Future Directions in Rotorcraft Technology at Ames Research Center

Edwin W. Aiken Robert A. Ormiston Larry A. YoungNASA-Ames Aeroflightdynamics Directorate (AMRDEC) NASA-Ames

U.S. Army Aviation and Missile Command

Army/NASA Rotorcraft Division Ames Research Center Moffett Field, CA 94035-1000

Abstract

Members of the NASA and Army rotorcraft research community at Ames Research Center have developeda vision for “Vertical Flight 2025”. This paper describes the development of that vision and the steps beingtaken to implement it. In an effort to realize the vision, consistent with both NASA and Army Aviationstrategic plans, two specific technology development projects have been identified: one focused on apersonal transportation system capable of vertical flight (the “Roto-Mobile”) and the other on smallautonomous rotorcraft (which is inclusive of vehicles which range in grams of gross weight for “Micro-Rotorcraft” to thousands of kilograms for rotorcraft uninhabited aerial vehicles). The paper provides astatus report on these projects as well as a summary of other revolutionary research thrusts being plannedand executed at Ames Research Center.

Introduction and Background

Through a synergy of Army AeroflightdynamicsDirectorate and NASA Ames Research Centerresources, the Army/NASA Rotorcraft Division(Ref.1) leads the Nation in both aeromechanics andflight control and cockpit integration technologydevelopment and insertion for military and civilhelicopters, tiltrotor aircraft, and other advancedrotary-wing aircraft. The Division also provides theU.S. rotorcraft industry, Department of Defense,and other Government agencies with the technicalexpertise required to produce and field safe,affordable, and effective all-weather rotorcraftsystems.

The strategic plan for the NASA AeroSpaceTechnology Enterprise (Ref. 2) is driven by a set ofthree “Pillar Goals” and ten “TechnologyObjectives”. Breakthroughs in rotorcraft andvertical flight technology can have significant

Presented at the American Helicopter Society 56thAnnual Forum, Virginia Beach, VA, May 2-4, 2000.Copyright © 2000 by the American HelicopterSociety, Inc. All rights reserved.

impacts on two of the Pillar Goals: “Global CivilAviation” and “Revolutionary Technology Leaps”.“NASA must pursue high-risk research to provideneeded technology advances for safer, cleaner,quieter, and more affordable air travel” (Ref. 3) in afuture environment in which the demand for airtravel is projected to triple in 20 years. NASA iscommitted to the Technology Objective of: “Whilemaintaining safety, triple the aviation systemthroughput, in all weather conditions, within 10years”. NASA’s AeroSpace Technology charter isalso to explore high-risk, revolutionary technologyareas that can result in a major expansion ofpersonal mobility (“a seamless transportationsystem with tremendous doorstep-to-destinationspeeds” – Ref. 4) and provide significant increasesin socio-economic opportunity, both in the U.S. andworldwide. This effort also includes providing forrevolutionary new development tools for aerospacesystems that will accelerate the application oftechnology advances.

The U.S. Army, in conjunction with the industryand the user community, has developed a top-downstrategic planning approach (the TechnologyDevelopment Approach, or TDA) and an associatedTechnology Area Plan (TAP) to guide the DoD

Science and Technology (S&T) efforts for rotary-wing vehicles. The strategic guidelines for this planare: aggressively pursue S&T efforts aimed at high-payoff goals, establish common goals andtechnology transfer with industry, and emphasizeaffordability and dual-use technology (emphasis bythe authors). As a result, although clearly focusedon maintaining U.S. military capability andreadiness, the attributes of the Army S&T Programshow considerable commonality with the NASAAeroSpace Technology Enterprise goals.

The development of a strategic vision for thetwenty-first century for integrated Army and NASAvertical flight research and technology representsboth an appealing opportunity and a dauntingchallenge. The extremely productive, 35-year-oldAgreement for Joint Participation in AeronauticalTechnology between NASA and the Army MaterielCommand has spawned many significantaccomplishments in rotorcraft technology, includingmomentous advances in tiltrotor technology(focused on the joint development of the XV-15tiltrotor research aircraft with Bell Helicopter – Ref.5). This Joint Agreement offers the opportunity forfurther revolutionary breakthroughs. The challengeis, while maintaining a commitment to both NASAand Army research goals, to devise a “strategicimplementation plan” consistent with the researchgoals of both organizations which provides both avision and a roadmap to achieve that vision.Although it is important to define and adhere to a“process” to develop the strategic implementationplan, it is also important not to get lost in theprocess.

Before the beginning of great brilliance, theremust be Chaos.

Before a brilliant person begins something great,he must look foolish to the crowd.

The I Ching (“Book of Changes”, ca 1000 BC)

Revolutionary developments in vertical flighttechnology, achievable under the auspices of theJoint NASA/Army Agreement, can solve criticalnational problems. Significant improvements in theU.S. transportation system can be achieved bysafely alleviating airport congestion and delays inall weather conditions and by providing expandedaccess to the air transportation system. In addition,new and emerging defense needs --includingcounter-terrorism, urban warfare, and disaster relief-- require the development of highly-mobile forces

with efficient logistics delivery and a rapid growthin the application of Information Technology on thebattlefield. Finally, advances in vertical flighttechnology can serve to meet the increasing demandfor rapid-response public service operations such asemergency medical service, search and rescue, andlaw enforcement.

Given this challenge, this paper provides anoverview of advanced research thrusts, describes thedevelopment of a vision for Future Directions inVertical Flight Technology at Ames ResearchCenter, and describes in more detail two focus areasspecifically identified by the vision process fortechnology development.

Advanced Research Thrusts

The Army/NASA Rotorcraft Division is engaged ina broad program of rotorcraft research andtechnology development. The elements of thisprogram have the potential to advance helicopterand tiltrotor technology far beyond current levels.Within the aerospace spectrum, rotorcrafttechnology is relatively immature and therefore thefuture holds great potential if critical fundamentaland advanced technology is aggressively pursued.Both the current immaturity and the future potentialof rotorcraft technology stem from the same keyfactor - the fundamental complexity of rotorcraftaerophysics. In years past, the difficulties presentedby these complex problems afforded only limitedprogress and incremental gains. Today, however,the situation is changing. New technologies,including composites, smart materials, and sensorswill enable fundamentally new approaches to rotordesign. With enormous new computer capabilities,we are on the threshold of solving scientific riddlesof rotorcraft aerodynamics that have plaguedtechnologists since the origin of the industry. Andemerging information technology will provide thecapability to integrate active controls, flightcontrols, and a myriad of other capabilities to enablehelicopters and rotorcraft to interface with users andperform missions in completely new ways. Theresult will be nothing less than quantum leaps intechnology and a metamorphosis of rotorcraft intovehicles with unprecedented new capabilities.

This section of the paper provides an overview ofthese current revolutionary research thrusts. Thissection also provides a basis for the followingsection that looks toward a vision of vertical flight

concepts in 2025. It is useful to outline briefly theorganizational elements of the Division and someof the major investment plans before undertakingthis overview. The two technical branchescomprising the Army/NASA Rotorcraft Divisionare the Flight Control and Cockpit IntegrationBranch and the Aeromechanics Branch.

The Flight Control and Cockpit Integration Branchis responsible for the development and insertion ofadvanced controls, guidance, and displaytechnology for rotorcraft and powered-lift aircraft,and for the integration of these technologies toachieve safer and more effective pilot-vehicleperformance. The research efforts include (1)definition of design and certification criteria, (2)investigation of operational problems includingtheir human factors elements, (3) development ofdisplay and advanced control concepts for bothpiloted and remotely-piloted rotorcraft, and (4)development of automation, guidance, navigation,and displays for rotorcraft operating in all-weather,near-terrain conditions. In researching anddemonstrating these technologies, the branchemploys an array of analytical, simulation, andflight research tools.

