X-31A quasi-tailless evaluation

The X-31A is a single seat, single engine airplane designed to explore maneuverability at angles of attack up to 70 degrees. It is a delta wing and canard design which incorporates multi-axis thrust vectoring to augment conventional flight control surfaces. The U.S. Joint Advanced Strike Technology (JAST) program tasked the X-31 International Test Organization (ITO) to obtain flight test data on the use of thrust vectoring for control augmentation of tailless airplanes. This was due to the JAST Program's requirement to investigate the feasibility of future strike aircraft designs with reduced, or no vertical tail. The X-31 flight control system software was modified to incorporate a quasi-tailless mode whereby the directional stability provided by the aircraft's vertical tail was incrementally reduced. A flight test evaluation was conducted to evaluate the use of thrust vectoring to augment an aircraft that was made directionally unstable via the conventional flight control system. The evaluation was centered around high gain mission tasks including precision approach and air-to-ground attack maneuvers. The tasks were: 1) Fixed reticle capture of the Adaptable Target Lighting Array System (ATLAS). This system consists of an array of ground fixed lights for air-to-ground target capture tasks. 2) Mission representative air-to-ground delivery profiles. 3) Nominal and offset approaches using the U.S. Navy's Fresnel precision approach glide path indication system. The fact that the airplane was not designed for the air-to-ground or for the carrier based role made the task development challenging. This paper discusses the flight test approach, buildup and conduct, test results and design considerations for future tailless aircraft.