ICAS-90-2.2.1 NASA Programs in Advanced Sensors and Measurement Technology for Aeronautical Applications Bruce A. Conway NASA Langley Research Center Hampton, Virginia 23665-5225 Abstract There are many challenges facing designers and operators of our next-generation aircraft in meeting the demands for efficiency, safety, and reliability which are will be imposed. This paper discusses aeronautical sensor requirements for a num of research and applications areas pertinent to the demands listed above. A brie overview will be given of aeronautical research measurements, along with a discussion of requirements for advanced technology. Also included will be descriptions of emerging sensors and instrumentation technology which may be exploited for enhanced research and operational capabilities. Finally, renewed emphasis of the National Aeronautics and Space Administration in advanced senso and instrumentation technology development will be discussed, including projec of technology advances over the next 5 years. Emphasis on NASA efforts to mor actively advance the state-of-the-art in sensors and measurement techniques is timely in light of exciting new opportunities in airc development and operation. An up-to-date summary of the measurement techno programs being established to respond to these opportunities is provided. I. Introduction In the development of new classes of aircraft, from the evaluation of design con in laboratories or flight to the application of sensors and instrumentation for r flight operations, incorporation of highly-capable sensors or measurement instrumentation is crucial to the research or operational mission success. Typi sensors to support research applications or flight operations have been "off-the-shelf" or developed in a short period of time in response to an urgent n The fast pace of today’s technology advances and the research programs undertak to enhance or exploit them have necessitated earlier attention to measurement technology to fully capitalize on the improved capabilities. In some cases, exist technology can be readily extended to address new application regimes, as for example with the enhancement of microphone technology for applications to high-temperature, high-frequency regimes associated with aerothermodynamics measurements. In other cases, emerging technology can be focussed to address critical safety and reliability problems; use of fiber optic techniques for struc monitoring of aircraft and spacecraft is one promising utilization of such a technology. However, unless a concerted, coordinated effort is made early in the planning stages (for a new aircraft project) to address the measurement requirements (and available technology or capabilities) associated with developm test and operations, implementation of downstream project milestones can be delayed for lack of appropriate instrumentation or measurement techniques. "Hurry-up" development must then occur, which may result in inefficiencies, add costs, and ultimately project delays. The U. S. hypersonics research and technolo program, including the National Aerospace Plane program, is an example where sensors and measurement technology had not kept pace with advances in computational aerodynamics, structures, materials, and propulsion technology. A preferable philosophy involves the "up-front" inclusion of measurement technolo requirements early in the development phases of an aircraft program, particular those developments where new regimes of speed, physical environment, or aerodynamic and structural characteristics are to be encountered and dealt with A key part of the mission of the U. S. National Aeronautics and Space Administra (NASA) is to undertake research and development in support of the nation’s mili and civil aeronautics and space endeavors.(1) In support of this role, NASA cond research and technology efforts in several associated disciplines, including electronics, sensors, and instrumentation techniques and systems. Agency progr are aimed at advancing the state-of-the-art and exploiting these and other adva for the furtherance of aerospace technology. In aeronautical sensors and measurement technology, various aeronautics discipline programs sponsor resear that is focussed to meet discipline-specific needs, including schedules. There hav been in the past, and are beginning to be re-established, more generic research activities not necessarily having a specific discipline or vehicle focus, but rathe which have broad potential applications. The following sections in this paper w address aeronautical research measurements in view of the reawakening interest longer-range research. The requirements for advanced technology to meet sever current and future needs, and the emerging sensor and instrumentation technolog being developed to address these requirements will be discussed in the context o NASA’s research and technology programs in aeronautical sensors. The paper will conclude with presentation of potential future sensors research and applications activities. II. Aeronautical Research Measurements In the conduct of ground-based (for example, wind tunnel) and flight research te measurement of many different parameters may be required. The number, freque resolution, and precision