The Aeromechanics Branch is responsible foraeromechanics research activities that directlysupport the Department of Defense and the U.S.rotorcraft industry. Branch programs address allaspects of rotorcraft that directly influence vehicleperformance, structural and dynamic response,external acoustics, vibration, and aeroelasticstability. The programs are both theoretical andexperimental in nature. Advanced computationalmethodology research using computational fluiddynamics and multidisciplinary comprehensiveanalyses seeks to improve the understanding of thecomplete rotorcraft operating environment and todevelop analytical models to predict rotorcraftaerodynamic, aeroacoustic, aeroelastic, anddynamic behavior. Experimental research seeks toobtain accurate data to validate these analyses,investigate phenomena currently beyond predictivecapability, and to achieve rapid solutions to flightvehicle problems. Databases from the flight andwind tunnel experimental programs are validated,documented and maintained for the benefit of theU.S. rotorcraft technology base.

Substantial investments and future plans arecurrently in place to develop and validate the keytechnologies for helicopters, tiltrotor aircraft, andother advanced rotary-wing concepts. Newsophisticated research test stands and facilities have

been, or are being, developed. Among these newresearch facilities are the Tilt Rotor AeroacousticModel (TRAM) and the Large Rotor Test Apparatus(LRTA), Ref. 6. The Army/NASA RotorcraftDivision continues to rely on, and strongly support,the NASA National Full-scale AerodynamicsComplex, the Vertical Motion Simulator, and theArmy Flight Project Office at Ames ResearchCenter. The Division plays a major advisory, andparticipatory, role in National RotorcraftTechnology Center (Ref. 7) projects and continuesto make significant contributions to the NASAAviation System Capacity Program’s Short Haul(Civil Tiltrotor) project (Ref. 8).

The following discussion outlines some of therevolutionary research opportunities in advancedrotorcraft technology being pursued within theArmy/NASA Rotorcraft Division.

High Lift Airfoils and the Stall Free Rotor

Unlike fixed wing aircraft, helicopter rotors havetraditionally relied upon relatively simple airfoilsbecause of the conflicting aerodynamicrequirements, aeroelastic constraints, and the needfor structural simplicity and operational reliability.As a consequence the significant performancebenefits of high-lift airfoils that are taken forgranted by fixed-wing aircraft designers have notbeen exploited for rotorcraft. In view of thepotential benefits, there is increasing interest indeveloping variable geometry airfoils andaerodynamic flow control technologies forrotorcraft. If sufficiently successful, such airfoilsmay enable a reduction in rotor solidity with aconsequent increase of rotor efficiency and/orenable an increase in the high-speed maneuvermargin of the helicopter.

Fig. 1 - High-lift airfoil design using OVERFLOWCFD code.

There are several opportunities and potentialapproaches. Recent studies (Ref. 9) have begun toexplore the application of advanced airfoil designmethodology including Navier-Stokes CFD codes todesign of multi-element airfoils with leading edgeslots, Fig. 1. The unique rotorcraft challenge is toachieve the high lift of a slotted airfoil to delay stallon the helicopter rotor retreating blade withoutincurring a significant drag penalty at the low anglesof attack of the advancing blade in high-speedforward flight.

Another high-lift airfoil approach is to introduce avariable geometry, such as a deployable leadingedge slat or variable leading edge contour. Variableleading edge camber or droop would be especiallybeneficial since no drag penalty would be incurredfor azimuthal locations operating at low lift,although such concepts pose demanding actuatorand structural requirements. Finally, newapproaches to active flow control based on low-mass flow, oscillatory blowing concepts haveemerged. So-called virtual jets energized by smartmaterial actuators may be a possible implementationapproach.

There are far reaching ramifications for thistechnology. If the stall angle of attack of rotorblade airfoils can be extended beyond the range ofhelicopter blade pitch angle control inputs, arevolutionary new concept will become a reality -the Stall-Free Rotor. The implications forrotorcraft, beyond the performance, blade loads, andvibration benefits, will be of great significance byremoving retreating blade stall as a major flyingqualities constraint and contribute to theachievement of helicopter carefree maneuvering.

The Jet Smooth Ride and Whisper Quiet Rotor

Smart structure active control rotor concepts basedon a coalescence of smart materials, active controls,and information technology have the potential torevolutionize future rotorcraft by reducing vibration,acoustic signature, and improving missionperformance. Decreasing vibration will produce amajor increase in time-between-overhaul of aircraftcritical components while reducing structuralfatigue, boosting avionics and subsystemsreliability, and bringing safer and longer service lifefor rotor blades, drive shafts, engines, transmissions,and airframes. A smoother ride will also reducefatigue and improve aircrew effectiveness.Attacking the problem at the source by integratingon-blade control surfaces, smart structures, sensors,and microelectronics, into a sophisticated active

control system will open the door to multiplebenefits for rotorcraft. Plans include wind-tunneltesting of Mach-scaled rotors with active on-bladecontrols and smart material actuators to exploreadvanced design configurations beyond exploratorysmall-scale model experiments (Ref. 10) depicted inFig. 2.

Fig. 2 - AFDD 7.5-ft experimental rotor with on-blade elevons and smart material piezo-ceramic

actuator to reduce vibratory blade loads.

Individual Blade Control

The Division is also pursuing individual bladecontrol (IBC) approaches in full-scale rotor tests.Under the Rotorcraft Algorithm Development andIntegrated Control Laws (RADICL) program, theU.S. Army, Sikorsky, ZF Luftfahrttechnik, andNASA Ames Research Center seek to developintegrated control laws to suppress both noise andvibration without losing helicopter performance.The RADICL program approach will replace theUH-60 rotor blade pitch links with servo-actuatorsand perform tests in the Ames 40- by 80-Foot WindTunnel similar to the BO 105 rotor (Ref. 11).Promising controller algorithms will also be

simulated in the Ames Vertical Motion Simulator todetermine the effect of IBC on handling qualitiesand flight dynamics. A potential flight test programwill also be considered.

Tiltrotor Active Rotor Control

The Army/NASA Rotorcraft Division has also madesubstantial progress recently to demonstrate thepotential of higher harmonic control (HHC) fortiltrotor rotor noise reduction. The most recenteffort has been an 80-by-120 Foot Wind Tunnel testin the Ames National Full-scale AerodynamicsComplex (NFAC) with an XV-15 isolated tiltrotor,for both 3- and 4-blade rotors in low-speed,helicopter-mode flight, Fig. 3. This wind tunneleffort was conducted in support of the Short HaulCivil Tiltrotor program, which is an element of theNASA Aviation System Capacity program.Dramatic noise reduction results (both on- and off-peak BVI descent conditions) on the order of 7-12dB were measured.

Fig. 3 - XV-15 Aeroacoustic Test

The Swashplateless Rotor

Smart structures and active control technologiesalso have the potential to change the fundamentaldesign approach and configuration of the rotor hub,blades, and controls first originated with theinvention of the helicopter in the late 1930s.Ultimately, active on-blade control concepts mayeliminate the conventional controls such as the

swashplate, pitch links, and hydraulic flight controlactuators, significantly reducing complexity of thehelicopter and improving weight, maintenance andreliability. A notional concept including on-bladeactive control is depicted in Fig. 4.

Eliminate ActuatorsEliminate Torque Tube,

Lag Damper

Eliminate Swashplate

Distributed Trailing Edge Control Elements

Fig. 4 - Swashplateless rotor concept with integralairfoil actuator.

Advanced Actuator Technologies

Active on-blade control and active fiber compositesfor integral twist rotor blades are advancing smartmaterial structures and actuator technologies. Othersmart material technologies are being pursued tointegrate the actuation material into the airfoilstructure to provide continuous airfoil trailing edgecamber change as an alternative to discrete hingedcontrol surfaces. Several innovative concepts inthese areas are supported by Army and NASA SBIRfunding. One of the actuator approaches beinginvestigated is an electromagnetic actuator for on-blade controls depicted in Fig. 5 that has been whirltested on a full-scale OH-58 rotor (Ref. 12).

Fig. 5 - Heliflap™ electromagnetic actuator for on-blade control whirl tested on OH-58 rotor.

Active Control of Rotorcraft Aeroelastic Stability.

