Jerald M. Vogel

Modeling and Control of Scramjet-Powered Hypersonic Vehicles: Challenges, Trends, & Tradeoffs

In this paper, we provide an overview of scramjet-powered hypersonic vehicle modeling and control challenges. Such vehicles are characterized by unstable non-minimum phase dynamics with significant coupling and low thrust or FER (normalized fuel equivalency ratio) margins. Recent trends in hypersonic vehicle research is summarized. To illustrate control system design issues and tradeoffs, a generic nonlinear 3DOF longitudinal dynamics model capturing aero-elastic-propulsive interactions for wedge-shaped vehicle is used. The model is analyzed over a broad range of hypersonic flight conditions (i.e. operating points). The paper highlights how vehicle level-flight static (trim) and dynamic properties change over the trimmable air-breathing corridor (∼ Mach 4.75-12.6, 70-115 kft). Particular attention is paid to thermal choking constraints imposed on control system design as a direct consequence of having a finite FER margin. The dependence of FER margin on altitude, Mach, and the bow flow turning angle is discussed. The latter depends on Mach, altitude, angle-of-attack (AOA), and forebody flexing. It is (briefly) discussed how FER margin can be estimated on the basis of Mach, altitude, and AOA if a flexing upper bound is assumed. The implication of this state-dependent nonlinear FER margin constraint as well as that of the right half plane (RHP) zero, associated with the elevator-flight path angle (FPA) map, on control system bandwidth (BW) and FPA tracking are discussed. It is argued that while the non-minimum phase zero limits the achievable closed loop FPA BW, FER coupling into FPA can be used to address this. This, however, is limited by FER margin limits and may impose constraints on the size of the FPA (and velocity) commands that can be followed. This is particularly important because the vehicle is inherently unstable which implies a closed loop system (with a finite downward gain margin) that can become destabilized if driven sufficiently deep into control saturation. A consequence of this is that designers must take note of the fact that FPA commands which are sufficiently large and/or rapid may be impossible to follow with the desired level of fidelity. This is quantified within the paper. Speed command following issues are also discussed.

Constraint enforcement for scramjet-powered hypersonic vehicles with significant aero-elastic-propulsion interactions

In this paper, we examine the control of a scramjet-powered hypersonic vehicle with significant aero-elastic-propulsion interactions. Such vehicles are characterized by open loop unstable non-minimum phase dynamics, low frequency aero-elastic modes, significant coupling, and hard constraints (e.g. control surface deflection limits, thrust margin). Within this paper, attention is placed on maintaining acceptable closed loop performance (i.e. tracking of speed and flight path angle commands) while satisfying hard control surface deflection constraints as well as stoichiometrically normalized fuel-equivalency-ratio (FER) margin constraints. Control surface constraints are a consequence of maximum permissible aerodynamic loading. FER margin constraints are a consequence of thermal choking (i.e. unity combustor exit Mach number) and the fact that thrust loss may not be captured for FER greater than unity. Such limits are particularly important since the vehicle is open loop unstable and “saturation” can result in instability. To address these issues, one can design conservative (i.e. less aggressive or lower bandwidth) controllers that maintain operation below saturation levels for anticipated reference commands (and disturbances). Doing so, however, unnecessarily sacrifices performance - particularly when small reference commands are issued. Within this paper, the above issues are addressed using generalized predictive control (GPC). A 3DOF longitudinal model for a generic hypersonic vehicle, which includes aero-elastic-propulsion interactions, is used to illustrate the ideas.

Control-relevant modeling, analysis, and design for scramjet-powered hypersonic vehicles

Within this paper, control-relevant vehicle design concepts are examined using a widely used 3 DOF (plus flexibility) nonlinear model for the longitudinal dynamics of a generic carrot-shaped scramjet powered hypersonic vehicle. Trade studies associated with vehicle/engine parameters are examined. The impact of parameters on control-relevant static properties (e.g. level-flight trimmable region, trim controls, AOA, thrust margin) and dynamic properties (e.g. instability and right half plane zero associated with flight path angle) are examined. Specific parameters considered include: inlet height, diffuser area ratio, lower forebody compression ramp inclination angle, engine location, center of gravity, and mass. Vehicle optimizations is also examined. Both static and dynamic considerations are addressed. The gap-metric optimized vehicle is obtained to illustrate how this controlcentric concept can be used to “reduce” scheduling requirements for the final control system. A classic inner-outer loop control architecture and methodology is used to shed light on how specific vehicle/engine design parameter selections impact control system design. In short, the work represents an important first step toward revealing fundamental tradeoffs and systematically treating control-relevant vehicle design.

