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NASA NTRS1988

Rotorcraft aeroelastic stability

Theoretical and experimental developments in the aeroelastic and aeromechanical stability of helicopters and tilt-rotor aircraft are addressed. Included are the underlying nonlinear structural mechanics of slender rotating beams, necessary for accurate modeling of elastic cantilever rotor blades, and the development of dynamic inflow, an unsteady aerodynamic theory for low-frequency aeroelastic stability applications. Analytical treatment of isolated rotor stability in hover and forward flight, coupled rotor-fuselage stability in hover and forward flight, and analysis of tilt-rotor dynamic stability are considered. Results of parametric investigations of system behavior are presented, and correlation between theoretical results and experimental data from small and large scale wind tunnel and flight testing are discussed.

Ormiston, Robert A.↗

NASA NTRS1987

Prediction of aeroelastic response of a model X-wing rotor

The rotorcraft dynamics analysis was used to predict the aeroelastic responses of a representative X-wing model with a 10 ft diameter rotor. The aeroelastic methodology used and the tests and assumptions involved are reviewed. Results are reported on the findings concerning control power and higher harmonic control in hover, transition flight, vibratory loads at forward speed, and responses in conversion. It is concluded that the analysis can give satisfactory predictions of X-wing behavior.

Dopher, Robert↗

NASA NTRS1988

Aeroelastic forced response analysis of turbomachinery

An introduction is given to the research activity that is underway to enable the prediction of turbomachinery aeroelastic forced response. An effort is being made to assemble a computer program (FREPS) which incorporates the aeroelastic structural models, unsteady aerodynamic models, and forcing function models. The structural and aerodynamic models are currently well developed. The forcing function models are at a primitive level. A significant activity has begun to identify the forcing functions due to stator-rotor aerodynamic interaction.

Smith, Todd E.↗

NASA NTRS1988

Application of unsteady aerodynamic methods for transonic aeroelastic analysis

Aerodynamic methods for aeroelastic analysis are applied to various flow problems. These methods include those that solve the three dimensional transonic small disturbance (TSD) potential equation, the two dimensional (2-D) full potential (FP) equation, and the 2-D thin layer Navier-Stokes equations. Flutter analysis performed using TSD aerodynamics show that such methods can be used to analyze some aeroelastic phenomena. For thicker bodies and larger amplitude motions, a nonisentropic FP method is presented. The unsteady FP equation is modified to model the entropy jumps across shock waves. The conservative form of the modified equation is solved in generalized coordinates using an implicit, approximate factorization method. Pressures calculated on the NLR 7301 and NACA 64A010A airfoils using the nonisentropic FP method are presented. It is shown that modeling shock generated entropy extends the range of validity of the FP method. A Navier-Stokes code is correlated with pressures measured on a supercritical airfoil at transonic speeds. When corrections are made for wind tunnel wall effects, the calculations correlate well with the measured data.

Whitlow, Woodrow, Jr.↗

NASA NTRS1988

Aeroelastic response of metallic and composite propfan models in yawed flow

An analytical investigation of aeroelastic response of metallic and composite propfan models in yawed flow was performed. The analytical model is based on the normal modes of a rotating blade and the three dimensional unsteady lifting surface aerodynamic theory including blade mistuning. The calculated blade stresses or strains are compared with published wind tunnel data on two metallic and three composite propfan wind tunnel models. The comparison shows a good agreement between theory and experiment. Additional parametric results indicate that blade response is very sensitive to the blade stiffness and also to blade frequency and mode shape mistuning. From these findings, it is concluded that both frequency and mode shape mistuning should be included in aeroelastic response analysis. Furthermore, both calculated and measured strains show that combined blade frequency and mode shape mistuning has beneficial effects on response due to yawed flow.

Kaza, Krishna Rao V.↗

NASA NTRS1987

Prediction of aeroelastic response of a model X-wing rotor

The Rotorcraft Dynamics Analysis was used to predict the aeroelastic responses of a representative X-Wing model with a 10-ft diameter rotor. The aeroelastic methodology used and the tests and assumptions involved are reviewed. Results are reported on the findings concerning control power and higher harmonic control in hover, transition flight, vibratory loads at forward speed, and responses in conversion. It is concluded that the analysis can give satisfactory predictions of X-Wing behavior.