One of the major constraints on the design of manyrotorcraft - especially for advanced configurations -

is the ever-present risk of aeroelastic,aeromechanical, or aeroservoelastic instabilitiesinvolving the rotor blade, flight control system,drive train, or rotor-body coupling. Inability tocontrol these instabilities results in degradedmission performance, reduced payload, increasedweight and cost and sometimes results in theaddition of auxiliary dampers. In some cases, suchinstabilities render promising advanced rotorcraftdesigns unfeasible. Solutions involve automaticcontrols with new computer and sensor technologiesto enable sufficient reliability, fail safety, and costeffectiveness to solve the problem. Otherapproaches involve more effective auxiliary dampermaterials, hybrid structural materials with dampingmaterial in composite blade structures and hybridstructures using passive or active smart materials.Expected achievements include damperlessbearingless rotors, soft-inplane tiltrotors, 4- and 5-blade tiltrotors, and high-speed tiltrotorconfigurations with aerodynamically efficient wingsections.

Rotorcraft Design Methodology - InformationTechnology Revolution

Timely, low risk, cost effective design of modernrotorcraft and system upgrades requires accurate,robust, and reliable analytical prediction methods.Moreover, accurate design methods are essential todevelop future advanced civil and military rotorcraftthat maximize mission performance capability -limitations in design methodology result in sub-optimum designs and reduced mission performance.

The last twenty-five years have seen developmentof increasingly sophisticated and detailed analyticalmodels of rotorcraft phenomena including majoradvances in CFD to model complex aerodynamicphenomena, sophisticated structural mechanicstheory, and the development of comprehensiveanalysis codes that integrate aerodynamics,structural dynamics, propulsion, and flight controlsystems together in complete software packages.However, the prediction of essential aeromechanicsdesign characteristics (performance, loads,vibration, stability, acoustics, and flight dynamics)does not yet satisfy the accuracy and reliabilityneeds of rotorcraft designers. This results ininefficient designs, excessive testing and high risk,which greatly increase development costs.

There are a number of reasons for this situation.Accurate prediction of rotorcraft aeromechanics isinherently difficult. Rotorcraft encompass multipledisciplines and distinctly different phenomena and

computational approaches. Aerophysics areinsufficiently understood to formulate accurate mathmodels. One of the most critical, yet leastunderstood is rotorcraft aerodynamics includingstall, compressibility, flow separation, wakes, andvortices. Accuracy requirements are high -designers need not only first order effects, buthigher order unsteady effects as well. Criticalanalyses involve limit conditions where stall andcompressibility dominate. It will not be possible toadequately support the designer until accuratemodels for rotorcraft aeromechanics phenomena areachieved.

Many research efforts are underway includingadvanced CFD methods, hybrid aerodynamicapproaches, multidisciplinary comprehensiveanalyses and experiments to acquire data to explorecritical fundamental physical phenomena andvalidate design methodology codes. Two examplesof this research will be briefly described.

Optimum Rotor Performance with AdvancedDesign Methodology. Development of optimumrotor configurations is exploiting efficient newaerodynamic design methodology based on hybridcomputational fluid dynamic (CFD) codesincorporat ing vort ici ty embedding forcomputational efficiency.

Fig. 6 - Advanced design methodology improvesrotor hover performance.

Application of this analysis capability permits thepractical optimization of subtle refinements in rotorblade tip shape, twist, taper, sweepback, andanhedral to reduce rotor power requirements, Fig. 6.Typical results for an Apache-sized helicopterindicate a 1000-lb increase in hover thrust for thesame installed power.

Numerical Simulations of Rotors and Wakes onParallel Computers. New approaches aim toaccurately predict the rotary-wing vortex wakes, aproblem that has plagued rotor designers for the past70 years. The rotor wake problem is currentlyamong the least understood and most importantfactor that drives rotorcraft design and performanceand is the most challenging problem in rotorcraftCFD. Typically, numerical dissipation causes thecomputed vortical wakes diffuse too rapidly. Newapproaches will leverage the power of large-scaleparallel computers and will employ several body-fitted structured grid systems attached to the movingrotor blades. These rotating grids move throughoverset Cartesian background grids that capture therotor wake system. The flow solver will be an MPI-parallel version of NASA’s OVERFLOW Navier-Stokes solver for which Meakin et al. (Ref. 13) havedemonstrated good parallel performance for anunsteady simulation of the V-22 tilt rotor in high-speed cruise. This V-22 simulation used a total of268 grid components and close to 28 million gridpoints. Particle traces from the rotor blades areshown in Fig. 7. Larger calculations will beundertaken (up to 100 million grid points), with thehope that the computed rotor performance resultswill show grid independence. Calculations will beused as a stepping stone towards running muchlarger problems on NASA’s Information PowerGrid. Ultimately, the goal is to dramaticallyimprove the ability to model rotor wake systems.

Fig. 7 - V-22 calculations point to advances inrotorcraft CFD predictions.

Aeromechanics Technology Impact

The synergistic impact of combining advanced rotorblade airfoils, active control, and swashplatelessrotor technologies will be very large. Even for aconstrained example, retrofitting such a rotor to acurrent utility helicopter would increase range andpayload 86% and 55% respectively compared to thebaseline vehicle. Figure 8 indicates the

contributions of each of the individual technicaldisciplines to the total increased range.

Rotor L/D

Hub Drag Figure of Merit

Blade Loading

Rotor Flight Control Wt

Vibration

Lag Damping

0% 5% 10% 15% 20% 25% 30%

Range Increase Breakdownby Rotor/Flight Control Technologies

Range Increase

Fig. 8 - Impact of aeromechanics technologies fortypical utility aircraft mission.

Helicopter Active Control Technology (HACT)

Future military rotorcraft will require majorimprovements in all-weather/night missionperformance, maneuverability/agility, flight safety,and reduced accident rate. At the same timesignificant improvement in operation and supportcosts is needed, as well as reduced developmenttime and cost. The Division is actively participatingin the HACT program, a joint activity of theAeroflightdynamics Directorate and the AviationApplied Technology Directorate. The HACTprogram will demonstrate integrated, state-of-the-artrotorcraft flight control technologies withexploitation of advanced fixed-wing hardwarecomponents and architectures. The objective is todemonstrate, through simulation and flight test,second-generation rotorcraft digital fly-by-wire/light-control systems with fault-tolerantarchitectures, including carefree maneuvering; task-compliant control law; and integrated fire, fuel, andflight control capabilities. There are major technicalbarriers such as the lack of knowledge of optimalrotorcraft response types; inadequate techniques forsensing envelope limits, cueing the pilot, or limitingpilot inputs; inadequate air vehicle math modelingfor high-bandwidth flight control; inadequate flightcontrol system design, optimization, and validationtechniques; and lack of knowledge as to theoptimum functional integration of flight control,weapon systems, and pilot interface. Thesechallenges will be met by implementing state-of-the-art rotary wing flight control technologies in an

advanced flight demonstration rotorcraft, exploitingadvanced fixed-wing flight control architectures andfly-by-light hardware, and optimizing control lawsfor all parts of the flight envelope. Envelope cueingand limiting techniques will be implemented will toachieve carefree maneuvering, Fig. 9. To helpoptimize the design process as well as improve theproduct, it will be necessary to use advanced mathmodels and rotorcraft flight control designtechniques for analysis and simulation.

Load Factor

Hub Moment

Main Rotorover/under speedflappingblade overstressstall

Rearward Speed

Tail Rotorloss of effectivenessgearbox over torqueblade overstress

Forward SpeedControlauthorityactuator reversibility

Engine/DriveTrainover torqueover temperatureN1, N2 speeds

Settling withPower

Fig. 9 - Multiple Limiting Factors and FlightConditions Impact Rotorcraft Carefree Maneuvering

Flight Control Design Cycle Technologies

Unique capabilities and expertise within the AmesResearch Center Flight Control and CockpitIntegration (ARH) branch are playing a critical rolein aircraft design cycle reduction based ondevelopment and widespread application of the

CONDUIT, RIPTIDE, and CIFER® tools.

