Aerodynamic Evaluation and Optimization of the Houck Joined Wing Aircraft

Abstract

Air Force Research Laboratories (AFRL) tasked the United States Air Force Academy (USAFA) Department of Aeronautics with investigating the aerodynamic characteristics of the Houck Joined Wing Concept Aircraft in their sear ch for a long range, long endurance Unmanned Aerial Vehicle (UAV) . Because lift induced drag entails a large penalty on aircraft performance, the Houck aircraft connects the upper and lower wings , removing the wingtips in an attempt to eliminate these eff ects . The first version of the design was evaluated at USAFA in Fall 2006. A erodynamic performance and static stability characteristics we re analyz ed and compared to a conventional mono -wing NASA Orbital Space Plane model . Testing showed that the Houck Joined Wing had a somewhat shallow drag polar, indicating a possible reduction of drag due to lift. However, the Joined Wing had a 7 times higher value for minimum drag coefficient when compared to the Space Plane. The Joined W ing reached static longitud inal stability at angles of attack above 9° but was unable to trim for the configuration eval uated. In addition, it was unstable in roll at low angles of attack. Roll stability improved with increasing angle of attack, be coming the most stable at 15°. The Joined W ing was unstable in yaw for all angles of attack except at 0° angle of attack at Mach numbers of 0.2 and 0.3. Due to its comparable D L C C 2 3 ratio to the Space Plane , the Houck aircraft configuration may have potential as a long e ndurance UAV with a reciprocating engine if operated at high lift coefficient s. No potential was found for improving range or endurance for jet powered flight or range for reciprocating engine power. The Joined Wing’s initial design configuration was ma tched as closely as possible to Mr. Houck’s original design. However, using general engineering rules of thumb and a University of Missouri study on the biplane design, adjustments were made in an attempt to optimize the Joined Wing’s performance. Change s considered include d a negative decalage angle, a taper ratio less than one, increased gap, decreased wing sweep, and / or decreased stagger. Making all these changes simu ltaneously would not allow the e ffects of each to be seen; consequently, gap and de calage angle were chosen to be varied on a modified Joined Wing model. The increased gap (4.75 in) with -1.5° decalage angle proved to be the optimum design configuration tested, producing higher lift coefficients and a more shallow drag polar. None of t he J oined Wing configurations evaluated showed significant potential for performance improvement over a monoplane. In fact , the test model produced the best results with the top wing removed altogether.