1,721,136 research outputs found
Nonlinearities in Flutter Evaluation
No full textFlutter and other aeroelastic instability phenomena have had a significant influence on airplane development and the airworthiness criteria governing the design of civil airplanes.
Flutter analyses are usually carried out with the hypothesis of structural linearity, but experience shows that a satisfactory description of the flutter phenomena cannot be made apart from nonlinearities. Nonlinearities can be lumped, typical of control surface mechanisms or distributed, such as those associated with riveted/bolted connections.
Because of the huge amount of riveted/bolted joints almost everywhere in the structure, their contribution to nonlinearities can be considered as “distributed” and consequently their effect on normal modes can be neglected. On the contrary experience has taught that lumped nonlinearities strongly affect the aeroelastic stability.
The present paper is focused on one of the approaches that can be followed to investigate the flutter behaviour in frequency domain when in presence of nonlinearities in the control circuit mechanism.
The I-23 aircraft (Institute of Aviation, Poland) has been used as a test-case of the methodology.
In particular the Harmonic Balance technique has been exploited to perform “pseudo-linear” flutter analyses with varying control circuit stiffness.
Thanks to a Dynamic Substructuring approach the pseudo-linear flutter analyses have been conducted with a considerable computational time saving because all nonlinearities have been considered at the boundaries between substructures. This way of modelling allows not to recalculate the Generalized Aerodynamic Forces at every changing of control circuit parameters.
Finally a set of nonlinear time analyses, by means of a nonlinear State Space representation, have been performed in order to “validate” the Harmonic Balance technique
Pre-test Analyses for Flight Flutter Test of an EASA CS-23 Aircraft
No full textSmall aircraft manufacturers are typically constrained by stringent budget requirements. Among all the cost headings the certification process cost is a non-negligible part, especially as regards the tests.
Flight Flutter Test is a crucial means to state flutter clearance. A fundamental issue connected to this test is the choice of good excitation devices or maneuvers in order to excite the flutter mode. Different types of excitation means are available, each one more or less suitable to the aircraft which will be tested. The choice of the best excitation device among the various available solutions basically depends upon the flutter mode frequency to be excited as well as time and budget constraints. If a rational analysis able to predict the reliability of the excitation is available, significant time/cost expenses reduction can be achieved.
In this paper a pre-test analysis aimed at correctly exciting the flutter mode during Flight Flutter test on an EASA CS-23 aircraft is presented. The Aeroelastic Dynamic Response under external forces (State Space representation and modal approach) has been investigated to assess the reliability of two different types of excitation devices: the control surface pulse (with zero additional cost) and the pyrotechnical thruster (bonker).
Actually no pre-Flight Flutter Test rational analyses are strictly required by Airworthiness Requirements, but it would be desirable if such an approach was followed, especially in the viewpoint of a better addressing of the test and of course the cost reduction
Symmetric Quasi Steady Maneuver Load Alleviation – a method to predict the control surface efficiency when used as load alleviator
No full textThe main purpose of the Load Alleviation system on which this paper is focused is a drop in structural wing loads for improved performances, fatigue life, Operative Empty Weight reduction.
The driving idea is to perform the alleviation during a quasi-steady symmetric maneuver by means of a symmetric actuation of dedicated control surfaces. The control effectiveness of the load alleviator is evaluated, in terms of required deflection for a desired bending moment reduction in the wing, but keeping the same maximum vertical load factor attained during the unalleviated maneuver.
When high performance is important, internal structural loads (maneuver loads) reduction becomes significant, if the alleviated maneuver can be performed with the vertical load factor kept unvarying. Consequently evaluations on the load alleviator effectiveness in the early design process are appropriate to find the best set of controls.
The load alleviator deflection time history is evaluated a priori, without feed-back calculations, simply depending upon elevators time history, system characteristics and flight condition.