The Control-System Designer’s Unified Interface(CONDUIT) provides an environment for design,integration, and data resource management for theaircraft flight control system designer (Fig. 10). Itis a sophisticated "associate" that providescomprehensive analysis support and designguidance to a knowledgeable designer. Flightcontrol system design involves application ofcomprehensive specifications and sophisticated timeand frequency-domain evaluation techniques toensure desired performance and handling-qualitiesof highly-augmented modern combat aircraft and tominimize flight test tuning. Thus, the costs tocontinually retune control laws and handling-qualities predictions as updates in math models andhardware test data become available are prohibitive.CONDUIT drastically reduces the time and effortrequired for this process. The capabilities of theCONDUIT system have been evaluated through a

series of design problems based on the RASCAL,UH-60, and X-29 aircraft flight control systems andhave been widely used by the rotorcraft industry.

Flight Control Engineer

• Rapid setup by non-specialists• Concurrent engineering• Comprehensive FCS/HQ eval• Tradeoff/sensitivity studies• Automated FCS tuning• Graphical user interface

• Rapid setup by non-specialists• Concurrent engineering• Comprehensive FCS/HQ eval• Tradeoff/sensitivity studies• Automated FCS tuning• Graphical user interface

Capabilities

Payoffs

• Dramatic reduction in design cycle• Improved performance and HQ

• Dramatic reduction in design cycle• Improved performance and HQ

Handling-Qualities &Servoloop Specs

Airframe & FCS modeling

Hardware characteristics(sensors and actuators)

Performancecriteria

Flight/ground testing(system ID results)

Flight Control LawArchitecture

Fig. 10 - CONDUIT, Control Designer's UnifiedInterface, an environment for design, integration,

and data resource management.

RIPTIDE is a software simulation environment forReal time Interactive Prototype TechnologyIntegration Development Environment. Pilotedsimulation evaluation of a notional control lawdesign using a high-fidelity non-linear mathematicalmodel has always involved a complex and lengthyintegration process because math models andcontrol law design tools are not typically designedto work together. As a result, the integrationprocess has not lent itself to rapid prototyping,whereby ideas can be quickly implemented andtested without significantly lengthening thedevelopment cycle.

Fig. 11 - RIPTIDE allows concurrent research invarious disciplines.

RIPTIDE eliminates this lengthy integrationprocess. This is accomplished by treating eachcomponent of the simulation as an independentprocess and providing inter-process communicationthrough shared computer memory. Also, processtiming and scheduling ensures that the variouscomponents of the simulation are executed in theproper order. RIPTIDE is a prefect complement toCONDUIT and can also be used to concurrentlyconduct mathematical model development, controlsystem design, and cockpit-display-conceptsevaluation (Fig. 11). The environment makespiloted simulation readily available and eachdiscipline is able to take advantage of toolsdeveloped by the other disciplines.

C I F E R ® is an integrated facility for systemidentification based on a comprehensive frequency-response approach jointly developed by the U.S.Army/NASA and Raytheon, STX. Systemidentification is a procedure by which amathematical description of vehicle or componentdynamic behavior is extracted from test data andCIFER performs this function, without requiring atime-consuming modeling effort. Applications ofsystem identification results include validation andupdate of simulation models, handling-qualitiesanalyses and specification compliance, andoptimization of automatic flight control systems.The foundation of the approach is the high-qualityextraction of a complete multi-input/multi-output(MIMO) set of non-parametric input-to-outputfrequency responses. These responses fullycharacterize the coupled characteristics of thesystem. Army and NASA researchers havesuccessfully used this capability in numerousrotorcraft flight test programs, including:Comanche, OH-58, AH-64, SH-2G, KMAX, A300,B214ST, B206, BO105, UH60, and most recentlythe Northrop-Grumman VTUAV.

Advanced Technology Rotor Demonstrators

The Joint Transport Rotorcraft (JTR) is envisionedto be DOD's future cargo logistics transport VTOLair vehicle, e.g., helicopter, tilt rotor, or otheradvanced configuration. Two pre-cursor programsare currently in progress or being defined to validatetechnology for this new generation of advancedrotorcraft: the Variable Geometry AdvancedRotorcraft Technology (VGART) and the VariableGeometry Advanced Rotorcraft Demonstrator(VGARD) programs. The VGART program iscurrently underway with both Army and industrytechnology development efforts. It involvesdevelopment and testing of advanced rotorcraft

technology concepts with the intent to advancecritical sub-component technology for futuredevelopment of the JTR demonstrator aircraft. Thekey elements involve critical rotor componentevaluation and testing for reliability, affordability,and scalability of advanced technology, includingvariable geometry concepts. The VGARD programwill lead to a full-scale wind tunnel or flightdemonstration of VGART developments. TheArmy/NASA Rotorcraft Division is fully supportingVGART and VGARD.

Biomimetics Initiative

NASA sees considerable promise across the wholeof the agency for the application of a new designparadigm for aerospace systems. Biomimetics - tomimic life, to imitate biological systems, technologyinspired by biology. The concept of biology-inspired technical solutions may have application ina wide variety of areas including cross-platformtechnology transfer to the rotorcraft community.Although, this is a relatively new initiative, theDivision has begun to investigate potentiallyrelevant topic areas and some of these include: 1)development of micro-air vehicles, 2) bio-inspiredanalogues for control such as sensing, stabilization,and navigation, 3) reconfigurable or variablegeometry rotorcraft embracing adaptable smartskins and artificial muscle concepts for controlsurface actuation, 4) multiple vehicle coordinationand mission execution (swarms or teams, multipleplayers), and 5) health monitoring, control, andsensors.

Applications of Rotorcraft Technology inExtraterrestrial Environments

The Army/NASA Rotorcraft Division has begun toperform and sponsor conceptual design studies ofvertical lift planetary aerial vehicles -- withemphasis on Martian autonomous rotorcraftconcepts (Fig. 12). Reference 14 identified thatthere are three planetary bodies (other than Earth) inour solar system where vertical lift vehicles forplanetary science/exploration might be feasible:Mars, Venus, and Titan (a moon of Saturn).Further, reference 14 also emphasized that planetaryscience missions to the outer, gas-giant planets --where vertical lift capability is not required as theseplanets do not have surfaces to interact with in aconventional sense – could still benefit from thedevelopment of planetary aerial vehicles thatemploy rotary-wing-related technologies, such aspropeller design. The extreme range of planetary

atmospheric characteristics will dictate substantialinnovation in future rotary-wing technologies andvehicle design. Vehicle autonomy, in particular, isa critical enabling technology for vertical liftplanetary aerial vehicles because of missiondistances and the communication time lag betweenthe vehicle and mission control on Earth.

Besides the reference 14 study, other work onvertical lift planetary aerial vehicles is currentlyunderway within the Army/NASA RotorcraftDivision. A small number of university grants havebeen, or soon will be, initiated to study autonomoussystems technology for vertical lift planetary aerialvehicles. Conceptual design work is in processdeveloping proof-of-concept ground test hardwarefor the in-house Martian autonomous rotorcrafteffort. Finally, the year 2000 AHS Student DesignCompetition is currently underway and is focusedon the design topic of a Martian autonomousrotorcraft.

Fig. 12 - Vertical Lift Planetary Aerial Vehicles

Future Vertical Flight Vehicle Development andDemonstration

NASA, as part of its Flight Research R&T BaseProgram, has initiated in program year 2000 theRevolutionary Concepts (REVCON) project. Thisproject is intended to foster revolutionarytechnology leaps in aeronautics through anexpedited vehicle concept development cycle thatquickly (three to five years nominally) leads toflight test demonstrations of the targetedtechnologies. A key feature of the REVCONproject is the encouragement of substantive teamingpartnerships of industry, NASA/US Government

agencies, and academia in planning and executingthe proposed efforts. Because of the shortdevelopment cycle dictated by the REVCON projectrequirements, uninhabited aerial vehicles for theflight test demonstrations are included in many ofthe proposals submitted to the project. Thesevehicles demonstrate varying levels of flightautonomy. Many innovative rotary-wing concepts,to date, have been proposed for the NASAREVCON project. The large number and variety ofrotorcraft proposals to the REVCON project send aclear message that innovation is alive and well inthe rotorcraft community.

Although the technology being developed underthese advanced Army and NASA research thrusts istruly revolutionary, there is a continuing need toenvision the world of the future to ensure that therotorcraft technology plan is appropriately focused.This requirement for a fresh look at the future led tothe development of “Vertical Flight 2025”.

