M.Sc Thesis

M.Sc StudentZiv Jamchi
SubjectWing Store Limit Cycle Oscillations Using Increased-
Order Modeling
DepartmentDepartment of Aerospace Engineering
Supervisor Professor Emeritus Karpel Mordechay
Full Thesis textFull thesis text - English Version


Limit cycle oscillations (LCO) have been a persistent problem on several fighter aircraft. The F-16 and F/A-18 encounter LCO at high subsonic and transonic speeds for different store configurations with AIM-9L missiles on the wingtips and heavy stores on the outboard pylons. The LCO response is characterized mainly by anti-symmetric motion of the wing and stores and a lateral motion of the fuselage and aircrew. This limit cycle behavior occurs in both level flight and during elevated aircraft load factor maneuvers.

Increased-Order Modeling (IOM) is a practical and efficient approach to modeling dynamic systems that are mostly linear, but their behavior may be significantly affected by local nonlinearities. The approach is based on augmenting a main linear block with nonlinear feedback loops that represent the important system nonlinearities.

The goal of this research was to develop an efficient tool for predicting wing-store LCO onset velocity and vibration frequency and amplitude in fighter aircrafts at the transonic flight regime. The Increased-Order Modeling (IOM) approach is used to model the LCO simulation for a generic fighter aircraft. The model is based on a linear flutter detection core model augmented by nonlinear feedback signals that correct the unsteady aerodynamics based on static CFD results.  The resulting LCO is presented and analyzed.