Sopher, Robert↗

NASA NTRS1988

Aeroelastic tailoring for oblique wing lateral trim

Composite material aeroelastic tailoring is presently explored as a means for the correction of the roll trim imbalance of oblique-wing aircraft configurations. The concept is demonstrated through the analysis of a realistic oblique wing by a static aeroelastic computational procedure encompassing the full potential transonic aerodynamic code FLO22 and a Ritz structural plate program that models the stiffness due to symmetrical-but-unbalanced composite wing skins. Results indicate that asymetric composite tailoring reduces the aileron deflection needed for roll equilibrium, and reduces control surface hinge moment and drag. Wing skin stresses are, however, very high.

Bohlmann, Jonathan D.↗

NASA NTRS1988

Structural optimization with aeroelastic constraints of rotor blades with straight and swept tips

This paper describes a study in which structural optimization techniques are used to minimize the n/rev vertical hub shears in forward flight, subject to aeroelastic stability constraints and frequency placement constraints. A special technique is used to build a sequence of approximate, inexpensive to solve optimization problems, the solutions of which converge to the solution of the exact, expensive to solve optimization problem. Blade configurations with both straight and swept tips, and single- and double-cell cross sections are analyzed. The results show that the approach used in this study is very efficient, and produces improved designs with a very small number of blade aeroelastic analyses.

Celi, R.↗

NASA NTRS1988

Aeroelastic analysis of hammerhead payloads

A launch vehicle aeroelastic stability method is presented in order to treat transonic flight problems that are characteristic of 'hammerhead' payload geometries. Navier-Stokes formulation-based CFD techniques are used to simulate the unsteady aerodynamic flowfields, and time-domain aeroelastic analyses are performed by coupling this flow solver with a modal representation of the vehicle's structural-dynamic characteristics. Numerical applications encompass the study of an Atlas-Able IV configuration in various flight conditions; good qualitative agreement with observations is obtained.

Azevedo, Joao Luis F.↗

NASA NTRS1987

Problems and progress in aeroelasticity for interdisciplinary design

Some problems and progress in the development of aerodynamic and aeroelastic computational capabilities are reviewed with emphasis on needs for use in current interdisciplinary design procedures as well as for stand-alone analyses. The primary focus is on integral-equation methods which are well suited for general, accurate, efficient, and unified treatment of flow around vehicles having arbitrary shapes, motions, and deformations at subsonic, transonic, and supersonic speeds up to high angles of attack. Computational methods for potential flows and viscous flows are discussed, and some applications are shown. Calculation of steady and unsteady aeroelastic characteristics of aircraft with nonlinear aerodynamic behavior is also addressed briefly.

Yates, E. Carson, Jr.↗

NASA NTRS1987

Constrained optimization techniques for active control of aeroelastic response

Active control of aeroelastic response is a complex problem in which the designer usually tries to satisfy many design criteria which are often conflicting in nature. To further complicate the design problem, the state space equations describing this type of control problem are usually of high order, involving a large number of states to represent the flexible structure and unsteady aerodynamics. Control laws based on the standard Linear - Quadratic - Gaussian method are of the same high order as the aeroelastic plant and may be difficult to implement in the flight computer. To overcome this disadvantage a new approach was developed for designing low-order optimized robust control laws. In this approach, a nonlinear programming algorithm is used to search for the values of control law design variables that minimize a performance index while satisfying several inequality constraints that describe the design criteria on the stability robustness and responses. The method is applied to a gust load alleviation problem and a stability robustness improvement problem of a drone aircraft.

Mukhopadhyay, Vivekananda↗

NASA NTRS1989

Transonic Unsteady Aerodynamics and Aeroelasticity 1987, part 1

Computational fluid dynamics methods have been widely accepted for transonic aeroelastic analysis. Previously, calculations with the TSD methods were used for 2-D airfoils, but now the TSD methods are applied to the aeroelastic analysis of the complete aircraft. The Symposium papers are grouped into five subject areas, two of which are covered in this part: (1) Transonic Small Disturbance (TSD) theory for complete aircraft configurations; and (2) Full potential and Euler equation methods.

Bland, Samuel R.↗

NASA NTRS1989

Unsteady aerodynamics and aeroelastic research at AFWAL

An efficient and accurate transonic unsteady aerodynamic method is needed for predicting flight loads, flutter and aeroelastic stability for advanced aircraft. There have been new developments and many improvements to older codes. XTRAN3S was improved and is a useful code for the near term. However, to predict the unsteady aerodynamics for high performance maneuvering aircraft, the Euler/Navier-Stokes codes must be extended and improved for complex 3-D configurations. The long term goal is development of Euler/Navier-Stokes unsteady aerodynamic methods for aeroelastic analysis.

