Flight Testing the Rotor Systems Research Aircraft (RSRA)

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NASA Tectm,cal Memorandum 86862 USAAVRADCOM TM 83·A·1

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Flight Testing the Rotor Systems

Research Aircraft (RSRA)

G. Warren Hall and Robert K. Merrill

(NASA-TM-85852) FLIGHT TESTING XHE ROXCB N84- 13J77 SYSTEMS .RESEA8Cli AIRCRAFT (858M (NASA) ~7 p BC A02/M£ AOJ eSCL Ole Unclas G3/0S 43197 October 1983 ..

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United States Army ijJ

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Aviation Research and National Aeronautics and Development Command Space Administration

SI. Louis, Missouri 63166 ~

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NASA Technical Memorandum 85852 USAAVRADCOM TM 83·A·1

Flight Testiflg the Rotor Systems

Research Aircraft (RSRA)

G. Warren Hall, Ames Research Center, Moffett Field, California Lt. Col. Robert K. Merrill, U. S. Army Aviation Engineering Flight Activit), I Edwards AFB, California

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National Aeronautics and United States Army Space Administration Aviation Research and Ames Research Center Development Command Moffett Field, California 94035 St. Louis, Missouri 6.3166

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FLIGHT TESTING THE ROTOR SYSTEMS RESEARCH AIRCRAFT (RSRA) by G. Warren Hall.

NASA Ames Research Center Moffett Field, California USA and Lt, Col. Robert K. Merrill US Army Aviation Engineering Flight Activity Edwards Air Force Base, California USA In the late 1960's, efforts to advance the state-of-the-art in rotor systems technology indicated a significant gap 0xisted between our ability to accurately predict the characteristics of a complex rotor system and the results obtained through flight verificatir.:. Even full scale wind tunnel efforts proved inaccurate because of the complex nature of a rotating, maneuvering rotor system. The key element missing, which prevented significant advances, was our inability to precisely measure the exact rotor state as a function of time and flight condition. Two Rotor Systems Research Aircraft (RSRA) were designed as pure rese~rch aircraft and dedicated rotor test vehicles whose function is to fill the gap between theory, wind tunnel testing, and flight verification. This paper describes the two aircraft, the development of the piloting techniques required to safely fly the compound helicopter, the government flight testing accomplished to date and proposed future research progr~ms,

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The Rotor Systems Research Aircraft Development Program is not a new program. As shown in Figure 1, the concept of providing a test bed aircraft for research in new candidate rotor systems and rotorcraft technology began in 1970. Developed by Sikorsky Aircraft under a joint Army/NASA contract, two aircraft were delivered to the government in 1979 and ure currently being flight test~d at NASA's Ames Research Center, Moffett Field, Califor- nia. While initial testing was conducted by Sikorsky I this paper is limited to describing the aircraft and government flight testing accomplished to date.

3 DECEMBER 1970 NASA/ARMY WORKING GROUP ESTABLISHED

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JOINT NASA/ARMY DEVELOPMENT AGREEMENT 1 NOVEMBER 1971

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1973 REQUEST FOR PROPOSAL (BELL/SI KORSKY) 1 MARCH

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6 NOVEMBER 1973 SI KORSKY AIJIIARDED CONTRACT

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12 OCTOBER 1976 FIRST FLIGHT - HELICOPTER

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10 APPI L 1978 FIRST FLIGHT - COMPOUND

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12 FEBRUARY 1979 HELICOPTER ARRIVES AT AMES RESEARCH CENTER

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29 SEPTEMBER 1979 COMPOUND A.RRIVES AT AMES RESEARCH CENTER

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23 FEBRUARY 1980 FIRS1' HI:LlCOPTER FLIGHT AT MnES RESEARCH CENTER - '.

25 NOVEMBER 1981 FIRst COMPOUND FLIGHT AT AMES RESEAPCH CENTER

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Fig. 1. RSRA Chronology , v.

In the late 60's, efforts to advance the state-of ... the-art in rotor systems technology, while meaningful in their own right; indicated that a significant gap existed between our ability to accurately'~predict the characteristics of a complex rotor system and the results obtained through flight verification. Even full scale wind tunnel efforts.proved inaccurate because of the complex nature of a rotating, maneuvering~otor system.

The key element missing, which prevented significant advances, was our inability to precisely measure the exact rotor state as a function of 97-2

time and flight condition. The "trial and error" method proved costly, both from the fiscal considerations and the extreme risk to flight crews.