“Vertical Flight 2025”: A StrategicVision for Revolutionary Flight

Concepts

In June 1999, members of the Ames rotorcraftcommunity gathered to brainstorm ideas for thefuture of Vertical Flight in the year 2025. Dr. L.S.“Skip” Fletcher, Ames Director of Aerospace,participated in this symposium and challenged allthe participants to visualize the world in 2025 and todetermine what new missions and markets could beopened by rotorcraft and other vehicles capable ofvertical flight. Fletcher then asked that the membersof this Rotorcraft “think tank” begin to define thetechnology barriers which would have to beovercome in order to realize this vision of “VerticalFlight 2025” and the research programs necessary toresolve them.

The outcome of this process was the recognitionthat there will be significant market potential fortwo very different classes of vertical flight vehicles:ultra-small-scale vehicles operating autonomouslyand larger-scale, “user-friendly” vehicles capable ofcarrying a significant payload.

Roto-Mobile

A potentially huge niche market for an applicationof rotorcraft technologies is the personal

transportation system: single- or multiple-passengervehicles with the ability to takeoff and landvertically and to be operated either autonomously ormanually with “car-like” controls. Significantmilitary advantages could be realized with suchvehicles, including rapid mobility with minimumcasualties by bypassing obstacles such as landmines, blocked roads, impassable bridges, and largebodies of water; rapid-response search and rescue,reconnaissance, and surveillance; and insertion ofSpecial Forces. These “Roto-Mobiles” could also beused to:

• offload highway systems to improve thecapacity of ground transportation systems,

• allow rapid, door-to-door transportation toand from airports,

• provide instant-response medical attention,• yield an order-of-magnitude decrease in

package delivery time,• provide transportation for our growing

communities of senior citizens,• provide a third dimension for the Sport

Utility Vehicle

Higher Capacity Utility Roto-Mobiles could bedeveloped and marketed to:

• assist in the construction and maintenanceof power lines, bridges, and multi-storybuildings

• replace ground vehicles for agriculturaltasks such as planting, spraying, andharvesting;

• make significant improvements in theproductivity of aquaculture;

• provide a s ta t ionary a i rbornecommunications facility or large radarplatform;

• participate in major chemical andbiological cleanups;

• detect and extract land mines; and• conduct search and rescue operations in

adverse weather conditions

Although many attempts have been made in the pastto tap this market by developing both civil andmilitary personal transportation systems, significanttechnical barriers have caused these efforts to beless than successful. With its expertise in rotorcraftaeromechanics and control, human factors, and airtraffic management, Ames is particularly well-qualified to participate in the advanced technologydevelopment required for a successful Roto-Mobile.

Small Autonomous Rotorcraft & Micro-Rotorcraft

The "Vision 2025" exercise confirmed thetremendous potential of developing smallautonomous rotorcraft and their associated enablingtechnologies. Small Autonomous Rotorcraft isenvisioned as a class of vehicles which range ingrams of gross weight for “Micro-Rotorcraft” tothousands of kilograms for conventional-sizedrotorcraft uninhabited aerial vehicles). Smallautonomous rotorcraft represents, therefore, a broadspectrum of vehicles that will have uniqueapplications, missions, and market potentialdepending, in part, on their size and payloadcapacity.

In particular, the potential applications for thesmallest of small autonomous rotorcraft -- or Micro-Rotorcraft -- are enormous in number. Theseinclude operations alone or in collaborative teamsfor:

• atmospheric sensing such as wind sheard e t e c t i o n a n d m e t e o r o l o g i c a lmeasurements;

• stealthy urban warfare surveillance;• public service applications such as

immigration, drug enforcement, and publicsafety;

• operations in contaminated environmentsunsuitable for humans, and

• planetary exploration as “astronautagents”.

Already prototype vehicles are being developed inthe two size extremes of small autonomousrotorcraft: rotary-wing micro air vehicles beingdeveloped in response to DARPA sponsorship androtorcraft UAVs being developed under Industryand U.S. Navy and Marine sponsorship.Nonetheless, opportunities also exist for vehiclesthat fall in the intermediate ranges of size andpayload capacity.

The common integral feature of small autonomousrotorcraft is the emerging field of 'intelligentsystems' and autonomous vehicle control. Just asadvances in more traditional rotorcraft technologies-- such as composite materials and turbine enginepropulsion -- have radically changed the nature ofrotorcraft over the past thirty years, autonomoussystem technology will have a correspondingtransformative effect. Finally, because of the verysmall scale of some of the small autonomousrotorcraft ("micro-rotorcraft"), there will be uniquetechnical challenges for these vehicles in the area of

materials and structures, propulsion, aerodynamics,control of flight, and ground and flight testing.

As lead Center for Information Technology andRotorcraft, Ames is well-positioned for its role inthe development of the high-payoff technologiesimplicit in the development of small autonomousrotorcraft.

Challenges for Implementing the Vision

Implementation of the “Vertical Flight 2025” visionis as critical as, if not more so than, developing thevision itself. As with the results of any creativethinking, there are always obstacles to be dealt withbefore reaping the potential benefits. To overcomethe technical and socioeconomic barriers implicit inthe development of rotorcraft personaltransportation systems ("roto-mobiles") and smallautonomous rotorcraft, carefully selected andplanned research projects must be initiated andexecuted. Given the current limitations in bothfinancial and human resources available to theAmes rotorcraft community, it is important to makereasoned decisions about the initiation of new, boldresearch projects such as the ones identified in the“Vertical Flight 2025” process. Our currentcommitments, both to existing NASA and U.S.Army research programs, must continue to be theprimary focus of our research portfolio.

Therefore, in addition to being creative inestablishing a vision, creativity is also required in itsimplementation. Government researchers must beprepared to operate in ways that might be veryunfamiliar, and even uncomfortable, to them. Small,focused, highly-competent teams made up ofmembers from many different scientific andengineering disciplines, working acrossorganizational boundaries, will be required toperform in a “Skunk Works” fashion. New, efficientways of working with small, creative private sectorcompanies must be invented. Universities mustparticipate as equal partners. In these lean budgettimes, “creative financing” must be employed. Newbudget sources for “venture capital” must beidentified. Short-term projects that address problemsthat can actually be solved must be included in theresearch portfolio. Affordable projects thatconvincingly demonstrate the attributes of thesenew vehicle concepts, such as a new operationalcapability, safe operations, and reliable automation,must be designed and carried out. But, above all, itis important for these research teams to:

• Do fabulous work and be known around theworld for (their) innovativeness.

• Attract exciting people -- more than a few ofwhom are a little offbeat.

• Raise hell, constantly question "the waythings are done around here," and never, everrest on (their) laurels. (Today's laurels aretomorrow's compost.)

Tom Peters (author of “In Search of Excellence”)

The following sections of the paper provideadditional details on the design and technologyopportunities and challenges implicit in thedevelopment of rotorcraft personal transportationsystems and small autonomous rotorcraft, as well asprovide a report on the efforts made to date torealize the vision of “Vertical Flight 2025”.

Vision 2025: The “Roto-Mobile” – APersonal Transportation System

Introductory Remarks

The concept of a simple flight vehicle for personaltransportation has been an enduring dream for manyyears, Fig. 13. To be truly revolutionary, such avehicle must be compact, economical, safe, easy tooperate, environmentally friendly, and particularlysignificant, runway independent. Aside from theability to takeoff and land vertically, this impliescapabilities heretofore unattainable in a flightvehicle. However, with today's rapidly advancingtechnology, such a vehicle may well be achievable.For example, consistent with the Division's VerticalFlight 2025 strategic vision effort described earlier,the concept of revolutionary flight vehicles that cansignificantly improve the short haul and commuter

Fig. 13 –Rotorcraft Personal Transportation SystemConcept

transportation capacity (throughput) of the countryis gaining increasing acceptance by aerospaceleaders (Ref. 4).

