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Research has been conducted in recent years to determine the dynamic behavior of aircraft in unusual flight attitudes, particularly at very high angle-of-attack or post-stall conditions. The possibility that future advanced fighter aircraft will have the ability to perform controlled maneuvers at such attitudes is indicated by the current military aircraft flying qualities specification, MIL-STD-1797. As it becomes more important to understand the dynamics of aircraft at such flight conditions, the need for a meaningful and useful assessment of aircraft departure resistance in varying attitudes will increase proportionally. This thesis surveys some of the measures of departure susceptibility currently in use and examines a candidate for a new departure resistance criterion which offers distinct advantages over the traditional metrics. The new departure resistance criterion, called DP_SSV is essentially a measure of how much uncertainty the nominally stable plant can tolerate before being driven unstable. DP_SSV is calculated using structured singular values. In this thesis, DP_SSV is calculated over various flight conditions for a typical high-performance fighter aircraft which is represented by a full six degree of freedom, nonlinear simulation. The results are compared with those obtained by using a traditional departure susceptibility metric and by examining the eigenvalues of linearized forms of the aircraft model. The new criterion DP_SSV is shown to provide more information about the departure susceptibility of an aircraft than C_{ηβ_DYN} traditional metric, and to produce results in good agreement with the eigenvalue analysis of the stability of the aircraft for the conditions studied. The interpretation of DP_SSV is discussed, and suggestions for future investigation are also presented. / Master of Science

In this thesis, the Leishman-Beddoes method of determining airloads for an airfoil undergoing dynamic stall is studied over a range of Mach numbers. To validate the method for conditions where little experimental data is available, a computational fluid dynamics solver is utilized to provide airload predictions for comparison to the Leishman-Beddoes results. It is found that even for high Mach numbers the Leishman-Beddoes method provides reliable predictions for lift coefficient. However, at the higher Mach numbers pitching moment is sometimes overpredicted at high angle of attack. This is seemingly due to an inability to accurately determine the center of pressure in the high speed unsteady flow environment.

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Thesis (M.S.)--West Virginia University, 2005. / Title from document title page. Document formatted into pages; contains xiv, 86 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 73-76).

An engineering model designed for the analysis of high angle-of-attack flight characteristics is developed and applied to the problem of aircraft poststall departure. The model consists of an aerodynamics package used interactively with a six-degree-of-freedom flight simulator. The aerodynamics are computed via a nonlinear lifting line theory with unsteady wake effects due to a discrete, nonplanar vortex system. A fully configured aircraft (main wing, horizontal tail and vertical fin) is mathematically constructed by modeling all lifting surfaces with bound, discrete vortex segments and associated control points; vehicle geometric influence on high angle-of-attack flight characteristics is included through complete variability in the relative locations, orientations and sizes of the flight surfaces. This aircraft model is “flown” through prescribed maneuvers by integrating the equations of motion. Selected results of trajectory simulations presented for a typical general aviation aircraft provide the following insights to wing-drop departure subsequent to stall. The abruptness of poststall roll-off depends on the presence of flight asymmetries at the stall break and the rate of stall penetration. Such out-of-trim flight conditions induce asymmetric wing panel unstall subsequent to deep stall penetration resulting in large wing-drop-producing roll moments. However, the abrupt departure from symmetric flight conditions is also found to be mathematically possible. This is a consequence of multiple lifting line solutions which exist for bound vortex systems assigned the lift properties of airfoils having stall discontinuities. The dynamic model is well suited to the prediction of departure resistance benefits realized through passive aerodynamic modifications, for example, drooped leading edge outboard wing panels. The model can also be applied to the generation of dynamic stability derivatives by analytically simulating forced oscillation test procedures. / Ph. D.

The ability of twelve distortion indices to predict stalling behavior in aircraft gas turbine engines was investigated using J85-GE-13 turbojet engine data, TF30-P-3 turbofan engine data, and modified T64-GE-6B compressor test-rig data. The indices were tested for correlation capability with constant speed loss in stall pressure ratio, constant mass loss in stall pressure ratio, and engine speed where appropriate. Predictive indices/models were compared directly with experimental data. In addition, the concept of including the effects of compressor dynamic response by modifying the inlet total pressure profile rather than the index was investigated. This was done by evaluating the accuracy of parallel compressor theory and two simple AP/P indices first using measured inlet total pressure data and then using modified or "effective" inlet total pressure profiles. A procedure was developed for deriving the effective inlet total pressure distribution from the measured distribution. / Master of Science

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A one-dimensional, stage-by-stage axial compression system mathematical model has been constructed which can describe system behavior during post-stall events such as surge and rotating stall. The model uses a numerical technique to solve the nonlinear conservation equations of mass, momentum, and energy. Inputs for blade forces and shaft work are provided by a set of quasi-steady stage characteristics modified by a first order lagging equation to simulate dynamic stage characteristics. The model was validated with experimental results for a three-stage, low-speed compressor and a nine-stage, high-pressure compressor. Using these models, a parametric study was conducted to determine the effect of inlet resistance, combustor performance, heat transfer, and stage characteristic changes due to hardware modification on post—stall system behavior. / Ph. D. / incomplete_metadata

Unsteady flow separation represents a highly complex and important area of study within fluid mechanics. The extent of separation and specific time scales over which it occurs are not fully understood and has significant consequences in numerous industrial applications such as helicopters, jet engines, hydroelectric turbines and wind turbines. A direct consequence of unsteady separation is the erratic movement of the separation point which causes highly dynamic and unpredictable loads on an airfoil. Current computational models underestimate the aerodynamic loads due to the inaccurate prediction of the emergence and severity of unsteady flow separation especially in response to a sudden change in the effective angle of attack. To capture the complex flow phenomena over wind turbine blades during stall development, a scaled three-dimensional non-rotating blade model is designed to be dynamically similar to a rotating full-scale NREL 5MW wind turbine blade. A time-resolved particle image velocimetry (PIV) investigation of flow behavior during the stall cycle examines the processes of stall development and flow reattachment. The flow fields are examined through the application of Eulerian techniques such as proper orthogonal decomposition and empirical mode decomposition to capture unsteady separation characteristics within the flow field. Then, for a higher order description, coherent structures such as vortices and material lines are resolved to fully characterize the flow during a full pitching cycle in a Lagrangian framework. The Eulerian and Lagrangian methods described in the present analysis is extended to investigate the spanwise characteristics within the root section of a three dimensional airfoil. Furthermore, statistical information of the separation point is pursued along four spanwise positions during two cases of unsteady separation. The results of the study describe a critical role of surface vorticity accumulation in unsteady separation and reattachment. Evaluation of the unsteady characteristics of the shear layer reveal evidence that the build-up and shedding of surface vorticity directly influence the dynamic changes in separation point. The quantitative characterization of surface vorticity and shear layer stability enables improved aerodynamic design, but also has broader implications on the larger discipline of unsteady fluid dynamics.