of measurements can vary widely in practice, depending the research objectives. The ability to make some desired measurements may b compromised by constraints such as environment (high or low temperatures, for example), physical limitations (model size, clearances available, or requirements non-intrusion), transducer or instrumentation system bandwidth limitations (frequency response characteristics), and, in some cases, an incomplete understanding of physical processes involved in an observed phenomena (making i difficult to develop a measurement transducer concept). As might be imagined, hypersonics-related research has a large amount of measurement difficulty associated with it, while measurement of parameters related to subsonic aeron is much more straightforward. Table 1 illustrates this dichotomy by listing se key measurement parameters associated with several aerodynamic regimes, along with an indication of the maturity of technology to make the required measurem There is a relationship among aeronautical measurements made to satisfy the requirements of (1) ground-based research and testing, (2) flight research and testing, and (3) operational applications (both ground and flight). Oftentimes t solution to a measurement problem in a ground research facility may be extended application on a full-scale aircraft or remotely-piloted vehicle, and then made p the aircraft’s instrument complement. The obvious inference from the existence this relationship is the strong potential influence that mature technology which supports aircraft operations has on availability of measurement techniques for additional research in the particular regime. Also, a lack of such operational applications may infer that a definitive program to develop required sensors and measurement technology should be carried out. The dearth of measurement techniques in a given regime or discipline (as indicated, for example, in Table 1) generally necessitates the application of advanced technology. The next section addresses some of the drivers and requirements for advanced technology to solve critical aeronautical measurement problems. III. Requirements for Advanced Technology As other aeronautical disciplines mature, driven by changing mission requiremen economics, or as a result of conscious fundamental research efforts, the need fo measurement technology to keep pace grows acute. For instance, in hypersonics technology, great strides are being made in high-temperature structural materi capabilities, with carbon-carbon composites being a leading example of advanced technology. However, as noted in the preceding section (Table 1) there is a lack sensors or measurement techniques to make high-fidelity temperature measurem in the ranges expected to be encountered in hypersonic flight. Similarly, the ab to measure skin friction in the boundary layer of a hypersonic vehicle necessitat sensor operation in the same high-temperature ranges. In fact, all in situ measurements will involve hostile environments in hypersonics, including those m in and around the propulsion system. In another regime, that of low-disturbance flow (such as might be sought in so- "quiet wind tunnels"), it becomes necessary to make measurements with minima no disturbance to the airflow. This same constraint applies to sensors used in for the determination of parameters associated with laminar flow. Low-forward-velocity flight also has characteristics that make it more difficu accurately measure the parameters of angle-of-attack and velocity using conventional sensor techniques. And, although the phenomena have been known fo awhile, accurate detection and warning of meteorology-related hazards such as shear, wake vortices, and clear air turbulence is not yet routine. The above-cited cases are but a few examples of areas where measurement technology must be improved to support advances in other aspects of aeronautic technology, including enhanced aviation safety. A major impetus for enhanced measurements capability is the dramatically-increased power of digital compute used to model aerodynamic, structural, and propulsion quantities. The computat fluid dynamics and computational structural mechanics programs run by today’s supercomputers can calculate parameters such as temperatures, pressures, and flow velocities to fine numerical and spatial precision. The valid of the computer software codes used in these computations requires measureme that are of comparable precision. The need for such precision directly leads to t requirements for advanced sensor and instrumentation technology to achieve it. Satisfying these requirements is a key problem of today’s research efforts, and be a crucial aspect of tomorrow’s aeronautical research and applications. The following section discusses emerging sensors and instrumentation technology in several areas, which may be useful in satisfying many of the advanced measurem requirements in aerospace research and applications. IV. Emerging Sensors and Instrumentation Technology Technology advances supporting aeronautical sensor and instrumentation techniq have been made on several fronts. Achievements in high-temperature materials metallic and non-metallic), lasers and electro-optics (including fiber-optics), high-speed