Representation Of Aerodynamics Research Articles

Nonlinear aeroelastic modeling and comparative studies of three degree of freedom wing-based systems

The effects of structural and aerodynamic nonlinearities are investigated on an aeroelastic system with three degrees of freedom, those being plunge, pitch, and flap motions using two different aerodynamic load representations. Stall effects are introduced using quasi-steady and unsteady aerodynamic representations. The effects of the unsteadiness of the flow and stall effects on the type of Hopf bifurcation and the limit cycle oscillations of the system are explored. A comparative study between the unsteady and quasi-steady representations from a nonlinear perspective is also carried out. Results from the linear analysis show that for this specific aeroelastic system, its linear characteristics are dependent on the aerodynamic representation and the system’s design. Additionally, the quasi-steady approximation of the aerodynamic loads results in an inaccurate prediction of the linear flutter speed. Furthermore, the nonlinear analysis shows the existence of an instability shift from the supercritical to the subcritical type, depending on the value of the stall coefficient, as well as that of the cubic nonlinearities, and is more dominantly for the case when the quasi-steady representation is considered. Results show that, even for small reduced frequencies and small angles of attack scenarios, the unsteady representation of aerodynamic loads is critical to correctly predict the flutter speed, instability type, and system’s dynamics.

Use of Compliant Hinges to Tailor Flight Dynamics of Unmanned Aircraft

The nexus of advanced manufacturing methods, computer-aided design tools, and modern structural-analysis software has enabled the design and fabrication of structurally complex wing structures with unique features. This is particularly true for small unmanned aircraft, in which discrete structural hinges can easily be integrated into the overall vehicle design. This paper examines the use of discrete structural hinges for tailoring the low-frequency flight dynamics of the vehicle. For sufficiently soft discrete structural hinges, substantial coupling between flexible and rigid modes occurs, leading to the potential to modify the flight dynamic behavior through structural flexibility. Using a multibody flight dynamics simulation tool with a nonlinear lifting-line aerodynamic representation, different structural-hinge elastic properties, orientation, and location on the aircraft are examined. The results for a small unmanned aircraft indicate that flexibility mostly affects the longitudinal modes and associated handling qualities of the vehicle. Changes in the short-period and phugoid modes due to flexibility caused by a set of discrete structural hinges are often antagonistic. A structural-hinge stiffness above a certain critical value tends to improve short-period flying qualities; below this value, the short-period flying qualities degrade. In addition to the stiffness characteristics of the discrete structural hinge, orientation of the hinge has a significant effect on flight dynamics. Orienting the structural hinge to produce pitch–flap coupling can triple the short-period-mode damping, which could improve a nominally Level 3 rated aircraft to a Level 1 rated aircraft.

Aeroelastic behavior of long-span suspension bridges under arbitrary wind profiles

The aeroelastic behavior of suspension bridges under generic temporal and space wind distributions is studied employing a parametric one-dimensional structural model coupled with a strip-based indicial representation of the unsteady aerodynamic loads. The aeroelastic response to spatially nonuniform gusts is also investigated. The equations of motion linearized about the prestressed aerostatic equilibrium are expressed in terms of incremental kinematic variables. The aerodynamic loading characteristics of the deck cross sections of the Runyang Suspension Bridge (over the Yangtze river in China) are evaluated by means of computational fluid dynamics simulations performed to extract the aeroelastic derivatives from which the indicial aerodynamic representation is obtained. The aeroelastic equations of motion are then reduced to the state-space ordinary differential form by using the Faedo–Galerkin approach. The reduced-order bridge dynamics and the description of the aerodynamic loading via the added aerodynamic states are solved simultaneously by using a time-integration numerical scheme. Dependence of the flutter condition and the transient response to gusts is shown through systematic parametric studies.

