High-performance Aircraft Research Articles

Robust kernel-based model reference adaptive control for unstable aircraft

In this article, a robust kernel-based model reference adaptive control is proposed for an unstable nonlinear aircraft. The heart of the proposed kernel-based model reference adaptive control scheme comprises an offline neural identifier and an online neural controller. In the offline neural identifier, the kernel-based unified extreme learning machine algorithm is used to identify the aircraft model with the available input–output data in a finite time interval. The finite time interval is selected to avoid the response of the unstable aircraft growing unbounded. In the kernel-based unified extreme learning machine, the hidden layer feature mapping is determined by the kernel matrix. However, the unified extreme learning machine is a batch learning algorithm and is not suitable for the online control learning. To solve the problem, a recursive version of the unified extreme learning machine is developed in this study. Based on a given reference model and the identified model, the recursive version of the unified extreme learning machine algorithm is applied to construct the online control law to compensate for the changes in the aircraft dynamics or characteristics. The performance of the proposed kernel-based model reference adaptive control scheme is validated through the simulation studies of a locally nonlinear longitudinal high-performance aircraft. Simulation studies are also compared with a model reference adaptive control based on the back-propagation algorithm and a model reference adaptive control based on the basic extreme learning machine algorithm in terms of the identification and tracking abilities. The results show that the proposed kernel-based model reference adaptive control can achieve better identification and tracking performance.

Multiplicative Error State Kalman Filter vs Nonlinear Complimentary Filter for a High Performance Aircraft Attitude Estimation

<p>Modern control law designs increasingly use aircraft attitude information to improve aircraft manoeuverability. Attitude information allows for gravity term compensations in the longitudinal as well as lateral directional control laws of a typical fighter aircraft. Methodologies and comparisons of multiplicative error state Kalman filter (MEKF) and nonlinear complimentary filter for estimation of attitudes of a high performance aircraft using its onboard autonomous sensors is presented. Shows a problem in pitch angle estimation beyond ± 80 deg in the MEKF and a solution is proposed for the same for the first time. Also presents novel aiding sensor modelling for the implementation of attitude heading reference system for this class of aircraft for the first time. The filter formulations are evaluated using full range manuoevering real flight test data.</p>

Neural mechanisms underlying catastrophic failure in human–machine interaction during aerial navigation

Objective. We investigated the neural correlates of workload buildup in a fine visuomotor task called the boundary avoidance task (BAT). The BAT has been known to induce naturally occurring failures of human–machine coupling in high performance aircraft that can potentially lead to a crash—these failures are termed pilot induced oscillations (PIOs). Approach. We recorded EEG and pupillometry data from human subjects engaged in a flight BAT simulated within a virtual 3D environment. Main results. We find that workload buildup in a BAT can be successfully decoded from oscillatory features in the electroencephalogram (EEG). Information in delta, theta, alpha, beta, and gamma spectral bands of the EEG all contribute to successful decoding, however gamma band activity with a lateralized somatosensory topography has the highest contribution, while theta band activity with a fronto-central topography has the most robust contribution in terms of real-world usability. We show that the output of the spectral decoder can be used to predict PIO susceptibility. We also find that workload buildup in the task induces pupil dilation, the magnitude of which is significantly correlated with the magnitude of the decoded EEG signals. These results suggest that PIOs may result from the dysregulation of cortical networks such as the locus coeruleus (LC)—anterior cingulate cortex (ACC) circuit. Significance. Our findings may generalize to similar control failures in other cases of tight man-machine coupling where gains and latencies in the control system must be inferred and compensated for by the human operators. A closed-loop intervention using neurophysiological decoding of workload buildup that targets the LC-ACC circuit may positively impact operator performance in such situations.

