Evolutionary design of a full-envelope full-authority flight control system for an unstable high-performance aircraft

The use of an evolutionary algorithm in the framework of H1 control theory is being considered as a means for synthesizing controller gains that minimize a weighted combination of the infinite norm of the sensitivity function (for disturbance attenuation requirements) and complementary sensitivity function (for robust stability requirements) at the same time. The case study deals with a complete full-authority longitudinal control system for an unstable high-performance jet aircraft featuring (i) a stability and control augmentation system and (ii) autopilot functions (speed and altitude hold). Constraints on closed-loop response are enforced, that representing typical requirements on airplane handling qualities, that makes the control law synthesis process more demanding. Gain scheduling is required, in order to obtain satisfactory performance over the whole flight envelope, so that the synthesis is performed at different reference trim conditions, for several values of the dynamic pressure, used as the scheduling parameter. Nonetheless, the dynamic behaviour of the aircraft may exhibit significant variations when flying at different altitudes, even for the same value of the dynamic pressure, so that a trade-off is required between different feasible controllers synthesized at different altitudes for a given equivalent airspeed. A multiobjective search is thus considered for the determination of the best suited solution to be introduced in the scheduling of the control law. The obtained results are then tested on a longitudinal non-linear model of the aircraft

Solution of Low-Thrust Lambert Problem with Perturbative Expansions of Equinoctial Elements

A method for solving the so-called low-thrust Lambert problem is proposed. After formulating it as a two-point boundary value problem, where initial and final positions are provided in terms of equinoctial variables, a first-order perturbative approach is used for investigating the variation of orbital elements generated by the low-thrust propulsion system, which acts as a perturbing parameter with respect to the zero-order Keplerian motion. An implicit algebraic problem is obtained, which allows for the determination of the low-thrust transfer trajectory that drives the equinoctial parameters from the initial to the final values in a prescribed time. Three test cases are presented, which demonstrate the flexibility of the method for different mission scenarios: an interplanetary transfer from Earth to Mars, a spiral multirevolution transfer from low Earth orbit to the International Space Station, and a maneuver to a highly elliptical orbit with large plane change

Fatigue damage modelling of PEEK short fibre composites

Abstract In the present research the fatigue damage evolution of a PEEK based composite was studied. Examined material consisted of a PEEK matrix reinforced by carbon micro-fibres with addition of fillers such as graphite and PTFE. Fatigue tests in load control were carried out up to 106 cycles at different stress level, with cycle ratio R = 0 and frequency 10 Hz. The damage evolution was evaluated by defining damage parameter based on elastic modulus reduction observed under cyclic loading. The cyclic damage evolution of PEEK based composite, which is a function of applied stress level, presented significantly different damage stages. In order to reproduce fatigue damage kinetic observed experimentally a phenomenological modelling approach was then implemented into a finite element code, based on fatigue damage model recently proposed for short fibre reinforced thermoplastic composites

Effects of fiber layout on strength and failure of 3D printed notched composites

This study investigates the effect of printing strategies on the strength of additively manufactured notched fiber reinforced composite specimens. Specimens with varying notch geometries (two radii and two opening angles) and fiber layouts (unreinforced, unidirectional, quasi-isotropic and concentric) were 3D printed and tested under tension. Digital image correlation provided surface strain field data. Results showed that fiber deposition patterns significantly impact notch sensitivity, failure loads and mechanisms, with notch geometry being of secondary importance. The unidirectional layout achieved the highest strength but with progressive failure, while quasiisotropic specimens failed abruptly from the notch. The concentric layout shielded the notch region but induced premature failure away from the notch due to transverse stress. Stress concentration factor approaches, which work well for conventional laminates, have limitations for 3D printed composites due to local differences and complex interactions. Optimizing fiber deposition, instead of geometry, emerges as a promising design route. Combining unidirectional and contouring algorithms may improve performance. However, further studies utilizing multiscale modelling and local failure analyses are needed to fully understand failure mechanisms and guide optimal notch designs for 3D printed composites. With improved understanding and design methods, 3D printing promises to unlock new possibilities for structurally efficient notched composite parts

Influence of micro-notches on the fatigue strength and crack propagation of unfilled and short carbon fiber reinforced PEEK

