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Nissim, E.

Publications and source records attributed to Nissim, E..

At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Improvement of method for experimental determination of flutter speed by parameter identification

The method investigated in the current research work is based on the identification of the equations of motion during test flights, followed by the solution of these identified equations to numerically compute the flutter dynamic pressure. The current research work is aimed at overcoming two main difficulties which arise when: (1) A continuous system is truncated into a system with a finite number of degrees of freedom. (2) The desired range of frequencies is wide; in the latter case, numerical difficulties occur that can only partly be overcome by giving relatively more weight to the high frequency modes. Results pertaining to the above first point will be presented under section entitled 'Truncation Effects', and results pertaining to the above second point will be presented under section entitled 'Frequency Range Effects'. The numerical example used herein consists of a continuous simply supported beam. It is further assumed that the object of the identification procedure is to identify the beam's first seven modes.

Nissim, E.↗
NASA NTRS1991

Placing Control Surfaces To Suppress Aeroelastic Flutter

Method for assigning positions of control surfaces to suppress aeroelastic flutter involves analysis of transfer of energy. Neither depends on nor prescribes specific form of control law. Assuming suitable control law available by whatever means, shows how to determine where on aircraft or airfoil control surface(s) maximally effective in suppressing flutter.

Nissim, E.↗
NASA NTRS1989

Effect of control surface mass unbalance on the stability of a closed-loop active control system

The effects on stability of inertial forces arising from closed-loop activation of mass-unbalanced control surfaces are studied analytically using inertial energy approach, similar to the aerodynamic energy approach used for flutter suppression. The limitations of a single control surface like a leading-edge (LE) control or a trailing-edge (TE) control are demonstrated and compared to the superior combined LE-TE mass unbalanced system. It is shown that a spanwise section for sensor location can be determined which ensures minimum sensitivity to the mode shapes of the aircraft. It is shown that an LE control exhibits compatibility between inertial stabilization and aerodynamic stabilization, and that a TE control lacks such compatibility. The results of the present work should prove valuable, both for the purpose of flutter suppression using mass unbalanced control surfaces, or for the stabilization of structural modes of large space structures by means of inertial forces.

Nissim, E.↗
NASA NTRS1989

Method for experimental determination of flutter speed by parameter identification

A method for flight flutter testing is proposed which enables one to determine the flutter dynamic pressure from flights flown far below the flutter dynamic pressure. The method is based on the identification of the coefficients of the equations of motion at low dynamic pressures, followed by the solution of these equations to compute the flutter dynamic pressure. The initial results of simulated data reported in the present work indicate that the method can accurately predict the flutter dynamic pressure, as described. If no insurmountable difficulties arise in the implementation of this method, it may significantly improve the procedures for flight flutter testing.

Nissim, E.↗
NASA NTRS1989

Control surface spanwise placement in active flutter suppression systems

All flutter suppression systems require sensors to detect the movement of the lifting surface and to activate a control surface according to a synthesized control law. Most of the work performed to date relates to the development of control laws based on predetermined locations of sensors and control surfaces. These locations of sensors and control surfaces are determined either arbitrarily, or by means of a trial and error procedure. The aerodynamic energy concept indicates that the sensors should be located within the activated strip. Furthermore, the best chordwise location of a sensor activating a T.E. control surface is around the 65 percent chord location. The best chordwise location for a sensor activating a L.E. surface is shown to lie upstream of the wing (around 20 percent upstream of the leading edge), or alternatively, two sensors located along the same chord should be used.

Nissim, E.↗
NASA NTRS1989

Design of control laws for flutter suppression based on the aerodynamic energy concept and comparisons with other design methods

The aerodynamic energy method is used in this paper to synthesize control laws for NASA's Drone for Aerodynamic and Structural Testing-Aerodynamic Research Wing 1 (DAST-ARW1) mathematical model. The performance of these control laws in terms of closed-loop flutter dynamic pressure, control surface activity, and robustness is compared against other control laws that appear in the literature and relate to the same model. A control law synthesis technique that makes use of the return difference singular values is developed in this paper. it is based on the aerodynamic energy approach and is shown to yield results superior to those given in the literature and based on optimal control theory. Nyquist plots are presented together with a short discussion regarding the relative merits of the minimum singular value as a measure of robustness, compared with the more traditional measure of robustness involving phase and gain margins.

Nissim, E.↗
NASA NTRS1989

Effect of control surface mass unbalance on the stability of a closed-loop active control system

An inertial energy approach similar to the aerodynamic energy method for flutter suppression is used to investigate the effects of mass-unbalanced control surfaces on the stability of a closed-loop system. It is demonstrated that a spanwise section for sensor location can be obtained which ensures minimum sensitivity to the mode shapes of the aircraft. Leading-edge control is characterized by a compatibility between inertial stabilization and aerodynamic stabilization that trailing-edge control lacks.

Nissim, E.↗
NASA NTRS1989

Method for experimental determination of flutter speed by parameter identification

A method for flight flutter testing is proposed which enables one to determine the flutter dynamic pressure from flights flown far below the flutter dynamic pressure. The method is based on the identification of the coefficients of the equations of motion at low dynamic pressures, followed by the solution of these equations to compute the flutter dynamic pressure. The initial results of simulated data reported in the present work indicate that the method can accurately predict the flutter dynamic pressure, as described. If no insurmountable difficulties arise in the implementation of this method, it may significantly improve the procedures for flight flutter testing.