An application on a business aircraft has been performed (not neglecting structural flexibility) for two kinds of symmetric maneuvers. By adopting the ailerons as load alleviators, the maximum bending reduction at the wing root is about 37 percent, with a maximum aileron deflection less than 12 deg
Load Alleviation on a Joined-Wing Unmanned Aircraft
No full textIn this paper a method of alleviating wing structural load of a flexible aircraft during a symmetric balanced maneuver is presented. An application on the unmanned aircraft in development at the Italian Aerospace Research Center, high altitude performance demonstrator, characterized by a joined-wing configuration, is illustrated. This load alleviation technique enables a desired value of the bending moment on a fixed wing control station to be obtained. The load reduction is achieved by deflecting a suitable set of flight-control surfaces, by always keeping the vertical load factor constant to preserve the maneuvering performance. The main hypotheses are: significant aeroelastic effects, linear behavior of aerodynamics and structure, and unvarying tensor of inertia under structural deflections. High-altitude performance demonstrator is a scaled performance demonstrator of an 80m-wing span high-altitude and long endurance unmanned aircraft in a joined-wing configuration. The advantages in terms of performance, fatigue life extension, and weight reduction can be achieved from the integration of an onboard load alleviation system. The results show that the attainable value of load alleviation in terms of bending moment reduction at the wing root is 37%. Moreover, the test-case analyses show that the maximum value of the alleviation increases with respect to the dynamic pressure although the load distribution varies because of significant aeroelastic effects
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Gravity gradiometers for planetary geodesy: requirements and concept for a space instrument
Full text linkThe measurement of the gravitational field of Solar System bodies is becoming ever and ever crucial in the physical description of their composition, state and evolution. Indeed, many planetary processes at large scale are ruled by their internal structure, where surface
and tectonic features are mainly the result of heat exchanges from the interior to the surface. Gravity field measurements are one of the observational methods to investigate those processes and to place constraints on the structure of the planetary interiors and on the
formation and geologic evolution of a planet. The retrieval of the spherical harmonic coefficients used to describe the gravitational field of a body gives insights into e.g. its polar oblateness, moment of inertia and deviations from hydrostatic equilibrium. With geologic assumptions and other remote sensing data, significant geophysical parameters, related e.g. to crust and mantle density and thickness, core size and structure, mantle/core coupling can be obtained. These parameters are used in planetary models to address topics such as planets differentiation, thermal evolution, characteristics and composition of the interiors. Moreover, the internal structure can be further investigated (wherever possible)
through seismometers on the surface, exploiting the analysis of seismic waves travelling through the interior (as performed by Apollo missions EASEP and ALSEP packages and currently by Mars Insight).
Until now, the Radio-Tracking technique (RT), part of the Radio Science (RS) observations, jointly with POD (Precise Orbit Determination), has been de-facto the main technique for gathering this type of information. It has been implemented in several deep-space missions, such as Magellan (Venus), MRO (Mars), Cassini (Saturn), Messenger (Mercury), Juno (Jupiter), and, in the forthcoming future, BepiColombo (Mercury) and JUICE (Jupiter and its moons).
Concerning scientific targets of interest, it needs to be highlighted that gravity field models are available (section 2), besides the Earth and the Moon, just for few planetary bodies such as the terrestrial planets Mercury, Venus and Mars. However, often such models are restricted only to large spatial resolutions, about one or more hundreds of kilometers, not enough to understand the geophysical processes that have driven formation and evolution of those bodies. The accuracy of these models is good enough as well but just for the lower part of the gravity field spectra, where a sufficient signal-to-noise ratio is achieved.
Moreover, there is much more lack of data for the external planets, where only few gravity field parameters have been derived for some of the gaseous planets and their main moons.
Any improvement on those targets, with a special attention to Venus, Mars and Galilean moons, would be very helpful in understanding their interior and the geophysical and geological processes that operated on them.
To answer the need for higher space resolution and accuracy in planetary gravity fields, two different approaches can be pursued:
1. to improve the measurement performance of the instrumentation used for RS; in these experiments the gravity field to be studied is inferred by the orbit of a spacecraft (that can be considered a ‘proof mass’ falling in the overall external gravity field) and an accelerometer is used to measure the Non-Gravitational Perturbations (NGP) perturbing the spacecraft free-fall, i.e. its motion from a pure (in principle) geodesic of space-time. An improvement of the accelerometer performance and its integration within an enhanced tracking system used to measure the spacecraft position and velocity, are needed conditions to improve the performance of gravity field reconstruction.