Short carbon fiber reinforced (SCFR) PEEK is a highly attractive material for lightweight structures; improving knowledge about the influence of local imperfections on its fatigue behavior is essential for the design of real components. To this aim, fatigue strength and crack propagation of two grades of SCFR PEEK and neat matrix were investigated by testing at different stress levels specimens with a micro-notch consisting of a small blind hole (range diameter 0.1–1 mm). Overall, the presence of a micro-notch resulted in a decrease of fatigue strength compared to un-notched condition, but with different sensitivity and crack propagation patterns; while a higher fiber volume fraction enhanced fatigue strength and resistance to crack propagation, the combination of a lower fiber content and inclusion of additive particles had a negative effect. Crack propagation in the notched region was also evaluated. The average values of Paris' law exponential coefficients were similar and within the range of literature values, without apparent correlation with reinforcement type. Preliminary investigations in the presence of the smallest micro-notches seem to indicate the presence of a threshold size below which the influence of a small notch is comparable with that of material inherent defects, but further testing is necessary

Ranking and Repulsing Supermartingales for Reachability in Probabilistic Programs

Computing reachability probabilities is a fundamental problem in the analysis of probabilistic programs. This paper aims at a comprehensive and comparative account on various martingale-based methods for over- and under-approximating reachability probabilities. Based on the existing works that stretch across different communities (formal verification, control theory, etc.), we offer a unifying account. In particular, we emphasize the role of order-theoretic fixed points---a classic topic in computer science---in the analysis of probabilistic programs. This leads us to two new martingale-based techniques, too. We give rigorous proofs for their soundness and completeness. We also make an experimental comparison using our implementation of template-based synthesis algorithms for those martingales

Future large-scale water-Cherenkov detector

MEMPHYS (MEgaton Mass PHYSics) is a proposed large-scale water-Cherenkov experiment to be performed deep underground. It is dedicated to nucleon decay searches and the detection of neutrinos from supernovae, solar, and atmospheric neutrinos, as well as neutrinos from a future beam to measure the CP violating phase in the leptonic sector and the mass hierarchy. This paper provides an overview of the latest studies on the expected performance of MEMPHYS in view of detailed estimates of its physics reach, mainly concerning neutrino beams

Study of the performance of a large scale water-Cherenkov detector (MEMPHYS)

MEMPHYS (MEgaton Mass PHYSics) is a proposed large-scale water Cherenkov experiment to be performed deep underground. It is dedicated to nucleon decay searches, neutrinos from supernovae, solar and atmospheric neutrinos, as well as neutrinos from a future Super-Beam or Beta-Beam to measure the CP violating phase in the leptonic sector and the mass hierarchy. A full simulation of the detector has been performed to evaluate its performance for beam physics. The results are given in terms of "Migration Matrices" of reconstructed versus true neutrino energy, taking into account all the experimental effects.Comment: Updated after JCAP's referee's comment

A hepatic scaffold from decellularized liver tissue: Food for thought

Allogeneic liver transplantation is still deemed the gold standard solution for end-stage organ failure; however, donor organ shortages have led to extended waiting lists for organ transplants. In order to overcome the lack of donors, the development of new therapeutic options is mandatory. In the last several years, organ bioengineering has been extensively explored to provide transplantable tissues or whole organs with the final goal of creating a three-dimensional growth microenvironment mimicking the native structure. It has been frequently reported that an extracellular matrix-based scaffold offers a structural support and important biological molecules that could help cellular proliferation during the recellularization process. The aim of the present review is to underline the recent developments in cell-on-scaffold technology for liver bioengineering, taking into account: (1) biological and synthetic scaffolds; (2) animal and human tissue decellularization; (3) scaffold recellularization; (4) 3D bioprinting; and (5) organoid technology. Future possible clinical applications in regenerative medicine for liver tissue engineering and for drug testing were underlined and dissected

Single axis pointing for underactuated spacecraft with a residual angular momentum

The problem of aiming a generic body-fixed axis along an inertially fixed direction is dealt with for an underactuated spacecraft in the presence of a non-zero residual angular momentum, when only two reaction wheels can exchange angular momentum with the spacecraft platform. An analytical condition for the feasibility of the desired pointing is derived first, together with a closed-form solution for the corresponding attitude with zero platform angular rate. A nonlinear controller is then developed in the framework of singular perturbation theory, enforcing a two-timescale response to the system. Convergence to the desired attitude, when the pointing direction falls within admissible limits, is then proved for rest-to-rest maneuvers and randomly generated initial tumbling conditions for a configuration representative of a small-size satellite