When an aircraft veers upwards too much, the decrease in lift and increase in drag may cause the vehicle to suddenly plummet. Known as a stall, this phenomenon has prompted many drone manufacturers to err on the side of extreme caution when they plan their vehicles’ autonomous flight movements. For vertical takeoff and landing (VTOL) tail-sitter drones, most manufacturers program the aircraft so that the vehicle body turns very slowly whenever it transitions from hover to forward flight and vice versa.

Researchers have created a trajectory planner that significantly shortens the time it takes for VTOL tail-sitter drones to make this crucial transition. The trajectory planner was designed for a quadrotor biplane tail-sitter used to test new design features and study fundamental aerodynamics.

VTOL tail-sitters typically rely on a heuristic-based approach whenever they transition between hover and forward flight, where they follow a very slow but very safe predetermined set of actions. In contrast, the trajectory planner can find the optimum sequence of flight movements for these transitions that tailor to each situation. Researchers discovered the availability of these more agile maneuvers when they modeled the unique interaction between the wake of the vehicle's rotors and the aerodynamics of its wings.

If the vehicle is hovering, the wings are pointed upwards and the rotors are spinning above it constantly; to start moving it forward, one would be dragging the wing effectively flat against the air. In reality, because of the air being blown down onto the wing, it's actually not seeing much drag. As a result of this extra downwash from the rotors, VTOL tail-sitters can handle a more aggressive transition between hover and forward flight than one would have assumed.

Through simulation, the researchers found that the incorporation of rotor on wind wake interference into the trajectory planner enabled the drone to transition into hover and land in half as much time as compared to the conventional approach. The team believes that the trajectory planner may eventually allow the drone to intelligently switch between hover and forward flight as it navigates across dense or urban areas.

The incorporation of more sophisticated flight models in the trajectory planner will provide the drone with a better understanding of the complex aerodynamic environment as it moves; for instance, if there was a building in the way, would it make more sense to fly over the building or around the building?

Once the trajectory planner undergoes more simulation trials, the researchers plan to hook the software up to hardware models to ensure a high level of robustness before they commence flight tests. A faster, more efficient transition between hover and forward flight will eventually help the Army develop new vehicles for intelligence, surveillance, and reconnaissance missions as well as aerial resupply operations.

For more information, contact DEVCOM, Army Research Laboratory Public Affairs at 703-693-6477.


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This article first appeared in the March, 2021 issue of Tech Briefs Magazine.

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