Each candidate rotor system required the development or modification of an airframe to match the system under study. Every rotor system reseat"ch effort, in fact, became a total aircraft development program. There was a general reluctance to depart very far from proven, successful conceJ>ts to investigate innovative, yet unproven rotor systems. Designed as a pure re.search aircraft, the RSRA is a dedicated rotor te!>t vehicle whose function is to fill the gap between t.heory, wind tunnel testing, and flight verification. The term "flying wind tunnel" has been coined to describe perhaps the most sophisticated and complex vehicle to join the research aircraft inventory.

A research vehicle, in order to be useful, must have certain basic capabilities. A flight test envelope must exist which will encompass the expected envelopes of future rotor systems under all flight conditions.

Versatility must be provided within the flight control system to exploit this envelope and to allow accurate, repeatable test results. Finally, a measurement and data acquisition system must be provided to accurately record desired flight parameters such that useful data analysis can be performed in a comprehensive manner.

To provide the required performance capability, the test configurations of the RSRA consist of a compound helicopter, (Figure 2) and a helicopter mode and, if necessary,removing the rotor from the HELICOPTER compound allows the RSRA to be flown as a fixed wing airplane.

The fixed wing config- uration primarily provides a flyback capability should it become necessary to sever an unstable rotor system in flight.

Fig. 2. RSRA Flight Configurations 97-3

The primary research configuration is tho compound helicopter.

Common to both configurations, however, is the basic helicopter fuselage which incorporates an 5-61 rotor, transmission, drive t't'ain and tail totor, driven by tl'.'O GE 1'-58 engines. These proven systems and the S-61 rotor provide an adequate pcrfo'rmance envelope and ~~apabi1i ty to allow a thorough systems integration flight evaluation and development of an adequate reference data base for comparison with future rotor designs.

The compound configuration has a 45-foot \'Iing which can change its angle of incidence between 15 degrees leading edge up and 9 degrees le~1.ding edge down. This capability allows one to vary the required rotor thrust from zero, to that which would be representative of a much heavier aircraft.

The wing contains conventional ailerons and high frequency flaps, both of which can be operated by an on-board digitalcCliilputer.

The tail section contains a conventional helicopter tail rotor, a lower horizontal all flying stabilator; an upper fixed horizontai tail plane ,I a conventional rudder and twc lO'rge aft mounted drag brakes. All of these controls can be operated from the cockpit controls or by the electronic flight control system.

Auxiliary power plants are mounted on either side of the fuselage.

These GE TF-34 high by-pass turbojet engines are used to offset the drag effects of a candidate rotor system. The compound configuration weighs approximately 27,000 pounds which means, with its present rotor system, it is unable to hover and has a minimum speed of 40 knots.

Si)lCe the winged configuration precludes hover, the second configuration of the aircraft is the pure helicopter. This configuration fills the perfonnance gap and allows complete investigation of the candidate rotor system in the hover and low speed r~gime. In building tip to the full compound configuration> the basic airframe was first flown as a helicopter to check out those systems common to both prior to adding the compound unique systems. We have been able to continuously fly the helicopter to maintain our flight proficiency, and to use it as a compound simulator in developing and practicing our compound techniques. Seventy knot roll-on landings in a helicopter are exciting, but proved to be excellent training in the technique~ and crew coordination required for flying the compound.

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or- PC' .. ,~ QUALITY MECHANICAL The helicopter also TORQUE LINKAGE incorporates a unique Active Isolation and Balance System (Figure 3). This system was designed to provide an acceptable ai'l'frame vibration level and . I', rotor load measurement capability over the \~ wide range of rotor excitation frequencies AUXILIARY UNIT that might be experi- enced with different LOAD CELLS experimental rotor systems.

Fig. 3. Rotor Force anti ,"'oment System: Active Isolation a"d Balance System Configuration The RSRA flight control system was designed to provide versatility for the flight research task and is tl'u1y unique. With the increased number of control surfaces available in the compound I a means of "control sharing" had to be incol'Porated. This was accomplished by providing a , p control phasing unit, or CPU. The CPU allows the pilot to select the proportion of control inputs to be made by the fixed or rotary wing control surfaces. The CPU is available for all three primary control axes. This means the pilot can select full rotary wing or full fixed wing control or any combination of the two. The capability to vary the control phasing was demonstrated during the government flight tests.

