
Despite the success of the P.1127 flight program,
the British Royal Air Force did not consider the
aircraft as a serious strike aircraft, citing an
unacceptably small payload capability and low engine
thrust. Aggravating the lack of interest, in March
1961 NATO requested proposals for a new V/STOL
close-support fighter with supersonic speed capability.
The Hawker design team responded with the P.1154,
a configuration with twice the thrust, twice the
speed, twice the weight, and twice the performance
of the P.1127. While pursuing the P.1154, Hawker
continued demonstrations of the subsonic P.1127
and kept the program alive.
Langley chief test pilot Jack
Reeder evaluated the P.1127 in 1962.
On June 13, 1962, Langley chief test pilot Jack
Reeder became the first foreign pilot to fly the
P.1127. Despite the fact that he had never flown
in the military service, Reeder was vastly experienced,
having flown over 177 different aircraft of which
7 were V/STOL research aircraft. His V/STOL flight
experience was unrivaled and Hawker eagerly awaited
his opinion of the P.1127. Reeder returned to the
U.S. filled with enthusiasm for the P.1127 and
he influenced many decision makers regarding the
potential of the aircraft for military applications.
The Labour government in the United Kingdom cancelled
the P.1154 program and instructed the frustrated
Royal Air Force to accept an upgraded version of
the subsonic P.1127—the Harrier. But first,
the P.1127 was developed into an interim version
known as the Kestrel. Nine Kestrel aircraft participated
in a unique international collaboration that was
designed to assess the practicality of V/STOL operations
in the field.
|
Contributions to
the Kestrel |
|
Under the leadership
of the MWDP, an agreement was signed in late 1961
by the United States, the Federal Republic of (West)
Germany, and the United Kingdom to test an improved
P.1127 concept in field conditions in the United
Kingdom. Changes made to the P.1127 to upgrade
it into the Kestrel included a new engine with
increased thrust, a new swept wing with more fuel
capacity than the P.1127 wing, a drooped horizontal
tail, and improved reaction controls. The flight-test
evaluations began in 1965. At the end of 9 months
of flight evaluations, the squadron pilots gave
glowing reports about the flying qualities of the
Kestrel.
Jack Reeder was an active participant in the
evaluation program, especially in discussions about
required improvements in the Kestrel handling qualities.
Following the international flight program, Reeder
persuaded officials to provide Langley with two
Kestrel aircraft for follow-on V/STOL research.
Langley test pilots Lee H. Person, Jr. and Perry
L. Deal flew the Kestrels (designated XV-6A) at
Langley under the leadership of Reeder.
Contributions of the Kestrel flight tests at
Langley had profound impact on the operational
usage of this unique vehicle in both the powered-lift
regime, as well as in conventional maneuvering
flight. Extensive flight evaluations of the efficiency
of existing Kestrel instrument approach procedures
led by researcher Samuel A. Morello identified
new methods to permit safer, more fuel-efficient
approaches and landings. However, the most valuable
contribution made by the Langley team was the flight
research and engineering development that permitted
the rotatable nozzles to be deflected in maneuvering
flight, thereby providing unprecedented maneuvering
for air-to-air combat.
In 1969, the Defense Department requested that
Langley review and comment on a report written
by Dr. John Attinello, an engineer of the Institute
of Defense Analyses, that favorably discussed the
potential of using thrust vectoring on P.1127-type
aircraft to enhance the maneuverability of fighters
in air combat. Although the application of vectoring
in forward flight (VIFF) was fundamentally attractive,
considerable engineering concern existed over potential
control requirements, stability characteristics,
and the physical well being of the engine in such
maneuvers.
The Langley team designed a flight-test program
to develop and evaluate the VIFF concept with the
Langley Kestrel. Person and Deal were assigned
as project pilots and Richard G. Culpepper was
assigned as project engineer. Hawker was initially
very skeptical and very concerned over the flight-test
objectives, with a special concern expressed over
the internal air ducts leading to the reaction
control “puffers” at the wingtips and
tail of the Kestrel. Operation of the reaction
controls at high speeds could result in the internal
ducts bursting. There was also concern about handling
problems, especially at high angles of attack.