Effects of bend-twist coupling on composite propeller performance

An investigation of a conventional propeller, made from composite materials, was conducted in which propeller characteristics were studied under quasi-static aerodynamic loading. Emphasis was placed on understanding the effects of bend-twist coupling of composite laminates on propeller performance. The classical blade element theory of propellers was used to calculate propeller characteristics and the aerodynamic force distribution acting on the propeller. The finite-element method was used to calculate the resulting deformation of the propeller blades. A simple algorithm was used in which propeller blade deformation and resulting changes in loading conditions were calculated repeatedly until equilibrium between deformation and loading was achieved. Results show that improvements in propeller performance are possible using the bend-twist coupling of composite laminates. Additionally, this study shows that the numerical approach developed by the authors is well suited for analyzing the performance characte...An investigation of a conventional propeller, made from composite materials, was conducted in which propeller characteristics were studied under quasi-static aerodynamic loading. Emphasis was placed on understanding the effects of bend-twist coupling of composite laminates on propeller performance. The classical blade element theory of propellers was used to calculate propeller characteristics and the aerodynamic force distribution acting on the propeller. The finite-element method was used to calculate the resulting deformation of the propeller blades. A simple algorithm was used in which propeller blade deformation and resulting changes in loading conditions were calculated repeatedly until equilibrium between deformation and loading was achieved. Results show that improvements in propeller performance are possible using the bend-twist coupling of composite laminates. Additionally, this study shows that the numerical approach developed by the authors is well suited for analyzing the performance characteristics of composite propellers.

Control-relevant modeling of hypersonic vehicles

Scramjet powered hypersonic vehicles represent the next critical step toward achieving NASAs vision for Highly Reliable Reusable Launch Systems (HRRLS), affordable space access, planetary re-entry systems, and global reach vehicles. The design of such vehicles is a very interdisciplinary and highly complex problem. As such, the development of varying fidelity mathematical models for assessing overall stability and performance during the design process is very important. In particular, developing “low-order” models with “sufficient fidelity” to capture control-centric phenomena becomes vital in early stages of vehicle design. Historically, the early stage vehicle design process never incorporated control related considerations. The design obtained by such practice is not optimal and can often lead to poor design from a stability and performance view point. This paper presents control-relevant modeling efforts which will facilitate quick iterative control analysis and design during early stages of vehicle design. The paper is intended to be of an introductory nature and presents the high level modeling framework and associated challenges. An example linear 6 DOF model with some representative analysis is also given to demonstrate the applicability of the tool suite.

Modeling and Analysis Framework for Early Stage Trade-off Studies for Scramjet-Powered Hypersonic Vehicles

NASA’s Hypersonic Project aims at development of tools and methods for designing the next generation of space access vehicles which are envisioned to be scramjet-powered hypersonic vehicles. Design of such vehicles is a very challenging problem. Dynamic models of these vehicles tend to be highly complex and multi-disciplinary due to strong interaction between aerodynamics, propulsion, structure, and controls. Historically, the vehicle design process never incorporated control-centric considerations in the early stages of vehicle design. The control considerations came after the aero, propulsion, and structural design. The design obtained by such practice is not optimal and can often tend to be inadequate from a stability and performance view point. As a result, developing ”low-order” models with ”sufficient fidelity” to capture mission-critical control-centric phenomena becomes vital in early stages of vehicle design. The current modeling methodologies lie at the two extreme ends of the spectrum in terms of level of fidelity in the models. Most of the archival papers use a simple wedge-shaped planar three degrees-of-freedom (3 DOF) configuration for developing early-stage engineering models for analysis. The design decisions based on these models tend to be far too unreliable. On the other hand, CFD- and FEM-based high order models and related analysis tends to be extremely time consuming and inefficient in early stages of the design. This paper describes the modeling and analysis toolsuite that is being developed to fill in this gap in the modeling capability. The paper describes the overall first-principle based modeling framework and associated modules. The utility of the

Control Centric Parametric Trade Studies for Scramjet-Powered Hypersonic Vehicles

