Flexible Multi-Body Design of a Morphing UCAV

Abstract

During the first Gulf War a major effort was carried out to find and destroy Iraqi Scud missiles. The McDonnell Douglas F-15E aircraft carried out the Scud hunting operation, which included a hunter and killer phase. This Scud hunting mission was not successful due to lack of evidence of any Scuds actually being destroyed. The endurance of the F-15E is sufficient for an attack aircraft, but not long enough for such a search and destroy mission. Even with mid-air refueling, the flight time is limited by the fatigue of the pilot. The high endurance of Northrop Grumman’s Global Hawk Unmanned Aerial Vehicle (UAV) along with a high-speed attack capability and sensors to identify Scud missiles is the key goal. The challenges of an Unmanned Combat Aerial Vehicle (UCAV) successfully meeting the requirements of both a low-speed, high-altitude loiter and a supersonic strike capable aircraft dictates a concept that requires significant geometric morphing. This resulted in a desired wing area change of 200%, a sweep angle change from 20 to 70 degrees, and an aspect ratio change from 3 to 7. The structural design challenges and solutions are presented that would satisfy such a requirement. The conceptual design model involves that of a coplanar joined wing, which used revolute and spherical joints to enable the necessary configuration change. Areas of concern included the amount of weight penalty involved with the addition of the joints and whether or not this weight penalty would be overcome by the benefit in fuel efficiency. Existing variable sweep wing aircraft that utilized joint mechanisms were researched to provide starting points for a morphing mechanism. The finite element based flexible multi-body analysis tool, DYMORE, was used to investigate joint loads and multi-body behavior for various configurations. Two models were tested, one which included only revolute joints, and the other included a combination of revolute and spherical joints. Joint materials were researched to reduce the weight and maximize strength of the mechanism as much as possible. The drop test simulation results, which was the most critical scenario, were used to size the joints via simple mechanics of materials assumptions. A modal analysis was performed as well to investigate the natural frequencies and critical modes that the aircraft would experience due to the significant geometrical change. The fact that the morphing combat aircraft is unmanned allows flexibility in pushing the envelope and increases the chance of success with such an exotic aircraft design.