pressure transducers, and photo-optics are being reported by researc in government, industry, and academia. NASA is involved in many of these areas achievement, either as contributing researchers, or through adaptation of the advances for improved measurement technology. This section will cover some of emerging technology areas highlighted by recent advances, with emphasis on the NASA is playing in their achievement and applications. High-Temperature Materials Because high-temperature operation is a key facet of many hypersonic aerodynam and propulsion research and technology measurement problems, there has been a concentration of effort recently in this area. For example, researchers D.Englund R.Seasholtz at NASA’s Lewis Research Center have investigated several alloy compositions for application to strain measurements at high temperatures.(2) O these alloys, PdCr, has been extensively characterized by J.Lei of Sverdrup Techno at NASA-Lewis and found to exhibit a high degree of repeatability and very low thermal output due to expansion from room temperature up to 700 (cid:176) C.(3) Use o strain gage materials is expected to significantly enhance research of turbine en hot sections, at temperatures up to 1000 xC, as well as find utility in strain measurements on models in high-temperature wind tunnels or arc jets. A problem encountered in attempting to use either conventional or high-temper strain gages in a hot environment is the reliability of the application or bonding particularly when newer surface materials such as composites are involved. Promising advances are being made by T.Moore at NASA-Langley, who has demonstrated application of several types of high-temperature strain gages on carbon-carbon surfaces with a SiC coating, with successful operation to 555 (cid:176) C.( Recently, successful application and tests at 833 (cid:176) C have been achieved. As mentioned, aerothermodynamic measurements on the surface of an aircraft m or flight vehicle in hypersonic flows can be difficult, particularly at the high frequencies characteristic of phenomena in hypersonic boundary layers. As temperature ranges extend outward, microphone sensors are being developed to capture the acoustic energy associated with, for example, transition from lamin turbulent boundary layer flow. Figure 1 depicts progress being made in this field A.Zuckerwar of NASA-Langley and F. Cuomo of the University of Rhode Island have recently developed a Ni 200 membrane, fiber optic microphone capable of operat 600 (cid:176) C.(5) This microphone has recently been successfully tested. Other emerging sensor and instrumentation technologies in the high-temperature materials area (some still requiring substantial work) include liquid crystals fo pressure or temperature measurements on aerodynamic surfaces and sputtered o vapor-deposited thin-film thermocouple and strain sensors.(2) Lasers and Electro-Optics A major constraint when making many aeronautical measurements is minimizing sensor’s influence on the parameters being measured. The field of non-intrusive measurements is thus aimed specifically at avoiding this conflict. Remote sensi techniques utilizing various forms and spectral regions of electromagnetic energ particularly suitable, with lasers as a well-known solution which is being rapidly improved. A flow visualization method which is proving increasingly effective is the so-ca laser light sheet. This technique utilizes a single cw laser with either a scannin galvanometer-driven mirror or a cylindrical lens to generate the "sheets" of ligh seeded flow. J. Franke, D. Rhodes, B. Leighty, and S. Jones of NASA-Langley have m several advances in this field in recent years.(6) The plane of the light sheet can moved along a model surface to visually follow the growth and burst of vortices illustrated in Figure 2. The laser light sheet nonintrusive flow visualization technique has been applied in supersonic flow experiments as well as subsonic flo An indication of the power of this method is its ability to delineate subtleties flow, such as the tertiary vortices easily visible in the photograph made from a television record of a supersonic (M = 2.4) experiment (Figure 3). Another group of related laser-based techniques for non-intrusive determination flow properties are laser transit anemometry (LTA), laser Doppler velocimetry ( and laser Doppler anemometry (LDA). These methods typically require that the be seeded in order that either time-of-flight or Doppler velocity components can detected via the laser beams; the resultant measurements are also point measurements, necessitating the scanning of the measurement volume to obtain complete mapping of the flow field. With advances being made in computational capability (speed, capacity) and detector algorithm sophistication, the relative new fields of particle image velocimetry (PIV) and Doppler global velocimetry (D are emerging; these can ascertain a planar flow velocity field in real- or near real-time. This is accomplished through a "double exposure" in the PIV case, or b selective filtering of a laser light sheet-illuminated flow-field plane in the DGV case. Both means can significantly reduce the amount of time needed to charact a flow field. Figure 4 illustrates a coarse PIV image of the flow over a standard airfoil, as compiled by W. Humphries of NASA-Langley. Laser-induced fluorescence (LIF) is another non-intrusive method of determining flow parameters over a wide range of airspeeds. LIF can be used to determine sp concentrations, making it particularly suitable for combustion flows. G. Laufer, Fletcher, and R. McKenzie of NASA-Ames Research Center have shown laser-induced fluorescence of O2, in combination with Raman scattering to be an accurate mea acquiring measurements of temperature, density, and their fluctuations due to turbulence in high-speed flows.