Stephen C. Smith

Aerodynamic and structural studies of joined-wing aircraft

A method for rapidly evaluating the structural and aerodynamic characteristics of joined-wing aircraft was developed and used to study the fundamental advantages attributed to this concept. The technique involves a rapid turnaround aerodynamic analysis method for computing minimum trimmed drag combined with a simple structural optimization. A variety of joined-wing designs are compared on the basis of trimmed drag, structural weight, and, finally, trimmed drag with fixed structural weight. The range of joined-wing design parameters resulting in best cruise performance is identified. Structural weight savings and net drag reductions are predicted for certain joined-wing configurations compared with conventional cantilever-wing configurations.

Computation of induced drag for elliptical and crescent-shaped wings

Recent interest in the induced drag characteristics of crescent-shaped wings has led to a closer look at the methods used for determination of induced drag from computational aerodynamic methods. Induced drag may be computed by integration of surface pressure or by evaluation of a contour integral in the Trefftz plane. A high-order panel method was used to study the induced drag of elliptical and crescent-shaped wings using both techniques. Induced drag computations using surface pressure integration were strongly affected by panel density and angle of attack. Drag computations for the crescent wing were especially sensitive to spanwise panel density because of the complex flowfield near the tip. Trefftz-plane results for the two wing planforms were not sensitive to panel density or angle of attack. The effect of correctly modeling the force-free, rolled-up wake geometry on the predicted span efficiency was demonstrated for both wing planforms. The span efficiency predicted from Trefftz-plane integration was about 0.97 for the elliptical wing and 0.99 for the crescent wing, both somewhat less than the classical theoretical maximum for planar wings. Most of the apparent drag reduction of the crescent wing claimed in previous studies was probably an artifact of the surface-pressure integration. The slightly higher span efficiency for the crescent wing was attributed to a more nearly elliptical spanwise lift distribution.

Wake Vortex Alleviation Flow Field Studies

Wake-vortex-allevialion research was conducted in the far-field vortex wake of a generic wing-tail aircraft model. The goats were to achieve accelerated vortex strength reduction and to map the conditions at which this reduction would occur. The wing-tail model was run in a water tow tank to generate a pair of unequal-strength counterrotating vortices on each side of centerline. Dye flow visualization provided physical insight into the nature of the vortex interactions, and three-component particle image velocimetry allowed quantification of key characteristics of the flowfield, including circulation, vorticity, vortex trajectory, and induced rolling moments. Experiments were conducted for a variety of model angles of attack, tail incidence angles, and tail spans

The design of a joined wing flight demonstrator aircraft

A joined-wing flight demonstrator aircraft has been developed at the NASA Ames Research Center in collaboration with ACA Industries. The aircraft is designed to utilize the fuselage, engines, and undercarriage of the existing NASA AD-1 flight demonstrator aircraft. The design objectives, methods, constraints, and the resulting aircraft design, called the JW-1, are presented. A wind-tunnel model of the JW-1 was tested in the NASA Ames 12-foot wind tunnel. The test results indicate that the JW-1 has satisfactory flying qualities for a flight demonstrator aircraft. Good agreement of test results with design predictions confirmed the validity of the design methods used for application to joined-wing configurations.

A closer look at the induced drag of crescent-shaped wings

Recent interest in the induced drag characteristics of crescent-shaped wings has led to a closer look at the methods used for determination of induced drag from computational aerodynamic methods. Induced drag may be computed by integration of surface pressure, or by evaluation of a contour integral in the Trefftz plane. A high-order panel method was used to study the induced drag of crescent and elliptical wings using both techniques. Induced drag computations using surface-pressure integration were strongly affected by panel density and angle of attack. Accurate drag computations for the crescent wing were obtained only when the spanwise as well as chordwise panel density was extremely high. Trefftz-plane results for the two wing planforms do not show this sensitivity to panel density or angle of attack. Span efficiencies of 0.994 for the crescent wing and 0.987 for the elliptical wing were computed by the Trefftz-plane technique. Substitution of a force-free, rolled-up wake geometry on the crescent wing did not change the pressure-integrated drag significantly. The slightly higher span efficiency of the crescent wing is attributed to a more nearly elliptical spanwise lift distribution. The chord distribution of the elliptical wing was modified to produce an elliptical span-loading on a wing with an unswept quarter-chord line. This wing demonstrated a span efficiency equal to that of the crescent wing.

