Aircraft Agility Research Articles

Thrust Vectoring Applied to Catastrophic Failure Prevention in Jet Transports

Thrust vectoring control (TVC) is generated by rapid deflections of the jetengine exhaust stream to create flight control moments for maximizing fighter aircraft agility, for flight safety of transport jets, and other applications. Here, a variable structure TVC for catastrophic failure prevention is developed. It is complicated by the fact that the six-degree-of-freedom aircraft model nonlinear in the states is, by the addition of TVC, also nonlinear in the controls. TVC is shown to save the aircraft in three cases of catastrophic failure of the conventional aerodynamic flight controls. The failures include two at cruise and one at take-off. The resulting TVC robustly tracks a desired smooth output trajectory back to safe trimmed cruise. The simulation took into account realistic magnitudes, rates and time lags for the TVC add-on patented kits (Gal-Or, 1998). By adding TVC kits to jet transports, at a relatively low cost, many crashes can be avoided.

Analysis of Aircraft Agility on Maximum Performance Maneuvers

We present an analysis of aircraft agility as an inverse simulation problem. The analytical framework is based on the differential geometry of the trajectory and on the complete set of motion equations for a rigid airframe. A local optimization method is adopted and the solution is obtained in terms of the trajectory properties subjected to assigned constraints. Three applications are presented, namely a rapid deceleration maneuver, a maximum performance turn, and a reverse turn. The results show that the method is flexible enough to deal with the maximization of the agility components, for some assigned characteristics of the trajectory, and to determine the control actions for maneuvers that are usually proposed for evaluating the aircraft agility

Minimum-Time Maneuvers of Thrust-Vectored Aircraft

The new aircraft technology of thrust vectoring (TV) enables the execution of new types of poststall maneuvers. Aircraft agility and e ight safety are greatly enhanced. The objective and main contribution of the present work are the suboptimal solution of minimum-time pitch-reversal (there and back ) and yaw-reversal standard agility comparison maneuvers (SACOM). It is demonstrated that, due to asymmetric and coupling moments at high ® and high ¯, pure pitch and yaw maneuvers are impossible; a full six-degree-of-freedom model must be employed. A novel parametric two-stage suboptimization algorithm was developed to copewith the complexity ofthe optimal control problem. All four aerodynamic and three TV controls are simultaneously active in the execution of a low g load, nearly pure 80-deg pitch-reversal and a 30-deg yaw-reversal SACOM of an F-15B aircraft.

The X-31 aircraft: Advances in aircraft agility and performance

The X-31 enhanced fighter maneuverability (EFM) demonstrator has pioneered agile flight in the post-stall flight regime and explored integrated multi-axis thrust vectoring across a broad flight envelope. Its maneuvering achievements include sustained flight up to 70 degrees angle of attack, velocity vector rolls in deep post-stall conditions, and post-stall turns from high entry to exit speeds with ultra low turning/transitional conditions. The concept of post-stall maneuverability was extensively studied in simulations preceding initiation of the X-31 program. These simulations provided a baseline for tactical utility demonstrations and vehicle design requirements. Post-stall maneuverability was not achieved without encountering and mitigating the effects of highly unsteady, asymmetric, vortex-dominated flow-fields associated with post-stall flight. Anomalies in vehicle response to control inputs were observed at high angles of attack, as were differences between simulator and actual flight parameters due to a misrepresentation of the effects of these complex flowfields. Some preliminary force and moment data for the X-31 configuration during dynamic maneuvers are provided to highlight the complex nature of the flowfield. The X-31 aircraft's enabling capabilities, including multi-axis thrust vectoring and integrated flight/propulsion control also provided performance enhancements across the entire flight envelope. In what were known as ‘quasi-tailless’ experiments, conventional aerodynamic control surfaces were used to reduce or eliminate the stabilizing influence of the vertical stabilizer, while the vehicle's multi-axis thrust vectoring capability was used for restabilization. Properly exploited, these technologies can lead to the reduction or elimination of traditional aerodynamic control surfaces, which provides profound improvements in vehicle range, weight, payload, and low observability. This review focuses on some of the principal aerodynamic issues encountered during the course of the X-31 program, the resulting lessons learned, and the derived results concerning the complex aerodynamic effects associated with exploiting this aircraft's unique capabilities. Suggested research and technology development issues which came to light during the X-31 program are also discussed.

