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This paper provides a historical perspective of the fundamental developments that have played a central role in rotary-wing dynamics and aeroelasticity and have had a major impact on the design of rotary-wing aircraft. The paper describes a historical progression starting with the classical flap-pitch problem that emulated fixed-wing behavior and describes the evolution of the dynamic and aeroelastic problems into those that are unique to rotorcraft, such as the flap-lag problem, the lag-pitch problem, and the coupled flap-lagtorsional problem. Subsequently, the coupled rotor/fuselage aeromechanical problems such as ground and air resonance are considered. A description of the evolution of the methodology used in the formulation and solution of these types of problems is also provided, emphasizing the structural and aerodynamic models required for their effective formulation and solution. The mathematical techniques used for solving the rotary-wing aeroelastic problems in hover and forward flight are also described. The primary emphasis of the paper is on aeroelastic stability, and aeroelastic response is only treated briefly. The paper focuses on contributions that have historical value because they represent landmark treatments. Because of the large amount of material available, an all-inclusive treatment of the research done in this field is impractical, and the paper has unavoidable omissions.

There are a variety of abnormal running conditions in rotating machinery that lead to rotor/stator interaction dynamics which, in turn, have a range of effects associated with them. One of these effects is steady vibration response at frequencies which are different from the excitation. This paper describes a mechanism of generating subharmonic vibration frequencies in both numerical simulation and measurements, which are obtained from a study of the relatively new problem of 'windmilling imbalance' in aero-engines. What is different from other nonlinear systems with, say, clearance or squeeze film dampers, is the richness of the frequency spectrum. [DOI: 10.1115/1.1467648]

Nonlinear finite-element models that predict the load-elongation behavior of single- and dual-bolted conical-head bolted lap joints are developed, and the load-elongation predictions are compared with experimental test data. The study is conducted for several, panel thicknesses with a 4.8 mm size fastener and 2024-T3 clad aluminum alloy panels. The model load-elongation predictions are in excellent agreement with experimental test data. The results show that (1) dual-bolted lap joint test results underpredict the strength of single-bolted lap joints; and (2) numerical models may reliably predict the load-elongation behavior of mechanically fastened lap joints at a fraction of the cost and time of experimental testing. DOI: 10.1061/(ASCE)0893-1321 (2002) 15:2(55) CE Database keywords: Aerospace engineering; Aircraft; Bolted joints; Finite elements; Nonlinear analysis.

A survey is conducted of CFD methods applied to the computation of high-lift multi-element configurations over the last 10-15 years. Both 2-D and 3-D configurations are covered. The review is organized by configuration, in an effort to glean useful insights with respect to particular successes or failings of CFD methods as a whole. In general, for both 2-D and 3-D flows, if certain guidelines regarding grid, transition, and turbulence model are followed, then surface pressures, skin friction, lift, and drag can be predicted with reasonably good accuracy at angles of attack below stall. Velocity profiles can generally be predicted in 2-D flow fields, with the exception of the slat wake, which tends to be predicted too deep by most CFD codes for a range of different configurations. CFD codes can usually predict trends due to Reynolds number in 2-D, but they are inconsistent in the prediction of trends due to configuration changes. On the whole, 2-D CFD is unreliable for predicting stall (maximum lift and the angle of attack at which it occurs); in most cases, maximum lift is overpredicted, but for some configurations the opposite occurs. However, there is some evidence that stall misprediction of nominally 2-D experiments may be caused by 3-D effects, which are obviously not modeled by 2-D CFD. In general, 3-D computations are also inconsistent with respect to computing stall, but there have been fewer of these applications to date. The paper concludes with a list of challenges that confront CFD at the start of the next decade, which should witness a dramatic increase in the number of CFD applications for 3-D high-lift configurations. Published by Elsevier Science Ltd.

The use of iteratively reweighted least squares (IRLS) for detecting design points where optimizations converge poorly is demonstrated. Because optimization error tends to be one-sided with poor results producing overweight designs, a nonsymmetrical version of IRLS (NIRLS) that accounts for the asymmetry in optimization errors is also developed. A parameterized Rosenbrock function problem, where a programming error caused poor optimizations, is used to demonstrate the techniques. The identified outliers were repaired by more accurate optimizations to improve response surface approximations. Structural optimization of a high-speed civil transport (HSCT) produced inaccurate wing structural weight. Optimization studies with various sets of convergence criteria showed that the optimization errors were due to incomplete convergence. The IRLS technique could identify many of the points with very large optimization errors, but the NIRLS techniques were much more reliable in this task. However, results of HSCT configuration optimizations using wing weight response surface models show that it may be preferable to identify and repair only a subset of the poorly converged optima.

It is well known that the weighted sum aggregate objective function fails to capture Pareto points that are located on a concave region of the Pareto frontier. Conditions are established for a general aggregate objective function to capture points on a concave Pareto frontier region. Specifically, these conditions are developed in terms of the relative orders of the Pareto frontier and aggregate objective function. These critical conditions can be used in practice to allow an aggregate objective function to capture any potentially desirable Pareto point. Conversely, failure to satisfy these critical conditions can represent a pivotal failure point of the general application of design optimization because we may fail to obtain the design we seek.

