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Aerospace, Volume 6, Issue 3 (March 2019)

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Cover Story (view full-size image) Today's aircraft manufacturers are eager to fulfill the societal demand for greener and more [...] Read more.
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Open AccessArticle Non-Linear Dynamic Inversion Control Design for Rotorcraft
Received: 12 February 2019 / Revised: 10 March 2019 / Accepted: 11 March 2019 / Published: 18 March 2019
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Abstract
Flight control design for rotorcraft is challenging due to high-order dynamics, cross-coupling effects, and inherent instability of the flight dynamics. Dynamic inversion design offers a desirable solution to rotorcraft flight control as it effectively decouples the plant model and effectively handles non-linearity. However, [...] Read more.
Flight control design for rotorcraft is challenging due to high-order dynamics, cross-coupling effects, and inherent instability of the flight dynamics. Dynamic inversion design offers a desirable solution to rotorcraft flight control as it effectively decouples the plant model and effectively handles non-linearity. However, the method has limitations for rotorcraft due to the requirement for full-state feedback and issues with non-minimum phase zeros. A control design study is performed using dynamic inversion with reduced order models of the rotorcraft dynamics, which alleviates the full-state feedback requirement. The design is analyzed using full order linear analysis and non-linear simulations of a utility helicopter. Simulation results show desired command tracking when the controller is applied to the full-order system. Classical stability margin analysis is used to achieve desired tradeoffs in robust stability and disturbance rejection. Results indicate the feasibility of applying dynamic inversion to rotorcraft control design, as long as full order linear analysis is applied to ensure stability and adequate modelling of low-frequency dynamics. Full article
(This article belongs to the Special Issue Rotorcraft)
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Open AccessArticle Energy Harvesting Performance of Plate Wing from Discrete Gust Excitation
Received: 30 December 2018 / Revised: 9 March 2019 / Accepted: 9 March 2019 / Published: 15 March 2019
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Abstract
Energy harvesting from aeroelastic response tends to have a wide application prospect, especially for small-scale unmanned aerial vehicles. Gusts encountered in flight can be treated as a potential source for sustainable energy supply. The plate model is more likely to describe a low [...] Read more.
Energy harvesting from aeroelastic response tends to have a wide application prospect, especially for small-scale unmanned aerial vehicles. Gusts encountered in flight can be treated as a potential source for sustainable energy supply. The plate model is more likely to describe a low aspect ratio, thin plate wing structure. In this paper, the Von Kármán plate theory and 3D doublet lattice method, coupled with a piezoelectric equation, are used to build a linear state-space equation. Under the load of “one-minus-cosine” discrete gust, the effects of flow speed and gust amplitude, thickness of piezoelectric ceramic transducer (PZTs) layers, and mounted load resistance are investigated. Results reveal that the PZTs layers on the wing root of the leading edge can obtain the highest electrical parameters. The flow velocity, thickness of the PZTs layers and load resistance are used to optimize energy harvesting data. Full article
(This article belongs to the Special Issue Aeroelasticity)
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Open AccessArticle Viability of an Electrically Driven Pump-Fed Hybrid Rocket for Small Launcher Upper Stages
Received: 31 January 2019 / Revised: 4 March 2019 / Accepted: 11 March 2019 / Published: 14 March 2019
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Abstract
An electrically driven pump-fed cycle for a hybrid rocket engine is proposed and compared to a simpler gas-pressurized feed system. A liquid-oxygen/paraffin-based fuel hybrid rocket engine which powers the third stage of a Vega-like launcher is considered. Third-stage ignition conditions are assigned, and [...] Read more.