The flight control system provided specifically for the research task is an electronic "fly-by-wire" system (Figure 4). The evaluation pilot flies the aircraft through electrical signals which are ultimately surruned int0 the primary mechanical control system. This summing can be either direct, through a force feel system, or through a digital computer.

Safety dictated one crew member must have direct positive override control 97-5

OF POOR QUALrtV capability of the aircraft at all times> thus I an esentially stundurd hydromechanical control system is also installed.

PilCH AO~~ The heart of the electronic flight control system is the -- TOfOOM Teledyne TDY-43 general

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purpose! flight quali fied, r ....--.!...l..U---, digital computer that can be r~y IIV WillE FLlGlH programmed in numerous ways COIIT110L S\VIISlfPLAI[ l'vSHM to provide changes in stabil- ity and control or force fcol system gains, By varying tllc computer program, the RBRA can be used as D 5 degre~ of PHIMAAY SAfE IV PILm freedom inflight simulator in studying the handling quali- ties associated with a research rotor. The computer also can be programmed to make various control inputs WING in a predetermined direction POSlll11N and magnitUde insuring repeat£lbi Ii ty of research Fig. 4. RSRA Flight Control System Con~lguratlon control inputs.

A third flight crewman, the research engineer, has direct access to the computer so that the software may be monitored and gains changed in flight.

The ultimate goal is to allow the electronic flight control system to be programmed to move the various control surfaces in such a way to collect in-flight rotor data in a manner similar to a wind tunnel program.

Just as the heart of any wind tunnel is its balance and data gathering system, the uniqueness of the RSRA is its force and moment measurement system allowing direct accurate inflight measurements of rotor forces and moments.

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OF POOR QU/\i..n"V The forces and moments generated by the main and tail rotors, auxili~ ary engines, and the wing are measured by a series of load celln and/or the Active Isolation and Balance system which are an integral part of the aircraft structure (Figure 5).

Fig. 5. RSRA Force and Moment Measurement Systems The measurement system was calibrated in Ames Research Center's Static Calibration Facility (Figure 6). The inflight data is recorded by an extensive onboard instru~ mentation system. A renl time telemetry system allows the test engineers on the ground to monitor critical flight parameters and aircraft limits.

Fig. 6. Flight Loads Measurement Systems Calibration Fixture 97-7

During the initial design conside't'at" ?ons, it was deemed necessary to provide the flight crew with an escape s'l~tem. This was especially true in the compound configuration where the proximity of the auxiliary engines, wing, and tail to the normal escape windows preclude a manual bail-out at even moderate airspeeds.

The Emergency Escape System (eBS) is p~rhnps tho most innovative system developeJ for the RSRA.

SEVERED/FRACTURED The r~sulting escape system (Figure 7) is truly an engineering masterpiece. It is a modification of the Stanley "Yankee" c::::::;:::::=:::l:l ext r ac t i 011 5 Y stem, used successfully in the T-28 and A-1 aircraft in Southeast Asia.

\ FLIGHT ENGINEER (FE) \ EVALUATION PILOT (EP) Fig. 7. RSRA Crew Escape System The "full escape" system consists of a dual, completely redundant network of pyrotechnic transfer lines to various components which: (1) sever the main rotor blades near the hub, (2) fracture the overhead canopies, (3) launch the tractor type main escape rockets to extract the 3 crew members and, (4) provide for a fully automatic parachute deploy- ment and seat separation.

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The Egress system was fully qualified through extensive componont and full-scale rocket-sled testing at Holloman AFB, New MQxico. In five sled tests at speeds ranging from 0 to 210 knots, thirteen oxtraetions of instrumented dummies resulted in a lOOt success rate.

The egress system is initjated by pulling upward on one of the handles mounted on the ft'ont edge of the pilot scats} the three crewmen are automatically retractod into their respective seat backs to ensuro proper egress positioning; the pilot's cyclic sticks aro hingo released at the base with a pyrotechnic pin; the rotor blades :;I.re sequentially explosively severed at predetermined azimuthal positions relative to the airplane to avoid striki.ng the tail or throwing a blade into the flight path; the canopies above the crewmen are explosively severed and frac- tured; the safety pilot and flight engineer's extraction rockets are launched; 1.4 seconds later the evaluation pilot's extraction rocket is fired (to prevent interference); whon tho extraction lines become taut, the crewmen are extracted in a :~~l).ndup position. 'rho recovery parachutQs a:re deployed from a container 011. tho bottom of the seat pan by a static line attached to tho aircraft. The seat assembly is released from the crewman by pyrotechnic-actuated mechanisms, allowing a normal descent nnd touchdown. - The escape system envelope is a function of forward spoed, roll angle; and rate of si.nk. In general, the helicopter configuration requires a higher minimum ~scape altitude and is more sensitive to roll and sink rate than the compound. The aircraft forward velocity limit for safe egress is limited by the close proximity of the flight engineer to the rotor engine inlets and by the ·clearance of the parachute deployment bag to the aircraft for the two pilots.

