The Society of Flight Test Engineers 
                                                   

 


 

ABSTRACT


Development and Flight Testing of Low Altitude, Long Endurance, Medium Size UAS

Ryan Lykins
Graduate Research Assistant, Department of Aerospace Engineering, University of Kansas

Shahriar Keshmiri
Assistant Professor, Department of Aerospace Engineering, University of Kansas

  The Meridian, is a low-wing, V-tail, diesel-driven, semi-autonomous aircraft designed and built by students at the University of Kansas/Center for Remote Sensing of Ice Sheets (CReSIS), which serves as a science platform to remotely measure the thickness of ice sheets in Greenland and Antarctica. The Meridian is a versatile and rugged aircraft, weighing 1, 100 pounds with a wingspan of 26.4 feet, designed to fly in both wheel and ski configurations and to carry a 165 pound science payload with a range of 950 nautical miles and an endurance of 12 hours. The Meridian has had five domestic flights in restricted airspace at altitudes between 1,000 and 4,350 feet above sea level, using both grass and paved runways. The Meridian flew in Antarctica in 2009, at sea level, using a hard packed ice runway in the wheel configuration with no radar antennas, flying autonomously for the first time. In 2011 the Meridian flew three flights in Greenland, at 8,300 feet above sea level, on a soft snow runway in the ski configuration with up to four flat plat radar antennas installed span-wise along the wing, remotely gathering ice thickness data for the first time in flight. The final flight of the Meridian took place in Antarctica in 2011 in the ski configuration with six radar antennas installed, demonstrating over-the-horizon communications through the Iridium satellite network for the first time while flying autonomously for over 45 minutes.
 
During the nine test flights of the Meridian both operational and technical issues were addressed, the use of a small-scale UAS with the same avionics package served as a test bed to minimize risk. As the Meridian is semi-autonomous, the aircraft must be manually piloted from a third-person perspective during takeoff and landing, requiring continuous monitoring and feedback of aircraft states to aid the pilot. Common pilot issues which have occurred are sharp bank angles exceeding 60° and 2g loading, incorrect trim settings resulting in 5.7g loading, and hard landings ranging between 3.3g and 4.3g loading. While in autonomous flight the controller maintains bank angles less than 40°, making less drastic maneuvers compared to manually piloted flight. Similarly, throttle commands, used to control airspeed, generated by the flight controller are on average less than the manual pilot inputs, reducing acceleration along the trajectory. 
  Continuity in team structure has also been a challenge, as the Meridian ground station and testing is conducted by students. To ease in the transition periods between students thorough training documentation has been compiled, including checklists, mission planning procedures, and aircraft systems description in order to reduce human error risks. Technical issues which were addressed during the testing and development of the Meridian included vibration isolation of the avionics package to increase Extended Kalman Filter accuracy, signal glitches inducing an engine kill event, flight controller induced oscillations, and ground pilot communication loss. High fidelity system identification has been performed to better characterize coupling effects due to the V-tail configuration and effects due to flow separation along the fuselage, in order to improve the flight controller and eliminate unwanted oscillations. Trade studies have been performed for different communication system frequencies and transmitting power to reduce the frequency of communication loss in flight.