Center of gravity effects using forced vibration response operational data

Joshua Joseph Cummins, Purdue University

Abstract

The center of gravity of a helicopter changes throughout the course of flight due to fuel burn off, loading and unloading of passengers and supplies, shifting of load in flight, etc. Operators are interested in developing methods for estimating aircraft center of gravity on the ground and in flight. Many of the current methods for estimating the center of gravity of an aircraft are time consuming, subject to human error, and primarily used on the ground. Methods that have been patented for tracking the center of gravity location in flight using neural network approaches have not previously illuminated the physical behavior of the aircraft that establish the basis for these data-driven approaches. Combinations of physics-based and data-driven methods are presented for estimating the center of gravity in this research. A simplified two degree-of-freedom model is developed to identify the effects of shifts in the center of gravity on the free and forced response of the bounce and pitch modes of vibration of a frame structure. Based on the results of the model, a spatial filter consisting of the modal operating shape in pitch is applied to measured frequency response function data on the frame structure to isolate the amplitude of the pitch mode of response, which is shown to be most sensitive to fore-aft shifts in the center of gravity. It is also shown that the Complex Mode Indicator Function can be used to estimate the center of gravity location by ensuring symmetry of cross terms in the mass and stiffness matrices representing the coupling between measurement degrees of freedom, which are assumed to be absolute coordinates. Tests on the frame are then conducted using a radio controlled model helicopter to supply rotordynamic excitation. It is shown that a similar trend is observed in the amplitude of response due to shifts in the center of gravity, but the trend reverses for unrealistically large changes in the center of gravity location.

Degree

M.S.M.E.

Advisors

Adams, Purdue University.

Subject Area

Aerospace engineering|Mechanical engineering

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