Flight through Narrow Gaps with Morphing-Wing Drones

Abstract

The size of a narrow gap traversable by a fixed-wing drone is limited by its wingspan. Inspired by birds, here, we enable the traversal of a gap of sub-wingspan width and height using a morphing-wing drone capable of temporarily sweeping in its wings mid-flight. This maneuver poses control challenges due to sudden lift loss during gap-passage at low flight speeds and the need for precisely timed wing-sweep actuation ahead of the gap. To address these challenges, we first develop an aerodynamic model for general wing-sweep morphing drone flight including low flight speeds and post-stall angles of attack. We integrate longitudinal drone dynamics into an optimal reference trajectory generation and Nonlinear Model Predictive Control framework with runtime adaptive costs and constraints. Validated on a 130 g wing-sweep-morphing drone, our method achieves an average altitude error of 5 cm during narrow-gap passage at forward speeds between 5 and 7 m/s, whilst enforcing fully swept wings near the gap across variable threshold distances. Trajectory analysis shows that the drone can compensate for lift loss during gap-passage by accelerating and pitching upwards ahead of the gap to an extent that differs between reference trajectory optimization objectives. We show that our strategy also allows for accurate gap passage on hardware whilst maintaining a constant forward flight speed reference and near-constant altitude.

Figures (8)

• Figure 1: A wing-sweep morphing drone flies through a narrow gap by temporarily sweeping in its wings.
• Figure 2: The 130 g morphing-wing drone platform leveraging wing-sweep morphing for narrow gap passage. (a) The outboard wing sweep is actuated by a servo-driven bar-linkage mechanism. The sweep angle $\Theta_w$ varies between -5° and 75° from the fully extended to the fully swept configuration respectively. (b) The side profile view shows the outboard flat-plate sweeping into the symmetric root airfoil.
• Figure 3: (a) Aerodynamic characterization setup using ATI Gamma load cell, a wind generator and a robot arm positioning the drone at angles of attack between -8° and 90°. We compare the derived aerodynamic model to measurements of (b) lift coefficients, (c) drag coefficients, and (d) pitch moment coefficients at a sweep angle of -5° (extended wing configuration) and 75° (fully swept wing configuration) at an inbound wind speed of 5 m/s. (e) Lift coefficient measurements and aerodynamic model predictions at wind speeds of 4 and 6 m/s at a wing sweep angle of 45° are additionally shown.
• Figure 4: Description of the morphing-winged drone narrow gap passage optimization problem: The drone approaches from level flight at a specified initial flight speed. During a chosen gap anticipation distance, the drone can vary its symmetric wing sweep, thrust and elevator inputs freely to preempt the narrow-gap passage (Phase 1). During a specified gap threshold distance, the symmetric wing sweep must remain in a fully swept actuator state and the drone must maintain an altitude corresponding to the center of the narrow gap (Phase 2). After gap passage, the drone is given a free distance to recover to a constant flight condition at the altitude corresponding to the gap center, with free choice of symmetric-wing sweep, elevator and thrust (Phase 3). In the final phase (Phase 4), the drone resumes steady flight at the initial launch altitude with a freely chosen constant velocity, attitude and wing sweep configuration.
• Figure 5: Optimization framework and control architecture, showing the generation of the reference trajectory using multi-phase optimization, the Nonlinear Model Predictive controller to generate symmetric sweep, motor thrust and elevator servo commands, and the low-level lateral attitude controller computing asymmetric wing sweep and rudder corrections to maintain lateral alignment to the gap.