NASA 1970-1990: Joint Program Momentum Peaks

During the early 1970s, the Ames Flight Simulator for Advanced Aircraft (FSAA) became operational and the first tilt rotor simulations were suc­cessfully accomplished. By 1975, the Army decided to augment the rotary wing flight dynamics research at Ames as NASA initiated the fabrica­tion of the Vertical Motion Simulator (VMS). This simulator, with very large vertical and horizontal motion capability, was a national asset well suited for rotary wing research.

At Langley, a major instrument flight rules (IFR) investigation was conducted under the VTOL Approach and Landing Technology (VALT) program. The VALT Boeing-Vertol CH-47 Chinook helicopter was the pri­mary research vehicle for exploring the control/display/task relationships. In addition, the Sikorsky SH-3 Sea King helicopter was used as a testbed for exploring the merits and defining the electro-optical parameter require­ments associated with advanced "real-world” display concepts. The objec­tive was to identify systems that might be capable of providing a pilot an "out-the-window display” during IFR flight conditions through the use of fog-cutting sensors or advanced computer-generated visual situation dis­plays. The VALT CH-47 flights were conducted at the Wallops Flight Center, where the NASA Aeronautical Research Radar Complex provided omni­directional tracking coverage. This facility permitted the investigation of a wide variety of approach trajectories and selection of any desired wind direction relative to the final approach heading. Computer-graphic dis­plays were generated on the ground and transmitted via video link to the aircraft for presentation in the pilots’ instrument panel. The integrated flight-test system permitted manual, augmented, or completely automatic control for executing flight trajectories that could be optimized from the standpoints of fuel, time, airspace utilization, ride qualities, noise abate­ment, or air traffic control considerations. Many concepts were explored in the IFR program, including flight director control/display concepts and signal smoothing techniques, which proved valuable in achieving fully automatic approach and landing capability.[293] Extensive flight demon­strations were conducted at Wallops Flight Center with the VALT CH-47 aircraft for Government and industry groups to demonstrate the new progress achieved in IFR approach and landing technology.

In structures technology, one of the important outcomes of the space program was the development and implementation of comprehensive computational finite element analyses. State-of-the-art finite element methodology was collected from among the large aerospace compa­nies and unified into the NASA Structural Analysis (NASTRAN) com­puter program. The basic development contract was managed by NASA’s Goddard Space Flight Center and then by Langley for improvements and distribution to approximately 260 installations. During the early 1980s, Langley played a key role in bringing advanced structural design capability into the helicopter industry. The contribution here was the onsite assignment of an experienced structural dynamics specialist at a prime manufacturer’s facility to guide the integration of the prelimi­nary static structural design methodology with rotor dynamic analysis methodology.[294] This avoided the tedious process of repeatedly freez­ing an airframe structural design effort and each time doing a separate dynamic analysis to determine if an acceptable dynamic response cri­terion was achieved.

During this period, the Army added to its already extensive helicop­ter crash-test activities by joining with NASA to crash-test the Boeing Vertol CH-47C helicopter in the Impact Dynamics Research Facility at Langley, which accommodated aircraft up to 30,000 pounds.[295] The facil­ity had been converted from a Lunar Landing Research Facility to a center for the study of crash effects on aircraft. A unique feature of this massive gantry structure was the capability to impact full-scale aircraft under free-flight conditions with precise control of attitude and velocity.

The ongoing rotary wing research began to expand in scope with the establishment of the Army co-located research groups at the three NASA Centers. At Ames, full-scale rotor wind tunnel testing continued at an increased pace in the 40- by 80-foot tunnel. In the 1970s, the wind tunnel tests included the Sikorsky Advancing Blade Concept (ABC) rotor. This rotor concept incorporated two counter-rotating coaxial rotors. The hingeless blades were very stiff to allow the advancing blades on both sides of the rotor disk to balance the opposing rolling moments thereby

The Sikorsky XH-59A Advancing Blade Concept helicopter was a joint test program between the Army, Navy, NASA, and Air Force. NASA

maintaining aircraft trim as airspeed is increased. Forward thrust is supplied by auxiliary propulsion rather than by forward tilt of the main rotor as in conventional helicopter designs.