Specialized System Identification for Parafoil and Payload Systems

There are a number of peculiar aspects to parafoil and payload systems thatmake it difficult to apply conventional system identification procedures used for aerospace systems. Parafoil and payload systems are unique because typically there is very little sensor information available, the sensors that are available are separated from the canopy by a complex network of flexible rigging, the systems are very sensitive to wind and turbulence, the systems exhibit a number of nonlinear behaviors, and the systems exhibit a high degree of variability from flight to flight. The current work describes a robust system identification procedure developed to address the specific difficulties posed by airdrop systems. By employing a two-phase approach that separately considers atmospheric winds estimation and aerodynamic coefficient estimation, a nonlinear, 6-degree-of-freedom dynamic simulation model is generated using only Global Positioning System data from the flight test. The key to this approach is the use of a simplified aerodynamic representation of the canopy, which requires identification of only the steady-state response to control input to completely define the dynamic model. The proposed procedure is demonstrated by creating a simulation model usingGlobal Positioning Systemdata fromactual flight tests. To validate the procedure, the dynamic response of the simulationmodel is then compared to inertial measurement unit data that were not used in any way to develop the simulation model, with excellent results.

A fluid structure coupling of the Ariane-5 nozzle section during start phase by detached eddy simulation

Concerning the requirements of future rocket technologies, providing a cost-efficient access to orbit as well as an increase in system reliability, a deeper insight into the unsteady phenomena during ascent of modern launchers is essential. Unsteady interactions and resonances of the turbulent separated launcher wake and the nozzle structure play an important role for the design of future main stage propulsion systems. The so-called buffeting coupling phenomenon is one of the main challenges during ascent. In the present study, a coupled simulation of the afterbody of the Ariane-5 launcher with a realistic structural and aerodynamic representation of different nozzle configurations is carried out. On the computational fluid dynamics side, unsteady detached eddy simulations are coupled with structural computations for different nozzle configurations. The essential features of the interaction process are well captured. The coupling algorithm is validated by a nonlinear supersonic panel flutter simulation with highly transient mechanical interaction.

A simple hydrologically based model of land surface water and energy fluxes for general circulation models

A generalization of the single soil layer variable infiltration capacity (VIC) land surface hydrological model previously implemented in the Geophysical Fluid Dynamics Laboratory general circulation model (GCM) is described. The new model is comprised of a two‐layer characterization of the soil column, and uses an aerodynamic representation of the latent and sensible heat fluxes at the land surface. The infiltration algorithm for the upper layer is essentially the same as for the single layer VIC model, while the lower layer drainage formulation is of the form previously implemented in the Max‐Planck‐Institut GCM. The model partitions the area of interest (e.g., grid cell) into multiple land surface cover types; for each land cover type the fraction of roots in the upper and lower zone is specified. Evapotranspiration consists of three components: canopy evaporation, evaporation from bare soils, and transpiration, which is represented using a canopy and architectural resistance formulation. Once the latent heat flux has been computed, the surface energy balance is iterated to solve for the land surface temperature at each time step. The model was tested using long‐term hydrologic and climatological data for Kings Creek, Kansas to estimate and validate the hydrological parameters, and surface flux data from three First International Satellite Land Surface Climatology Project Field Experiment intensive field campaigns in the summer‐fall of 1987 to validate the surface energy fluxes.

Transonic aeroelasticity analysis using state-space unsteady aerodynamic modeling

An aeroelastic analysis is conducted on a two-degree-of-freedom airfoil in transonic flow using a generalized state-space approximation for the unsteady aerodynamics. The aerodynamic representation is validated against computational fluid dynamic solutions for angle of attack oscillations up to Mach numbers of 0.875 and at reduced frequencies up to 1.0. Despite the inherent nonlinear nature of transonic flow, it is shown that a linear finite-state model with as few as eight states can provide a good approximation to the unsteady lift and moment behavior if appropriate allowance is made for Mach number effects on the airfoil's static lift curve slope and mean aerodynamic center. It is shown how the aerodynamic representation can be coupled to the structural equations of a typical airfoil section with bending and torsional degrees of freedom. The stability of the resulting aeroelastic system is determined by eigenanalysis. This aeroelastic analysis is shown to be in excellent agreement with calculations performed using more sophisticated unsteady aerodynamic theories.

