Coherent lidar solution for the HSCT supersonic engine inlet unstart problem

Atmospheric turbulence environments can adversely affect the operation of both commercial and military supersonic aircraft. Future aircraft designs, such as the High Speed Civil Transport will aim to alleviate the effects of supersonic engine inlet unstart. Fluctuations in air temperature, longitudinal and transverse velocity all can trigger inlet unstarts. With fore- knowledge of the turbulence, a feed-forward control system can be used to re-configure the propulsion systems to avoid unstarts. The same technology can be used to counteract gust effects to improve ride quality and reduce gust loads. A coherent lidar sensor is being developed to demonstrate that the atmospheric turbulence can be measured with sufficient reliability, fidelity, and pre-encounter time for these feed-forward control solutions. The NASA Airborne Coherent Lidar for Advanced In-flight Measurements (ACLAIM) program will develop and flight test a sensor on NASA research aircraft, including the SR-71, and investigate the atmospheric environment to establish the feasibility of a lidar sensor. The paper will present an overview of the ACLAIM program including: the scope and content of the program, lidar measurement challenges, atmospheric environment, technology choices, and anticipated problem areas.