Ghter Aircraft Research Articles

Wing Rock Suppression Using Recessed Angle Spanwise Blowing

Introduction O NE of the limitationsto combat effectivenessof Ž ghter aircraft is wing rock—a high angle-of-attack dynamic motion manifested primarily in a limit-cycle roll oscillationsphenomenon. It is an aerodynamicphenomenonthat causeslossof stabilityand control in the lateral/directional mode. The leading-edge vortices emanating from the high-performance Ž ghter aircraft are one of the main causes of wing rock at high angles of attack. An effective means of suppressing wing rock is through the direct control and manipulation of leading-edge vortices at these  ight regimes. The technique of “blowing”2i4 has been of greatest interest because it is found to be highly effective in controlling the vortex orientation. This Note involves the experimental implementation of one such blowing technique—recessed angle spanwise blowing (RASB), using a simple rule base controller to a slender delta wing model in a lowturbulence wind tunnel. Through interpretation of the observations and results obtained from the experiments,an analysis is conducted to assess the effectiveness of RASB in suppressing wing rock. The results are substantiated by the  ow visualization experiments and force balance experiments. The excellent correlationsof the results and observations demonstrate the effectiveness of the RASB technique in its stipulated role of wing rock suppression.

Using a Multiple-Model Adaptive Estimator in a Random Evasion Missile/Aircraft Encounter

The terminal phase (end game)of an encounter between an air-to-air missileequipped with an activemonopulse radar seeker and an evading e ghter aircraft, possibly employing electronic countermeasures in the form of electronic jinking, is addressed. The missile uses a guidance law derived from linear differential game theory, which is implemented by using a multiple-model adaptiveestimator (MMAE). TheMMAE identie es theevasion strategy of theaircraft, which consists of thecombination of evasion maneuverand electronic jinking. An extensivenumerical study is used to demonstrate the viability of the concept. In comparison with a previously proposed mixed strategy guidance methodology, the new MMAE-based approach leads to a substantial improvement in the guaranteed single-shot kill probability for the missile. ODERN air-to-air missiles, designed to intercept highly maneuverable aircraft equipped with electronic countermeasures (ECM), have to operate in a highly uncertain environment. This paper is concerned with the terminal phase (end game) of an encounter between a missile equipped with an active monopulse radar seeker and an evading e ghter aircraft having the option to use ECM. The terminal homing phase of the interception starts when the active seeker of the missile locks on its target, generating also a warning signal (including some threat identie cation ) in the aircraft. This warning is the only information the pilot has. At the moment when the warning is received, the pilot starts to execute a sequence ofperiodicalevasivemaneuvers.Simultaneously,toenhanceitssur

Flight Testing of Reconfigurable Control Law on the X-36 Tailless Aircraft

Piloted simulation and e ight-test results from the evaluation of a recone gurable e ight control law for the X-36 tailless e ghter aircraft are presented. The recone gurable control law, which is based on dynamic inversion in an explicitmodelfollowing framework, employs an onlineneural network to adaptively regulatetheerror in the plant inversion, which may be due to modeling uncertainties, failures, or damage. Simulated actuator failures, which causedeffectorstobelockedatprescribed positions,wereinjectedtoevaluatethestability andhandling qualitiesof therecone gurablecontrol law underfailures. Thecontrol law was not given any knowledgeof the actuator failure, and fault detection logic was not employed. Piloted simulation testing was used to compare the recone gurable control law to the X-36 dynamic inversion control laws under simulated actuator failures. The recone gurable control law showed improved handling qualities and departure resistance relative to the baseline X-36 control laws.