Real-time prediction of unsteady aerodynamics: Application for aircraft control and manoeuvrability enhancement

The capability to control unsteady separated flow fields could dramatically enhance aircraft agility. To enable control, however, real-time prediction of these flow fields over a broad parameter range must be realized. The present work describes real-time predictions of three-dimensional unsteady separated flow fields and aerodynamic coefficients using neural networks. Unsteady surface-pressure readings were obtained from an airfoil pitched at a constant rate through the static stall angle. All data sets were comprised of 15 simultaneously acquired pressure records and one pitch angle record. Five such records and the associated pitch angle histories were used to train the neural network using a time-series algorithm. Post-training, the input to the network was the pitch angle (alpha), the angular velocity (dalpha/dt), and the initial 15 recorded surface pressures at time (t (0)). Subsequently, the time (t+Deltat) network predictions, for each of the surface pressures, were fed back as the input to the network throughout the pitch history. The results indicated that the neural network accurately predicted the unsteady separated flow fields as well as the aerodynamic coefficients to within 5% of the experimental data. Consistent results were obtained both for the training set as well as for generalization to both other constant pitch rates and to sinusoidal pitch motions. The results clearly indicated that the neural-network model could predict the unsteady surface-pressure distributions and aerodynamic coefficients based solely on angle of attack information. The capability for real-time prediction of both unsteady separated flow fields and aerodynamic coefficients across a wide range of parameters in turn provides a critical step towards the development of control systems targeted at exploiting unsteady aerodynamics for aircraft manoeuvrability enhancement.

Comparing aircraft agility using Mahalanobis distances

neering, edited by M. Gad-el-Hak, Vol. 46, Springer-Verlag, Berlin, 1989, pp. 365-428. Brandon, J. M., and Shah, G. H., Unsteady Aerodynamic Characteristics of a Fighter Model Undergoing Large Amplitude Pitching Motions at High Angles of Attack, AIAA Paper 90-0309, Jan. 1990. Del Frate, J. H., and Zuniga, F. A., In-Flight Field Analysis on the NASA F-18 High Alpha Research Vehicle with Comparisons to Ground Facility Data, AIAA Paper 90-0231, Jan. 1990; see also NASA TM 4193, May 1990. Hebbar, S. K., Platzer, M. F., and Cavazos, O. V., Pitch Rate/ Sideslip Effects on Leading-Edge Extension Vortices of an F/A-18 Aircraft Model, Journal of Aircraft, Vol. 29, No. 4, 1992, pp. 720723; see also AIAA Paper 91-0280, Jan. 1991. Hummel, D., and Redeker, G., Ueber den Einfluss des Aufplatzens der Wirbel auf die aerodynamische Beiwerte von Deltafluegeln mit klinen Seitenverhaeltnis beim Scheibeflug, Jahrbuch der W.G.L.R., 1967, pp. 232-240. Verhaagen, N. G., and Naarding, S. H. J., and Numerical Investigation of over a Sideslipping Delta Wing, Journal of Aircraft, Vol. 26, No. 11, 1989, pp. 971-978. Grismer, D., Nelson, R., and Ely, W., An Experimental Study of Double Delta Wings in Sideslip, AIAA Paper 91-3308, Sept. 1991. O'Leary, C. O., and Weir, B., The Effect of the Foreplanes on the and Dynamic Characteristics of a Combat Aircraft Model, AGARD-CP-465, Oct. 1989. Hebbar, S. K., Platzer, M. F., and Kwon, H. M., Static and Dynamic Tunnel Visualization Studies of a Canard-Configured X-31A-Like Fighter Aircraft Model, AIAA Paper 91-1629, June 1991. Hebbar, S. K., Platzer, M. F., and Kwon, H. M., Vortex Breakdown Studies of a Canard-Configured X-31A-Like Fighter Aircraft Model, Journal of Aircraft, Vol. 30, No. 3, 1993, pp. 405-408. Kim, C. H., Flow Visualization Studies of a Sideslipping, Canard-Configured X-31A-Like Fighter Aircraft Model, M.S. Thesis, Naval Postgraduate School, Monterey, CA, Dec. 1991. Hebbar, S. K., Platzer, M. F., and Kirn, C. H., Water Tunnel Visualization of Dynamic Effects During Sideslipping of a CanardConfigured Fighter Model, Proceedings of the 5th Asian Congress of Fluid Mechanics (Taejon, Korea), Vol. 2, 1992, pp. 1161-1164.

Fighter agility metrics, research and test

This paper presents the results of an analysis of proposed metrics to assess fighter aircraft agility. A novel framework for classifying these metrics is developed and applied. A set of transient metrics intended to quantify the axial and pitch agility of fighter aircraft is evaluated with a high fidelity, nonlinear generic simulation of the F-18 Hornet. Test techniques and data reduction methods are proposed, and sensitivities to pilot introduced errors during flight testing is investigated. Results indicate that the power onset and power loss parameters are promising candidates for quantifying axial agility, while maximum pitch-up and pitch-down rates are for quantifying pitch agility.

Qualitative and quantitative comparison of government and industry agility metrics

Government and industry have proposed various agility metrics in an effort to define and measure aircraft agility. Each of the agility metrics proposed measure different quantities related to aircraft performance and each, if designed to, would increase aircraft performance. The intent of this paper is not to choose the best or most correct agility metric but to compare their similarities and differences qualitatively and quantitatively compare their sensitivity to variations in agility-related design parameters.