The Small Aircraft Transportation System (SATS)concept being led by NASA/FAA/DOT seeks toadvance transportation infrastructure and vehicletechnologies to satisfy a significant portion of the21st century transportation demand of the nationand relieve pressure on existing ground and airsystems (Ref. 15). In recent years, noted aerospacetechnologists have recognized growingtechnological capability converging with the visionspecifically for vertical flight vehicles (Refs. 16 and17). The concept Roto-Mobile class of verticalflight vehicle is intended to represent a means forstudying, advancing, and applying technologiespotentially capable of realizing such a personaltransportation vehicle.

Previous attempts to develop revolutionary personalaircraft comprise a variety of vehicles - the roadableairplane or flying car, individual lift devices orsmall flying platforms, and numerous smallhelicopters and VTOL aircraft concepts (Ref. 18).None of these devices has yet come close toachieving the dream of a personal transportationsystem. However, valuable experience has beengained toward the realization of the Roto-Mobileconcept. This experience will be used as a basis todefine a representative configuration and to identifythe technical issues needing priority attention.Currently, the Roto-Mobile is envisioned withducted fan(s) for lift and propulsion in view ofgeneral appeal, aerodynamic efficiency, andmoderate downwash and noise. A rotor may requireless power, but a compact, enclosed thruster will besafer, more compatible in confined spaces, andenjoy broader user acceptance.

To be more specific about the requirements, theRoto-Mobile is conceived as a revolutionary single-or multi-passenger vehicle able to 1) take off andland vertically from confined spaces and 2) operatesemi-autonomously or with simple "car-like"controls. Such a vehicle must have unprecedentedreliability and fail-safety and this will requiresimplicity, excellence in design, and the utmost intechnology innovation. To be affordable, both interms of acquisition and operating cost, it must beaerodynamically efficient, designed for ease ofmanufacture, and successful enough to gain costsavings from mass production. And finally, it mustbe practical, convenient, and easy to use in ways farbeyond present day experience such as semi-autonomous or semi-automatic operation, providing

operators with personally adaptive controls anddisplays to navigate from designated landing andparking areas through controlled 3-D pathways inthe sky.

The implied premise behind such a revolutionaryconcept is that recent and anticipated technologicaladvances in a variety of disciplines may becombined with earlier concepts to overcomeprevious limitations and deficiencies. These newtechnologies include advanced computers,information technology, flight control systems,avionics and sensors, powerplants, and structuralmaterials. Revolution in these disciplines will notautomatically generate a revolutionary flight vehiclebut will provide new opportunities for creativethinking, inventiveness, and enterprise to realizesuch a vehicle. The result will be nothing less thana reshaping of the way aviation impacts our way oflife for transportation, recreation, and defense.

In this section of the paper, we consider some of theopportunities and discuss issues that must beaddressed to make the Roto-Mobile concept areality. We will also briefly describe technicalactivities underway at Ames Research Center toaddress technology needs as well as cooperativeactivities aimed at furthering this technology.

Schroeder's recent historical overview (Ref. 19)provides an up-to-date survey of the broad spectrumof individual flying platforms, including briefdescriptions of approximately 30 different verticaltakeoff and landing aircraft that have undergonesome degree of flight testing. This survey outlinesthe prevalent approaches along with theiradvantages and disadvantages. The manifoldproblems illuminated a variety of technical issuesthat must be resolved before the Roto-Mobileconcept can succeed.

Zimmerman (Ref. 20) developed the flying platformconcept at NACA after hypothesizing that naturalkinesthetic balancing reactions of a standingoperator would stabilize a thrust device attachedbeneath his feet. A flurry of interest occurredduring the 1950s when military interest generatedsupport for a variety of experimental developments.Many of these configurations were conventionallow-disc-loading rotors, but other types includedducted fan platforms, jet platforms, and evenhydrogen peroxide rocket belts. The rotor platformshad the highest hovering efficiency but were lesscompact and compatible with confined areaoperation than ducted fans. Jet and rocket deviceswith poor propulsive efficiency suffered from

limited endurance and generated very high noiselevels. Although there were some areas of success,the group of vehicles as a whole experiencedsignificant technical obstacles that were largelyunsolved. The main problems included marginalstability, lack of adequate control and handlingqualities, and gust response sensitivity. Otherproblems included limited maneuver capability,pilot induced oscillations, and very low trimmedflight speed. Of course only limited stability andcontrol augmentation systems were incorporated inthese vehicles so lack of success is not surprising.Given adequate aerodynamic control power andmodern flight control system technology, acceptableflying qualities should be achievable.

Technical Challenges

A number of critical technical challenges that mustbe faced in several key technical discipline areaswill be briefly discussed, including the potential oftoday's technologies and some suggestions onpossible conceptual approaches.

Flight Control and Handling Qualities. Perhaps thesingle most important requirement for the Roto-Mobile concept is providing ease of control for theoperator - and in ways that go far beyond thetraditional meaning of handling qualities. Aminimum requirement for a traditional flight vehiclewould be satisfactory controllability by a skilledpilot and with proper application of modern flightcontrol technology this capability should beavailable. However, to be a revolutionary vehicle,the Roto-Mobile should be flyable by another classof operator, requiring skills no more difficult tolearn than those needed to drive a car. This willpresent challenges on several levels. It will benecessary to understand what this means in terms ofvehicle requirements, and moreover, what "car-like"means when applied to a vehicle that operates inthree dimensions. At one extreme it could meanfully automatic control with the operator merelyselecting a destination and turning over full controland navigation to the vehicle. However, this isprobably not desirable. To develop a sense ofconfidence in the vehicle, achieved throughmastering control of the device, the operator willlikely desire some degree of control over bothmaneuver and navigation. The key will be todetermine the automatic flight control requirementsneeded to engender this sense of confidence. Theappropriate degree of maneuvering control, ease ofuse, and navigability may vary according to theflight regime and piloting task. At the least, such

capabilities must be highly intuitive and not requirethe operator to undergo lengthy training to become ahighly skilled "pilot." This requirement will placenew demands on understanding flying qualitiesrequirements and configuring control systemarchitectures. One conceptual framework for thedesired control functionality has already beenproposed by Drees (Ref. 16).

Human Factors. Closely associated with the need todevelop the appropriate flight control characteristicswill be the need to examine human factorsrequirements; that is, the unique ergonomics anddisplays required for a VTOL aircraft that mustoperate safely. The human factors issues for theRoto-Mobile operator will present significantchallenges.

Air System Operation. Requirements for bothsafety and the goal of helping to meet nationaltransportation needs will require the Roto-Mobile tohave revolutionary airspace operability. In thissense, the challenges for the Roto-Mobile conceptgo beyond the scope of issues addressed by earlierindividual flying platform development projects -the significant fact is that the concept of arevolutionary vehicle cannot be viable unless the"infrastructure interface" requirements are satisfied.That is, a VTOL aircraft must operate safely andnavigate within a controlled airspace environment.Navigation may be greatly facilitated by newtechnologies such as GPS systems. A successfulvehicle will proliferate in large numbers and willrequire new approaches to air traffic control,separation, and collision avoidance, well beyond thechallenges facing the current aircraft population. Anumber of these issues are being addressed in thecontext of the SATS program (Ref. 15) to advancethe National Airspace System infrastructureincluding "Smart" airports, EnRoute and TerminalFree Flight, and satellite-based communications,navigation, and surveillance. For militaryapplications, numerous navigation and controlissues will need to be addressed to insure missioneffectiveness and reduced vulnerability.

Safety and Reliability Characteristics. A revolutionin flight vehicles will require an unprecedentedlevel of intrinsic vehicle safety and reliability, inaddition to safe operation of the vehicle airspacesystem. Vehicle safety and reliability involve theairframe structure, propulsion, flight controleffectiveness, and reliability of electronics includingavionics, sensors, and computers. Backup safetyrecovery systems need to be considered. Advancedtechnology offers many benefits for safety. Gas

turbine powerplants offer high reliability althoughfew candidates are currently available for personaltransportation-sized vehicles. Internal combustionengines are not normally considered highly reliable,however they can be made extremely reliable ifdesigned accordingly. For example, multiplepowerplant redundancy, derating to increasereliability and operating life, and multiple redundantelectronic ignition and fuel injection systems willsubstantially increase reliability and safety.Advanced health and condition monitoring and faultdetection systems will provide increased warning ofimpending failures. Structural reliability must beassured through intelligent design backed up bythorough qualification testing. Composite materialsoffer increased reliability, benign failure modes, andon-condition inspectability. Subsystem componentssuch as actuators, flight control hardware, fuelsystems, can be improved with design excellence,redundancy, derating, and thorough qualificationtesting. In the event a failure occurs, ballisticparachute recovery devices are available for lightaircraft and other concepts may be developed thatmay ultimately offer backup safety devices effectivethroughout the entire flight envelope.

