Wing Tilt Research Articles

Design and aerodynamic analysis of a VTOL tilt-wing UAV

The aerodynamic design and analysis of an Unmanned Air Vehicle, capable of vertical take-off and landing by employing fixed four rotors on the tilt-wing and two rotors on the tilt-tail, will be presented in this study. Both main wing and the horizontal tail can be tilted 90°. During VTOL, transition and forward flight, aerodynamic and thrust forces have been employed. Different flight conditions, including the effects of angle of attack, side slip, wing tilt angle and control surfaces deflection angle changes, have been studied with CFD analysis. For a Tilt-Wing UAV, there are challenges like high non-linearity, vulnerability to disturbances during hover and transition, cross-coupling between three axes, very low time to double amplitude of the open-loop system. Therefore, the aerodynamic analysis has been done in detail considering especially the challenges in transition phase of the flight envelope.

Control Augmentation System Design for Quad-Tilt-Wing Unmanned Aerial Vehicle via Robust Output Regulation Method

Quad-tilt-wing (QTW) configuration is a promising one for airplanes to achieve vertical take-off and landing (VTOL), which has a pair of tandem wings that can tilt from vertical to horizontal positions. This unique configuration allows the aircraft to perform a VTOL as well as fly at high cruising speed. For that reason, QTW unmanned aerial vehicles (UAV) have vast potential in civil applications. However, designing a controller to stabilize and control the attitude of QTW UAV is a challenging mission since the aerodynamic characteristics of the aircraft greatly change in accordance with the wing tilt angle. Based on the previous work, this paper examines the possibility for improving the tracking performance of the QTW UAV by the application of the modified robust output regulation method. An experiment was conducted to examine the control performance of the designed controller.

The role of surface diffusion and wing tilt in the formation of localized stacking faults in high In-content InGaN MQW nanostructures

Yellow and green emitting multiple quantum well structures are grown on nanostripe templates with {10-11} facets. SEM and cathodoluminescence measurements show a correlation between rough surface morphology near the bottom of the stripes and non-radiative recombination centers. Transmission electron microscopy (TEM) analysis shows that these surface instabilities are a result of stacking faults generated from the quantum well (QW) regions near the bottom of the pyramid that propagate to the surface. HRTEM images show that the stacking faults are I1 type which is formed by removal of one half basal plane to relieve the compressive strain in the InGaN QW. Thicker QWs near the bottom as a result of growth rate enhancement due to the surface diffusion of the precursors from the mask regions cause increased strain. Additionally, the compressive strain induced by the bending of the nanostructure towards the growth mask further increases the strain experienced by the QW thereby causing the localized defect generation.

Flight Controller Design and Demonstration of Quad-Tilt-Wing Unmanned Aerial Vehicle

This paper concerns the flight controller design of aircraft that has significant changes in aerodynamic characteristics and addresses the flight controller design of a quad-tilt-wing unmanned aerial vehicle that has vertical takeoff and landing as well as high-speed cruise capabilities. The quad-tilt-wing unmanned aerial vehicle has a tandem wing that tilts between vertical to horizontal positions. This configuration change significantly alters its aerodynamic characteristics, and therefore requires a gain-scheduled flight controller. The gain-scheduled flight controller, which consists of a gain-scheduled stability augmentation system, a gain-scheduled control augmentation system, and a gain-scheduled turn coordinator, is also required to be robust against modeling errors because the motion dynamics of the quad-tilt-wing unmanned aerial vehicle are inherently unstable at almost all wing tilt angles and the precise aerodynamic characteristics are not available. To this end, this paper proposes a design method for structured gain-scheduled flight controllers by combining a conventional gain-scheduled controller design method and multiple-model approach to satisfy the robustness requirement, and it designs a gain-scheduled flight controller for the quad-tilt-wing unmanned aerial vehicle. The effectiveness of the method is demonstrated by flight tests during which the quad-tilt-wing unmanned aerial vehicle successfully achieved full conversion flight: that is, vertical takeoff, accelerating transition, high-speed cruise, decelerating transition, and vertical landing, as well as super-short takeoff.

Extractor X — Autonomous Tilt Rotor UAV

This paper looks into conceptualization, implementation and validation of the potential role that hybrid quad tilt rotor unmanned aerial vehicles (UAV) may undertake in the near future, especially so in military applications. A tilt rotor is designed, built and analyzed with wind tunnel data, theoretical calculations and flight log data. In addition, the paper will discuss the flight mechanics involved during its transition from hover to forward flight. As the tandem wing tilt rotor encompasses two unconventional designs, numerous mathematical models were developed in the design of the UAV.

Recent progress of GaN growth on maskless chemical-etched grooved sapphire substrate

A new GaN lateral epitaxial overgrowth (LEO) technique using a chemical-etched grooved sapphire substrate is demonstrated here. The substrate fabrication, selective growth mechanism and dislocation reduction mechanism are discussed. By applying this technique, improvement in the quality of wing tilt free GaN LEO films and enhancement of GaN LEDs can be achieved.

