Capt Randel Gordon [1] and Maj Paul Blue [2]
Department of Aeronautics and Astronautics
[1] AFIT/TPS Student, AFIT/ENY
[2] Thesis Advisor, AFIT/ENY
Unlike the current generation of land roving robotic explorers, the National Aeronautics and Space Administration (NASA) is developing a new series of relatively large sailplane-like aircraft that will deploy from deep space planetary probes during atmospheric entry. These airborne robotic explorers will rely on soaring techniques to extract energy from the atmosphere to reduce their on board power requirements, while enhancing their endurance and range. As a result, these vehicles will be able to devote more of their limited payload to science and engineering instead of propulsion.
In an effort to advance the state-of-the-art in soaring for full size aircraft, the Air Force Institute of Technology, in collaboration with United States Air Force Test Pilot School (USAF TPS), the National Aeronautics and Space Administration Dryden Flight Research Center (NASA DFRC), and the Air Force Research Laboratory Aerospace Vehicles Technology Assessment and Simulation Branch (AFRL/VACD), launched an effort to demonstrate the energy benefits of dynamic soaring for manned sailplanes. The primary goal of this research was to develop, test, and analyze optimal dynamic soaring trajectories in order to prove or disprove the viability of dynamic soaring for enhancing a full size aircraft’s total energy, using a manned sailplane as a demonstration platform.
Using soaring techniques with a manned sailplane is not a new concept. Since the dawn of the 20th century, the sport of soaring has relied on three primary techniques in order to enhance the energy state of sailplanes. These techniques, known as thermalling, ridge soaring, and wave soaring, are all categorized as static soaring techniques. Static soaring is a technique that uses a vertical component of the air’s velocity to help the sailplane remain aloft. Dynamic soaring, however, is fundamentally different from static soaring. In dynamic soaring, energy is extracted from the atmosphere by flying in strictly horizontal wind shears. By maneuvering through these wind shears correctly, an aircraft’s lift vector is rotated so a component of lift acts like a thrust force, which makes it possible to increase both the aircraft’s kinetic and potential energy. Like static soaring, dynamic soaring is not new. This theory was first proposed by Physics Nobel Laureate Lord Rayleigh in 1883. Furthermore, seabirds like the albatross have demonstrated the viability of dynamic soaring by staying aloft for hundreds of kilometers over the ocean by adding energy to their flight without flapping their wings. Radio controlled glider enthusiasts also use dynamic soaring techniques and can propel their gliders to sustained speeds of over 200 miles per hour. However, the energy states of a full size sailplane had never before been quantified during dynamic soaring maneuvers.
Research
This research began with a point mass optimization study of an L-23 Super Blanik sailplane. The goal of this study was to develop and analyze optimal dynamic soaring trajectories for full size sailplanes. The point mass aircraft equations of motion were used with dynamic optimization to develop a maneuver that extracts the greatest amount of energy based on the horizontal wind shear conditions; this maneuver is called the hairpin. Two additional maneuvers were developed for comparison: the anti-hairpin was designed to show that an energy penalty occurs when flying the opposite maneuver of the hairpin, and the third maneuver is a baseline without wind shear, which provides a reference to compare the hairpin and anti-hairpin against.
The results from the dynamic optimization portion of the research were then used to build a prototype dynamic soaring simulator, which was implemented in AFRL/VACD’s Large Amplitude Multi-Mode Aerospace Research Simulator (LAMARS). The experience gained from this simulation was then used to build a full 6-Degree of Freedom sailplane flight simulator at the NASA DFRC APEX Flight Simulator facility. This flight simulator helped to refine flight test maneuvers, and to evaluate the hardware and software developed specifically for the dynamic soaring flight tests.
Flight tests were then performed in an instrumented LET L-23 Super Blanik Sailplane equipped with advanced electronic displays, navigation sensors, and weather monitoring equipment. The test sailplane was flown at very high speeds and low altitudes in boundary layer wind shears and temperature inversions present over the Rogers Dry Lake Bed at Edwards Air Force Base in California.
Results
This project was the first of its kind to show that manned sailplanes could extract energy from horizontal wind shears. Extensive modeling, optimization, and simulation built a solid foundation upon which to conduct flight test. As a result, the flight test program was a resounding success as all test points were flown and all objectives were met. The flight test results were in accordance with dynamic soaring theory and closely matched those predicted by dynamic optimization and simulation. This effort laid a foundation for dynamic soaring with full size aircraft on which future research can be conducted using different types of wind shears, different classes of sailplanes, and different maneuvers. The results of this project will be featured in an upcoming high definition National Geographic special on bird flight to be aired in January 2007.