NASA also tested a full-scale semispan wing-pylon-rotor of the Bell Helicopter tilt rotor design.[296] This test was followed by a similar entry of a semispan setup of a Boeing Vertol tilt rotor concept. During this period, improvements were made in the 40- by 80-foot Full-Scale Tunnel to upgrade the research capability. Its online data capability was aug­mented by introducing a new Dynamic Analysis System for real-time analysis of critical test parameters. A new Rotor Test Apparatus (RTA) was added to facilitate full-scale rotor testing. With this new equip­ment in place, a Kaman Controllable Twist Rotor (CTR) was first inves­tigated in 1975.

In the early 1970s, the modest in-house research funding level for rotary wing projects led to seeking other sources within the new, more elaborate financial system of NASA. It turned out that contracting out – of-house research had become a staple of the rapidly growing procure-

ment system.[297] This offered the opportunity to begin to solicit, select, and fund small supporting research contracts to augment the in-house rotary wing work categorized as Research and Technology Base. In the Flight Research Branch at Langley between 1969 and 1974, over 77 con­tractor reports (CR) and related technical papers were published. The performing organizations included industry and university research departments. The research topics included analytical and experimental investigations of rotor-blade aeroelastic stability, blade-tip vortex aero­dynamics, rotor-blade structural loads prediction, free-wake geometry prediction, nonuniform swash-plate dynamic analysis program, rotor – blade dynamic stall, composite blade structures, and variable geome­try rotor concepts, In the mid 1970s, this entry into contracted research to augment in-house work was further augmented by teaming of NASA and Army rotary wing research at the three NASA Centers. Finally, proj­ects between NASA, the Army, and contractors evolved into major joint efforts in Systems Technology and Experimental Aircraft during the following decade.

The mid-1970s brought two major rotary wing experimental aircraft programs, both jointly funded and managed by NASA and the Army. At Langley, the Rotor Systems Research Aircraft (RSRA) program was launched. This was a new approach to conducting flight research on helicopter rotor systems.[298] Two vehicles were designed and fabricated by Sikorsky Aircraft. The basic airframe, propulsion, and control systems of the two RSRA vehicles were those of the Sikorsky S-61 Sea King heli­copter. In addition, the RSRA incorporated a unique rotor force balance system and isolation system, a programmable electronic control system, a variable incidence wing with a force balance system, drag brakes, and two TF34 auxiliary thrust turbofan engines. As a unique safety feature, the three-member-crew ejection system incorporated automatic bal­anced sequencing of explosive separation of the test rotor-blades as the first step in permitting the rapid ejection of the pilot, copilot, and test engineer. After design and fabrication at Sikorsky, the first of two RSRA vehicles made its first flight in 1976. After initial tests of the helicopter configuration, flight-testing was continued at the NASA Wallops Flight

Center with the Langley-Army project team and contractor onsite sup­port. Acceptance testing was completed by the Langley team, which was then joined by Ames flight-test representatives in anticipation of pend­ing transfer of the RSRA program to Ames.

At Ames, a NASA-Army program of equal magnitude was launched to design and fabricate two XV-15 Tilt Rotor Research Aircraft (TRRA). In this case, the program focused on a proof-of-concept flight investiga­tion. This concept, pursued by rotary wing designers since the early 20th century, employs a low-disk-loading rotor at each wingtip that can tilt its axis from vertical, providing lift, to horizontal, providing propulsive thrust in wing-borne forward flight. The TRRA contract was awarded to Bell Helicopter Textron. Late in the program, as the XV-15 reached flight status, the United States Navy added funding for special mission – suitability testing. Eventually, XV-15 testing gave confidence to tilt rotor advocates who successfully pushed for development of an operational system, which emerged as the V-22 Osprey.

The RSRA and TRRA experimental aircraft programs together rep­resented a total initial investment of approximately $90 million, ($337 million in 2009 dollars), shared equally by NASA and the Army. The size and scope of these programs were orders of magnitude beyond previ­ous NACA-NASA rotary wing projects. This represented a new level of

The NASA-Army Sikorsky S-72 Rotor Systems Research Aircraft in flight at NASA’s Ames Research Center. NASA.

resources in rotary wing research for NASA and with it came consider­ably more day-to-day visibility within the NASA aeronautics program.

The bicentennial year of 1976 also marked a year of major orga­nizational change in NASA rotary wing research. As part of an over­all Agency reassessment of the roles and missions of each Center, the Ames Research Center was assigned the lead Center responsibility for helicopter research. An objective of the lead Center concept was to con­solidate program lead in one Center and, wherever possible, combine research efforts of similar nature. As a result, all rotary wing flight test, guidance, navigation, and terminal area research were consolidated at Ames, which brought these research activities together with the exten­sive simulation and related flight research facilities. Langley retained supporting research roles in structures, noise, dynamics, and aero – elasticity. The realignment of responsibilities and transfer of flight research aircraft caused unavoidable turbulence in the day-to-day conduct of the rotary wing program from 1976 to 1978. However, the momentum of the program gradually returned, and the program grew to new levels with NASA and Army research teams at Ames, Langley, and Glenn working to carry out their responsibilities in rotary wing research.