While studies and limited expe:rimentation with helicopter escape systems had been conducted, until the development of the RSRA, no complete system had been designed, fabricated, tested and declared qualified and operational.

In the compound configuration we also have the option of severing the rotor blades only, allowing a return to base as a fixed wing airplane.

The "Blades Only" mode is initiated by a handle located on the overhead console between the two pilots. The blade severance is the same as in the full egress system, however an additional system is provided to ensure the rotary wing engines continue to run, following rotor blade severance, , 97-9

since they are rcquir~d to drive the main and accessory goal' boxes to provido electrical and hydraulic power, 'rhe taU rotor continues to run.

Wi th tho rotor system force mOilSHl'Omen'c s),stem calibratod, and with tho aircraft in tho holicoptot configul·utlon, the RSRA conducted its first research program. llighly accurate vel'tical drug information Was obtained 'in hover and at low speed allowing determination of rotor down ..

wash and fusol~'!lge interference effects. Measuroml.lnts never before possible in actual flight were achieved. This also allowed us to increaso oLlr lonl of proficiency in preparation for oLlr first flight in the full compound, With tho completion and analysis of the data from the Vertical Drag Progr.am, the aircraft waS reconfigured as a full compound and a program designed to fully document baseline data, resolve structural problems found during the contract,or flight test program and expand the envelope to the full research capnbili ties of the ui rcraft bogan. We hOod many factors on our side in preparing foX' flying the compound configuration: 1. We had been extensively flying a program on the RSRA helicopter and at iJvery opportunity \vere practicing compound techniques; 2. \Ve had developed n high fidelity, £ompound helicopter simulation on the Flight Simulator for Advanced Aircraft at Ames; and 3. The corporate memory, from several years previous was still present in flight test engineers, pilots, and other personnel associated with the program.

'takeoff Technique: Previous experience making rolling takeoffs in the helicopter and with the concurrent development of a high fidelity simulation indicated a takeoff technique that was a compromise between that used for a tail dragger fixed wing aircraft and a rolling takeoff for n heavy weight helicopter would be required. Several factors were to play nn important role in the selection of our final technique: • High power settings of 65% fan speed or greater on the TF-34 engines at zero velocity caused structural damage to the lower horizontal tail because of exhaust and bypass air impingement.

• There were several unknown factors about directional control in the 40 to 70 knot range.

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• 'i'he inability to sot a precise power setting on the TP .. 34 twist throttles because of hysteresis and high forces in tho throttle system, • And finally, the presence of the qui to largo collective to pitch coupling observed both in the simulator and during our helicopter flights.

As a result of a lot of thought, helicopter flying~ and a series of simulation sessions, the following techniquo \'/a5 developed: The sequence £01' takeoff was: • Establish the cyclic in the near center position using the cockpit control position indicators keeping the collective full down until the takeoff roll begins.

• Slowl)' advance the TF-34 engines to takeoff power as the aircraft rolls forward.

• As takeoff power is stabilized, collective is increased slowly to arrive at 40 percent collective position and 70 knots simultaneously.

With the 10 degree incidence on the wing, the aircraft simply flias off the rum~ay in the 3 point attitude I'lith only minimal control input required to maintain that attitudrJ.

Acceleration to 90 knots occurs quite rapidly. and without changing power i a 5 light aft cyclic movement results in a 90 knot climb.

Up and away flight is ver~' comfortable, much more so than with the helicopter. The compound is more stable and exhibits less vibration.

All flight cards were extremely busy, with the goal of accomplishing as much as possible in th~ short flight time available to this four engine aircraft .

.!!anding Technique: Again, a wide range of options were available. If the compound was to be flown as a pure helicopter, a run-on landing was the way to go. The jets would be reduced to idle on the base leg and collective and cyclic would be the primary controls for glidepath and airspeed control.