(7) Application of LIF to hypersonic flows is proceeding at both Langley and Ames. Reliable tunable solid-state lasers is one example of emerging technology applica to aerodynamic measurements. The lighter weight and lower power requirements comparison to dye or gas lasers used for several years, holds great promise for enhanced laboratory operations such as Doppler velocimetry, laser-induced fluorescence, and laser transit anemometry, as well as enabling similar measurements on flight vehicles for the first time. One promising use of laser technology is in the so-called aviation meteorology f providing an ability to measure wind velocity in such meteorology-related pheno as wind shear, wake vortices, and clear-air turbulence (see Figure 5). M. Storm an Rohrbach of NASA-Langley have demonstrated for the first time single-longitudinal-mode lasing of Ho:Tm:YAG using a laser diode pump.(8) This demonstration, at a wavelength of 2.091 fm, paves the way for potential applic to coherent Doppler measurements in the eye-safe region of the spectrum. The to achieve the requisite lasing with an all-solid-state system also holds promise other applications in laboratory and ground-based systems. Solid-state lasers are also seeing increased use in such techniques as Rayleigh and Raman scattering in supersonic flows. For example, B. Shirinzadeh, M. Hillard, and Exton of NASA-Langley have utilized a frequency-doubled Nd:YAG pulsed laser (at nm) to obtain simultaneous Rayleigh and Raman scattering in Langley’s M = 6 wind tunnel to infer density and pressure in the tunnel flow. Their experimental set- shown in Figure 6, has enabled the demonstration that Rayleigh scattering for ga density measurements in the free stream of a supersonic tunnel can be seriously hampered by flow-generated interference (which they have postulated results fr nucleation process). Another solid-state laser application is in the Coherent Anti-Stokes Raman Spectroscopy (CARS) technique; R. Antcliff, O. Jarrett, T. Chitsomboon, and A. Cutler at NASA-Langley have utilized a 10 Hz pulsed Nd:YAG l to pump broadband dye lasers which produce the CARS probe beams. In recent experiments, two beams were generated to simultaneously examine oxygen and nitrogen temperature and density in a supersonic combustor flow.(9) The use of optical fibers is becoming more commonplace in aeronautical measure applications. They are being employed in harsh environments, such as those associated with turbine engines, for quantities such as temperature(10) and sha position (via a wavelength-division multiplexed optical position transducer(11)) for qualitative purposes such as combustor viewing.(2) Fiber optics have recent been applied in CARS measurements, to remotely route Raman signal energy throug harsh vibration environments to the receiving photodiodes and electronics. Othe examples of emerging fiber optics technology use is in so-called smart structures Rogowski of NASA-Langley has, in collaboration with other Langley and U.S. Air Fo Astronautics Laboratory investigators, been conducting research into the use of embedded fiber optic sensors in aerospace materials for strain measurements.(12)(13) Nondestructive evaluation and damage detection are oth functions emerging as amenable to implementation via fiber optics/smart struc Photo-Optics and Thermography Classical photo-optic instrumentation techniques are seeing reemergent applica to many of the nonintrusive measurement problems of today’s aeronautical resea programs. In addition to the requirements for avoidance of flow interference, t techniques enable the determination of parameters in harsh environments. The ballistic range is one example of a research facility needing such nonintrusive measurements. A. Strawa and J. Cavolowsky of NASA-Ames have been developing instrumentation for use in the Ames Research Center’s Hypervelocity Free-Flight Aerodynamic Facility.(14) One technique is the use of holographic interferomet Ames system employs dual-plate, double-pass interferometry to infer the densi ratio across a standoff shock wave. Figure 7 is an interferogram of a free-fligh model in the ballistic range portion of the Ames facility. Application of close-range photogrammetry to determine wind tunnel model deformation is being carried out. A. Burner, W. Snow, W. Goad, and B. Childers of NASA-Langley have developed a digital video model deformation system for application in the U.S. National Transonic Facility (NTF).