At Ames, the testing of full-scale rotor systems continued at an increasing pace in the 40 by 80 Full-Scale Tunnel. In 1976, the Controllable Twist Rotor concept was tested again, this time with mul­ticyclic control. "Two-per-rev” (two control cycles per one rotor revolu­tion), "three-per-rev,” and "four-per-rev” cyclic control was added to the CTR’s servo flap system to evaluate the effectiveness in reducing blade stresses and vibration of the fuselage module. Both favorable effects were achieved with only minor effect on the rotor power requirements. The Sikorsky S-76 rotor system was tested in 1977 in a joint NASA – Sikorsky investigation of tip shapes. This was followed by a joint NASA – Bell investigation of the Bell Model 222 fuselage drag characteristics. In 1978, the NASA-Army XV-15 Tilt Rotor Research Aircraft arrived from Bell Helicopter for full-scale wind tunnel tests prior to initiation of its own flight tests. The wind tunnel tests revealed a potential tail struc­tural vibration problem that would be further explored in flight follow­ing the strengthening of the empennage attachment structure. The next rotor test was the Kaman Circulation Control Rotor (CCR) in 1978.[299]

A new concept was introduced based on technology developed at the David Taylor Ship Research and Development Center (since 1992 the Carderock Division of the Naval Surface Weapons Center). The Kaman rotor utilized elliptical-shaped airfoils with trailing edge slots. Lift was augmented by blowing compressed air from these slots. The need for mechanical cyclic blade feathering to provide rotor control was elimi­nated replaced by cyclic blowing. The wind tunnel testing investigated the amount of blowing control necessary to maintain forward flight. In 1979, the Lockheed X-Wing Stoppable Rotor was tested in the 40 by 80 Full-Scale Tunnel. This concept, funded by the Defense Advanced Research Projects Agency, also incorporated a circulation control con­cept. The X-Wing rotor was designed to be stoppable (and startable) at high forward flight speed while still carrying lift. Since two of the four blade trailing edges become leading edges when stopped, provisions were made to provide for separate blowing systems for the leading and trail­ing edges of the blades. When operating as a fixed X-Wing aircraft, air­craft roll and pitch control were provided by differential blowing from the aft edges of opposing, nonrotating blades serving as swept forward and aft wings. The wind tunnel tests of the 25-foot-diameter rotor suc­cessfully demonstrated the ability to start and stop the rotor at speeds of approximately 180 knots (maximum tunnel speed).

The Boeing Vertol Bearingless Main Rotor (BMR) was tested in 1980.[300] The BMR used elastic materials in the construction of the rotor hub rather than mechanical bearings for articulation. Such designs have aeroelastic stability characteristics different from conventional mechan­ical systems. Therefore, the wind tunnel tests investigated the degree of stability present and established appropriate boundaries for safe flight. In addition, in 1980, the Sikorsky Advancing Blade Concept (ABC) coax­ial rotor was again tested in the 40 by 80 Full-Scale Tunnel.[301] In this entry, the full-scale rotor was tested atop a configuration replica of the actual XH-59A flight-test aircraft. This testing focused on an investiga­tion of the drag characteristics of the rotor shaft and hubs of the coaxial rotors. In an effort to reduce the drag, tests were made with the actual fuselage modeled and the actual flight demonstrator hardware compo-

nents utilized to explore several inter-rotor fairing configurations. (In 2008, Sikorsky Aircraft unveiled a new technology demonstrator aircraft incorporating the advancing blade concept identified as the X2. In this design forward thrust is provided by a pusher propeller installation.)

In 1984, Ames shut down the 40- by 80-foot facility for tunnel mod­ification to upgrade the 40- by 80-foot section to a speed capability of 250 knots and add a new 80 by 120 leg to the tunnel facility capable of speeds to 80 knots. The upgraded facility, known as the National Full – Scale Aerodynamics Complex (NFAC), reopened in 1987 and would have been operated by NASA until 2010. However, budgetary pressures forced its closure in 2003. Four years later, in 2007, the United States Air Force’s Arnold Engineering Development Center (AEDC) upgraded key operating systems and reopened the facility under a 25-year lease with NASA. The anticipated majority customer for this national asset was seen to be the United States Army, in collaboration with NASA, in support of rotary wing research.