Wing tilt investigations on GaN epilayer grown on maskless grooved sapphire by MOCVD

GaN epilayer is grown on maskless periodically grooved sapphire by metal organic chemical vapor deposition (MOCVD) in this article. Wing tilt is detected by high resolution X-ray rocking curve. Inhomogeneous deformations between the wing and mesa regions are found by atomic force microscopy (AFM) characterization. The stress distribution is investigated using finite-element simulations. Inhomogeneous stress distribution in the mesa and wing regions is shown, which is also confirmed by micro-Raman spectroscopy. The results show that the wing tilt in GaN layers grown on maskless periodically grooved sapphire mainly originates from the different deformations for mesa and wing region caused by inhomogeneous stress distribution.

Rocking Curve Imaging Studies of Laterally Overgrown GaAs and GaSb Epitaxial Layers

X-ray rocking curve imaging technique was used to study crystallographic perfection of laterally overgrown epitaxial structures. We focus on rocking curve imaging studies of Si-doped GaAs and GaSb laterally overgrown layers grown by liquid phase epitaxy on SiO2 masked GaAs and GaSb/GaAs substrates, respectively. High spatial resolution offered by rocking curve imaging technique allows studying the effect of laterally overgrown epitaxial wing tilt towards the mask. Distribution of tilt magnitude over the area of laterally overgrown epitaxial stripes is easily determined. In heteroepitaxial GaSb/GaAs laterally overgrown epitaxial structures local mosaicity in the wing area was detected. Since individual grains are clearly visible on rocking curve imaging maps, their size and relative misorientation can be determined.

X-ray diffraction micro-imaging of strain in laterally overgrown GaAs layers. Part I: analysis of a single GaAs stripe

Spatially resolved X-ray diffraction (SRXRD) is used for micro-imaging of strain in GaAs:Si layers grown by liquid phase epitaxial lateral overgrowth (ELO) on SiO2-masked GaAs substrates. We show that laterally overgrown parts of the layers (wings) are tilted towards the underlying mask. By SRXRD mapping local wing tilt is easily distinguished from macroscopic sample curvature. The direction of the tilt and distribution of tilt magnitude across the width of each layer can also be readily determined. This allows measuring of the shape of the lattice planes in individual ELO stripes. Downward wing tilt disappears completely when the mask is removed by selective etching. Then residual strain in ELO layers is exposed. In particular, upward tilt is found in free-standing ELO wings. Numerical simulations show that this phenomenon is caused by different concentrations of silicon dopant in vertically and laterally grown parts of the layer.

X-ray diffraction micro-imaging of strain in laterally overgrown GaAs layers. Part II: analysis of multi-stripe and fully overgrown layers

As a development of our previous analysis of a single GaAs stripe, in this work spatially resolved X-ray diffraction (SRXRD) was applied for micro-imaging of strain in partially and fully overgrown GaAs layers grown by epitaxial lateral overgrowth (ELO) on SiO2-masked GaAs substrates. We show that a standard X-ray diffraction providing information integrated over many stripes leads to overestimation of lattice misorientation in such samples. On the contrary, the SRXRD allows precise measurements of local wing tilts of individual ELO stripes with micrometer-scale spatial resolution. A complex strain field was found in fully overgrown GaAs ELO samples. With the use of the SRXRD technique, the distribution of this strain was measured that allowed reproducing the shape of deformed lattice planes both inside the epitaxial layer as well as in the substrate underneath.

Epitaxial lateral overgrowth of a-plane GaN by metalorganic chemical vapor deposition

We report on epitaxial lateral overgrowth (ELO) of (112¯0) a-plane GaN by metalorganic chemical vapor deposition. Different growth rates of Ga- and N-polar wings together with wing tilt create a major obstacle for achieving a smooth, fully coalesced surface in ELO a-plane GaN. To address this issue a two-step growth method was employed to provide a large aspect ratio of height to width in the first growth step followed by enhanced lateral growth in the second by controlling the growth temperature. By this method, the average ratio of Ga- to N-polar wing growth rate has been reduced from 4–6 to 1.5–2, which consequently reduced the wing-tilt induced height difference between the two approaching wings at the coalescence front, thereby making their coalescence much easier. Transmission electron microscopy showed that the threading dislocation density in the wing regions was 1.0×108 cm−2, more than two orders of magnitude lower than that in the window regions (4.2×1010 cm−2). However, a relatively high density of basal stacking faults of 1.2×104 cm−1 was still present in the wing regions as compared to c -plane GaN, where they are rarely observed away from the substrate. Atomic force microscopy (AFM) measurements showed two orders of magnitude higher density of surface pits in the window than in the wing regions, which were considered to be terminated by dislocations (partial ones related to stacking faults and full ones) on the surface. The existence of basal stacking faults was also revealed by AFM measurements on the a-plane ELO sample after wet chemical etching in hot H3PO4∕H2SO4 (1:1). The extensions of Ga-polar wings near the meeting fronts were almost free of stacking faults. The improvement of crystalline quality in the overgrown layer by ELO was also verified by near field scanning optical microscopy and time-resolved photoluminescence measurements; the former showing strongly enhanced luminescence from the wing regions, and the latter indicating longer decay times (0.25 ns) compared to a standard a-plane GaN template (40 ps).

