Aerospace, Volume 2, Issue 3 (September 2015) – 8 articles , Pages 376-554

Morphing technology offers a strategy to modify the wing geometry, and the wing planform and cross-sectional parameters can be optimised to the flight conditions. This paper presents an investigation into the effect of span and camber morphing on the mission performance of a 25-kg UAV, with a straight, rectangular, unswept wing. The wing is optimised over two velocities for various fixed wing and morphing wing strategies, where the objective is to maximise aerodynamic efficiency or range. The investigation analyses the effect of the low and high speed velocity selected, the weighting of the low and high velocity on the computation of the mission parameter, the maximum allowable span retraction and the weight penalty on the mission performance. Models that represent the adaptive aspect ratio (AdAR) span morphing concept and the fish bone active camber (FishBAC) camber morphing concept are used to investigate the effect on the wing parameters. The results indicate that generally morphing for both span and camber, the aerodynamic efficiency is maximised for a 30%–70% to 40%–60% weighting between the low and high speed flight conditions, respectively. The span morphing strategy with optimised fixed camber at the root can deliver up to 25% improvement in the aerodynamic efficiency over a fixed camber and span, for an allowable 50% retraction with a velocity range of 50–115 kph. Reducing the allowable retraction to 25% reduces the improvement to 8%–10% for a 50%–50% mission weighting. Camber morphing offers a maximum of 4.5% improvement approximately for a velocity range of 50–90 kph. Improvements in the efficiency achieved through camber morphing are more sensitive to the velocity range in the mission, generally decreasing rapidly by reducing or increasing the velocity range, where span morphing appears more robust for an increase in velocity range beyond the optimum. However, where span morphing requires considerable modification to the planform, the camber change required for optimum performance is only a 5% trailing edge tip deflection relative to cross-sectional chord length. Span morphing, at the optimal mission velocity range, with 25% allowable retraction, can allow up to a 12% increase in mass before no performance advantage is observed, where the camber morphing only allows up to 3%. This provides the designer with a mass budget that must be achieved for morphing to be viable to increase the mission performance. Full article

Methods of observing birds in flight now include training them to fly under known conditions in wind tunnels, and fitting free-flying birds with data loggers, that are either retrieved or read remotely via satellite links. The performance that comes to light depends on the known limitations of the materials from which they are made, and the conditions in which the birds live. Bird glide polars can be obtained by training birds to glide in a tilting wind tunnel. Translating these curves to power required from the flight muscles in level flight requires drag coefficients to be measured, which unfortunately does not work with bird bodies, because the flow is always fully detached. The drag of bodies in level flight can be determined by observing wingbeat frequency, and shows C_D values around 0.08 in small birds, down to 0.06 in small waders specialised for efficient migration. Lift coefficients are up to 1.6 in gliding, or 1.8 for short, temporary glides. In-flight measurements can be used to calculate power curves for birds in level flight, and this has been applied to migrating geese in detail. These typically achieve lift:drag ratios around 15, including allowances for stops, as against 19 for continuous powered flight. The same calculations, applied to Pacific Black-tailed Godwits which start with fat fractions up to 0.55 at departure, show that such birds not only cross the Pacific to New Zealand, but have enough fuel in hand to reach the South Pole if that were necessary. This performance depends on the “dual fuel” arrangements of these migrants, whereby they use fat as their main fuel, and supplement this by extra fuel from burning the engine (flight muscles), as less power is needed later in the flight. The accuracy of these power curves has never been checked, although provision for stopping the bird, and making these checks at regular intervals during a simulated flight was built into the original design of the Lund wind tunnel. The Flight programme, which does these comparisons, also had provision for including contributions due to extracting energy from the atmosphere (soaring), or intermittent bounding flight in small birds (Passerines). It has been known for some time that the feathered surface allows the bird to delay or reverse detachment of the boundary layer, although exactly how this works remains a mystery, which might have practical applications. The bird wing was in use in past times, when birds were still competing with pterosaurs, although these had less efficient wings. The birds that survived the extinction that killed the pterosaurs and dinosaurs have (today) an automatic spherical navigator, which enables them to cross the Pacific and find New Zealand on the other side. Bats have never had such a device, and pterosaurs probably did not either. Animals, when seen from a zoological point of view, are adapted to whatever problems they had to deal with in earlier times. Full article

The idea of morphing is drawing extensive attention in aerospace technologies. Several different approaches like span, camber, twist, and sweep are finding applications. In this work, the concept of a trailing edge control surface which is capable of performing decamber morphing is explained. The upper and lower parts of the control surface undergo different chordwise elongations and the difference between these displacements gives rise to either camber or decamber morphing. The necessary force is achieved by the help of servo actuators. During the design, the structural analyses were done to determine the best viable options for the number of servo actuators, the location of the servo actuators, and the material properties used in the control surface. The control surface was designed of aluminum, composite and compliant materials hence was called a hybrid one. The structural analyses were conducted by using ANSYS^® Workbench v14.0 package program. After finding the best viable design, which was made for in vacuo condition, the proposed design was also verified under the simulated aerodynamic loading. The aerodynamic loads were obtained from CFD analyses which were done with SU2 V3.2.3 open-source flow solver. Full article