Self-similarity of level-based wavepacket modeling of high-performance military aircraft noise

Construction of the equivalent acoustic source model for high-performance military aircraft noise presented in this paper begins with a decomposition of sound levels, from ground-based microphones 11.7 m from a high-performance military aircraft, into portions matching the similarity spectra associated with large-scale and fine-scale turbulent mixing noise (LSS and FSS). [Tam et al. AIAA Paper 96-1716 (1996)]. A Fourier transform of the spatial distribution of the decomposed levels yields frequency-dependent wavenumber spectra [Morris, Int. J. Aeroacoust. 8, 301-316 (2009)]. Each LSS-educed wavenumber spectrum corresponds to a wavepacket, consisting of a spatially varying amplitude distribution with a constant axial phase relationship. This wavepacket model produces a directional, coherent sound field. However, the asymmetry in the LSS-educed wavenumber spectra may indicate that a nonuniform phase relationship is a more physical choice that could be correlated to the axial variation in convective speed. The FSS component is modeled with a line of incoherent monopoles whose amplitude distribution is related to the FSS-educed wavenumber spectrum. While this level-based model is limited, the addition of the FSS-related source yields a better match to the noise environment of a high-performance military aircraft than is found using a single coherent wavepacket. [Work supported by ONR.]

Wavepacket source modeling of high-performance military aircraft jet noise from cross-beamforming analysis

Wavepacket models provide a convenient representation of jet noise source phenomena because of the extended, partially correlated nature of the turbulent mixing noise. When treated as an equivalent source model, they are useful to estimate features of both the radiated noise as well as the source characteristics to assist in jet noise reduction efforts. In this study, advanced cross-beamforming techniques are applied to measurements in the vicinity of a high-performance military aircraft. These results are then decomposed into an azimuthally-averaged multi-wavepacket representation of the data, which can then be treated as an equivalent source. Estimates of the field levels and coherence properties using the equivalent source are compared with measurements, and results from the multi-wavepacket model show good agreement with benchmark measurements over a range of frequencies that contribute significantly to the overall radiation. The capabilities and limitations of the model to estimate field properties are quantified with respect to benchmark measurements in the mid and far fields as a function of frequency. Results indicate that the multi-wavepacket representation is an improvement over single-wavepacket models, which do not incorporate spatiotemporal features of the radiation. [Work supported by ONR and USAFRL through ORISE.]

Near-field array measurements of F-35 aircraft noise

Noise source measurement and modeling efforts are being conducted for current and future generations of high-performance military aircraft. A near-field microphone array was used in a noise measurement of F-35A and F-35B aircraft at Edwards Air Force Base in 2013 in order to extract source models. The two aircraft measured were tethered to the ground on a concrete run-up pad. The array consisted of 76 sensors, spanned 32 m, and was placed on the ground approximately 7 m from the jet plume. The engines operated over a full range of power settings from idle through full afterburner. In preparation for the execution of source modeling efforts, these near-field data are explored and compared to far-field data published previously [James et al., AIAA Paper 2015-2375]. [Work supported by USAFRL through F-35 JPO.]

Robust aircraft control based on UDE theory

In this work, the Uncertainty and Disturbance Estimation (UDE) approach is employed for the design of a robust flight controller for high performance aircraft. The UDE estimated uncertainty is used to robustify an input–output linearization-based controller designed for nominal system. The UDE theory employing a first-order filter and α-filter is used for the estimation of the composite uncertainty. To address the issue of output derivatives, an observer which too employs the UDE estimated uncertainties for the purpose of robustness is designed. Closed-loop stability for the overall controller-observer system is established. Uncertainties and disturbances in terms of parametric variations, wind gust, unmodeled actuator dynamics and measurement noise are considered to evaluate and compare performances of the two filter-based designs. Further, a performance comparison with some of the existing designs is carried out and the results are presented to demonstrate the efficacy of the proposed work. A notable feature of the proposed design is that the approach neither requires an accurate plant model nor any information about the uncertainties and disturbances.

Modeling and simulating the dynamics of the “Death Star” shotgun target

Modern competitive shooting is a strenuous test of the human perceptual and motor systems. Like driving a race car or piloting a high performance aircraft, speed shooting matches require precise, rapid movements coordinated using a careful mixture of planning and reaction. In some disciplines of competitive shooting, this mixture is further complicated by complex moving targets. This paper develops a set of state space equations for a moving, reactive steel target used in professional 3-Gun competition. The equations for target motion are nonlinear, time-varying, and chaotic in certain regions of the state space. Once derived, the equations for target motion are validated against motion extracted from video. Then, the calibrated equations are implemented in real-time on a portable, laser-activated simulator. Applications of the simulation environment to marksmanship training and human motor control studies are also discussed.

