Modeling, Planning, and Control for Hybrid UAV Transition Maneuvers

TL;DR

This work addresses the challenge of designing safe, robust transition maneuvers for hybrid tail-sitter UAVs in autonomous package delivery. It develops a reduced-order planar tailsitter model and a nonlinear geometric controller, then proposes two trajectory-generation approaches—constant horizontal acceleration and prescribed AoA—to achieve constant-altitude transitions. The analysis reveals a bifurcation in the equilibrium angle of attack causing abrupt pitch jumps during transitions, and initial constant-acceleration attempts demonstrate feasibility limits near a_v ≈ 3.82, underscoring stability barriers. A prescribed AoA approach partially mitigates these issues but yields long transition times and mid-transition instabilities, guiding future work toward 2-D planning, enhanced stabilization (e.g., energy shaping), and prop-wash integration for safe, real-time tailsitter transitions with practical delivery applications.

Abstract

Small unmanned aerial vehicles (UAVs) have become standard tools in reconnaissance and surveying for both civilian and defense applications. In the future, UAVs will likely play a pivotal role in autonomous package delivery, but current multi-rotor candidates suffer from poor energy efficiency leading to insufficient endurance and range. In order to reduce the power demands of package delivery UAVs while still maintaining necessary hovering capabilities, companies like Amazon are experimenting with hybrid Vertical Take-Off and Landing (VTOL) platforms. Tailsitter VTOLs offer a mechanically simple and cost-effective solution compared to other hybrid VTOL configurations, and while advances in hardware and microelectronics have optimized the tailsitter for package delivery, the software behind its operation has largely remained a critical barrier to industry adoption. Tailsitters currently lack a generic, computationally efficient method of control that can provide strong safety and robustness guarantees over the entire flight domain. Further, tailsitters lack a closed-form method of designing dynamically feasible transition maneuvers between hover and cruise. In this paper, we survey the modeling and control methods currently implemented on small-scale tailsitter UAVs, and attempt to leverage a nonlinear dynamic model to design physically realizable, continuous-pitch transition maneuvers at constant altitude. Primary results from this paper isolate potential barriers to constant-altitude transition, and a novel approach to bypassing these barriers is proposed. While initial results are unsuccessful at providing feasible transition, this work acts as a stepping stone for future efforts to design new transition maneuvers that are safe, robust, and computationally efficient.

Figures (15)

• Figure 1: A brief selection of tailsitter aircraft showcasing the state of the art in hybrid UAV design over the years; (a) Lockheed XVF Pogo (1954); (b) Stone et. al (2008,Stone:design); (c) Phillips et. al. (2017,Phillips:design); (d) Gu et. al (2019,Gu:design).
• Figure 2: The Quadrotor Biplane Tailsitter (QBiT) configuration used as a motivating example for studying hybrid VTOL transition maneuvers. Attached to the QBiT is a body-fixed frame that is located in reference to a fixed inertial frame. For this project, only planar motion in the $\boldsymbol{\hat{i_2}}$-$\boldsymbol{\hat{i_3}}$ plane is considered, with changes only in the pitch axis by angle $\theta$. This CAD model was provided courtesy of Dr. Michael Avera from the United States Army Research Laboratory.
• Figure 3: A side view, here defined in the y-z plane that shows (a) the reference frames $\mathcal{I}, \mathcal{B}, \mathcal{C}$ and $\mathcal{E}$ that are used to describe the vehicle's dynamics and orient different airflow contributions over the wing; (b) a free body diagram of the QBiT during transition flight.
• Figure 4: (a) Lift, (b) Drag, and (c) Pitching Moment aerodynamic coefficients from -180$\degree$ to 180$\degree$ for a symmetric NACA 0015 airfoil at $Re = 160,000$. Data was taken from Sandia National Laboratories wind tunnel test dataSheldahl:aero_data and fitted with a cubic spline to maintain smoothness in the aerodynamics.
• Figure 5: (a) Equilibria angles of attack associated with a desired airspeed characterized by the dimensionless parameter $a_v$. The equilibria are colored by stability criterion described in Equation \ref{['eq:routh_hurwitz_conditions']} derived by Pucci Pucci:controller_nonlinear. The bifurcation region is bounded by dashed lines, and a sample of three equilibria $\alpha =\{3.63\degree, 12.8\degree, 17.4\degree\}$ are selected for $a_v = 2.5$; (b) The unstable equilibria are observed to coincide with the stall region associated with a NACA 0015 symmetric airfoil.