Lee Person and Jack Reeder with
the two Kestrel (XV-6A) aircraft assigned to Langley.
The initial flight trials were conducted in straight
and level flight, with Person carefully evaluating
internal duct pressures, angle of attack, engine
exhaust gas temperatures, and handling qualities.
The flight envelope was gradually opened up to
250 knots, then up to 450 knots, until the nozzles
could be deflected from the horizontal (cruise)
position downward through 90 deg to the breaking
stop position. In level flight, the deceleration
of the aircraft was extremely high and Person reasoned
that the abrupt change in speed could be used to
force an enemy pilot to over shoot and become the
target. The Kestrel also experienced a nose-up
trim change when VIFF was used, but forward stick
could be used to maintain attitude. During simulated
air combat maneuvers, Person would rotate the nozzles
all the way down, roll the wings into the turn
and create a very rapid, very high decelerating
turn. Other Langley pilots who chased the Kestrel
in a Langley T-38 aircraft (including Robert A.
Champine, noted X-1 pilot) observed that the Kestrel
appeared to “turn a square corner”
and added their enthusiasm to that of the test
crew.
This initial exploration of VIFF was conducted
by Langley from January 1970 to the end of June
1970.
Following the completion of the flight trials
of VIFF in 1970, attempts to obtain a more modern
Harrier aircraft for follow-on VIFF experimentation
met with great disappointment because only six
development batch Harriers were available, and
all six were heavily involved in development flying
in the United Kingdom. Former astronaut Neil A.
Armstrong, then serving as Deputy Associate Administrator
for Aeronautics at NASA Headquarters, used his
influence to obtain a British Harrier and have
it modified for VIFF research by NASA. A joint
VIFF program between NASA and the Royal Aircraft
Establishment was initiated in 1972, and flying
in the United Kingdom continued through 1976. Results
obtained in flight evaluations against a variety
of high-performance adversary aircraft and analyses
of evasive maneuvers provided by VIFF against enemy
ground-to-air and air-to-air missiles resulted
in overwhelming support for VIFF as a valuable
tool for the AV-8 pilot.
As far as the U.S. Marine Corps is concerned,
the engineering contribution of NASA was invaluable
in developing and proving the VIFF concept (ref.
1).
Other studies of the Kestrel included wind-tunnel
tests of a large, powered model in the Langley
V/STOL Tunnel (later renamed the Langley 14- by
22-Foot Subsonic Tunnel) by a team led by Richard
J. Margason with the objective of establishing
wind tunnel to flight correlation. Although the
test results were published in a NASA report, the
flight correlation effort was not undertaken (ref.
6).
|
Contributions to
the AV-8 |
|
The U.S. Marine
Corps has unquestionably been the strongest supporter
of the Harrier concept. Most aircraft development
programs, however, are driven by politics, service
rivalries, and many factors other than technology.
In the fall of 1972, the U.S. Navy issued a request
for proposals of the next generation V/STOL aircraft.
Unfortunately, the list of candidates did not include
any further development of the Harrier. Instead,
the Navy favored the North American Rockwell XFV-12A
supersonic fighter design. The XFV-12A used a thrust
augmentation scheme that diverted the total exhaust
flow of the main engine and ejected it through
a venetian blind arrangement in the wings to give
vertical-lift capability. The concept was considered
by many at Langley to be very risky when compared
with the proven Harrier approach, but the Navy
was prepared to fully fund the development of the
aircraft and close out further development of the
Harrier. Two activities subsequently transpired
that resulted in Langley contributions to the AV-8
program. First, Langley supported test and analysis
of the XFV-12A. Second, Langley contributed airfoil
design methods and wind-tunnel databases that played
a key role in the development of the second-generation
AV-8B.