Flight Performance Efficiency. Flight performancewill benefit significantly from improvements inaerodynamics, structures, propulsion, and flightcontrol systems. For the reasons noted above, thebasic Roto-Mobile concept will likely embodyducted fans for lift and propulsion in hover andforward flight. Application of advancedaerodynamic analysis methods, including CFD, andappropriate wind tunnel and free flight testing willhelp insure optimum performance capabilities formultiple operating conditions. Sufficientaerodynamic control power will be essential toovercome extremely limited forward flight speedcapability of the ducted fan stand-on platform.Additionally, reducing the drag of the cylindricalshroud of the simple ducted fan will be needed toenable reasonable forward speeds with a fixed duct.At the present time it is unclear whether the fanducts should be tilted to achieve forward flight, orwhether a vane system (louvers) should be used forthrust deflection. In view of the cascading effects ofincreasing complexity, the appropriate configurationtypes will need to be determined in relation tomission requirements. Current powerplanttechnology should be sufficient to enable reasonableperformance, however tailoring powerplants tospecific size and operational requirements of theRoto-Mobile concept would undoubtedly enhanceperformance considerably. Similarly, advancedcomposite structures offer significant benefits that

should be exploited to enhance viability of theconcept.

Environmental Impacts. Duct downwash, engineand ducted fan noise, and engine pollution areimportant considerations for success of the concept.One benefit of the ducted fan is relatively lowdownwash provided the disk loading is keptreasonably low. Engine noise will require mufflersystems and ducted fan propellers will need tooperate at relatively low tip speeds to be acceptable.Finally, engine exhaust pollution, especially fortwo-cycle reciprocating engines will have to beaddressed particularly if such vehicles do indeedprogress to the mass market.

Ames Activities

In view of the timeliness of the technologyopportunities and the potential mission applications,the Army/NASA Rotorcraft Division has initiatedseveral technical activities to study the Roto-Mobileconcept and strengthen the technical base inpreparation for possible future activities. These willbe briefly described below.

NASA Ames Research center has entered into aNon-Reimbursable Space Act Agreement withMillennium Jet, Inc. to "assess the validity andfeasibility of a personal rotorcraft for potentialaircraft development to meet future nationalrequirements to increase use of rotary wing aircraftand reduce the cost of air travel." Millennium Jet,Inc. (MJI) has designed, fabricated, and has initiatedground testing of a prototype individual flightvehicle called the SoloTrekTM XFVTM Exo-SkeletorFlying Vehicle.

The SoloTrekTM VTOL configuration (Ref. 21) isdesigned to transport a single operator in thestanding position with a pair of fixed-pitch ductedfans mounted above the operator's shoulders anddriven by a two-cycle, four-cylinder engine (Fig.14). Under the terms of the agreement, MJI willprovide engineering design analyses and conceptualdesigns and results of MJI analyses and tests whileNASA will contribute government requirements,engineering and mission assessments, and results ofselected NASA analyses and tests. MJI and NASAwill jointly assess the adequacy of various designsand testing processes.

The Division is also initiating research involvingsmall-scale wind tunnel testing of ducted fans toextend the experimental database for axial andoblique operating conditions and to study details of

Fig. 14 - SoloTrekTM XFVTM Exo-Skeletor FlyingVehicle.

duct flow separation and aerodynamic efficiency forvariations in duct and fan design characteristics.Most importantly, this information will be used tocompare with and refine aerodynamic analysismethods, including computational fluid dynamics(CFD) for ducted fan configurations. In the area offlight control and stability, preliminary simplifiedmodels will be employed to study the handlingqualities and control system requirements of theRoto-Mobile concept and attempt to find ways ofovercoming the problems that have plagued suchconcepts in the past.

Vision 2025: Small AutonomousRotorcraft

Introductory Remarks

In addition to the vision of rotary-wing vehicles asbeing key components in a Personal TransportSystem, the other major strategic vision forrotorcraft as identified by senior technologistswithin the Army/NASA Rotorcraft Division is thedevelopment and use of small autonomousrotorcraft. Small autonomous rotorcraft are definedfor the purposes of this paper to be a class ofvehicles that ranges in size from very small rotary-wing micro air vehicles that weigh only a few grams

– referred herein this paper as “micro-rotorcraft”(Fig. 15) -- to larger, more conventional, rotorcraftuninhabited aerial vehicles (UAVs) that have grossweights in the thousands of kilograms.

Fig. 15 -- Various Notional Micro-RotorcraftConcepts.

What is the underlying rationale for thedevelopment of small autonomous rotorcraft? Firstand foremost, the "intelligent systems" capabilityinherent in small autonomous rotorcraft reduces oreliminates the human element in execution of thesevehicles missions/applications -- thus significantlyreducing operator cost and personal risk.

In a general sense, small autonomous rotorcraft canbe seen to hold great promise in three primary areas:covert surveillance applications; mobile userinteraction and utility functions; andimplementation of rapid deployment of ‘distributed’processes. Vehicle sizes and payload capacity forsmall autonomous rotorcraft is dependent upon thespecific missions/applications being considered.This will be discussed in further detail.

Covert Surveillance. Covert surveil lanceapplications could benefit from the inherent lowobservability of small autonomous rotorcraft --particularly micro-rotorcraft. “Distributed”

missions/applications could call for the use ofcollaborative teams of vehicles, while doing so in acost-effective manner. The mobility of smallautonomous rotorcraft could enhance the flexibilityof intelligence gathering versus remote, ornonmobile, sensors. Use of multiple distributedvehicles would minimize the impact of individualvehicle detection/compromise to the overall covertoperation. Small vehicles are potentially easier toinsert into a target territory unobtrusively. Forexample, small vehicle size could enable manualtransport/fielding ( a micro-rotorcraft size vehiclecould easily be carried by one or twosoldiers/operators/operatives). Very small,lightweight, and robust vehicles could alternativelypotentially be dispersed efficiently be specialized'carrier' vehicles. Finally, on the larger end of thesmall autonomous rotorcraft spectrum, the vehiclescould be self-deployable from a home base.

Mobile User Interaction and Utility Function. It iscurrently not unusual for people to carry one ormore digital electronic devices for day-to-daypersonal, professional, or recreational use. Thefuture promises an explosion of personal digitaldevices. Even wearable computers are seriouslybeing studied. Use of small autonomous rotorcraftfor personal interactive services are a natural logicalextrapolation of this trend. Micro-rotorcraft, inparticular, could operate in close proximity to andinteract with a human being to provideunprecedented personal utility and function.“Distributed” processing/sensing across multiplevehicles could provide enhanced utility withoutincreasing any individual device/vehicle’sweight/size. Further, vehicle mobility wouldreduce, or eliminate, the manual burden oftransporting this functionality (versus hand-carriedor wearable devices). For example, smallautonomous rotorcraft could be used as flying‘personal’ video cameras for high-endconsumers/world-travelers. Other examplesinclude: improved Secret Service protection forhigh-level politicians/dignitaries; improvedmanagement of prisoner release programs; personalsecurity; truly secure communication through actualphysical transfer of information by means of micro-rotorcraft acting as ‘couriers.’ On the other sizeextreme of the small autonomous rotorcraftspectrum for 'mobile user' applications is the semi-autonomous flight control of rotorcraft personaltransport systems ("rotor-mobiles").