Hyperoxia and hypergravity are independent risk factors of atelectasis in healthy sitting humans: a pulmonary ultrasound and SPECT/CT study.

Aeroatelectasis has developed in aircrew flying routine peacetime flights on the latest generation high-performance aircraft, when undergoing excessive oxygen supply. To single out the effects of hyperoxia and hypergravity on lung tissue compression, and on ventilation and perfusion, eight subjects were studied before and after 1 h 15 min exposure to +1 to +3.5 Gz in a human centrifuge. They performed the protocol three times, breathing air, 44.5% O2, or 100% O2 and underwent functional and topographical imaging of the whole lung by ultrasound and single-photon emission computed tomography combined with computed tomography (SPECT/CT). Ultrasound lung comets (ULC) and atelectasis both increased after exposure. The number of ULC was <1 pre protocol (i.e., normal lung) and larger post 100% O2 (22 ± 3, mean ± SD) than in all other conditions (P < 0.001). Post 44.5% O2 differed from air (P < 0.05). Seven subjects showed low- to medium-grade atelectasis post 100% O2 There was an effect on grade of gas mixture and hypergravity, with interaction (P < 0.001, respectively); 100% O2, 44.5% O2, and air differed from each other (P < 0.05). SPECT ventilation and perfusion were always normal. Ultrasound concurred with CT in showing normal lung in the upper third and ULC/atelectasis in posterior and inferior areas, not for other localizations. In conclusion, hyperoxia and hypergravity are independent risk factors of reversible atelectasis formation. Ultrasound is a useful screening tool. Together with electrical impedance tomography measurements (reported separately), these findings show that zones with decreased ventilation prone to transient airway closure are present above atelectatic areas.

Derivative skewness values of shock-containing noise waveforms

Acoustic shocks present in military aircraft noise are often referred to as crackle, a component of jet noise that can significantly alter perception and annoyance. Quantifying the shock content within a waveform is important for gauging possible effects that these shocks may have. In the past, the derivative skewness has been used to quantify the steepening of waveforms throughout propagation, representing average behavior throughout the waveform. Though past use has been mostly qualitative, recent work has given physical meaning to derivative skewness values of sinusoids and provided recommendations on sampling rate and thresholds that can indicate significant shocks within the waveform. Using these recommendations, the derivative skewness of high-performance military aircraft noise is estimated for various locations and engine conditions. Derivative skewness values are compared with shock counting schemes based on various criteria. The comparisons show a strong relationship between high derivative skewness values and the strength and number of shocks within a waveform.

Military jet noise source imaging using multisource statistically optimized near-field acoustical holography

The identification of acoustic sources is critical to targeted noise reduction efforts for jets on high-performance tactical aircraft. This paper describes the imaging of acoustic sources from a tactical jet using near-field acoustical holography techniques. The measurement consists of a series of scans over the hologram with a dense microphone array. Partial field decomposition methods are performed to generate coherent holograms. Numerical extrapolation of data beyond the measurement aperture mitigates artifacts near the aperture edges. A multisource equivalent wave model is used that includes the effects of the ground reflection on the measurement. Multisource statistically optimized near-field acoustical holography (M-SONAH) is used to reconstruct apparent source distributions between 20 and 1250 Hz at four engine powers. It is shown that M-SONAH produces accurate field reconstructions for both inward and outward propagation in the region spanned by the physical hologram measurement. Reconstructions across the set of engine powers and frequencies suggests that directivity depends mainly on estimated source location; sources farther downstream radiate at a higher angle relative to the inlet axis. At some frequencies and engine powers, reconstructed fields exhibit multiple radiation lobes originating from overlapped source regions, which is a phenomenon relatively recently reported for full-scale jets.

Quadspectral nonlinearity analysis of military jet aircraft noise waveforms

Understanding the impact of jet noise can be improved by quantifying the nonlinearity in a signal with a single-microphone measurement. Based on the quadspectral Morfey-Howell indicator, a nonlinearity gain factor called ν N has been derived from an ensemble-averaged, frequency-domain version of the generalized Burgers equation [Miller et al., AIP Conf. Proc. 1685, 090003 (2015)]. This gain factor gives a quantitative expression for the change in sound pressure level spectrum over distance. Past results show that ν N accurately characterizes nonlinear evolution of waves in simulation and model-scale jet data. Here, noise waveforms from a high-performance military jet aircraft are characterized using the ν N indicator; results are compared with those from other indicators that have been used previously (e.g., derivative skewness, time-waveform steepening factor, etc.). Far field results show that the nonlinear gains at high frequencies (>1 kHz) tend to balance the absorption losses, thus establishing the c...