Elimination of Crystallographic Wing Tilt of Canti-BridgedEpitaxial Laterally Overgrown GaN Films by Optimizing Growth Procedure

Canti-bridged epitaxial lateral overgrowth (CBELO) of GaN is performedby metalorganic chemical vapour deposition (MOCVD) on maskless V-groovedsapphire substrates prepared by wet chemical etching with different mesawidths. The wing tilt usually observed in ELO is not found in the CBELOGaN with wide mesa widths, while it can be detected obviously in the GaNwith narrow mesa widths. The wing tilt of CBELO GaN grown on a groovedsapphire substrate with narrow mesa can be controlled by adjusting thethickness of the nucleation layer. The dependence of the wing tilt onthe nucleation layer thickness is studied. Cross-sectional scanningelectron microscopy is used to characterize the geometry of the wingregions, and double crystal x-ray diffraction is used to analyse thestructural characteristics and to measure the magnitude of thecrystalline wing tilt. It is found that the crystalline wing tilt can beeliminated completely by first growth of a thin nucleation GaN layerthen the CBELO GaN. Possible reason and the origin of the wing tilt inCBELO GaN films are also discussed.

Spatial variation of optical and structural properties of ELO GaN directly grown on patterned sapphire by HVPE

A 275 µm thick GaN layer was directly grown on the SiO2-prepatterned sapphire in a home-built vertical hydride vapour phase epitaxy (HVPE) reactor. The variation of optical and structure characteristics were microscopically identified using spatially resolved cathodeluminescence and micro-Raman spectroscopy in a cross section of the thick film. The D 0 XA line with the FWHM of 5.1 meV and etch-pit density of 9 × 106 cm−2 illustrated high crystalline quality of the thick GaN epitaxial layer. Optically active regions appeared above the SiO2 masks and disappeared abruptly due to the tapered inversion domains at 210–230 µm thickness. The crystalline quality was improved by increasing the thickness of the GaN/sapphire interface, but the region with a distance of 2 µm from the top surface revealed relatively low quality due to degenerate surface reconstruction by residual gas reaction. The x-ray rocking curve for the symmetric (0 0 2) and asymmetric (1 0 2) reflections also showed good quality and a small wing tilt of the epitaxial lateral overgrowth (ELO) GaN.

Characterization of growth defects in thin GaN layers with X‐ray microbeam

AbstractThe spatially resolved distribution of strain, misfit and threading dislocations, and crystallographic orientation in uncoalesced GaN layers grown on Si(111) by maskless cantilever epitaxy or by pendeo epitaxy on SiC was studied by white beam Laue X‐ray microdiffraction, scanning electron and orientation imaging microscopy. Tilt boundaries formed at the column/wing interface depending on the growth conditions. A depth dependent deviatoric strain gradient is found in the GaN. The density of misfit dislocations as well as their arrangement within different dislocation arrays was quantified. Two different kinds of tilt (parallel and perpendicular to the stripe direction) manifested themselves by mutually orthogonal displacements of the (0006) GaN Laue spot relative to the Si(444) Laue spot. The origin of the tilts is discussed with respect to the miscut of the Si(111) surface and misfit dislocations formed at the interface. Regular oscillations of the conventional wing tilt were observed. Irregular crystallographic tilts fluctuations were found in the direction parallel to stripes. The amplitude of fluctuations is an order of magnitude smaller for layers with a lower defects density. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The origin of wing tilt for uncoalesced GaN grown on maskless grooved sapphire fabricated by wet chemical etching

In this paper, wing tilt was detected by high resolution X-ray rocking curve for uncoalesced GaN epilayer grown on maskless periodically grooved sapphire fabricated by wet chemical etching. Stress distribution was characterized by the frequency shift in micro-Raman spectroscopy measurement. It shows inhomogeneous stress distribution between the mesa and wing region, which is in agreement with the finite-element analysis simulations. Meanwhile, the wing tilt calculated by finite-element analysis agrees well with the rocking curve result. The results show that the different relaxation of stress causes inhomogeneous displacements between the mesa and wing region is the main reason of wing tilt formation.