A Helicopter Transmission Technology program was initiated at the Glenn Research Center to foster the application of an extensive tech­nology base in bearings, seals, gears, and new concepts specifically to helicopter propulsion systems.[302] Research continued at a growing pace. In order to upgrade the analytical methods for large spiral bevel gears, NASA supported the development and validation testing of finite ele­ment method computer programs by Boeing Vertol. The opportunity was taken to utilize the available aft transmission hardware assets, available from the canceled XCH-62 Heavy Lift Helicopter Program, for analyt­ical methods validation data. Another program at Glenn was the joint NASA-DARPA Convertible Engine Systems Technology (CEST) pro­gram. This program involved the modification of a TF34 turbofan engine to a fan/shaft engine configuration for use as a research test engine to investigate the performance, control, noise, and transient characteris­tics. The potential application of CEST was to the X-Wing vehicle con­cept by using a single-core engine to provide shaft power to a rotor in hover and low speed, and conversion capability to provide fan thrust for high speed, stopped rotor mode, and flight propulsion.

Ongoing research in helicopter handling qualities continued and expanded at the Ames Research Center. In 1978, one of these programs

provided essential simulation data on the effects of large variations in rotor design parameters on handling qualities and agility in helicopter nap-of – the-Earth (NOE) flight. The parameters investigated including flapping hinge offset, flapping hinge restraint, rotor blade inertia, and blade pitch – flap coupling. Experiments were carried out on the Ames piloted simula­tors to systematically study stability and control augmentation systems designed to improve NOE flying and handling qualities characteristics.

New efforts in computational analysis to increase rotor efficiency began at Ames. An analytical procedure was developed to predict rotor performance trends in relation to changes in the shape of the blade tips. The analytical procedure utilized two full potential flow-field computer programs developed for computation of the transonic flow field about fixed wings and airfoils. The analytical procedure rapidly became a use­ful tool for predicting aerodynamic performance improvements that may be achieved by modifying blade geometry. The procedure was guided by design studies and reduced the experimental testing required to select blade configurations. NASA continued the long-established tradition of fur­nishing excellent references for technical practice when, in 1980, research scientist Wayne Johnson, a member of the Army-NASA research team at Ames, published his book Helicopter Theory, a comprehensive state-of – the-art coverage of the fundamentals of helicopter theory and engineering analysis. The extensive bibliography of cited literature included an exten­sive listing of rotary wing technical publications authored by researchers from the NACA, NASA, the Army, industry, and academia.[303]

Research accelerated on advancing the ability of a helicopter to execute a radar approach. Civil weather/mapping radar could be used to provide approach guidance under instrument meteorological con­ditions (IMC) to select safe landing environments. Onboard radar sys­tems were widely used by helicopter operators to provide approach guidance to offshore oil rigs without the need for electronic naviga­tion aids at the landing site. For use over the water, the radar provided guidance and ensures obstacle awareness and avoidance, but involved very high pilot workload and limited guidance accuracy. For use over land, the ground clutter return made these approaches infeasible with­out more advanced radar systems. Two programs at Ames resulted from major advances in radar approaches. One program involved the

The NASA/Army/Bell XV-15 Tilt Rotor Research Aircraft in flight. NASA.

use of a video data processor in conjunction with the weather radar for overwater approaches. This system automatically tracked a des­ignated radar target and displayed a pilot-selected approach course. The second radar program involved the development of an innovative technique to suppress ground clutter radar returns in order to locate simple, low-cost radar reflectors near the landing site. This program was extended to provide the pilot with precision localizer and glide – slope information using airborne weather radar and a ground-based beacon or reflector array.