Static aeroelastic characteristics of circulation control wings

Circulation control airfoils can develop lift coefficients far in excess of conventional airfoils through the use of tangential blowing and thus have potential applications for V/STOL aircraft. The static aeroelastic effects of circulation control on swept wings are examined using two analytical models: a simple two-degree-of-freedom model with linearized aerodynamics and an elastic beam model coupled with nonlinear two-dimensional airfoil data. The static divergence instability and a circulation control reversal phenomenon are investigated through the use of lift and control effectiveness ratios. Effects of wing sweep angle, elastic axis location, blowing level, and spanwise blowing distribution are presented. Linear, nonlinear incompressible, and nonlinear compressible aerodynamic representations are compared. Significant differences were observed between the linear and nonlinear aerodynamic results. Tailoring of the spanwise blowing distribution is shown to be an effective means of improving undesirable aeroelastic characteristics. The results indicate that the aeroelastic behavior of circulation control wings can be quite different from that of conventional wings.

Flutter Analysis of Missile Control Surfaces Containing Structural Nonlinearities

Missile control surfaces often contain nonlinearities which affect their performance characteristics and flutter boundaries. Analysis techniques accounting for these nonlinearities and an understanding of their potential influence on the flutter mechanism greatly increase the efficiency of the control surface design process. This paper presents such analysis procedures and discusses their application to the investigation of the dynamics of missile control surfaces containing structural freeplay-type nonlinearities. The problem discussed deals with a missile control surface exposed to subsonic flow. Nonlinearities are associated with freeplay in the root support stiffness. Definition of the loads acting on the control surface uses a simplified aerodynamic representation. The basic assumption of this approach is that the lift force is proportional to and in phase with the torsional, or pitch, motion. Both rigid and flexible control surface configurations have been examined with nonlinearities in either one or both of the root pitch or roll degrees-of-freedom.

Air-to-Air Combat Tactics Synthesis and Analysis Program Based on an Adaptive Maneuvering Logic

Abstract Two digital computer programs synthesizing optimal maneuvers in one-on-one air-to-air combat situations are described. The method develops intelligently interactive maneuvers without relying on human pilot experience. One program drives one of the interacting aircraft, thus replacing one of the human pilots on the NASA Langley Research Center's Differential Maneuvering Simulator, this in real time. The other program operates in a normal batch processing mode. Both programs use the same technique which maps the physical situation of the two aircraft into a quantized, abstract situation space. The outcome in this situation space is predicted for several trial maneuvers, a value is associated with the outcome of each trial maneuver, and finally, the maneuver with the highest predicted value is executed. These programs, operating with six degrees of freedom and realistic aerodynamic representation for both aircraft, provide a means for objective evaluation of weapons systems and pilot performance.

A Comparison of Methods for the Analysis of Wing-Tail Interaction Flutter

Results of an analytical investigation of the aeroelastic stability of variable sweep aircraft, specifically with respect to wing-tail interaction flutter, are presented. Three aerodynamic representations are employed: 1) modified two-dimensional strip theory containing no wingtail aerodynamic interaction, 2) vortex lattice theory containing aerodynamic interaction of wing on tail only, and 3) subsonic kernel function theory containing a complete evaluation of wing-tail aerodynamic interaction. The capability of the interaction methods is established by an application to an experimental flutter model where wing-tail aerodynamic interaction is known to be of importance. The aerodynamic methods are then applied to a particular high performance variable sweep aircraft. The basic flutter mechanisms for this hypothetical aircraft are generated by wing-fuselage mechanical interaction and are predicted by wing aerodynamics alone. Component aerodynamics on wing and tail without interaction do not predict the mechanism. Aerodynamic interaction causes the reappearance of flutter at a velocity 30% lower than generated by the wing alone.