Pitch Divergence Suppression of a Subscale Wing in Ground Effect (WIG) Aircraft

1. Pitch Divergence Suppression of a Subscale Wing in Ground Effect (WIG) Aircraft 56th Annual AIAA Southeastern Regional Student Conference April 4-5, 2005 Robert Love Auburn University

2. What is a WIG Aircraft? • An aircraft which flies over a mostly level surface at a height lower than half of the span to use advantageous ground effect conditions

3. Advantages of WIG Aircraft • Chord Dominated • “RAM” effect increases Lift • Span Dominated • Reduction of wing tip vortices dramatically lowers induced drag • Therefore high L/D ratios

4. History of WIG Aircraft • Designs are extremely varied • Early Designs • U. S. “Spruce Goose” • Russian “Erkanoplans” • The KM, Lun and Orlyronk in the (1960’s) • PAR motors, strait wing • Amphistar • The Lippisch Design, single motor • Airfisch 3 • L-325 Flarecraft

5. What is being done now? • Australia • FS-8 (with Singapore) • Incat Wing (trimaran with WIG support) • China • TY-1 • XTW-4 • United States • Boeing Pelican • Aerocon Atlantis 1 • Germany • Hoverwing • X-114

6. Introduction • Divergence due to ground effect is well known in other fields • Longitudinal Stability a historic problem for WIG aircraft • Sudden pitch and height changes cause divergence • Contributors • High thrust line, throttle cut too quickly, lack of inherent stability, wrong CG, slowness/inability to respond to pitching motions • Caused loss of many aircraft, reputation as unreliable

7. Previous Approaches • Structural Fixes • Large Tail Wing, Canards, slats, elevators, the Lippisch design of the wing, S-shaped airfoils • Disadvantages include large amounts of drag and little effectiveness • Tweaking Dynamic Characteristics • Movement of the center of pitch, center of gravity, and aerodynamic center • Some success, but dependent on careful balancing

8. The Aircraft Model • Based off of Graham Taylor’s MK5 WizzyWIG XGE plans • Materials Used • Balsa wood • Carbon fiber motor mounts • Bonding with Cyano-Acrylate Resin and Hysol 9433 • Covered with model skinning material and flashing tape • Hardware • 3 Astro 020 Direct Drive Brushless motors • 3 Lithium Polymer Batteries • 2 servos, 1 JR DS368 and 1 Futaba FP-S-14B • 1 Cirrius micropiezo MPG-10 gyroscope • Overall Size • 2.5 lbs, 3.5 ft long, 11.5 in high, CG at 1/3rd of chord • Main wing 17.5 in span by 17 in chord

9. The Aircraft Model • Notable Features • PAR motor mount to serve as an elevator (-5° to 40°) • Canard wing • Large tail wing • Upward slope of body in front and back • Flat main wing with sponsons • Center of Gravity Location and connection to rig at this location

10. Experimental Procedure • First Flight-free flight on January 27, 2005 experienced divergence at low speed • Whirl test rig made to test the longitudinal stability of the aircraft in a stable environment • Test settings • With and without maximized gain pitch rate feedback stabilization • Full and Half Elevator Deflection • Throttle setting at 2.5, 3, and 4 of 6 • Digital Video analyzed with ImagePro Analysis software • velocity, divergence times, and body pitch attitudes

11. Results • Effect of Rate Stabilization on Divergence Times for Full Elevator Deflection

12. Results

13. Results • Divergence prevention by pitch rate feedback system for speed of 29.7 ft/s without gyroscope and 33.0 ft/s with gyroscope, at throttle 3 settings

14. Results • Overall View of the Effectiveness of the Pitch Divergence Suppression at Half Elevator with Pitch Rate Feedback System

15. Summary of Results • Longitudinal instability for full elevator • Divergence was not preventable through pitch rate stabilization with a gyroscope • Longitudinal instability for half elevator • Suppressed at speeds lower than 26 ft/s indicated by divergence taking three times longer than without stabilization • Prevented completely at speeds higher than 30 ft/s through pitch rate stabilization

16. Significance • Increased thrust available to overcome “hump drag” due to higher allowable elevator settings • Increased stability for transitioning between modes • Increased maneuverability to avoid obstacles • Increased reaction time for pilot or control system to prevent divergence as it starts to occur • Increased “pitch stiffness” of aircraft without substantial drag penalties from large tail or canard wings • Increased safety margin • Simplified design while providing a solution to problem

17. Conclusion • Divergence of a subscale wing in ground effect aircraft was able to be suppressed or prevented using a pitch rate feedback system at speeds from 20 ft/s to 45 ft/s for an elevator disturbance which normally would cause divergence.

18. Thanks • To Auburn University and Dr. Ron Barrett for support and technical advice • To Christoph Burger and Adam Chesler for lab help and construction advice • To Graham Taylor for the WIZZYWIG plans and technical advice • To all the other employees of the Adaptive Aerostructures Lab for their occasional helping hands and encouragement

19. References • 1. Online. “Divergence”. 2005. April 1, 2005. http://www.hypercraft-associates/divergence/divergence.htm. • 2. Online. The Wig Page. “Wing in Ground Effect Aerodynamics.” 2005. February 14, 2005. http://www.se-technology.com/wig/index.php. • 3. Online. “Wing in Ground Effect Aerodynamics.” 2005. March 18, 2005. http://www.aerospaceweb.org/question/aerodynamics/q0130.shtml. • 4. Online. 2005. March 18, 2005. http://foxxaero.homestead.com/indrad_044.html. • 5. Taylor, G. K., “Are you missing the boat? The Ekranoplan in the 21st Century Its Possibilities and Limitations”. February 2002, 18th Fast Ferry Conference, 2002.

20. Questions?