Naval Air Systems Command requested Langley support
for the XFV-12A Program. This support included
testing a free-flight model in the Langley Full-Scale
Tunnel and a spin model in the Langley 20-Foot
Vertical Spin Tunnel and conducting a remarkable
hovering test evaluation of an XFV-12A prototype
at the Langley Impact Dynamics Research Facility
(IDRF). This facility, previously known as the
Langley Lunar Landing Facility, had been used to
train astronauts for the reduced gravity levels
of the moon’s environment. Interest in using
the IDRF was stimulated by the difficulty of mounting
the XFV-12A airframe on a more conventional pedestal
mount for the hover test.
The results of the free-flight model tests in
1974 in the Full-Scale Tunnel indicated that the
projected thrust augmentation for the XFV-12A was
considerably less than expected, and the thrust
available for vertical flight was insufficient
to permit powered-lift flights. Although the configuration
flew well in conventional wing-borne flight, the
Langley team expressed grave concern over the deficient
V/STOL capability of the free-flight model.
In early 1978, tethered hover tests of the full-scale
XFV-12A on the IDRF were carried out by a joint
team of NASA, Navy, and Rockwell personnel. Richard
G. Culpepper served as the lead Langley engineer
for the investigation. The IDRF had under-gone
major modifications to permit static and dynamic
tethered hover tests for powered V/STOL aircraft.
During 6 months of tests, it became apparent that
major deficiencies existed in the XFV-12A for hovering
flight, including marginal vertical thrust. Although
the augmentation of flow at the wing augmentors
was as predicted, large losses in the internal
ducting and corners of the propulsion system seriously
degraded the net thrust to the extent that only
75 percent of the weight of the vehicle could be
supported in attempts to hover. The results of
the tests at Langley influenced the Navy’s
decision to cancel the XFV-12A Program.
Langley researcher William Newsom
with the XFV-12A free-flight
model with augmentor doors on wing and canard open.
XFV-12A aircraft mounted for
tethered hover flights on Langley Impact Dynamics
Research Facility.
Meanwhile, faced with the potential end of the
Harrier program, McDonnell Douglas and its partners
launched a major redesign effort to provide a significant
improvement in the V/STOL capability of the AV-8A.
McDonnell Douglas engineers drew on two fundamental
research efforts at Langley to assist them in redesigning
the AV-8A into the AV-8B. Under the leadership
of Richard E. Kuhn, Langley researchers in the
Langley V/STOL Tunnel conducted systematic wind-tunnel
studies of the aerodynamic interactions that occur
between rotatable fuselage-mounted nozzles and
a high wing with a trailing-edge flap. The test
variables included a range of geometric relationships
between these components and showed that the resulting
total lift for an aircraft similar to the Harrier
could be significantly impacted (both favorably
and unfavorably) by the positioning of these elements.
Drawing on this database, the McDonnell Douglas
engineers arrived at the current AV-8B wing-nozzle-flap
configuration, which resulted in an increase of
more than 6,000 lb of lift beyond that produced
by the AV-8A arrangement. In addition, McDonnell
Douglas used methods that had matured from the
research of Dr. Richard T. Whitcomb for supercritical
airfoils. The resulting AV-8B wing design has a
thicker wing with better performance at high speeds,
better fuel consumption, and provides an increase
in internal fuel capacity of over 40 percent.
After the decision was made by NASA Headquarters
in 1973 to consolidate all V/STOL research under
the leadership of the NASA Ames Research Center,
additional wind-tunnel and V/STOL flight research
on the Harrier was conducted by Ames, and close
NASA involvement in the AV-8 Program continues
today.
Langley continues to support the program in areas
unique to Langley expertise and facilities. For
example, spin tunnel tests were conducted at Langley
for the AV-8B in 1984, as the external configuration,
armament, and other important factors changed in
the AV-8 fleet.
|
