Recent Flight-Test Results of Optical Airdata Techniques

Rodney K. BogueNASA Dryden Flight Research FacilityP.O. Box 273Edwards, California 93523-0273ABSTRACTOptical techniques for measuring airdata param-eters have been demonstrated with promising re-sults on high-performance fighter aircraft. Thesesystems can measure the airspeed vector, andsome are not as dependent on special in-flight cali-bration processes as current systems. Optical con-cepts for measuring freestrearn static temperatureand density are feasible for in-flight applications.The best feature of these concepts is that the airdatameasurements are obtained nonintrusively, and forthe most part well into the freestream region of theflow field about the aircraft. Current requirementsfor measuring airdata at high angle of attack, andfuture need to measure the same information at hy-personic flight conditions place strains on existingtechniques. Optical technology advances show out-standing potential for application in future programsand promise to make common use of optical con-cepts a reality. This paper summarizes results fromseveral flight-test programs and identifies the tech-nology advances required to make optical airdatatechniques practical.INTRODUCTIONThis document is an overview of recent test re-sults of optically based airdata measurement. Asbackground for the overview, the measurementproblem is defined, existing techniques are as-sessed, and the shortcomings are identified. Fourdifferent activities involving the flight testing of op-tical airdata measurement techniques are reviewedand conclusions are developed. In summary, con-clusions are drawn and technology advances are1identified that will be needed to bring optical airdatasensing into common, everyday use for military andcommercial applications.THE MEASUREMENT PROBLEMReference freestream airdata is important inflight testing and in flight operations for many rea-sons. The correlation of wind tunnel and flight-testresults relies on freestream airdata for comparison.Most aerodynamic flight data obtained from differ-ent aircraft or from the same aircraft on differentflights must be normalized using freestream airdataas the normalizing term. Flight research demandshigh-quality data and for this reason, the airdata in-formation in research situations must be accurate.In many flight-control systems, airdata is used as ascaling factor and must therefore be highly robustto assure reliable flight vehicle operation. Acquir-ing airdata for hypersonic flight is difficult. Conven-tional pressure-based airdata sensing is impracticalbecause of the extreme environment. Because ofthe important role that it plays in flight research andin operational control of aircraft systems, much ef-fort is expended to obtain accurate, repeatable, andreliable airdata.To identify the freestream situation, the followingparameters must be known:Airspeed- this vector quantity is most often specifiedby the airspeed magnitude with two flow directionangles as follows:Flow angles defined in Fig. 1 as: _Angle of attack (,_) -arctan (u/V)