An electrically driven pump-fed cycle for a hybrid rocket engine is proposed and compared to a simpler gas-pressurized feed system. A liquid-oxygen/paraffin-based fuel hybrid rocket engine which powers the third stage of a Vega-like launcher is considered. Third-stage ignition conditions are assigned, and engine design and payload mass are defined by a proper set of parameters. Uncertainties in the classical regression rate correlation coefficients are taken into account and robust design optimization is carried out with an approach based on an epsilon-constrained evolutionary algorithm. A mission-specific objective function, which takes into account both the payload mass and the ability of the rocket to reach the required final orbit despite uncertainties, is determined by an indirect trajectory optimization approach. The target orbit is a 700 km altitude polar orbit. Results show that electrically driven pump-fed cycle is a viable option for the replacement of the conventional gas-pressurized feed system. Robustness in the design is granted and a remarkable payload gain is achieved, using both present and advanced technologies for electrical systems. Full article
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Open AccessArticle Plasma-Assisted Control of Supersonic Flow over a Compression Ramp
Received: 13 February 2019 / Revised: 6 March 2019 / Accepted: 7 March 2019 / Published: 12 March 2019
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Abstract
This study considers the effect of an electric discharge on the flow structure near a 19.4° compression ramp in Mach-2 supersonic flow. The experiments were conducted in the supersonic wind tunnel SBR-50 at the University of Notre Dame. The stagnation temperature and pressure [...] Read more.
This study considers the effect of an electric discharge on the flow structure near a 19.4° compression ramp in Mach-2 supersonic flow. The experiments were conducted in the supersonic wind tunnel SBR-50 at the University of Notre Dame. The stagnation temperature and pressure were varied in a range of 294–600 K and 1–3 bar, respectively, to attain various Reynolds numbers ranging from 5.3 × 105 to 3.4 × 106 based on the distance between the exit of the Mach-2 nozzle and the leading edge of the ramp. Surface pressure measurements, schlieren visualization, discharge voltage and current measurements, and plasma imaging with a high-speed camera were used to evaluate the plasma control authority on the ramp pressure distribution. The plasma being generated in front of the compression ramp shifted the shock position from the ramp corner to the electrode location, forming a flow separation zone ahead of the ramp. It was found that the pressure on the compression surface reduced almost linearly with the plasma power. The ratio of pressure change to flow stagnation pressure was also an increasing function of the ratio of plasma power to enthalpy flux, indicating that the task-related plasma control effectiveness ranged from 17.5 to 25. Full article
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Open AccessArticle Verification of Boundary Conditions Applied to Active Flow Circulation Control
Received: 30 November 2018 / Revised: 2 March 2019 / Accepted: 4 March 2019 / Published: 8 March 2019
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Abstract
Inclusion of Active Flow Control (AFC) into Computational Fluid Dynamics (CFD) simulations is usually highly time-consuming and requires extensive computational resources and effort. In principle, the flow inside of the fluidic AFC actuators should be incorporated into the problem under consideration. However, for [...] Read more.
Inclusion of Active Flow Control (AFC) into Computational Fluid Dynamics (CFD) simulations is usually highly time-consuming and requires extensive computational resources and effort. In principle, the flow inside of the fluidic AFC actuators should be incorporated into the problem under consideration. However, for many applications, the internal actuator flow is not crucial, and only its effect on the outer flow needs to be resolved. In this study, the unsteady periodic flow inside the Suction and Oscillatory Blowing (SaOB) actuator is analyzed, using two CFD methods of ranging complexity (URANS and hybrid RANS-LES). The results are used for the definition and development of the simplified surface boundary condition for simulating the SaOB flow at the actuator’s exit. The developed boundary condition is verified and validated, in the case of a low-speed airfoil with suction applied on the upper (suction) side of the airfoil and oscillatory blowing applied on the lower (pressure) side, close to the trailing edge—a fluidic Gurney flap. Its effect on the circulation is analyzed and compared to the experimental data. Full article
(This article belongs to the Special Issue 8th EASN-CEAS Workshop on Manufacturing for Growth and Innovation)
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Open AccessArticle Effect of Piezo-Embedded Inverted Flag in Free Shear Layer Wake
Received: 21 January 2019 / Revised: 20 February 2019 / Accepted: 25 February 2019 / Published: 7 March 2019
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Abstract
The use of flexible inverted piezo embedded Polyvinylidene Difluoride (PVDF) as a simultaneous energy harvester and as a wake sensor is explored. The oscillation amplitude (characterized by voltage output) and oscillation frequency of the piezo-embedded PDVF was quantified in the wake of a [...] Read more.