A class of lower-upper symmetric Gauss-Seidel implicit weighted essentially nonoscillatory (ENO) schemes for solving the two- and three-dimensional compressible Navier-Stokes equations with pointwise version of Baldwin-Barth one-equation turbulence model is presented (Baldwin, B. S., and Barth, T. J., 'A One-Equation Turbulence Transport Model for High Reynolds Number Wall Bounded Flows,' AIAA Paper 91-0610, 1991). A weighted ENO (WENO) spatial operator is employed for inviscid fluxes and central differencing for viscous fluxes. A numerical flux of the WENO scheme in flux limiter form is adopted, which consists of first-order and high-order fluxes and allows for a more flexible choice of first-order dissipative methods. The computations are performed for the two-dimensional turbulent flows over NACA 0012 and Royal Aircraft Establishment 2822 airfoil and the three-dimensional turbulent flow over an ONERA M6 wing. The present solutions are compared with experimental data and other computational results and exhibit good agreement.

Five low fuel use and emissions airplane design concepts being worked on by NASA and Boeing Co. are discussed.

A study of aircraft structures is presented with reference to the use of aeroservoelastic analysis. The performance of design optimization is examined and verified experimentally.

A study of the development of a design method for subsonic flows on aircraft is presented. The effect of this inverse efficient aerodynamic design is described and examined using two- and three-dimensional examples.

The constant emphasis on specialization produces university graduates who do not or cannot look at problems broadly. As a result, engineers, scientists and executives--indeed graduates in all fields--including the supposedly broad-based humanities--often cannot solve problems that require knowledge outside of their specializations. Or their narrowness causes them to commit embarrassing blunders that could be avoided if they took a broader view. The case of the British Westland Lysander P12 Ground Strafer aircraft illustrates the problem of narrow thinking. Very little direct information is available on this ingenious but obscure prototype airplane, but by examining many peripheral matters we can determine not only why the P12 was built but also how it was built. Further, we can also determine why it failed. Had the initial designers approached the problem in a broad way, and using information that was then available, they would have seen in advance that the project would fail. The case is instructive as an industrial problem, but it also demonstrates the value of global thinking methodology.

Research is presented concerning the behavior of integrally stiffened curved aluminum panels which were subjected to an axial compressive load whose loading and boundary conditions were varied.

Research is presented concerning the use of a parametric study to analyze the damage growth retardation characteristics of graphite-epoxy panels constructed with Kevlar stitch lines. The progressive failure characteristics are discussed.

Research is presented concerning the aeroelastic analysis of an entire aircraft flying in a subsonic manner. The amplitude of limit-cycle oscillations, flutter speed and nonlinearities on the trim are discussed.

A new study investigates the flow mechanisms present in subscale experiments by analysing rotary balance data on a generic aircraft model.

Genetic algorithms (GAs) are used in many areas of aerospace engineering. They are effective in exploring the design domain and have the capability to identify promising design variables set. GA capability can handle continuous and discrete variables, providing high flexibility for the genetic optimization procedure in aircraft configuration.

An efficient computational method was developed to define aerodynamic loads on maneuvering flexible flight vehicles. It is based on the conventional Navier-Stokers/Euler code, and the computational fluid dynamics (CFD) code was supplemented with a modal structural model. Normal modes, used for the representation of the structure, were accurate, producing an interface between the CFD and the structure.

A research program aimed to develop computational techniques to minimize noise transmission into aircraft fuselage interiors, using finite element methods and boundary element methods. A structure was optimized for minimum noise at certain points in the interior, and it was found that varying the stiffener sizes over the cylinder is more important than optimizing the shape of the cross sections.

Recent advances in derivative alloys offer useful improvements in product performance, quality and reliability. They can be applied to aging aircraft problems, reducing ownership costs. Older aircraft can be retrofitted, achieving new durability and damage-tolerance. A generic material substitution system is required for rapid implementation of the best material options.

A study of four-ply-thick Kevlar-epoxy panels loaded by in-plane shear involved a finite element analysis to determine the initial linear buckling load and geometrically nonlinear responses. Post-buckling strength of the panels was determined, along with panel fatigue life of the solid and sandwich panels.

Results describe models based on three-dimensional Reynolds-averaged Navier-Stokes equations and one-equation turbulence for analysis of the unstructured mesh Navier-Stokes flow solver in compressible and incompressible forms. Optimization data for both flow forms are presented.

Six types of stiffener terminations for stiffened graphite/epoxy panels were examined. Stiffener flanges can be terminated abruptly or by tapering their thickness, and flange length also varies. The results suggest that failure depends on the stringer configuration of the panel. Interlaminar stresses and bending must be taken into account to predict the failure of thin skin panels, while interlaminar stress, in plane shear strains and axial strains should be taken into account to predict the failure of thick-skin panels.

A method to undertake structural optimization with static aeroelastic considerations is presented, where aerodynamic loads are provided by nonlinear computational aerodynamics schemes. A modification of a computational aerodynamics code originally design for solving Euler of Navier-Stokes equations was used to incorporate aeroelastic effects. The method was used with a wing-fuselage-elevator transport aircraft model.

The transition from wood and fabric airplanes to all metal aircraft was one of the most influential advances in aeronautical development. Metal enabled engineers to extend performance parameters, but many key innovations were developed to take advantage of metal, but emerged independently of the construction material, first appearing in wooden airplanes.