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If it was going to be flown as a pure fixed wing airplane, a fixed collective with cyclic controlling airspe~d and TF~34 engine modulation providing glidepath control would be best. At altitude we were ablo to reduce to approach speeds, and had the opportunity to try both techniques. As the simulation had indicated, the fixed wing tuchnique proved to be bost.

The approach spoeds selected were 110 knots downwind, 100 knots on base leg, and 90 knots on final. This provided an adequate margin above r,tall on the wing, which we were able to monitor with an angle of attitude indicator, and, with the use of flaps to increase drag, put us in a "straight line" region of TF-34 thrust modulation to control glidepath.

After several landings, it was determined that by maintaining a 20 percent collective setting and the airspeeds mcntiol'ed above, crossing the fence at 90 knots with a flare at the bottom put the aircraft in u slight tail low attitude at 70 knots. After we began moving the wing in flight, we found that a 7-1/20 wing incidence put us in precisely a 3 point attitude with jets at idle and 70 knot touchdown target. There is no tendency to float or enter any kind of rIO in ground effect. If, in rare occasions, one misjudges the loca- tion of the wheels and ends up slightly high prior to touchdown, a slight downward pressure on the collective provides the flexibility to recover gracefully.

Wing Movement Deyeloprnent: During the contractor flight test program the variable incidence wing had not been moved in flight. Thus, it was important to determine very ea:rly what effect varying the wing incidence would have on the structural loads and aircraft handling qualities.

The initial movement of the wing in flight proved to be rather benig)). Essentially, the fuselage rotates about the wing and the angle of attack remains constant. The wing to elevator interconnect practically negates the requirement for a cyclic trim change.

The only noticeable effect on the handling qualities with a. change in wing incidence is a new flight attitude in cruise and quite different power requirements for the auxiliary engines during turning flight. The higher wing incidence angles require considerably more power in a turn to maintain a constant speed.

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The greatest influence of wing incidence is on where the maximum stresses occur during high speed flight. Much of the flight test effort has been devoted to mapping the structL\ral loads as a function of wing incidence and collective setting.

The limiting structural loads at high speed occur either in the main rotor blades or in the upper horizontal stabilizer. At high wing incidence angle, the horizontal stabilizer reaches its endurance level around 180 knots. At low wing incidence angle, the main rotor blades reach their endurance levels at about the same speed. A lowering of the collective tends to reduce the rotor loads.

To dattl, the structural loads on the aircraft have been mapped at wing angles of 0, 5, 7-1/2, and 10° from 50 to 180 knots at several collect.Lve settings. We have explored flight techniques which enable us to minimize cumulative damage on components in transitioning from one flight region to another. All of these techniques and the knowledge gained to this point will be used in designing programs for future rotor systems to be studied with the aircraft.

The future of the RSRA is bright. Initial experimental rotor system proposals have been submitted and evaluated, and award of the contract and fabrication should begin this year (1983).

The initial system will

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be a 4-bladed rotor BEARINGLESS . \ -........."-.

COMPOSITE HUB (Figure 8) with the LOW ORAG ROTOR capability of varying "'-....--·Huii- ____ CONTROL

--- ~ - POWER COMPOSITE

blade geometry and ., _ FLEXBEAM

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NO BLAOE . , ~ inertia, to study, RETENTION BEARINGS A"ROEI.ASTIC and more importantly, LAG DAMPER~' . ,;" TAILORING to measure the effects REDUCED. • ROTOR' LOW DRAG of pure rotor design VIBRATION . PITCH CONTROL CONCEPT and design changes on the airframe and rotor TIP combination, holding ADVANCED TIP SHAPE other variables con- LOW NOISE

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stant in a manner which has not been Fig. 8. Proposed Four-Bladed Rotor possible to date.

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OR1C»t~At. PAG~ ~S OF POOFSQ!JAhJTY.

Fig. 9. Artist Concept of Proposed X·Wing Configuration Another major use of the aircraft is tho development and inflight demonstration of the "X-wing" c'~.ncept (Figure 9). One proposal is to reconfigure the pure helicopter as a compound aircraft and install this innovative and highly experimental system for flight test.

The RSRA gives the flight test community a versatile tool. It is truly a "flying wind tunnel" and \",ill play an important role in advancing the state of the art in helicopter rotor systems design. The government flight tests have explored all capabilities of the RSRA and we feel it ready ttl commence the research for which it was designed.

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