Conference Paper

Load Alleviation Control using Dynamic Inversion with Direct Load Feedback

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Abstract

This paper addresses the use of dynamic inversion with direct load feedback to provide combined load alleviation and flight control of rotorocraft. The method is applied to a compound utility rotorcraft with similar airframe properties as a UH-60A along with a lifting wing. The controller makes use of flaperons and horizontal stabilizer in addition to the conventional main rotor / tail rotor blade pitch controls to track pilot commands while also minimizing pitch link loads. The nonlinear simulation is developed in FLIGHTLAB ® with structural models of the rotor blades and control system. This model must be linearized to a linear time-invariant (LTI) system to support linear Dynamic Inversion control design. The vehicle dynamics and critical fatigue load are modeled with a linear time-periodic (LTP) model which is converted via harmonic decomposition into a high-order LTI model. This model is then reduced to design controllers across a range of airspeeds. The controllers are tested both in linear model simulations and using the full nonlinear FLIGHTLAB ® model. The results show that the load alleviating controller achieves significant reduction in the pitch link peak-to-peak loads with minimal change in response characteristics, indicating that load alleviation can be achieved with no degradation in handling qualities.

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... 23,[25][26][27], to design load alleviation control (LAC) laws (Refs. [28][29][30], to predict and avoid flight envelope limits (Refs. [30][31][32], and to examine the stability of flapping-wing flyers (Refs. ...
... [28][29][30], to predict and avoid flight envelope limits (Refs. [30][31][32], and to examine the stability of flapping-wing flyers (Refs. 33). ...
... However, the reliance on state transition matrices to approximate LTP systems with higher-order LTI systems was recently relaxed by a numerical method that originated from the rotorcraft community known as "harmonic decomposition" [33,34]. High-order LTI approximate models obtained using harmonic decomposition have been used to study the interference effects between higher-harmonic control (HHC) and the aircraft flight control system (AFCS) [33,[35][36][37], in the design of load alleviation control (LAC) laws [38][39][40], and in the prediction and avoidance of flight envelope limits [40][41][42]. When coupled with a harmonic balance scheme, harmonic decomposition can also be used to compute the periodic solutions for flight vehicles with NLTP dynamics [30]. ...
... However, the reliance on state transition matrices to approximate LTP systems with higher-order LTI systems was recently relaxed by a numerical method that originated from the rotorcraft community known as "harmonic decomposition" [33,34]. High-order LTI approximate models obtained using harmonic decomposition have been used to study the interference effects between higher-harmonic control (HHC) and the aircraft flight control system (AFCS) [33,[35][36][37], in the design of load alleviation control (LAC) laws [38][39][40], and in the prediction and avoidance of flight envelope limits [40][41][42]. When coupled with a harmonic balance scheme, harmonic decomposition can also be used to compute the periodic solutions for flight vehicles with NLTP dynamics [30]. ...
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