Huttsell, L. J.↗

NASA NTRS1989

Static aeroelasticity of a composite oblique wing in transonic flows

One aircraft configuration that shows great promise in achieving high performance is that of an asymmetrically swept wing. When compared to conventional swept wings, these advantages include higher lift to drag ratios and reduced takeoff and landing speeds, which translate into greater performance in terms of fuel comsumption, loiter time, and range. However, the oblique wing has a number of disadvantages because of its asymmetric configuration. The question is how to best achieve maximum stability and roll equilibrium without compromising performance. Using aeroelastic tailoring to enhance aeroelastic stability and control has been demonstrated in several analyses, especially for the forward swept wing. The advantages and disadvantages for the oblique wing configuration are discussed.

Bohlmann, Jonathan D.↗

NASA NTRS1988

Survey of Army/NASA rotorcraft aeroelastic stability research

Theoretical and experimental developments in the aeroelastic and aeromechanical stability of helicopters and tilt-rotor aircraft are addressed. Included are the underlying nonlinear structural mechanics of slender rotating beams, necessary for accurate modeling of elastic cantilever rotor blades, and the development of dynamic inflow, an unsteady aerodynamic theory for low frequency aeroelastic stability applications. Analytical treatment of isolated rotor stability in hover and forward flight, coupled rotor-fuselage stability are considered. Results of parametric investigations of system behavior are presented, and correlations between theoretical results and experimental data from small- and large-scale wind tunnel and flight testing are discussed.

Ormiston, Robert A.↗

NASA NTRS1989

Effects of nonlinear aerodynamics and static aeroelasticity on mission performance calculations for a fighter aircraft

During conceptual design studies of advanced aircraft, the usual practice is to use linear theory to calculate the aerodynamic characteristics of candidate rigid (nonflexible) geometric external shapes. Recent developments and improvements in computational methods, especially computational fluid dynamics (CFD), provide significantly improved capability to generate detailed analysis data for the use of all disciplines involved in the evaluation of a proposed aircraft design. A multidisciplinary application of such analysis methods to calculate the effects of nonlinear aerodynamics and static aeroelasticity on the mission performance of a fighter aircraft concept is described. The aircraft configuration selected for study was defined in a previous study using linear aerodynamics and rigid geometry. The results from the previous study are used as a basis of comparison for the data generated herein. Aerodynamic characteristics are calculated using two different nonlinear theories, potential flow and rotational (Euler) flow. The aerodynamic calculations are performed in an iterative procedure with an equivalent plate structural analysis method to obtain lift and drag data for a flexible (nonrigid) aircraft. These static aeroelastic data are then used in calculating the combat and mission performance characteristics of the aircraft.

Giles, Gary L.↗

NASA NTRS1989

Aeroelastic modeling for the FIT (Functional Integration Technology) team F/A-18 simulation

As part of Langley Research Center's commitment to developing multidisciplinary integration methods to improve aerospace systems, the Functional Integration Technology (FIT) team was established to perform dynamics integration research using an existing aircraft configuration, the F/A-18. An essential part of this effort has been the development of a comprehensive simulation modeling capability that includes structural, control, and propulsion dynamics as well as steady and unsteady aerodynamics. The structural and unsteady aerodynamics contributions come from an aeroelastic mode. Some details of the aeroelastic modeling done for the Functional Integration Technology (FIT) team research are presented. Particular attention is given to work done in the area of correction factors to unsteady aerodynamics data.

Zeiler, Thomas A.↗

NASA NTRS1988

Application of unsteady aerodynamic methods for transonic aeroelastic analysis

Aerodynamic methods for aeroelastic analysis are applied to various flow problems. These methods include those that solve the three-dimensional transonic small disturbance (TSD) potential equation, the two-dimensional full potential (FP) equation, and the thin-layer Navier-Stokes equations. Flutter analysis performed using TSD aerodynamics shows that such methods can be used to analyze some aeroelastic phenomena. For thicker bodies and larger amplitude motions a nonisentropic full potential method is presented.

Whitlow, Woodrow, Jr.↗