The reasons for the difficulty in simulating accurately strong 3-D shock wave/turbulent boundary layer interactions (SBLIs) and high-alpha flows with classical turbulence models are investigated. These flows are characterized by the appearance of strong crossflow separation. In view of recent additional evidence, a previously published flow analysis, which attributes the poor performance of classical turbulence models to the observed laminarization of the separation domain, is reexamined. According to this analysis, the longitudinal vortices into which the separated boundary layer rolls up in this type of separated flow, transfer external inviscid air into the part of the separation adjacent to the wall, decreasing its turbulence. It is demonstrated that linear models based on the Boussinesq equation provide solutions of moderate accuracy, while non-linear ones and others that consider the particular structure of the flow are more efficient. Published and new Reynolds Averaged Navier–Stokes (RANS) simulations are reviewed, as well as results from a recent Large Eddy Simulation (LES) study, which indicate that in calculations characterized by sufficient accuracy the turbulent kinetic energy of the reverse flow inside the separation vortices is very low, i.e., the flow is almost laminar there. Full article

This article describes an image processing algorithm to identify the size and shape of a spherical reflecting celestial body prominently depicted in images taken from a spacecraft with an optical camera, with the purpose of estimating the relative distance between target and observer in magnitude and direction. The approach is based on the fact that in such images, the pixels belonging to the target’s hard edge have the highest values of the image derivative; therefore, they are easily recognizable when the image is processed with a gradient filter. Eventual extraneous points polluting the dataset (outliers) are eliminated by two methods applied in sequence. The target center and radius are estimated by non-linear least squares using circular sigmoid functions. The proposed image processing has been applied to real and synthetic Moon images. An error analysis is also performed to determine the performance of the proposed method. Full article

Recently, the use of graphene as a conductive nanofiller in the preparation of inorganic/polymer nanocomposites has attracted increasing interest in the aerospace field. The reason for this is the possibility of overcoming problems strictly connected to the aircraft structures, such as electrical conductivity and thus lightning strike protection. In addition, graphene is an ideal candidate to enhance the anti-corrosion properties of the resin, since it absorbs most of the light and provides hydrophobicity for repelling water. An important aspect of these multifunctional materials is that all these improvements can be realized even at very low filler loadings in the polymer matrix. In this work, graphene nanoflakes were incorporated into a water-based epoxy resin, and then the hybrid coating was applied to Al 2024-T3 samples. The addition of graphene considerably improved some physical properties of the hybrid coating as demonstrated by Electrochemical Impedance Spectroscopy (EIS) analysis, ameliorating anti-corrosion performances of raw material. DSC measurements and Cross-cut Test showed that graphene did not affect the curing process or the adhesion properties. Moreover, an increment of water contact angle was displayed. Full article

An unmanned aerial ad hoc network (UAANET) is a special type of mobile ad hoc network (MANET). For these networks, researchers rely mostly on simulations to evaluate their proposed networking protocols. Hence, it is of great importance that the simulation environment of a UAANET replicates as much as possible the reality of UAVs. One major component of that environment is the movement pattern of the UAVs. This means that the mobility model used in simulations has to be thoroughly understood in terms of its impact on the performance of the network. In this paper, we investigate how mobility models affect the performance of UAANET in simulations in order to come up with conclusions/recommendations that provide a benchmark for future UAANET simulations. To that end, we first propose a few metrics to evaluate the mobility models. Then, we present five random entity mobility models that allow nodes to move almost freely and independently from one another and evaluate four carefully-chosen MANET/UAANET routing protocols: ad hoc on-demand distance vector (AODV), optimized link state routing (OLSR), reactive-geographic hybrid routing (RGR) and geographic routing protocol (GRP). In addition, flooding is also evaluated. The results show a wide variation of the protocol performance over different mobility models. These performance differences can be explained by the mobility model characteristics, and we discuss these effects. The results of our analysis show that: (i) the enhanced Gauss–Markov (EGM) mobility model is best suited for UAANET; (ii) OLSR, a table-driven proactive routing protocol, and GRP, a position-based geographic protocol, are the protocols most sensitive to the change of mobility models; (iii) RGR, a reactive-geographic hybrid routing protocol, is best suited for UAANET. Full article

Cognitive radar (CRr) is a recent radar paradigm that can potentially help drive aerospace innovation forward. Two specific platforms of cognitive radar used for target identification are discussed. One uses sequential hypothesis testing (SHT) in the receiver processing and is referred to as SHT-CRr and the other one uses maximum a posteriori (MAP) and is referred to as MAP-CRr. Our main goal in this article is to make a practical comparison between SHT-CRr and MAP-CRr platforms in terms of transmission energy efficiency. Since the performance metric for the SHT-CRr is the average number of illuminations (ANI) and the performance metric for MAP-CRr is the percentage of correct decisions (\(P_{cd}\)), a direct comparison between the platforms is difficult to perform. In this work, we introduce a useful procedure that involves a metric called total transmit energy (TTE) given a fixed \(P_{cd}\) as a metric to measure the transmit energy efficiency of both platforms. Lower TTE means that the platform is more efficient in achieving a desired \(P_{cd}\). To facilitate a robust comparison, a transmit-adaptive waveform that consistently outperforms the pulsed waveform in terms of both \(P_{cd}\) and ANI is needed. We show that a certain adaptive waveform called the probability weighted energy signal-to-noise ratio-based (PWE-SNR) waveform outperforms the pulsed wideband waveform (i.e., flat frequency response) in terms of ANI and \(P_{cd}\) for all ranges of transmit waveform energy. We also note that the \(P_{cd}\) performance of SHT-CRr can be drastically different from the probability threshold (i.e., the probability value that is used to stop radar illumination for the purposes of classification), which is critically important for CRr system designers to realize. Indeed, this fact turns out to be key in accomplishing our goal to compare SHT-CRr and MAP-CRr in terms of transmit energy efficiency. Full article