Innovators at NASA’s Dryden Flight Research Center have patented a guidance and control method that, in combination with relevant sensors, can increase range and flight time for unmanned aerial vehicles (UAVs). The method detects and exploits buoyant plumes of air known as thermals. Detecting and soaring in thermals enables UAVs to optimize flight performance, increase speed, extend flight duration and range, and reduce energy consumption. Designed to augment the UAV’s primary power and propulsion system (typically battery or solar based), the technology significantly increases flight duration and range. For example, a UAV with a typical endurance of two hours can gain up to twelve hours of flight time per mission. Although piloted gliders and low-powered aircraft have systems to detect thermals, Dryden’s guidance and control system is the first ever to be used by UAVs.
Atmospheric convection thermals occur when the air near the ground becomes less dense than the surrounding air as a result of heating or humidity changes at the Earth’s surface. Thermal height varies greatly depending on climate and season but typically falls between 1,600 feet and 12,000 feet above ground level. Sailplane pilots primarily rely on thermals to travel great distances and soar for long periods of time, and large birds soar extensively while searching for food and vary their migration paths and flight times in response to thermal activity. Many UAVs have similar sizes and wing loadings to soaring birds and manned sailplanes, so taking advantage of thermals could similarly increase UAV flight duration and range.
Dryden innovators developed a set of algorithms designed to autonomously detect and utilize thermals. The system is hosted on the aircraft’s autopilot. Inputs consist of static pressure, airspeed, throttle command, and aircraft latitude and longitude. Estimates of the total energy rate and total energy acceleration of the aircraft are used by thermal identification equations to determine the radius, vertical velocity, and position of the thermal. The circle guidance calculates the steady state turn rate, position error, and velocity error for tracking a circular path inside the thermal. The controller attempts to drive position and velocity errors to zero and respond to changes in energy acceleration. Simple mode logic based on an aircraft’s energy state determines when the aircraft should be searching for thermals (by flying waypoints) and when it should circle within a thermal.
Because UAVs are small and lightweight, they cannot accommodate large engines, battery packs, or solar arrays sufficient to reliably power the aircraft for extended flights. For this reason, most UAVs are limited to approximately two hours of flight time, which can be insufficient for some missions critical to their use. Mission profiles that could allow small UAVs to take advantage of thermals include remote sensing, surveillance, atmospheric research, communications, law enforcement, forest-fire monitoring, land management, and border control activities, among others. This Dryden-derived technology can significantly increase flight duration and range for UAVs.
Dryden has one patent issued (U.S. Patent No: 7,431,243→ ) for this technology.
This technology is part of NASA’s Innovative Partnerships Office, which seeks to transfer technology into and out of NASA to benefit the space program and U.S. industry. NASA invites companies to consider licensing the Improved Guidance and Control for Autonomous Soaring UAV technology (DRC-006-001) for further development and commercial applications.