Rapid Deployment of Distributed Processes. Largeareas can be surveyed or secured more quickly andcomprehensively with multiple small autonomous

rotorcraft carrying multiple types of sensors and/or‘active elements.’ This task can be accomplishedmost effectively with small, low-cost vehicles thatcan be rapidly and easily dispersed from 'carrier'vehicles, and are considered ultimately to beexpendable. Examples could include: chemicalspill surveys and clean-ups (timed release ofpetrochemical-eating bacteria as one example);surveillance for treaty violations of weapons ofmass destruction (chemical, biological, andnuclear); plant/building physical security; bordersecurity; planetary exploration; revealing,documenting, and perhaps preventing potentialcrimes against humanity (via rapid insertion ofactive, intrusive, pervasive, and visible intelligenceassets primarily comprised of micro-rotorcraft).Inevitably trade-offs must be made between largersizes of small autonomous rotorcraft for improvedcapacity to carry payloads, to survey greater areas,or to stay longer on station for observations, versussmaller vehicles that could be more easily dispersedin larger numbers.

Table 1 is an qualitative assessment of the spectrumof small autonomous rotorcraft sizes and weightclasses applicable for specific mission/applications(consistent with the three general application areasnoted above). Though there is plenty of room forreasoned argument for whether a particular vehiclesize can perform a given mission, this tablehopefully provides a good starting point for follow-on discussion.

Most of the original interest in fixed-wing androtary-wing micro air vehicles is due to theadvocacy and support of DARPA. Reference 22summarizes that interest for anticipated future DoDmissions. Through DARPA support, a number ofmicro air vehicle concepts have been taken to proof-of-concept flight demonstrations. DARPA hasimposed a maximum vehicle size limit of 15 cm intheir ongoing micro air vehicle studies. But it is notclear that, in the long run, that this design-by-dictateapproach is best. There are at least two differentperspectives in the development of rotary-wingmicro air vehicles. One perspective, seems to beabout pushing the frontiers of micro-machinetechnology rather than being mission/applicationdriven. However, at this early stage ofinvestigation, both technical approaches(technology- versus mission- or application-driven)should be pursued.

Though a number of rotary-wing micro air vehicle(Ref.s 23-24) and rotorcraft UAV (Ref.s 25-28)development efforts are currently underway within

Table 1 -- Vehicle Size versus Types of Mission (APartial List)

Types of Missions <0.1m

(<0.1kg)

0.1 - 1m

(0.1 -10kg)

1-3m

(10-100kg)

3-10m

(50-1000kg)

RotorDiameter > 10m(VehicleMass >1000kg)

Covert Surveillance - Intelligence orreconnaissance inBuilding/StructureInteriors - Insertion/action insensitive open (outside)environment - Border/campboundary protection(discovery orobservation onlymoderately critical) - Medium AltitudeSurveillance (discoveryor observation notcritical)Mobile UserInteraction and UtilityFunction - Personal aerialcamera - Professional MediaAerial PlatformPersonal or professionallevel security - Personalized courierservice

- Personal transportautonomous support for‘automobile-like’controlRapid Deployment ofDistributed Processes - Urban police security

- Hazardous materialtracking and clean-upaction - International tribunalsurveillance anddocumentation

- Border patrol

industry and academia, the promise of this vehicleshas not yet been realized and many opportunitiesremain to develop enabling technologies for thisbroad class of 'small autonomous rotorcraft.' Amission-oriented perspective to vehicledevelopment for small autonomous rotorcraft willlikely point to the need to focus on intermediatevehicle sizes in addition to rotary-wing micro airvehicles and conventional rotorcraft UAVs.

Technical Challenges

It is important to ask what are the technical issuesfor small autonomous rotorcraft -- particularly formicro-rotorcraft.

Low Reynolds number aerodynamics and lowaspect ratio lifting surfaces will present challengesto develop vehicles with acceptable performancecharacteristics. Further, there is little empiricalinformation/insight into the design of very smallvehicles. The closest analog to small autonomousrotorcraft are hobbyist, or radio-controlled,helicopters. But the anticipated missions orapplications for small autonomous rotorcraft requirevehicle performance and system capability wellbeyond 'hobbyist' or ‘industrial’ radio-controlledhelicopters. Nonetheless, some lessons learnedfrom the hobbyist world might be applicable tosmall autonomous rotorcraft development.

Another set of technical challenges involveproviding adequate onboard resources to performpractical missions/application (including providingadequate fuel/power to fly reasonable ranges andendurance, adequate payload fraction for missionpackage for sensors and downlink telemetry, high-speed, multiprocessor flight/mission computerarchitectures compatible with both low- and high-levels of autonomy. Low-cost manufacturabilitywill be essential to small autonomous rotorcraft (alarge number of these vehicles will be required ascompared to larger vehicles, and these vehicles willlikely be considered ‘disposable’). Minimum man-in-the-loop effort is also essential for operating andinterrogating these very small (and potentially largenumbers of) rotorcraft vehicles.

Finally, vehicle range and endurance, particularlyfor vehicles powered by electric propulsion, will bea major issue. Current state of the art motor andbattery technology limits electric-powered hobbyisthelicopters less than a half-hour of flight at best.Endurance limitations of individual micro-rotorcraft, for example, can be compensated for byusing multiple platforms to accomplish a givenmission -- such that a constant cycle of vehiclescould either be ‘recharging’ themselves orperforming the station keeping mission. As thesevehicles are very small and low-cost, such aconstant re-supply/mission sortie cycle should beviable for even simple missions. One possibility toexpedite such a constant re-supply/mission sortiecycle strategy would entail having a ‘perch’ for aswarm of micro-rotorcraft which would incorporate

an induction-plate electrical charger. Anotherpossibility is to have the vehicles drop to the groundand recharge via solar cells before returning to flightand the mission.

Powerful market and technological forces will helpinfluence the successful development of smallautonomous rotorcraft. The fast-paced world ofconsumer electronic/digital products is driving thedevelopment of new advanced battery and/or fuel-cell technologies, micro-mechanical sensors, high-speed, low-power, distributed micro-processors, andother miniature electronic components necessary forsmall autonomous rotorcraft. Finally, there hasbeen a renaissance recently as to automatedreasoning software development and robotics.NASA research into information technology andintelligent systems research has seen significantincreases in the past few years. NASA has a long-term strategic goal to develop robotic systemsostensibly for space and planetary science missionsbut with potential broad application – includingautonomous rotorcraft. Further, this NASAstrategic goal includes studies into distributedrobotic ‘colonies’ with multiple, and heterogeneousgroups of, ‘agents' (Ref.s 29-36). All of this workin automated reasoning, robotics, robotic colonies,and distributed processing will have significantimplications for the successful introduction of smallautonomous rotorcraft.

Ames Activities

The first tentative steps towards to making thisvision of small autonomous rotorcraft a pervasivepart of our technological society are already beingtaken by the Army/NASA Rotorcraft Division. TheNASA Intelligent Systems (IS) program and theRotorcraft Division have initiated a cooperativeproject studying autonomous rotorcraft technology.Further, an in-house effort to develop variousinnovative vehicle configurations for micro-rotorcraft is also underway. These initial effortswill hopefully lead to comprehensive researchprograms that will examine the broad spectrum ofsmall autonomous rotorcraft and, therefore, enablethe strategic vision outlined in this paper.

Concluding Remarks

Imagine it's the year 2025. In the sky above aredozens of miniature robotic helicopters measuring

only two to three inches in size darting about as youstroll to your one-person "Roto-Mobile" to beginyour daily commute to your downtown office.

Sound farfetched? The Army/NASA RotorcraftDivision doesn’t necessarily think so. SeniorDivision management and technical staff recentlyengaged in an effort to forecast the future ofrotorcraft and other vehicles with a vertical flightcapability. During this brainstorming session, itbecame apparent that there will be significantmarket potential for two very different classes ofvertical flight vehicles: ultra-small-scale vehiclesoperating autonomously and larger-scale, 'user-friendly' vehicles capable of carrying a significantpayload. The Army/NASA Rotorcraft Division istaking steps to formalize and implement thisstrategic vision of rotorcraft technology for thefuture.

This paper also highlights other ongoingrevolutionary research thrusts within theArmy/NASA Rotorcraft Division at Ames ResearchCenter. This summary of revolutionary projects isnot a comprehensive list but gives a broadperspective of the active and innovative researchbeing conducted by the Army/NASA RotorcraftDivision.

References

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