Vehicle Design of a Sharp CTV Concept Using a Virtual Flight Rapid Integration Test Environment

In an effort to improve the overall aerospace vehicle design process, a design environment that merges technologies from piloted simulations, computational fluid dynamics, wind-tunnel and flight test data is currently under development at NASA Ames Research Center. The specific objective of this project, entitled Virtual Flight Rapid Integration Test Environment, was to assess the role that piloted simulations can play in the conceptual design of advanced vehicles. As a result, a conceptual design study of a Crew Transfer Vehicle was undertaken to demonstrate this rapid turn-around process. This process included aerodynamic models generated from computational fluid dynamic methods, data validation from wind-tunnel testing, and a high fidelity pilot-inthe-loop motion-based flight simulation. These vehicles were designed using multi-point numerical optimization methods coupled with an Euler flow solver. A low-speed wind-tunnel test was conducted to validate the low-speed aerodynamics database. A piloted simulation experiment was conducted to evaluate the low-speed handling qualities of the various configurations in the approach and landing phase. Six astronaut pilots evaluated each of the configurations using Cooper-Harper ratings. The knowledge gained from the simulation data and pilot evaluations was quickly returned to the design team. From these findings, a new configuration was developed and cycled back through the simulation evaluation. This paper will summarize the design process of the Virtual Flight Rapid Integration Test Environment and discuss the results of the design study including the piloted simulation experiment. Introduction The Virtual Flight Rapid Integration Test Environment (RITE) project was initiated to develop an * Aerospace Engineer, Senior Member † Aerospace Engineer, Associate Fellow ‡ Research Scientist information technology process to rapidly and easily merge data from computational fluid dynamics (CFD), wind tunnel, and/or flight test into a real-time, piloted flight simulation. The process then cycles the knowledge gained from the simulation back into the design process. To accomplish this, a new engineering design environment was constructed that combined these various data generation methods and test environments within one infrastructure. The goal of this project was to develop such a design environment to improve current design methodologies and to reduce design cycle time. Current design environments do not allow data transfer and integration to take place easily between the different technologies during the preliminary design. By providing an infrastructure that brings together all these technologies, designers will be better equipped with higherfidelity tools and methods, including simulation studies, which will lead to higher-fidelity preliminary designs. The main advantage of conducting piloted simulation studies early is to identify problems and deficiencies in aerodynamic performance, vehicle stability and control, and guidance and navigation, which can be addressed in the preliminary design phase. Simulation studies also allow for the opportunity to develop preliminary control systems early in the developmental phase. Historically, the outer mold lines of a design are defined before simulation studies and control system development can begin which may lead to expensive and complex control systems and less than optimal vehicle performance. The Space Shuttle Orbiter, which was designed in the 1970’s, is an example of such developmental problems. More recent design studies have used simulation tools in the design phase but without an infrastructure and process in place to facilitate and expedite its use. 3 Presently, the Virtual Flight RITE project has demonstrated the rapid and seamless integration of CFD, flight, and wind-tunnel data into a simulation database. During an early phase of the project, the Space Shuttle Orbiter was selected as the baseline configuration for this re-design demonstration. The radius and length of the nose of the Orbiter were altered as the design parameters. This phase of the project led AIAA Atmospheric Flight Mechanics Conference and Exhibit 5-8 August 2002, Monterey, California AIAA 2002-4881 Copyright

Lift-enhancing surfaces on several advanced V/STOL fighter/attack aircraft concepts

An analysis of the relative influences of for-ward lift-enhancing surfaces on the overall lift and drag characteristics of three wind-tunnel models representative of V/STOL fighter/attack aircraft is presented. Two of the models are canard-wing configurations and one has a wing leading-edge extension (LEX) as the forward lifting surface. Data are taken from wind-tunnel tests of each model covering Mach numbers from 0.4 to 1.4. Overall lift and drag characteristics of these models and the generally favorable interactions of the forward surfaces with the wings are highlighted. Results indicate surface that larger LFXs and canards generally give greater lift and drag improvements than ones that are smaller relative to the wings.