Tzlil Nahom, M.Sc. (Graduated)
Wind-Tunnel Study of the ARMA Flutter Prediction Method
An experimental study of flutter prediction via Autoregressive Moving-Average system identification and the use of a linear stability parameter. System identification at subcritical (pre-flutter) conditions is based on measured structural responses to excitation by natural air turbulence and is, therefore, attractive for flutter flight testing. The current study investigates the application aspects of the methodology in a dedicated wind tunnel experiment. An elastic wing was designed and manufactured using rapid prototyping, and tested in a subsonic wind tunnel all the way to flutter. Structural responses were recorded by accelerometers, strain gauges, and by fiber-optic sensors (measuring strains). The data was processed using signal processing techniques, such as filtering and averaging, and used for system identification and flutter prediction. The study focuses on the prediction characteristics and accuracy, method applicability with various dynamic data, and signal processing techniques.
Kobi Cohen, M.Sc. (Graduated)
Comparison of Laboratory Testing Techniques for Replicating In-Flight Dynamic Loads
Modern weapon systems used on high-performance aircraft have complex electronic assemblies that are required to operate in a challenging dynamic environment throughout their complete life cycle. The design of these components depends on knowledge of the environmental loads acting on the weapon system during operational conditions. These include aircraft noise induced by engines, gunfire, etc., mechanical loads induced by aircraft structural response to flight conditions (transferred to the system through the pylons), and aerodynamic noise induced by the turbulent boundary layer and shock-boundary layer interaction. The latter is considered to be the most significant source of the in-flight vibratory environment. Aerodynamic noise is expressed as pressure fluctuations in a broadband spectrum (typically up to 10kHz). These loads cause the system to vibrate in a broad spectrum.
The development of weapon systems involves extensive laboratory testing, designed to substantiate the durability of the hardware in flight-like environmental loads. Various types of laboratory tests are used as experimental simulations of in-flight loads, from direct mechanical vibration tests through acoustic tests in reverberant chambers, or progressive wave tubes. The question of interest is which testing method, in terms of boundary conditions and excitation type (mechanical or acoustic), is most adequate to accurately simulate the response of components to flight loads.
To answer that question, a weapon system was tested in mechanical vibration test as well as in acoustic test, in an attempt to replicate the system’s acceleration response that was measured in captive flight. In the mechanical vibration test the system was excited by electro-dynamic shakers, whereas in the acoustic test, the excitation was done by loudspeakers in a reverberant chamber. The responses from each test were studied for their frequency content, and compared to the responses measured in captive flight.
Itzik Mizrahi, MSc (Graduated)
An investigation of Wing Elasticity Effects on Store Separation
The study explores wing elasticity effects on the store separation process, based on computational fluid dynamics simulations. The nominal test case is that of an unmanned aerial vehicle that carries two identical stores, without fins or control surfaces, on two wing stations in a symmetric configuration. The stores are ejected during straight and level flight at 0.35M, 2500m. Simultaneous, time-accurate analysis of the dynamic aeroelastic wing response and the store’s trajectory reveals that the most significant aeroelastic effect is a roll motion developed by the store. This roll motion is due to misalignment of the ejection force vector and the store’s center of gravity, due to the wing’s static and dynamic elastic deformations. The second part of the study presents a parametric study of the effects of various structural and configurational parameters on the wing’s response, and consequently on the store’s rolling motion. A more flexible wing, a heavier store, a larger ejection force, or a shorter ejection period, all result in increased store rolling. Ejection of a store from an asymmetric configuration (of a single store) resulted in a larger store roll than the symmetric ejection. The dynamic wing response plays a significant role in creating the force misalignment that generates the store roll. Hence the latter is also dependent on the relation between the ejection period and the structural frequencies of the wing.