2. to introduce innovative measurement concepts, allowing to overcome some of the bottlenecks of the current methods (non-continuous monitoring, field attenuation with the altitude, disturbances mitigation, etc. In a roadmap definition, one of the more promising is the gravity gradiometry technique, which would allow to directly sense the gravity field by measuring the gravity gradients, and not just indirectly, as for RT, through monitoring the spacecraft gravitational perturbations. Unlike the radio-tracking, spacebased gravity gradiometry has still to unfold its potentialities; indeed, the ESA’s GOCE mission is the first and unique till now that has flown a gravity gradiometer to explore Earth’s gravity in 2009-13. The planetary gradiometry still awaits achievements outside the Earth System.
Satellite gradiometry refers to the measurement of acceleration differences, ideally in all three spatial directions, between the test-masses of an ensemble of accelerometers inside one satellite. The differentiation of gravity accelerations allows to highlight small-scale surface and sub-surface features, making such a technique, differently wrt RT, inherently sensitive to medium and large degrees (i.e. high resolutions) of the spherical harmonic representation of the gravity field. Therefore, the use of gradiometry would allow to improve the gravity field knowledge by measuring medium and large degrees, filling the gap above depicted and fostering the investigation on the structure and evolution of the planets.
The activity of this PhD Thesis starts from the definition of the planetary gravity field state of the art and the identification of the needs of the scientific community to improve the planetary bodies knowledge. Based on this result, a selection of targets of interest will be operated. A
review of the gravity field measurement techniques will be carried out, identifying advantages and drawbacks, pointing out innovative techniques such as gradiometry. On the basis of these activities, a series of numerical simulations will be implemented to produce the time series of gradiometric signals foreseen in a set of case studies. The choice of the case studies will be based on the preliminary studies about the science needs. The main outcome will be a set of requirements to be matched by a typical gradiometric instrument/mission, aiming at fulfilling the scientific needs. An important requirement would be, for instance, the typical instrument sensitivity and spectral band, as well as the expected
acceleration or gravity gradient amplitude of a signal sensed with a reasonable signal-tonoise ratio. Different scenarios will be simulated on the basis of the science needs.
In chapter 2 the gravity field is faced from the theoretical point of view and a snapshot of the current understanding of gravity field of planetary bodies is carried out. At last, science needs are identified and planetary bodies of interest are selected.
In chapter 3 measurement techniques of the gravity field are described, focusing the attention on the gravitational gradiometry. Advantages and drawbacks are considered. Moreover, spaceborne, airborne and groundborne gradiometric instruments have been identified and analysed to identify the current state of the art.
In chapter 4 gravity mission needs are identified in terms of science and mission requirements. Afterwards, a matlab code developed to compute the gravity gradient signal expected in some case studies is described and evaluated. At last, analysis of ways to increase the sensitivity of gradiometers is carried out.
In chapter 5, based on analysis of previous chapters, an instrument concept is introduced and analysed to match the requirements identified. The basic performance are derived, discussed and compared to the signal that is expected to be measured according to the computation carried out with the matlab code. Future work foresees to further develop the concept and to further deep the analysis of the identified gradiometer configurations
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
DESIGN OF A COMBINED PNEUMATIC AND MAGNETORHEOLOGICAL ACTUATOR FOR PROPORTIONAL MOTION CONTROL
No full textThis paper shows a solution based on the use of magnetorheological fluids, aimed at conferring proportional controllability to standard on-off pneumatic actuators. The central idea is based on adding a magnetorheological fluid brake on a standard pneumatic device, in order to achieve position control of the system. The goal is to design a small and cheap magnetorheological fluid brake for automotive applications . Several concept of brake are presented, using the fluid in shear and flow mode. The final configuration involves not only mechanical architecture, but also an investigation on the coil shape and the related magnetic field
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