Structural stability margin criteria for accelerated clearance of developmental flights

Sura, NK and Patel, Vijay V and Deodhare, Girish and Chetty, Shyam (2008) Structural stability margin criteria for accelerated clearance of developmental flights. In: Proceedings of the International Conference on Aerospace Science and Technology (INCAST 2008-117), 26-28 Jun 2008, Bangalore, India.

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Inertial motion sensors in the flight control system pick up high frequency signals due to the flexible modes in addition to the rigid body responses and could lead to instability at the structural frequencies when the rigid body stability augmentation feedback loops are closed. To attenuate the effects of this structural coupling, notch filters are designed and placed in series with the inertial sensors in each of the feedback paths. During the initial phase of control law design and development, aircraft structural response data is generally not available. Therefore, while designing the rigid body control laws, an additional budget is allocated for phase lags introduced by the structural notch filters. Once the structural coupling tests are carried out on the aircraft, structural filters are designed in order to meet the certification requirements. During developmental flight tests with multiple prototype vehicles there are likely to be variations in the structural response characteristics as the production processes are maturing and also, due to minor updates to the aircraft standard of preparation and addition of different external stores. This calls for the redesign of structural filters and subsequent onboard software development and testing which is a lengthy process. Hence, in order to meet project schedules without compromising safety, the clearance procedure was modified so that it could be efficiently used for flights during the development phase. This paper presents a revised clearance procedure based not only on values of the nominal stability margins, but also on the sensitivity of structural stability margins to the modal frequency perturbation. This change in the clearance philosophy was considered acceptable for the developmental phase of flights but for the final production vehicle standard one would need to strictly follow the MIL-F 9490 D Standard guidelines. This paper discusses the changes carried out to the clearance procedure, the rationale behind the changes made, and also gives directions for future research in robust stabilization of structural modes as well as notch filter design.

Item Type: Conference or Workshop Item (Paper)
Additional Information: Copyright for this article belongs to National Aerospace Laboratories
Uncontrolled Keywords: Inertial motion sensors;Flight control system
Subjects: AERONAUTICS > Aeronautics (General)
Depositing User: Ms. Alphones Mary
Date Deposited: 23 Mar 2009
Last Modified: 27 Aug 2015 06:37

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