Conceptual Design of a Variable Air Inlet, JAS 39 Gripen

Sammanfattning: The JAS 39 Gripen currently has a static air inlet designed for cooling the engine bay. This inlet has been developed over the years and has consisted of several different solutions. In this master’s thesis an investigation of past and current designs has been conducted in order to develop new concepts with a variable solution. Since the static inlet is designed for a worst case scenario, long duration of flight at low altitudes and high atmospheric temperatures, the cooling produced is not always needed and instead causes an unnecessary amount of drag. Developing concepts with variable solutions would alter the caption area depending on flight case and thereby decrease drag accordingly. Both aerodynamic and airworthiness properties are important to consider when developing concepts of an air inlet. Initially a concept generation was conducted in order to create possible concepts for implementation. 14 concepts were generated, evaluated and analyzed resulting in two concepts chosen based on their superior aerodynamic and airworthiness properties. Screening was done with both a Pugh matrix as well as a scoring matrix. One of the selected concepts has a similar design to the current scoop inlet, but with a variable solution. The second is a variable NACA duct design. An alternative solution briefly discussed was that of a static design, with different caption areas, attached to the service door depending on location of operation and flight conditions but is only theoretical at this point. The advantage of implementing the NACA concept would be the decrease in drag since it is a flush inlet. The weight and radar cross section (RCS) are both considered to be low when compared to the scoop concept. Shortcomings of the NACA concept would include the uncertainty in performance concerning cooling and producing sufficient mass flow, as well as the uncertainty in case of bird strike. The scoop concept is similar to the current design and therefore is a verified design with satisfactory bird strike properties as well as having a high probability of producing sufficient mass flow. When compared to the NACA duct it has disadvantages concerning drag, weight, RCS and foreign object damage.