Nissim, E.↗
NASA NTRS1989

Modeling of aerodynamic forces in the Laplace domain with minimum number of augmented states for the design of active flutter suppression systems

A method is proposed by which an aeroservoelastic problem is brought to a state-space form with a minimum number of augmented aerodynamic terms. The examples treated in this work relate to NASA's Drone for Aerodynamic and Structural Testing-Aerodynamic Research Wing 1 (DAST-ARW1) and to the YF-17 fighter model. It is shown that in all cases considered, the method yields a very good accuracy regarding the flutter parameters and the dynamic behavior of the systems, using only two augmented aerodynamic states. The method should prove useful in the design of lower order control laws based on optimal control theory.

Nissim, E.↗
NASA NTRS1988

Control surface spanwise placement in active flutter suppression systems

A method is developed that determines the placement of an active control surface for maximum effectiveness in suppressing flutter. No specific control law is required by this method which is based on the aerodynamic energy concept. It is argued that the spanwise placement of the active controls should coincide with the locations where maximum energy per unit span is fed into the system. The method enables one to determine the distribution, over the different surfaces of the aircraft, of the energy input into the system as a result of the unstable fluttering mode. The method is illustrated using three numerical examples.

Nissim, E.↗
NASA NTRS1985

Optimization of cascade blade mistuning. II - Global optimum and numerical optimization

The values of the mistuning which yield the most stable eigenvectors are analytically determined, using the simplified equations of motion which were developed in Part I of this work. It is shown that random mistunings, if large enough, may lead to the maximal stability, whereas the alternate mistunings cannot. The problem of obtaining maximum stability for minimal mistuning is formulated, based on numerical optimization techniques. Several local minima are obtained using different starting mistuning vectors. The starting vectors which lead to the global minimum are identified. It is analytically shown that all minima appear in multiplicities which are equal to the number of compressor blades. The effect of mistuning on the flutter speed is studied using both an optimum mistuning vector and an alternate mistuning vector. Effects of mistunings in elastic axis locations are shown to have a negligible effect on the eigenvalues. Finally, it is shown that any general two-dimensional bending-torsion system can be reduced to an equivalent uncoupled torsional system.

Nissim, E.↗
NASA NTRS1985

Optimization of cascade blade mistuning. I - Equations of motion and basic inherent properties

Attention is given to the derivation of the equations of motion of mistuned compressor blades, interpolating aerodynamic coefficients by means of quadratic expressions in the reduced frequency. If the coefficients of the quadratic expressions are permitted to assume complex values, excellent accuracy is obtained and Pade rational expressions are obviated. On the basis of the resulting equations, it is shown analytically that the sum of all the real parts of the eigenvalues is independent of the mistuning introduced into the system. Blade mistuning is further treated through the aerodynamic energy approach, and the limiting vibration modes associated with alternative mistunings are identified.

Nissim, E.↗
NASA NTRS1980

Active controls for flutter suppression and gust alleviation in supersonic aircraft

Results of work done on active controls on the modified YF-17 flutter model are summarized. The basic derivation of a suitable control law is discussed. It is shown that discrepencies found between analysis and wind tunnel tests originate from the lack of proper implementation of the desired control law. Program capabilities are described.

Nissim, E.↗
NASA NTRS1980

Active controls for flutter suppression and gust alleviation in supersonic aircraft

Application is made in the present paper of the recently developed relaxed aerodynamic energy concept and synthesis techniques to the definition of appropriate active control systems for the low-speed flutter model of the B-2707-300 supersonic cruise airplane. The effectiveness of the resulting activated systems is analytically tested for flutter suppression, wing root bending moment alleviation, and ride control (fuselage accelerations). The results obtained indicate that considerable increase in flutter speeds can be obtained by the various control systems, using a single trailing-edge control. In all cases, the flutter suppression control system led to a substantial reduction in both wing root bending moments and in fuselage and wing accelerations.

Nissim, E.↗
NASA NTRS1979

Active external store flutter suppression in the YF-17 flutter model

A single activated trailing-edge (T.E.) control system (spanning 7% of each wing) is applied to the YF-17 flutter model with the object of suppressing the external store flutter of three different store configurations. The control law is derived by the use of the aerodynamic energy concept and its gains are maintained constant for all three configurations. The results obtained show that the activated T.E. control system leads to very significant increases in the flutter dynamic pressures Q(DF) of all three configurations; these increases range between 160-240% increase in Q(DF). These increases in Q(DF) are maintained over a very wide range of flight altitudes and flight velocities.

Nissim, E.↗
NASA NTRS1979

On single-degree-of-freedom flutter induced by activated controls

It is shown that activation of the trailing-edge control of an airfoil leads to single-degree-of-freedom type instabilities which span over a very wide region of reduced frequencies k, including high values of k (unlike the nonactivated system). These instabilities are shown to be sensitive to changes in pitching axis location, control deflection phase angle, and values of the reduced frequency. These sensitivities of the single-degree-of-freedom system cause the activated airfoil to be potentially sensitive to changes in flight conditions, and may be the source of the many difficulties encountered in suppressing classical multi-degree-of-freedom flutter by means of active controls. The results presented herein relate to zero Mach number and to a 20% trailing-edge control surface.

Nissim, E.↗
NASA NTRS1978

Comparative study between two different active flutter suppression systems

An activated leading-edge (LE)-tailing-edge (TE) control system is applied to a drone aircraft with the objective of enabling the drone to fly subsonically at dynamic pressures which are 44% above the open-loop flutter dynamic pressure. The control synthesis approach is based on the aerodynamic energy concept and it incorporates recent developments in this area. A comparison is made between the performance of the activated LE-TE control system and the performance of a TE control system, analyzed in a previous work. The results obtained indicate that although all the control systems achieve the flutter suppression objectives, the TE control system appears to be somewhat superior to the LE-TE control system, in this specific application. This superiority is manifested through reduced values of control surface activity over a wide range of flight conditions.

Nissim, E.↗