This article describes the theoretical derivation, implementation, and validation of a teetering rotor model for applications in flight dynamics simulations. The rotor formulation is generic and accounts for both positive and negative angular rotor speeds. The module is integrated within a generic multi-rotor/wing flight dynamics code implemented in MATLAB®/Simulink. The model is validated against flight-identified data for an ultralight coaxial helicopter. The impact of the rotor-on-rotor interactional aerodynamics is evaluated in trimmed flight in terms of spectral and frequency response analyses. Model-order reduction methods are investigated to guide the development of linearized models that are tractable for flight control design while still predicting the effect of rotor-on-rotor interactions on the vehicle flight dynamics. Model-following flight control laws based on these reduced-order linearized models are developed and tested on the nonlinear dynamics. Results indicate that incorporating rotor-on-rotor aerodynamic interactions significantly affects trim, dynamic response, and power requirements. These interactions introduce nonzero roll trim attitudes and increased collective and pedal inputs at low speeds due to asymmetric thrust distribution between rotors. The upper rotor generates higher thrust and inflow while the lower rotor operates in its wake, leading to over 60% higher total power consumption. Frequency response analysis shows improved gain and phase alignment with flight-identified data, particularly in roll, yaw, and heave responses above 0.4–0.5 rad/s.
Saetti et al. (2026) studied this question.