IFAC PapersOnLine 53-1 (2020) 75–80 ScienceDirect ScienceDirect Available online at www.sciencedirect.com 2405-8963 © 2020, IFAC (International Federation of Automatic Control) Hosting by Elsevier Ltd. All rights reserved. Peer review under responsibility of International Federation of Automatic Control. 10.1016/j.ifacol.2020.06.013 The large commercial aircraft, developed today by man- ufacturers, are categorized into Very Flexible Aircraft (VFA) for the motivation to generate high-altitude long- endurance (HALE) flights (Langford, 1989; Shearer and Cesnik, 2007; Su and S. Cesnik, 2011). Traditionally, the flight control law is designed based on the assumption that aircraft dynamics are a rigid-body model. Moreover, notch filters are designed based on the knowledge of the frequencies of the elastic modes to suppress the influence of the aeroelastic dynamics in the closed-loop response. For better fuel efficiency and maneuverability, the mass of the aircraft is reducing, and the aircraft structure becomes more flexible. This flexibility can have many adverse effects on the system performance as seen in the case of Helios prototype of NASA, which had crashed due to high wing deformations when it encountered low-level turbulence in 2003 (Noll et al., 2004). The lesson learned from the accident is that the model for control designs has to include body flexible effects (Gibson et al., 2011). In general, when the frequency separation between the rigid-body and the aeroelastic dynamics of a flexible air- craft are closer, the performance of the traditional con- troller becomes low and results in a stronger interaction between the flight control system and the structural modes due to higher flexibility. It can cause instabilities such as flutter, Limit Cycle Oscillation (LCO), and gust loads as they are adversely affecting the performance and stability of an aircraft. Thus, the identification of the flexibility effects on the aircraft aerodynamics and controller design for an aircraft having higher flexibility are challenging research areas (Majeed et al., 2012; Bucharles and Vacher, 1. INTRODUCTION 2002; Mohamed, 2017; Bialy et al., 2016; Yagil et al., 2017; Qu and Annaswamy, 2015; Gao et al., 2019; He et al., 2018). The flexible aircraft contain more states as the number of elastic states of a flexible aircraft depends on the number of elastic modes. In this regards, aircraft contain more flexible modes having a large number of dynamic states, at which the design of a flight control system is tedious work. Thus, a flight control law is derived from the reduced-order model of flexible aircraft. In the literature, there are various methods applied to de- sign flight control system such as Fuzzy control (Huˇ sek and Narenathreyas, 2016), optimization by Genetic algorithm (Bian et al., 2019), adaptive control (Lungu and Lungu, 2017), Dynamic inversion (Lungu and Lungu, 2016), neu- ral networks (Silvestre et al., 2016), and so on. Most of them are applied to rigid body dynamics. In this paper, we are describing the flexibility effects on aircraft model contain the four elastic modes and proposing a reduced- order controller. For the formulation of LPV (linear pa- rameter varying) system of flexible aircraft valid on the entire flight envelope, a variety of MOR techniques are carried out in Wang et al. (2016). Comparison study among the various approaches of model reduction is also made in Tantaroudas and Da Ronch (2017). Recently, the model projection method is applied to reduce the order of the system (Pagliuca and Timme, 2017), and flexible aircraft whose order of the short-period model is reduced in Avanzini et al. (2017). This paper uses a model reduction technique developed by Chidambara (Chidambara, 1969), and a flight controller is designed based on the reduced model of a flexible aircraft(Rao and Lamba, 1974). Main contribution of this paper are Keywords: Flexible aircraft, flight control, model reduction, observer design and optimal control Abstract: The interaction between the flight control system and its structural modes becomes stronger if the frequency separation between rigid body mode and flexible modes of flexible aircraft are closer, and it introduces higher flexibility effects on aircraft dynamics. In such a case, the design of a flight control law is not valid based on the assumption that aircraft dynamics are rigid body models. Moreover, an integrated aircraft model having a rigid body and elastic body modes contain a large number of states. Therefore, we have designed an optimal flight control law from a reduced-order model and realized with a flexible aircraft represented by a full order model. For this, the simplified model of the flexible aircraft is derived using the Chidambara technique, and Luenberger observer is applied to estimating the elastic states of a simplified model from the aircraft measurements. * Principal Scientist, Flight Mechanics and Control Division, CSIR-National Aerospace Laboratories, Bangalore, 560017, India (e-mail: majeed@nal.res.in). ** M.Tech Student, College of Engineering Trivandrum, Thiruvananthapuram-16, India (e-mail: madhavang.28@gmail.com) Majeed Mohamed * Madhavan G ** Reduced Order Model Based Flight Control System for a Flexible Aircraft © 2020, IFAC (International Federation of Automatic Control) Hosting by Elsevier Ltd. All rights reserved.