(15) Figure 8 shows the experimental configuration; the two CCD cameras are 36 inches apart, and each is approximately 72 inches from the center of the wing. The technique has been validated by measurements on a Boeing 767 model in the NTF. Infrared thermography is an example of emerging technology used in several aeronautical applications. It has been demonstrated effective in nondestructive evaluation of bonded or laminated space and aeronautical structures.(16) In particular, it shows promise in providing large area inspection capability for airliners as part of the U.S. aircraft structural integrity program. IR thermograp also has been demonstrated in surface temperature measurements on models in high-speed flows. There are indications that it can provide qualitative delineati flow transition regions on aerodynamic surfaces.(17) V. NASA Research and Technology Programs in Aeronautical Sensors The United States National Aeronautics and Space Administration supports aeronautics research and technology programs in several disciplines; among these structures and materials, aerodynamics, propulsion, flight controls, and human factors. NASA also sponsors research and project activities in vehicle-related classes such as rotorcraft, hypersonics, vertical/short takeoff and landing (V/S and high performance military aircraft. The agency participates with other organizations in such major programs as the National Aerospace Plane (NASP). In of these research programs and projects there are resources identified for gene nearer-term (up to 2 years or so) discipline-specific instrumentation developme In some cases, a discipline (such as aerodynamics) may sponsor sensors and measurement techniques research of a more fundamental or unfocussed nature to address longer-range needs (3 or more years in the future). An example of this t of research support was the development of instrument technology capable of operating in cryogenic environments, in parallel with the 5-year construction schedule of the National Transonic Facility. There is currently no separate aeronautical sensors program sponsored by NASA to provide generic long-range measurement technology, although proposals are under consideration by NASA management to reestablish this type of broadly-applicable technology activity. NASA’s aeronautical instrumentation and measurement technology program activ are conducted in four locations: the Langley Research Center in Hampton, Virginia Lewis Research Center in Cleveland, Ohio; and the Ames Research Center, in facilit at Moffett Field, California, and at the Dryden Flight Research Facility at Edward Force Base, California. The Langley and Ames-Moffett centers conduct research several measurement research and applications disciplines, while the effort at L is focussed primarily on propulsion applications and the Ames-Dryden is concerne principally with sensors and measurements for flight research. In a typical year the estimated funding from all discipline programs for aeronau sensors and measurements research in NASA is approximately US$10 million. This amount does not include those instruments or systems bought essentially "off-the-shelf" for direct application and use in a laboratory or flight experimen Additional resources (about US$3 M ) are made available throught the jointly-sponsored NASP program, specifically for instrumentation technology maturation directly applicable to hypersonics. In addition, other resources are m available by NASP technology maturation discipline teams such as aerodynamics a structures/materials. NASA shares this funding with other U.S. government and industry organizations. While there is a strong inhouse research program at NASA (as evidenced by the m contributions noted in the preceding section), much of the basic sensors researc conducted by, or in collaboration with, universities and non-profit research foundations. There is also industry participation in some of the measurements research, particularly in applications phases. NASA participates in a domestic technology transfer program, where several concepts annually are incorporated b commercial firms in their product line. Thus, results of aeronautical sensors and instrumentation research programs can be applied to non-aerospace uses. VI. Future Aeronautical Sensors Research and Applications A major aeronautics research focus in the United States as well as in other coun is hypersonic flight. The development of reliable single-stage-to-orbit transportation and the promise of the "Orient Express" will continue to require focus of measurements technology on the validation of the aerodynamic, aerothermodynamic, structures/materials, and propulsion concepts employed. K efforts will continue to be expended toward making measurements nonintrusivel high-temperature environments. High-temperature thermocouples and strain gages, capable of 1670x C operation, will be developed; a synergistic program ma well be the development of similar capabilities for the U.S. Exploration initiative which features aerobrakes in some orbital transfer vehicle concepts. Nonintrusi techniques will continue to be emphasized in hypersonic as well as in other speed regimes involved in aeronautical research. It is anticipated that two- and three-dimensional real-time flow visualization methods will be in heavy demand. all of these research thrusts, increased precision will be sought, in deference to