F-35B Lift Fan Inlet Development

Abstract

The lift fan air induction system for the Joint Strike Fighter STOVL variant has gone through an extensive development program, evolving from the side hinged bi-fold door concept used on the X-35B technology demonstrator aircraft to the current single piece aft hinged door design for the F-35B production aircraft. The history, challenges and solutions behind the development of the F-35B Lift Fan inlet are detailed in this paper. Details of the lift fan inlet down-select process will be included along with the events that led up to the discovery of inlet flow angularity and the subsequent redesign optimization process. The results from the various sub-scale and full scale testing efforts will be summarized. Lift Fan inlet pressure recovery and distortion predictions are compared to flight test results. I. Introduction HE Short Take-Off Vertical Landing (STOVL) version of the Joint Strike Fighter (JSF), designated F-35B Lightning II aircraft, contains a unique integration of the shaft driven lift fan to provide the vertical thrust required for STOVL operations. Oriented ninety degrees to the direction of conventional flight, the lift fan integration posed new challenges for the measurement and determination of lift fan performance and operability. Flight testing of the X-35B Concept Demonstrator Aircraft (CDA) indicated a redesign of the lift fan inlet was required to reduce lift fan blade stresses and increase the hardware life for the System Development and Demonstration (SDD) phase of the F-35 program. Initial subscale testing using traditional inlet instrumentation identified a promising lift fan inlet concept to be refined during SDD. Subsequent and more detailed testing indicated pressure measurements were susceptible to errors due to localized flow angularity, prompting the development of new measurements and methods to determine lift fan performance and operability. The close proximity of the inlet and lift fan turbo-machinery created a close coupled system that introduced aerodynamic losses not present in conventional inlet/engine installations. Undetected flow angularity induced losses developing downstream of the measured inlet Aerodynamic Interface Plane (AIP) seriously threatened the performance margins required for viable STOVL operations. Propulsion system installed performance was successfully recovered through a lift fan inlet redesign effort and was subsequently verified via full-scale system tests. This paper discusses the traditional process employed by airframe and engine manufacturers when determining inlet/engine aerodynamic compatibility and briefly touches on the history of events leading up to the discovery of significant flow angularity induced losses during the F-35 lift fan inlet development. The majority of this paper details the efforts made deriving the new inlet/engine methodology for the F-35 lift fan system and the correlation process used to determine the inlet induced losses and to quantify lift fan operability.