The use of flexible inverted piezo embedded Polyvinylidene Difluoride (PVDF) as a simultaneous energy harvester and as a wake sensor is explored. The oscillation amplitude (characterized by voltage output) and oscillation frequency of the piezo-embedded PDVF was quantified in the wake of a 2D NACA 0012 model and SD7003 model at a Reynolds number of 100,000 and 67,000, respectively. The performance of the sensor was also quantified in the freestream without the presence of the wing. In order to quantify the sensor response to angle of attack and downstream distance, the amplitude and frequency of oscillations were recorded in the wing wake. Increase in angle of attack of the wing resulted in increase in oscillation frequency and amplitude of the PVDF. The results also indicated that the inverted flag configuration performed better in the wake under unsteady conditions when compared to freestream conditions. The results from Particle Image Velocimetry indicated that the wake signature was not affected by the presence of the PVDF in the wake. The root mean square voltage contours in the wake of SD7003 airfoil show remarkable free shear layer wake features such as upper and lower surface stratification and downwash angle which shows the sensitivity of the sensor to the unsteadiness in the wake. The capability of this device to act as a potential energy harvester and as a sensor has serious implications in extending the mission capabilities of small UAVs. Full article
(This article belongs to the Special Issue Design and Analysis of Wind-Tunnel Models and Fluidic Measurements)
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Open AccessArticle Synergy Effects in Electric and Hybrid Electric Aircraft
Received: 3 December 2018 / Revised: 27 February 2019 / Accepted: 28 February 2019 / Published: 6 March 2019
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Abstract
The interest in electric and hybrid electric power system has been increasing, in recent times, due to the benefits of this technology, such as high power-to-weight ratio, reliability, compactness, quietness, and, above all, elimination of local pollutant emissions. One of the key factors [...] Read more.
The interest in electric and hybrid electric power system has been increasing, in recent times, due to the benefits of this technology, such as high power-to-weight ratio, reliability, compactness, quietness, and, above all, elimination of local pollutant emissions. One of the key factors of these technologies is the possibility to exploit the synergy between powertrain, structure, and mission. This investigation addresses this topic by applying multi-objective optimization to two test cases—a fixed-wing, tail-sitter, Vertical Take-off and Landing Unmanned Aerial Vehicle (VTOL-UAV), and a Medium-Altitude Long-Endurance Unmanned Aerial Vehicle (MALE-UAV). Cruise time and payload weight were selected as goals for the first optimization problem, while fuel consumption and electric endurance were selected for the second one. The optimizations were performed with Non-dominated Sorting Genetic Algorithm-II (NSGA-II) and S-Metric Selection Evolutionary Multiobjective Algorithm (SMS-EMOA), by taking several constraints into account. The VTOL-UAV optimization was performed, at different levels (structure only, power system only, structure and power system together). To better underline the synergic effect of electrification, the potential benefit of structural integration and multi-functionalization was also addressed. The optimization of the MALE-UAV was performed at two different levels (power system only, power system, and mission profile together), to explore the synergic effect of hybridization. Results showed that large improvements could be obtained, either in the first test case when, both, the powertrain design and the aircraft structure were considered, and in the optimization of the hybrid electric UAV, where the optimization of the aircraft flight path gave a strong contribution to the overall performances. Full article
(This article belongs to the Special Issue 8th EASN-CEAS Workshop on Manufacturing for Growth and Innovation)
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Open AccessArticle Comparison of Constrained Parameterisation Strategies for Aerodynamic Optimisation of Morphing Leading Edge Airfoil
Received: 31 January 2019 / Revised: 25 February 2019 / Accepted: 28 February 2019 / Published: 6 March 2019
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Abstract
In the context of ambitious targets for reducing environmental impact in the aviation sector, dictated by international institutions, morphing aircraft are expected to have potential for achieving the required efficiency increases. However, there are still open issues related to the design and implementation [...] Read more.