The 1980s brought several major accomplishments in the tilt rotor program.[304] The second XV-15 aircraft was brought to flight status and accepted by the Government after check flights and acceptance cere­monies at NASA’s Dryden Flight Research Center on October 28, 1980. It was then used for flight tests aimed at verifying aeroelastic stabil­ity, evaluating fatigue load reduction modifications, and expanding the maneuver envelope. Subsequently, this aircraft was ferried to Ames, where tests continued in the areas of handling qualities, flight con­trol, and expansion of the landing approach envelope. The first XV-15 aircraft was brought to flight status in late 1980, and initial work was

done on a ground tiedown rig to measure the downwash field and noise environment. Meanwhile, the second XV-15 participated in the Paris Air Show. The aircraft performed daily, on schedule, and received wide acclaim as a demonstration of new aeronautical technological achieve­ment. The XV-15 crew concluded each daily performance with a cour­teous "bow,” the hovering tilt rotor ceremoniously dipping its nose to the audience. After the flight demonstration in France and subsequent flights in Farnborough, England, the aircraft was returned to Ames for continued flight demonstration and proof-of-concept testing. The two vehicles achieved a high level of operational reliability, not the usual attribute of highly specialized research aircraft. One of the vehicles was returned to Bell Helicopter under a cooperative arrangement that made the aircraft available to the contractor at no cost in exchange for doing a number of program flight-test tasks, particularly in the mission suit­ability category. The overall success of the NASA-Army XV-15 (with a rotor diameter of 25 feet and a gross weight of 13,428 pounds) proof – of-concept program contribution is reflected in the application of the proven technology to the design and production of the new joint-ser­vice V-22 Osprey, (rotor diameter: 38 feet; gross weight: 52,000 pounds). The classic claim of research results having to endure a 20-year shelf life before actual engineering design application begins did not apply. It took only 5 years to move from achieving proof-of-concept with the XV-15 research aircraft to initiation of preliminary design of the oper­ational V-22 Osprey.

There has been over a half century of an unbroken series of NACA – NASA research contributions to tilt rotors since early XV-3 flight assess­ments and wind tunnel testing in the mid-1950s.[305] Since that beginning, NACA-NASA researchers have pursued many subject areas, includ­ing tilt rotor analytical investigations to solve a rotor/pylon aeroelas – tic stability problem, dynamic model aeroelastic testing in the Langley Transonic Dynamics Tunnel, analytical method development and verifi­cation, wind tunnel tests of full-scale rotor/wing/pylon assembles, XV-15 vehicle wind tunnel tests and flight tests, and detailed investigation of many other potential problem areas. This sustained effort and the robust demonstration and advocacy of the technology’s potential resulted in the XV-15 program being cited in 1993 as "the program that wouldn’t

die” in a University of California at Berkeley School of Engineering case study in a course on "The Political Process in Systems Architecture.”[306]

During the early 1980s, the rotary wing activity at Glenn Research Center increased with the addition of new transmission test facilities rated at 500 and 3,000 horsepower. Research progressed on traction drive, hybrid drive, and other advanced technology concepts. The prob­lem of efficient engine operation at partial power settings was addressed with initial studies indicating turbine bypass engine concepts offered potential solutions. Similar studies on contingency power for one – engine-inoperative (OEI) emergency operation focused on water injection and cooling flow modulation. Renewed efforts in aircraft icing included rotary wing icing research. A broad scope program was launched to study the icing environment, develop basic ice accretion prediction methods, acquiring in-flight icing data for comparison with wind tun­nel data from airfoil icing tests to verify rotor performance prediction methods. In addition, flight tests of a pneumatic deicing boot system were conducted using the Ottawa spray rig and the United States Army CH-47 in-flight icing spray system. In 1983, research testing began on the NASA-DARPA Convertible Engine System Technology program.[307] TF34 fan/shaft engine hardware with variable fan inlet guide vanes for thrust modulation was used to evaluate fan hub design and map the steady-state and transient performance and stability of the concept. New rotary wing efforts were also started in the areas of transmission noise, and flight/propulsion control integration technology.

Langley Research Center activity in rotary wing research increased substantially within the Structures Directorate, with focused programs in acoustics, dynamics, structural materials, and crashworthiness. This research was carried out in close association with the Army Structures Laboratory, now known as the Vehicle Technology Directorate (VTD). NASA and Army joint use of the Langley 4- by 7-meter tunnel for aero­dynamic and acoustic model testing became an important feature of the rotary wing program. Confirmed progress was achieved in airframe dynamic analysis methodology addressing the engineering manage­ment and execution of the efficient use of finite element methods for

simultaneous tasks of static and dynamic airframe preliminary design.[308] These techniques were demonstrated, publicly documented, and verified by comparison with shake test data for the CH-47 helicopter airframe. Other research related to helicopter dynamics included participation with the Army in a program to demonstrate the use of closed-loop multicyclic control of rotor-blade pitch motion for vibration reduction. The program involved flight-testing of an Army OH-6 helicopter by Hughes Helicopters.[309]

