Aerodynamically Controlled Expansion Nozzle for Short Takeoff and Vertical Landing Aircraft

Introduction T HE propulsion system of a short takeoff and vertical landing aircraft, such as the F-35 Joint Strike Fighter, must operate over a wide range of conditions, provide good fuel efficiency at cruise, and deliver high thrust in augmented mode for transonic acceleration and supersonic operation. The propulsion system must be efficient for both high-speed flight and during hover, while addressing the mechanical complexity required for conversion of the thrust stream from horizontal to vertical. A situation where the desire for mechanical simplicity and high propulsion performance are at odds arises in the exhaust system. The exhaust flow conditions in hover are dramatically different from that in transonic acceleration, yet the nozzle provides the same flowpath in both cases (Fig. 1). Specifically, the nozzle provides an internal expansion ratio of 1.3 to give good performance for the critical transonic acceleration portion of the mission. The same expansion ratio is present in hover and produces overexpansion of the flow, resulting in thrust loss. Reduction of the nozzle expansion ratio to 1.1, however, will increase the gross thrust coefficient C f g at hover from roughly 0.92 to at least 0.96 (Ref. 1). For the three mission points of cruise, transonic acceleration, and hover, nozzle pressure ratio (NPR) values are NPR = 3–4, 6–8, and 2, respectively. The total temperatures are 780 K for cruise and hover and 2000 K for transonic acceleration. An aerodynamic approach appears feasible for the provision of nozzle expansion control. The underlying principle is to displace the primary airflow away from the divergent flap to achieve an effectively smaller nozzle expansion area ratio. Examples of such techniques include the use of slots or vents to admit secondary air2 and the use of ejectors or injectors.3−5 Greathouse and Beale5 showed that the gross thrust coefficient increases with increasing secondary ejector flow, particularly at low