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This paper establishes the benefits of a truss-braced-wing transonic transport aircraft configuration compared to the cantilever-wing aircraft and to a strut-braced wing. Multidisciplinary design optimization is used to design aircraft with three wing configurations with increasing complexity of topology: cantilever, one-member truss (strut), and three-member truss. Three objective functions are studied: minimum takeoff gross weight, minimum fuel consumption and emissions, and maximum lift-to-drag ratio. A mission with a 7730 n mile range at a cruise Mach number of 0.85 is considered. The results show the significant advantage of strut and simple truss configurations over the conventional cantilever configuration. One comparison produces a reduction of 45% in the fuel consumption while decreasing the minimum takeoff gross weight by 15%. For a second comparison, the fuel weight is reduced by 33% with a decreased minimum takeoff gross weight of 19%. Very attractive vehicle performance can be achieved without the necessity of decreasing cruise Mach number. The results also indicate that a truss-braced wing has a greater potential for improved aerodynamic performance than other innovative aircraft configurations. Further studies will consider the inclusion of more complex truss topologies and other innovative technologies that are judged to be synergistic with truss-braced-wing configurations. DOI: 10.2514/1.47546

Accurate drag estimation is critical in making computational design studies. Drag may be estimated thousands of times during a multidisciplinary design optimization, and computational fluid dynamics is not yet possible in these studies. The current model has been developed as part of an air-vehicle conceptual-design multidisciplinary design optimization framework. Its use for subsonic and transonic aircraft configurations is presented and validated. We present our parametric geometry definition, followed by the drag model description. The drag model includes induced, friction, wave, and interference drag. The model is compared with subsonic and transonic isolated wings, and a wing/body configuration used previously in drag prediction workshops. The agreement between the predictions of the drag model and test data is good, but lessens at high lift coefficients and high transonic Mach numbers. In some cases the accuracy of this drag estimation method exceeds much more elaborate analyses. DOI: 10.2514/1.47557

A new noise metric is developed for clean-wing aerodynamic noise modeling that may be used in aircraft or wind turbine design studies involving aerodynamic noise. The method uses Reynolds-averaged Navier--Stokes calculations with a two-equation turbulence model to include the effects of the lift coefficient, flow three-dimensionality, and wing design parameters on the trailing-edge noise. These effects are not considered in the existing, relatively less expensive, semi-empirical noise prediction methods. The proposed noise metric is not the exact value of the noise intensity, but it is a relative noise measure suitable for design studies. The new noise metric is less expensive to compute compared to the full-noise calculations done with computational aeroacoustics methods, and it can be easily implemented using the solutions of Reynolds-averaged Navier--Stokes simulations that may have already been performed as part of the aerodynamic design and analysis. Parametric studies were performed to investigate the effect of the wing geometry and the lift coefficient on the noise metric. Two-dimensional studies were done using two subsonic (NACA 0012 and 0009) and two supercritical [SC(2)-0710 and -0714] airfoils. The energy-efficient transport wing (a generic conventional transport wing) was used for the three-dimensional study. Both two-and three-dimensional studies show that the trailing-edge noise increases significantly at high lift coefficients. The three-dimensional effects observed in the parametric wing study indicate the importance of calculating the noise metric with a characteristic velocity and length scale that vary along the span. DOI: 10.2514/1.29105

The modular transonic vortex interaction configuration was developed at the NASA Langley Research Center to investigate the aerodynamic characteristics of a generic fighter incorporating a chined fuselage and delta wing. Previous experiments showed that the fuselage and leading-edge vortex interactions are detrimental to the vehicle's aerodynamic characteristics for angles of attack greater than 23 deg at low angles of sideslip. This is largely due to abrupt asymmetric vortex breakdown, which leads to pronounced pitch-up and significant nonlinearities in lateral stability that could result in roll departure. An improved understanding of the exact origins of this nonlinear behavior would improve future fighter design, and predictive capabilities of such nonlinearities could drastically reduce the cost associated with flight testing new or modified aircraft. The nonlinearities experienced by the modular transonic vortex interaction configuration at a 30 deg angle of attack, Reynolds number of 2.68 x [10.sup.6], and Mach number of 0.4 are computed using delayed detached-eddy simulation. Computational predictions of rolling moment compare very well with previous wind-tunnel experiments at the same conditions, including the abrupt nonlinear increase in rolling moment as a function of sideslip angle at small sideslip angles. A detailed investigation of the computational fluid dynamic data confirms that this nonlinearity is due to a rapid change in the flowfield structures from symmetric to asymmetric vortex breakdown. DOI: 10.2514/1.40955