In the context of ambitious targets for reducing environmental impact in the aviation sector, dictated by international institutions, morphing aircraft are expected to have potential for achieving the required efficiency increases. However, there are still open issues related to the design and implementation of deformable structures. In this paper, we compare three constrained parameterisation strategies for the aerodynamic design of a morphing leading edge, representing a potential substitute for traditional high-lift systems. In order to facilitate the structural design and promote the feasibility of solutions, we solve a multi-objective optimisation problem, including constraints on axial and bending strain introduced by morphing. A parameterisation method, inherently producing constant arc length curves, is employed in three variants, representing different morphing strategies which provide an increasing level of deformability, by allowing the lower edge of the flexible skin to slide and the gap formed with the fixed spar to be closed by a hatch. The results for the optimisation of a baseline airfoil show that the geometric constraints are effectively handled in the optimisation and the solutions are smooth, with a continuous variation along the Pareto frontier. The larger shape modification allowed by more flexible parameterisation variants enables an increase of the maximum lift coefficient up to 8.35%, and efficiency at 70% of stall incidence up to 4.26%. Full article
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Open AccessArticle CFD-Based Aeroelastic Sensitivity Study of a Low-Speed Flutter Demonstrator
Received: 6 February 2019 / Revised: 22 February 2019 / Accepted: 1 March 2019 / Published: 6 March 2019
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Abstract
The goal of developing aircraft that are greener, safer and cheaper can only be maintained through significant innovations in aircraft design. An integrated multidisciplinary design approach can lead to an increase in the performance of future derivative aircraft. Advanced aerodynamics and structural design [...] Read more.
The goal of developing aircraft that are greener, safer and cheaper can only be maintained through significant innovations in aircraft design. An integrated multidisciplinary design approach can lead to an increase in the performance of future derivative aircraft. Advanced aerodynamics and structural design technologies can be achieved by both passive and active suppression of aeroelastic instabilities. To demonstrate the potential of this approach, the EU-funded project Flutter Free Flight Envelope Expansion for Economical Performance Improvement is developing an unmanned aerial vehicle with a high-aspect-ratio-wing and clearly defined flutter characteristics. The aircraft is used as an experimental test platform. The scope of this work is the investigation of the aeroelastic behaviour of the aircraft and the determination of its flutter limits. The modeling of unsteady aerodynamics is performed by means of the small disturbance CFD approach that provides higher fidelity compared to conventional linear-potential-theory-based methods. The CFD-based and the linear-potential-theory-based results are compared and discussed. Furthermore, the sensitivity of the flutter behaviour to the geometric level of detail of the CFD model is evaluated. Full article
(This article belongs to the Special Issue Aeroelasticity)
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Open AccessArticle To Burn-In, or Not to Burn-In: That’s the Question
Received: 7 January 2019 / Revised: 11 February 2019 / Accepted: 1 March 2019 / Published: 6 March 2019
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Abstract
In this paper it is shown that the bathtub-curve (BTC) based time-derivative of the failure rate at the initial moment of time can be considered as a suitable criterion of whether burn-in testing (BIT) should or does not have to be conducted. It [...] Read more.
In this paper it is shown that the bathtub-curve (BTC) based time-derivative of the failure rate at the initial moment of time can be considered as a suitable criterion of whether burn-in testing (BIT) should or does not have to be conducted. It is also shown that the above criterion is, in effect, the variance of the random statistical failure rate (SFR) of the mass-produced components that the product manufacturer received from numerous vendors, whose commitments to reliability were unknown, and their random SFR might vary therefore in a very wide range, from zero to infinity. A formula for the non-random SFR of a product comprised of mass-produced components with random SFRs was derived, and a solution for the case of the normally distributed random SFR was obtained. Full article
(This article belongs to the Special Issue Challenges in Reliability Analysis of Aerospace Electronics)
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Open AccessArticle Hybrid Rocket Underwater Propulsion: A Preliminary Assessment
Received: 28 January 2019 / Revised: 17 February 2019 / Accepted: 22 February 2019 / Published: 6 March 2019
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Abstract
This paper presents an attempt to use the hybrid rocket for marine applications with a 500 N class hybrid motor. A 5-port high density polyethylene (HDPE) fuel grain was used as a test-bed for the preliminary assessment of the underwater boosting device. A [...] Read more.