One of the more innovative approaches to research teaming was developed in the area of rotary wing noise. In 1982, discussions between the American Helicopter Society and NASA addressed the industry con­cern that the proposed rulemaking by Federal Aviation Administration would place the helicopter industry at a considerable disadvantage. The issue was based on the point that the state-of-the-art noise prediction did not allow the prediction of noise for new designs with acceptable confidence levels. As a result, NASA and the Society, joined by the FAA and the Helicopter Association International (HAI)—an organization of helicopter commercial operators—embarked on a joint program in noise research. Through the AHS, American helicopter manufacturers pooled their research with that of NASA under a 5-year plan leading to improved noise prediction capability. All research results were shared among the Government and industry participants in periodic techni­cal exchanges. Langley managed the program with full participation by Ames and Glenn Research Centers in their areas of expertise.

After delivery of the two RSRA vehicles to the Ames Research Center in the late 1970s, the helicopter and compound (with wing and TF34 turbofan engines installed) configurations were involved in an extended period of ground – and flight-testing to document the characteristics of the basic vehicles. This included extensive calibrations of the onboard load measurement systems for the rotor forces and moments; wing lift, drag, and pitching moment; and TF34 engine thrust. This work was fol­lowed by the initiation the research flight program utilizing the deliv­ered S-61 rotor system. In 1983, NASA and DARPA launched a major research program to design, fabricate and flight-test an X-Wing rotor on the new RSRA. The RSRA was ideally suited to the testing of new rotor

concepts, being specifically design for the purpose. One RSRA vehicle was returned to Sikorsky Aircraft for installation of an X-Wing rotor. This aircraft was eventually moved to NASA Dryden Flight Research Center at Edwards Air Force Base, CA, where final preparations were made for flight-testing. The second vehicle embarked on fixed-wing flight testing at the Dryden Center to expand and document the flight enve­lope of the RSRA beyond 200 knots, the speed range of interest in the start-stop conversion testing for the X-Wing rotor.

Contributions were beginning to emerge from the NASA-American Helicopter Society Rotorcraft Noise Prediction Program, the joint Government-industry effort initiated in 1983.[310] The four major thrusts were: noise prediction, database development, noise reduction, and crite­ria development. Fundamental experimental and analytical studies were started in-house and under grants to universities. In order to obtain high – quality noise data for comparison with evolving prediction capability, a wind tunnel testing program was initiated. This NASA-sponsored pro­gram was performed in 1986 in the Dutch-German wind tunnel (Duits – Nederlandse wind tunnel, DNW) using a model-scale Bo 105 main rotor. This program was performed with the support of the Federal Aviation Administration and the collaboration of the German aerospace research establishment. In these tests and in subsequent tests of the model in the DNW tunnel in 1988, researchers gained valuable insight into the aero – acoustic mechanism of blade vortex interaction (BVI) noise.

In regard to rotor external noise reduction, Langley researchers investigated the possibility of BVI noise reduction using active control of blade pitch. A model-scale wind tunnel test was conducted in the Langley Transonic Dynamics Tunnel (TDT) using the Aeroelastic Rotor Experimental System (ARES).[311] Results were encouraging and demon­strated noise level reductions up to 5 decibels (dB) for low and moderate forward speeds. A major contribution of the NASA-AHS program was the development of a comprehensive rotorcraft system noise prediction capability. The primary objective of this capability, the computer code named ROTONET, was to provide industry with a reliable predictor for

use in design evaluation and noise certification efforts. ROTONET was developed in several phases, with each phase released to Noise Reduction Program participants for testing and evaluation. Validation data from flight test of production and experimental rotorcraft constituted a vital element of the program. The first was of the McDonnell-Douglas 500E helicopter, tested at NASA’s Wallops Flight Facility. The second flight – test effort at Wallops, a joint NASA-Army program, was performed in

1987 using an Aerospatiale Dauphine helicopter, which had a relatively advanced blade design and a Fenestron-type (ducted) tail rotor. The year

1988 saw a joint NASA-Bell Helicopter effort in flight investigation of the noise characteristics the NASA-Army XV-15 Tilt Rotor Research Aircraft. The results indicated that while the aircraft seemed very quiet in the airplane mode, significant blade-vortex interaction noise was evi­dent in the helicopter mode of flight. NASA benefited from the inter­action with and participation in the variety of industry noise programs, which helped set the groundwork for subsequent joint participation in rotary wing research.[312]