Nonlinear Control Methods for High-Energy Limit-Cycle Oscillations

Limit-cycle oscillations, as they appear in high-performance e ghter aircraft, remain an area of scrutiny by the aerospace community. Tools for the simulation and prediction of the onset for limit-cycle oscillations have maturedsignie cantly.Surprisingly,lessprogresshasbeenmadeinactivecontrolmethodologiesfortheseinherently nonlinear dynamic phenomena. Even in the cases where it is known that limit-cycle oscillations may be observed in particular e ight regimes, and active control methodologies are employed to attenuate response, there are very few analytical results that characterize the stability of the closed-loop system. In part, this may be attributed to the dife culty in understanding the nature of the contributing nonlinear structural and nonlinear aerodynamic interactions that account for the motion. Recent progress made by the authors in the derivation, development, and implementation of nonlinear control methodologies appropriate for a class of e utter problems is reviewed. Both analytical and experimentalresultsaresummarized. Directionsforfuture study and, in particular,remaining technical barriers that must be overcome, are summarized.

Optimization for Fighter Aircraft Vertical-Plane Maneuvering Using Poststall Flight

A time-optimal, vertical-plane, evasive-offensive maneuver using poststall e ight is found for a e ghter aircraft similar to an F-16. An evader aircraft initially followed by a pursuer aircraft transfers position using the verticalplane maneuver, becoming the pursuer. The aircraft that was initially the pursuer is assumed to continue e ight at constant velocity and constant altitude. A direct numerical method, collocation with nonlinear programming, proveswellsuitedforsolvingthisproblem.Theevaderaircraft’ soptimaltrajectoryisfoundtobeacobramaneuver, one of the vertical poststall maneuvers. Allowing the evader to use thrust vectoring yields a small improvement in e nal time. Nomenclature CD = drag coefe cient CL = lift coefe cient D = drag force e = unit vector directed along the longitudinal axis of an evader g = gravitational acceleration h = altitude J = cost function L = lift force m = mass Nz = normal acceleration r = position vector of a pursuer relative to an evader S = wing area T = thrust t = time v = velocity x = downrange position a = angle of attack c = e ight-path angle q = air density s = thrust vectoring angle s LMT = absolute value of limitation of thrust vectoring angle Subscripts e = value for initial evader f = value at e nal time

Direct adaptive reconfigurable flight control for a tailless advanced fighter aircraft

SUMMARY This paper presents a direct adaptive recongurable #ight control approach and demonstrates its e!ectiveness via an application to an advanced tailless ghter aircraft. The recongurable control law is based on a dynamic inversion controller in an explicit model following architecture. An on-line neural network is used to adaptively regulate the error between the desired response model and the actual vehicle response. An on-line control allocation scheme generates individual control e!ector commands to yield the moments commanded by the controller, while prioritizing critical axes and optimizing performance objectives such as maneuver load alleviation. An on-line system identication module generates estimates of the vehicle’s stability and control derivatives for use in control allocation and command limiting. The recongurable control laws are demonstrated by comparing their performance to a dynamic inversion control law when unknown failure/damage are induced. Copyright ( 1999 John Wiley & Sons, Ltd.

Unified Approach for Roll Ratcheting Analysis

A great database has been generated in the Lateral High Order System(LATHOS)programtosupporthandling qualitiesinvestigations of e ghter aircraft in the roll axis. 5 This e ight test programalso includes data and an analysis of RR. It has been shown that a low roll mode time constant increases the potential for RR. Closed-loop models of pilot-aircraft systems are suitable tools for RR investigations. An early approach was made by using a modie cation of the crossovermodelthatindicatedclosed-looppilot-aircraftoscillations of2 to3 Hz (Ref.6).However, RRmay also occur in large transport aircraft because of aeroelastic effects such as those observed with a C-17 aircraft. 7 In this Note a unie ed approach for RR analysis is presented. A new four-body manipulation model including the control stick, the pilot’ s arm, torso, and hip is used within a closed-loop pilot-aircraft system. The model is successfully applied to an RR incident of the F-16 XL. A new RR prediction criterion based on stability margins in the frequency domain is presented. Background