This paper presents an overview of a modeling and analysis tool suite for supporting early stage design of scramjet powered hypersonic vehicles. This efiort is a part and parcel of NASA’s Hypersonic project aimed at development of tools and methods for designing the next generation of space access vehicles. The tool suite, known as Aerospace Vehicles Simulation and Analysis Program for Hypersonics (ASAP-Hypersonics), is designed to provide rapid modeling and analysis capability from a GNC view point. The primary focus of ASAP is to provide the capability to generate GNC-centric constraints for use in early stage vehicle design studies. The modeling and analysis capabilities of ASAP-Hypersonics are demonstrated using some example trade studies. ASAP-Hypersonics can be used in various modes of operation including modeling, dynamic analysis, control analysis, parametric trade studies, and vehicle optimization.

Aircraft Control Augmentation and Health Monitoring using Flush Air Data System Feedback

This paper describes the development of an innovative control architecture that integrates Flush-air data system with Reconfigurable Control to yield an intelligent prognostic on-line control augmentation and Health and Efficiency Monitoring (FaRCHEM) system. A proof-of-concept FaRCHEM system was developed using a Fouga (CM 170) aircraft as the test platform. The functionality of FaRCHEM system architecture was demonstrated through various simulations examples of different flight scenarios. In particular, the proof-of-concept study demonstrated the capability of the FaRCHEM system by simulating some example aircraft health degradation and/or failure situations and showing how the FaRCHEM system offers graceful recovery through pro-active corrective control actions. The proposed control architecture uses prognostic sensory feedback in the form of instantaneous pressure levels obtained from Flush Air Data System (FADS) in reconfigurable feedback control configuration to provide stability augmentation and/or performance enhancement. The instantaneous pressure signature of the aircraft is effectively used to assess the health of the aircraft. The FaRCHEM system provides valuable information to pilot about imminent anomalies in flight conditions that may lead to potential failures. This pilot alert system will enable pilot to take corrective action to avoid potential failure. The reconfigurable control system uses its control re-allocation capability to redistribute control signal in the event of any actuator/sensor failures. I. Introduction

Modeling and analysis of rotational freeplay nonlinearity of a 2D airfoil

Aeroelastic flutter is a dynamic instability of fluid-structural system in which the structure exhibits a sustained, often diverging oscillation. Flutter behavior is self-feeding and destructive. Nonlinearities such as freeplay in rigid-body rotational stiffness of the structural system can have an effect on the onset of flutter and its amplitude. In particular there is experimental evidence that as the amount of freeplay increases, the freestream velocity at which the flutter instability occurs decreases. In this paper, we develop a modeling framework that allows us to predict this dependence of flutter velocity on the freeplay parameter. We model the airfoil system with freeplay nonlinearity as a feedback interconnection of linear system and sector bounded nonlinearity. Freeplay in stiffness is practically approximated as a hyperbola nonlinearity. Eigenvalue analysis at equilibrium points is used to predict onset of flutter and characterize a Hopf bifurcation of the system from stable to limit cycle behavior. Spectral analysis us used to characterize the limit cycle behavior. This analysis indicates the flutter onset velocity to be a function of freeplay region length. Follow-on research correlating recently obtained wind tunnel results to a three-dimensional extension of the model is outlined.

Modeling and characterization of the impact of control surface free-play on flutter for an all moving surface

Dynamics of flutter is an important consideration in the design of aircraft structures. Flutter is an unstable self-excitation of the structure due to an undesirable coupling of structural elasticity and aerodynamics. Flutter is very difficult to predict and its occurrence can lead to catastrophic structural failure. The dynamics of flutter are affected by several factors including nonlinearities in structural stiffness, damping, and free-play in control surfaces. The free-play nonlinearity in control surfaces mechanisms is similar to the backlash in gears. Such nonlinearity introduces persistent limit cycle oscillations (LCOs) and significantly affects the onset of flutter. The impact of free-play on the flutter speed and frequency is not fully understood and is an active area of research. Historically, very conservative estimates have been used for the allowable free-play. The current military specification limit for free-play is based on the wind tunnel tests performed in 1950s at the Wright Air Development Center (WADC). The key contribution of this paper lies in gaining deeper understanding of free-play dynamics to enable more accurate modeling of free-play and predict its impact on flutter speed and frequency. The proposed modeling methodology is validated via close agreement of the simulation with WADC test data. Energy-based novel approach is presented for life cycle assessment and to predict flutter instability.