Daniel Kariv, M.Sc. (Graduated)
Dynamic Response of an Elastic Aircraft to Store Ejection
An experimental study of flutter prediction via Autoregressive Moving-Average system identification and the use of a linear stability parameter. System identification at subcritical (pre-flutter) conditions is based on measured structural responses to excitation by natural air turbulence and is, therefore, attractive for flutter flight testing. The current study investigates the application aspects of the methodology in a dedicated wind tunnel experiment. An elastic wing was designed and manufactured using rapid prototyping, and tested in a subsonic wind tunnel all the way to flutter. Structural responses were recorded by accelerometers, strain gauges, and by fiber-optic sensors (measuring strains). The data was processed using signal processing techniques, such as filtering and averaging, and used for system identification and flutter prediction. The study focuses on the prediction characteristics and accuracy, method applicability with various dynamic data, and signal processing techniques.
Leeran Yagil, MSc (Graduated)
A methodology is presented for performing trim optimization in highly flexible aircraft configurations by incorporating redundant, multiple control surfaces positioned on the trailing and leading edge of the wing, in order to minimize wing deflection. It employs the simplex linear programming algorithm and compares three different control surface configurations. The beneficial results from using multiple redundant surfaces are evident. It allows for an increase of the maximum load factor and, equivalently, a decrease of the minimum dynamic pressure in a trimmed flight, while maintaining a low wing deformation.
Etay Kantor, M.Sc. (Graduated)
Nonlinear Structural Modal Model for Aeroelastic Applications
The study revisits a novel methodology for the analysis of geometrically nonlinear structures by sub-structuring. In this methodology, the structure is divided into a few substructures. The deformation of each substructure is written in terms of a corotated system that is attached to the substructure and is expressed through modes computed using the fictitious mass method. The method is examined using a test case of a beam subjected to large follower and non-followers tip forces. It yields excellent results compared with a solution by a nonlinear finite-element software, by using a very small number of segments and modes. The method is appealing for aeroelastic applications, both because of its computational efficiency when compared to nonlinear finite-element analysis, and because it overcomes the need for a nonlinear finite element software.
Arik Drachinsky, M.Sc. (Graduated)
Limit Cycle Oscillation of a Pretensed Membrane Strip
A computational and experimental study of the nonlinear aeroelastic response of a pre-tensed, high aspect ratio, thin membrane strip. The goal is to derive and validate a computational model that can be used for the analysis and design of membrane strips, for the purpose of energy harvesting from flutter at low airspeeds. The mathematical model is based on a beam model, accounting for the additional stiffness due to pre-tension and large deformations. The aerodynamic model is a potential flow model. The equations of motion are written as a set of nonlinear ODEs, using Galerkin’s method, and are simulated numerically. The nonlinear aeroelastic model is used to study the oscillation characteristics of the membrane strip in the various stability regions, and the effect of pre-tension on the energy-harvesting potential.
Sonya Tiomkin, Ph.D. (Graduated)
During the last years, there is a growing interest in the implementation of membrane wings in micro air vehicles (MAVs), mainly due to their lightweight and passive shape adaptation to flight conditions. Since MAVs fly with relatively low velocity (around 10 m/s), the effect of atmospheric turbulence (gusts) on MAV performance can be considerable. One of the most desirable characteristics of MAV wings is the fast adaptation to gusts, in a manner that preserves the aerodynamic characteristics even at high induced angles of attack. The main objective of the study is to establish knowledge of membrane wing dynamic response to gust at low Reynolds numbers. The research goal is to gain a physical understanding of the shape adaptation and resulting aerodynamic response of the membrane.
Michael Iovnovich, Ph.D. (Graduated)
Aeroelastic Phenomena of a 3D Wing in the Vicinity of Shock-buffet Instability
Shock-buffet is the term used for the self-sustained, low frequency, large amplitude shock oscillations that are observed for certain combinations of Mach number, and steady mean angle-of-attack in transonic flows. Shock buffet degrades the performance of airfoils and wings, and it may also compromise flight vehicle safety. Moreover, the interaction of the shock buffet with the elastic structural motion of the airfoil or wing may induce aeroelastic responses that are highly undesirable from the point of view of structural integrity, aerodynamics, and flight handling qualities. Hence, there is a strong motivation to better understand, model, and ultimately control transonic aeroelastic phenomena in the vicinity of shock-buffet.