Wearable Accelerometers in High Performance Jet Aircraft.

Wearable accelerometers have become ubiquitous in the fields of exercise physiology and ambulatory hospital settings. However, these devices have yet to be validated in extreme operational environments. The objective of this study was to correlate the gravitational forces (G forces) detected by wearable accelerometers with the G forces detected by high performance aircraft. We compared the in-flight G forces detected by the two commercially available portable accelerometers to the F/A-18 Carrier Aircraft Inertial Navigation System (CAINS-2) during 20 flights performed by the Navy's Flight Demonstration Squadron (Blue Angels). Postflight questionnaires were also used to assess the perception of distractibility during flight. Of the 20 flights analyzed, 10 complete in-flight comparisons were made, accounting for 25,700 s of correlation between the CAINS-2 and the two tested accelerometers. Both accelerometers had strong correlations with that of the F/A-18 Gz axis, averaging r = 0.92 and r = 0.93, respectively, over 10 flights. Comparison of both portable accelerometer's average vector magnitude to each other yielded an average correlation of r = 0.93. Both accelerometers were found to be minimally distracting. These results suggest the use of wearable accelerometers is a valid means of detecting G forces during high performance aircraft flight. Future studies using this surrogate method of detecting accelerative forces combined with physiological information may yield valuable in-flight normative data that heretofore has been technically difficult to obtain and hence holds the promise of opening the door for a new golden age of aeromedical research.

Robust constraint backstepping control for high-performance aircraft with account of unsteady aerodynamic effects

This paper presents a robust constraint backstepping control scheme for high performance aircraft attitude tracking in the presence of physical constraints, model uncertainties and unsteady effects. The six-degree-of-freedom aircraft model with unsteady aerodynamics is first established, of which the characteristics are investigated through open-loop analysis. Based on the immersion and invariance approach, a state observer is developed to estimate the unmeasurable unsteady aerodynamic states. The unsteady effects are then compensated in the controller design where the bounds of estimate errors and model uncertainties are used to increase robustness. In addition, a command filter is introduced to impose physical constraints on states and control inputs and overcome the ‘explosion of terms’ problem. An auxiliary system of which the states are applied to the controller design is constructed as well to evaluate the constraint effects. Furthermore, stability of the closed-loop system and convergence of the tracking error are proven by Lyapunov stability theorem. Finally, several numerical simulations are performed to verify the effectiveness and robustness of the proposed control scheme.

Physical Condition Does Not Affect Gravity-Induced Loss of Consciousness during Human Centrifuge Training in Well-Experienced Young Aviators.

BackgroundConsensus on whether physical condition affects the risk of gravity-induced loss of consciousness (G-LOC) has not been reached, and most previous studies about the issue did not include well-experienced aviators. We compared the physical conditions of well-experienced young aviators according to the occurrence of G-LOC during human centrifuge training.MethodsAmong 361 young male aviators on active flight duty with experience in high performance aircrafts for at least 2 years, 350 had full data available and were reviewed in this study. We divided the aviators into the G-LOC group and the non-G-LOC group according to their human centrifuge training results. We then compared their basic characteristics, body composition, physical fitness level, and pulmonary function.ResultsTwenty nine aviators (8.3%) who experienced G-LOC during human centrifuge training in their first trials were classified into the G-LOC group. There was no difference in physical condition of aviators between the two groups.ConclusionsYoung aviators with experience in G-LOC showed no difference in physical condition such as muscle mass, strength, and general endurance from the aviators with no such experience. Although more studies are needed, physical condition does not seem to be a significant determinant of G-LOC among the experienced aviators.