Forward Flight Effects on Counterflow Thrust Vector Control of a Supersonic Jet

Introduction T HE beneŽ ts of active thrust vector control in high-performance Ž ghter aircraft, such as improved agility and maneuverability, are well established. Unfortunately, conventional thrust vectoring schemes, which rely on mechanical means to vector the jet thrust, have some associated drawbacks. They generally require complicated hardware, which can add to the aircraft weight; the dynamic performance of such systems is less than ideal; and there are heat transfer problems due to the hot exhaust jet impinging on the vectoring surface. Although recent advances have been made in improving mechanical systems, some of the mentioned issues remain a concern. In recent years, a  uidic-based thrust vectoring technique, counter ow thrust vector control (CFTVC), has been exploredwith promising results.2i4 These investigationshave convincingly demonstrated that CFTVC can be used to achieve single-axis pitch vectoring and multiaxis thrust vectoring for supersonic nozzles of various geometries and over a wide range of operating conditions. One of the primary advantages of CFTVC is the simplicity of hardware and excellent dynamic response. A comprehensive summary of these studies can be found in Refs. 2 and 4. The ultimate goal of our researchprogramis to developa CFTVC system that can be implemented in aircraft propulsion systems of the future. To this end one needs to consider real  ight effects, such as high jet exhaust temperatures and the in uence of forward  ight. Although high-temperatureeffects have been addressed to a limited extent in a previous investigation, almost nothing is known about forward  ight effects on CFTVC performance. In the present study, forward  ight effectsare simulatedby generatinga co owing stream around the periphery of the CFTVC system consisting of a rectangular Mach 1.4 jet. The in uence of the co owing stream on the system performance in terms of jet vector angle response is described in this Note.

Maneuvering Flight Control

The development of control laws for maneuvering e ight, specie cally, a high-amplitude velocity vector roll, is addressed. The plant model has seventh-order nonlinear dynamics with coupled pitch and lateral directional dynamics. Using time scale separation and pointwise linearization about nonequilibria trajectories, a receding horizon linear quadratic optimal control law with full state feedback is synthesized on-line. Also on-line, the pilot inputs are modie ed using linear programming to prevent actuator rate saturation over the optimization horizon. The nonlinear control law performance is demonstrated in a e ghter aircraft simulation with a rudder failure during a loaded roll maneuver. The performance of the controller during aggressive pitch and yaw maneuvers is also demonstrated. The work described in this paper is cone ned to model-based receding horizon optimal control law synthesis; one will rely on on-line system identie cation to provide the failed aerodynamic stability and control derivatives, thus achieving indirect adaptive control.

Aerodynamics Performance Prediction of Thrust-Vectoring Nozzles

This paper deals with a study that has been undertaken to analyze the aerodynamics performance of thrust-vectoring nozzles from an engineering design point of view. The rationale for the study is the existence of a trend within the aeronautics industry to provide Ž ghter aircraft with thrust-de ection capabilities. Regarding practical design, and taking into account the fact that these nozzles have additional degrees of freedom, efŽ cient prediction methodologies are needed to analyze the aerodynamics behavior associated with the many combinations of parameters that characterize these propulsive devices. The methodology presented is based on a combination of theoretical analysis and computational  uid dynamics in such a way that numerical simulation provides data which, after some theoretical work, are shaped in the fashion of generic design curves ready for use with practical problems. Details of the  ow solver as well as a summary of its validation campaign are also provided.

Multiple Control Surface Utilization in Active Aeroelastic Wing Technology

This study investigates the use of multiple control surfaces to effect roll trim of aircraft. Analytic models of a rectangular wing and a e ghter aircraft are used as examples for steady aeroelastic and antisymmetric trim analyses. The e nite element method is used to calculate structural deformations caused by steady aerodynamic input forces, which are generated by a linear panel method. Values of control surface effectiveness, calculated from the e exible rolling moment stability derivatives, are used to determine which control surfaces are most effective in achieving a desired roll maneuver. The roll trim equation of motion is solved for multiple control surface dee ections using an iterative technique that minimizes the control effort to achieve a specie ed roll rate. Roll trim utilizing multiple control surfaces, including leading-edge surfaces, is examined over a range of dynamic pressures. Aircraft models with reduced wing stiffness are investigated to determine if roll performance requirements can be achieved through the use of multiple control surfaces. The results of utilizing multiple control surfaces on e exible wings are examined to determine if weight savings can be achieved while maintaining a desired level of roll performance.