This paper investigates the resistance to a change in wing shape due to the aerodynamic forces. In particular, the work required by an airfoil to overcome the aerodynamic forces and produce a change in lift is examined. The relationship between this work and the total aerodynamic energy balance is shown to have significant consequences for transient changes in airfoil shape. Specification of the placement of the actuators and the actuator energetics is shown to be required for the determination of the airfoil shape change, requiring minimum energy input. A general simplified actuator model is adopted in this study, which assigns different values of actuator efficiency for negative and positive power output. Unsteady thin airfoil theory is used to analytically determine the pressure distribution and aerodynamic coefficients as a function of time for a ramp input of control deflection. This allows the required power and work to overcome the aerodynamic forces to be determined for a prescribed change in the airfoil camberline. The energy required for a pitching flat plate, conventional flap, conformal flap, and two variable camber configurations is investigated. For the pitching flat plate, the minimum energy pitching axis is shown to be dependent on the pitch rate and the initial angle of attack. The conformal flap is shown to require less actuator energy than the conventional flap to overcome the aerodynamic forces for a prescribed change in lift. The energy requirements of a variable camber configuration are shown to be sensitive to the layout of the variable camber device. The present analysis shows that the unsteady aerodynamic influence is important only for [[tau].sup.*] values less than five. For [[tau].sup.*] values larger than this, the present analysis reduces to the steady airfoil results of past studies. DOI: 10.2514/1.26423

A method has been developed to generate and use polynomial approximations to the range and cruise drag components in a highly constrained, multidisciplinary design optimization of a high-speed civil transport (HSCT) configuration. The method improves optimization performance by eliminating the numerical noise present in the analyses through the use of response surface methodology. In this implementation quadratic polynomials are fit within variable bounds to data gathered from a series of numerical analyses of different aircraft designs. Because the HSCT optimization process contains noise and suffers from a nonconvex design space even when noise is filtered out, multiple optimization runs are performed from different starting points with and without the response surface models in order to evaluate both their effectiveness as surrogate functions and as a design exploration tool. The alternative method used is variable complexity modeling (VCM). It is shown that response surface methodology facilitates design space exploration, allowing improvements in terms of both convergence performance and computational effort when multiple starting points are required, although using VCM usually produces better final designs.

Research is presented concerning the conceptual design of a strut-braced wing aircraft which has engines mounted at the tip of the wings or alongside the fuselage. The aerodynamics of strut-braced wings are discussed.

Two different applications of the reasonable design approach to the multidisciplinary optimization of a high-speed civil transport model are described. These include the creation of a response surface approximation for structural bending material weight, and the creation of response surface approximations for components of aerodynamic drag. The advantages of using reasonable surface models created via the reasonable design space approach are discussed.

The influence of the F-5A forebody on directional stability at high angles of attack has been simulated through computers over attack ranges of 10 to 45 degrees by using computational fluid dynamics while studying flowfields at low-speed high angles of attack. The CFL3D code uses a modified turbulence model to solve the Reynolds equations.

Analysis of flowfields over chine-shaped forebodies at low-speed high angle- of-attack conditions with sideslip involves application of computational fluid dynamics to determine stability-enhancing forebody geometries. Dynamic analyses of several forebody shapes to determine the shape that enhances directional stability at high angle of attack shows that the shape of the cross section such as broad chine-shapes promote stability, which is further improved by chines placed near the top of the cross section.