Crystallographic wing tilt and thermal-stress distribution of GaN laterally overgrown on maskless V-grooved sapphire substrate by metal-organic chemical vapor deposition

The crystallographic wing tilt, commonly observed for laterally overgrown GaN films, has been essentially overcome by the authors’ newly developed GaN lateral-epitaxial overgrowth (LEO) method or the so-called cantibridge epitaxy (CBE), which employs a maskless V-grooved c-plane sapphire as the substrate. The absence of the void between the coalesced wings in the CBE GaN films is found to be a key factor for the elimination of the wing tilt in the absence of any mask material. Finite element analysis is performed for voided and nonvoided CBE GaN samples. This analysis verified the correlation between the coalescence void and the crystallographic wing tilt. Thermal-stress distribution in the GaN film is presented as well. The agreement between the measured and simulated tilt angles denotes that the essence of the crystallographic wing tilt in the maskless LEO GaN films is not the same as that of the conventional LEO GaN film with a mask, which did not develop during the epitaxy process, but instead in the ...

Tilt and dislocations in epitaxial laterally overgrown GaAs layers

Transmission electron microscopy (TEM) and high-resolution x-ray diffraction were used to study the crystalline quality of GaAs layers grown by liquid phase epitaxial lateral overgrowth (ELO) on SiO2-masked (001) GaAs substrates. A low-angle grain boundary with a well-organized set of dislocations accommodating the misorientation of tilted ELO wings was found at the coalescence front of ELO layers. Similar behavior is often reported for GaN on sapphire ELO structures. Unlike the GaN case, however, no dislocations were found above edges of the mask, which is explained by the much smaller wing tilt angle in our case. The geometry of the dislocation network and analysis of thickness fringes on TEM images were used to measure misorientation of merging ELO wings. The values obtained are in good agreement with those determined by x-ray diffraction.

Defect reduction in (112¯) a-plane GaN by two-stage epitaxial lateral overgrowth

The authors report a two-stage epitaxial lateral overgrowth (ELO) method to get uniformly coalesced (112¯0) a-plane GaN using metal organic chemical vapor deposition by employing a relatively lower growth temperature in the first stage followed by conditions leading to enhanced lateral growth in the second. Using a two-stage ELO method the average Ga-polar to N-polar wing growth rate ratio has been reduced from 4–6 to 1.5–2, which consequently reduced the height difference between the two approaching wings at the coalescence front that resulted from the wing tilt (0.44° for Ga and 0.37° for N wings, measured by x-ray diffraction), thereby making their coalescence much easier. Transmission electron microscopy showed that the threading dislocation density in the wing areas was 1.0×108cm−2, more than two orders of magnitude lower than that in the window areas (4.2×1010cm−2). However, high density of basal stacking faults of 1.2×104cm−1 was still present in the wing areas as compared to c-plane GaN where they are rarely observed away from the substrate. Atomic force microscopy and photoluminescence measurements on the coalesced ELO a-plane GaN sample also indicated improved material quality.

Spatially resolved distribution of dislocations and crystallographic tilts in GaN layers grown on Si(111) substrates by maskless cantilever epitaxy

The spatial distribution of strain, misfit and threading dislocations, and crystallographic orientation in uncoalesced GaN layers grown on Si(111) substrates by maskless cantilever epitaxy was studied by polychromatic x-ray microdiffraction, high resolution monochromatic x-ray diffraction, and scanning electron microscopy. Tilt boundaries formed at the column/wing interface depending on the growth conditions. A higher lateral to vertical growth ratio suppressed sidewall deposition and was found to produce larger lattice tilts in the GaN films. Two kinds of crystallographic tilts are observed in the films. The measurements revealed that the free-hanging wings are tilted upward at room temperature in the direction perpendicular to the stripes. Finite element simulations of the thermally induced part of the wing tilt are presented. Moreover, a misorientation between the GaN(0001) and the Si(111) planes is observed in the parallel to the stripe direction. Its origin is discussed with respect to the strain of the epitaxial GaN on a miscut Si(111) surface and misfit dislocations formed at the interface.

X‐ray microdiffraction imaging investigations of wing tilt in epitaxially overgrown GaN

AbstractThe crystalline quality in epitaxially laterally overgrown (ELO) GaN and the amount of wing tilt is characterized on a local basis, with high spatial and angular resolution. A method of full‐field X‐ray microdiffraction imaging, termed rocking curve imaging, is used to record simultaneously a large set of local X‐ray diffraction profiles originating from sample surface areas of micrometer size. x –ω maps of diffracted intensity allow to quantify the amount of wing tilt in individual lateral ELO periods as well as to monitor the fluctuations of tilt between adjacent periods. Automated shape analysis of the full set of local rocking curves provides a means to quantitatively characterize the local crystalline perfection of GaN. The ELO window and wing regions can be clearly separated; comparison indicates an average improvement of crystal quality by a factor 3–4 due to the lateral overgrowth process. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)