Multivariate Spline-Based Adaptive Control of High-Performance Aircraft with Aerodynamic Uncertainties

In this paper, a new modular adaptive control system is presented to compensate for aerodynamic uncertainties in high-performance flight control systems. This approach combines nonlinear dynamic inversion with multivariate spline-based adaptive control allocation. A new real-time identification routine for multivariate splines is presented to compensate for aerodynamic uncertainties in the control allocation system. This method, indicated as spline-based adaptive nonlinear dynamic inversion, is applied to control an F-16 aircraft subject to significant aerodynamics uncertainties. Simulation results indicate that the new controller can tune itself each time a model error is detected and has superior adaptability compared to an ordinary polynomial-based adaptive controller. Multivariate splines have sufficient flexibility and approximation power to accurately model nonlinear aerodynamics over the entire flight envelope. As a result, the global model remains intact. Although a part of the model is being reconfigured using incoming observations, the remainder of the model remains unchanged and can be used as an a priori source of information. This prevents the occurrence of sudden fundamental changes in the global model structure, which are experienced when using ordinary polynomials.

Store-Induced Limit-Cycle Oscillations Due to Nonlinear Wing-Store Attachment

Several high-performance fighter aircraft exhibit store-induced limit-cycle oscillations, leading to pilot discomfort, potential structural fatigue, and flight envelope restrictions. The roles of various aerodynamic and structural factors causing the limit-cycle oscillation are not sufficiently understood, and their numerical exploration via time marching is computationally expensive. In this paper, the effects of nonlinear stiffness and damping in the wing-store attachments of the F-16 aircraft are examined, in the presence of steady flow aerodynamic nonlinearity, using the computationally efficient harmonic balance method. Structural mechanisms including cubic restoring force of both softening and hardening types, freeplay, and Coulomb friction are systematically evaluated, and the most likely among these are identified by comparing the computed limit-cycle oscillation results to flight data. An extension of the harmonic balance method to handle nonlinear unsteady aerodynamics along with structural nonlinearity is also proposed to enable rapid and accurate limit-cycle oscillation assessment of candidate store configurations.

Novel method for static pressure calibration using Unscented Kalman Filter

Static pressure plays an important role in modern fighters in terms of providing stability augmentation and control. Static pressure is measured by pitot static systems. They can be installed at different locations in the aircraft. Each of them measure different values and hence calibration of these pitot static systems is essential to produce reliable static pressure. This paper addresses a novel method for static pressure calibration using Unscented Kalman Filter (UKF). The method proposed here can be extended to Flush Air Data Systems (FADS). The technique is tested using the flight test data of a high performance aircraft.

Process capability and effect size of vacuum extraction shaped tube electrolytic drilling of Inconel alloy for high-performance cooling hole

Cooling holes are important structures in turbine blades for high-performance aircraft engines. Nickel-based superalloys such as Inconel 718 are widely used in manufacturing turbine blades. However, it is very challenging to manufacture cooling holes in Inconel alloys. Conventional machining process such as electrical discharge machining (EDM) and laser beam machining (LBM) are thermal processes, which lead to recast layer, heat-affected zone, and tensile residual stresses. Electrochemical machining is an alternative to machine cooling holes without thermal damage. This paper focuses on the process capability and effect size of vacuum extraction shaped tube electrolytic drilling (VE-STED) process to manufacture cooling holes of Inconel 718. The effect of key process parameters, i.e., applied voltage, electrolyte concentration, and tool feed rate on process efficiency, form accuracy, and process stability has been investigated. The effect size was determined using ANOVA.

Career perspective: Victor A. Convertino.

This review focuses on a career of unique opportunities to participate in various areas of research related to extreme physiology and medicine. My experience as a volunteer subject in exercise experiments conducted at NASA included the study of acute and chronic physiological responses and adaptations to exercise in environments of hypoxia, heat stress, and simulated microgravity (bed rest), and eventually to my doctoral work on mechanisms underlying expansion of plasma and blood volume with acute and repeated exercise and heat exposure. My career has taken me to research positions at NASA, the Stanford University School of Medicine, the University of Arizona, and the U.S. Air Force Research Laboratory before assuming my present location at the U.S. Army Institute of Surgical Research. As a result of these multiple experiences across a period of 45 years, I have had opportunities to translate basic research to astronauts, high-performance aircraft pilots, and critically ill patients who are challenged by conditions of extreme physiology and medicine.