This paper presents an attempt to use the hybrid rocket for marine applications with a 500 N class hybrid motor. A 5-port high density polyethylene (HDPE) fuel grain was used as a test-bed for the preliminary assessment of the underwater boosting device. A rupture disc preset to burst at a given pressure was attached to the nozzle exit to prevent water intrusion where a careful hot-firing sequence was unconditionally required to avoid the wet environment within the chamber. The average thrust level around 450 N was delivered by both a ground test and an underwater test using a water-proof load cell. However, it was found that instantaneous underwater thrusts were prone to vibration, which was due in part to the wake structure downstream of the nozzle exit. Distinctive ignition curves depending on the rupture disc bursting pressure and oxidizer mass flow rate were also investigated. To assess the soft-start capability of the hybrid motor, the minimum power thrust, viewed as the idle test case, was evaluated by modulating the flow controlling valve. It was found that an optimum valve angle, delivering 16.3% of the full throttle test case, sustained the minimum thrust level. This preliminary study suggests that the throttable hybrid propulsion system can be a justifiable candidate for a short-duration, high-speed marine boosting system as an alternative to the solid underwater propulsion system. Full article
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Open AccessArticle Structured Control Design for a Highly Flexible Flutter Demonstrator
Received: 28 January 2019 / Revised: 25 February 2019 / Accepted: 1 March 2019 / Published: 5 March 2019
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Abstract
The model-based flight control system design for a highly flexible flutter demonstrator, developed in the European FLEXOP project, is presented. The flight control system includes a baseline controller to operate the aircraft fully autonomously and a flutter suppression controller to stabilize the unstable [...] Read more.
The model-based flight control system design for a highly flexible flutter demonstrator, developed in the European FLEXOP project, is presented. The flight control system includes a baseline controller to operate the aircraft fully autonomously and a flutter suppression controller to stabilize the unstable aeroelastic modes and extend the aircraft’s operational range. The baseline control system features a classical cascade flight control structure with scheduled control loops to augment the lateral and longitudinal axis of the aircraft. The flutter suppression controller uses an advanced blending technique to blend the flutter relevant sensor and actuator signals. These blends decouple the unstable modes and individually control them by scheduled single loop controllers. For the tuning of the free parameters in the defined controller structures, a model-based approach solving multi-objective, non-linear optimization problems is used. The developed control system, including baseline and flutter control algorithms, is verified in an extensive simulation campaign using a high fidelity simulator. The simulator is embedded in MATLAB and a features non-linear model of the aircraft dynamics itself and detailed sensor and actuator descriptions. Full article
(This article belongs to the Special Issue Aeroelasticity)
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Open AccessArticle Electric VTOL Configurations Comparison
Received: 21 December 2018 / Revised: 13 February 2019 / Accepted: 22 February 2019 / Published: 28 February 2019
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Abstract
In the last ten years, different concepts of electric vertical take-off and landing aircrafts (eVTOLs) have been tested. This article addresses the problem of the choice of the best configuration. VTOLs built since the fifties are presented and their advantages, disadvantages, and problems [...] Read more.
In the last ten years, different concepts of electric vertical take-off and landing aircrafts (eVTOLs) have been tested. This article addresses the problem of the choice of the best configuration. VTOLs built since the fifties are presented and their advantages, disadvantages, and problems are discussed. Three representative eVTOLs, one for each main configuration, are compared on five main parameters and three reference missions. The parameters are disk loading, total hover time, cruise speed, practical range, and flight time. The performance of the eVTOLs on the urban, extra-urban, and long-range mission is evaluated computing the time and energy required. The results show that the best configuration depends on the mission. The multirotor is more efficient in hover. The vectored thrust jet is more efficient in cruise and has a higher range. The lift + cruise is a compromise. Full article
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Open AccessArticle Computational Analysis of Compressed Stiffened Composite Panels with Impact Damage
Received: 15 November 2018 / Revised: 9 February 2019 / Accepted: 15 February 2019 / Published: 27 February 2019
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Abstract
A complex modeling technique is presented in this paper for a numerical analysis of compressed stiffened composite panels with impact damage. The numerical technique is based on the LS-Dyna code application, which simulates both the dynamic deformation of the panel subjected to a [...] Read more.
A complex modeling technique is presented in this paper for a numerical analysis of compressed stiffened composite panels with impact damage. The numerical technique is based on the LS-Dyna code application, which simulates both the dynamic deformation of the panel subjected to a local impact and the quasi-static uniform compression of the panel within the local damage zone. The technique has been validated by both impact and compression experimental tests of the stiffened composite panel. The obtained numerical results show that impact damage to the composite panel can reduce the carrying capacity in more than 50% of damaged panels compared to undamaged panels. Full article
(This article belongs to the Special Issue Structural Stability of Aerospace Structures)
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