Antisymmetric Twist Research Articles

Study on fracture behavior of layered limestone under mixed mode I/III loading

Fractures in anisotropic rocks pose challenges in rock engineering. However, fracture characteristics of layered rocks under mixed mode I/III loading are scarcely investigated. In this study, pure mode I, III, and mixed mode I/III fracture tests were conducted using edge notch disc bend (ENDB) specimens made of layered limestone with different bedding angles (α = 0°, 30°, 45°, 60°, and 90°). The results showed that the equivalent fracture toughness (Keff) followed the trend: Mode I > Mixed Mode I/III > Mode III at α < 45°, while the opposite trend is observed at α ≥ 45°. Keff decreased and then increased with α under mode I and mixed mode I/III loading, increased first then decreased and finally increased with α under mode III loading. Conversely, the fracture energy trended: mode III > mixed mode I/III > mode I. The acoustic emission (AE) characteristics of the specimens were significantly influenced by the bedding angle α and loading mode as well as bedding strength. The AE signal for α = 0° was most conspicuous among the loading modes and followed by tensile/torsional damage and fracture along the bedding plane, and least when fracture only occurred along the bedding plane. The fracture trajectory depended on the combination of KIc, KIIIc, and bedding strength. Generally, in pure mode I loading, the failure mode of the specimen changed from complete layer penetration to along the bedding plane as α increased. In contrast, in pure mode III and mixed mode I/III loading, the step-like failure pattern of antisymmetric twist transformed along the bedding plane as α increased. Furthermore, the MTS criterion agrees better with the KIII/KIc test values for α = 30° and 90°, and the MTSED criterion agrees better with the test results for α ≤ 30°. Test results could provide new insights into the mixed mode I/III fracture mechanism of layered rocks.

Adverse-Yaw Control During Roll for a Class of Optimal Lift Distributions

Lifting-line theory has formed the basis of wing-design understanding for nearly a century. This theory has previously been used to develop a class of optimal lift distributions that minimize induced drag for finite wings under specific structural and aerodynamic constraints. This class of lift distributions was first introduced by Prandtl in 1933 and includes the elliptic lift distribution and the bell-shaped lift distribution, among others, as special cases. Here, it is shown that, for all lift distributions in this class except the elliptic lift distribution, an antisymmetric twist distribution can be added to the wing to control the adverse yawing moment during roll. The requisite amount of this additional twist distribution depends on the prescribed yawing-moment sign and magnitude, and it can be used to produce zero adverse yaw or even proverse yaw during roll. Of course, employing this methodology to control the roll–yaw coupling produces additional induced drag relative to the case without roll–yaw coupling control. This induced-drag penalty is studied in detail and shows that a symmetric lift distribution within this class can be selected to minimize induced drag for a desired roll–yaw coupling. Roll initiation and steady rolling rate are both considered. Results can be applied to the design of morphing aircraft to minimize the use of a vertical control surface for yaw control.

Analytic and computational analysis of wing twist to minimize induced drag during roll

Geometric and/or aerodynamic wing twist can be used to produce a lift distribution that results in a rolling moment. A decomposed Fourier-series solution to Prandtl’s lifting-line theory is used to develop analytic spanwise antisymmetric twist distributions for roll control that minimize induced drag on wings of arbitrary planform in pure rolling motion. Roll initiation, steady rolling rate, and the transition between the two are each considered. It is shown that if these antisymmetric twist distributions are used, the induced drag is proportional to the square of the rolling moment, and the induced drag during a steady rolling rate is equal to that on the wing at the same lift coefficient with no rolling rate or antisymmetric twist distribution. Results also show that if these antisymmetric twist distributions are used on straight, tapered wings without symmetric twist, any rolling maneuver for which the rolling rate and rolling moment have the same sign will always produce a yawing moment in the opposite direction. Computational results are also included, which were obtained using a gradient-based optimization algorithm in combination with a modern numerical lifting-line algorithm to find the optimum twist solutions. The resulting twist, induced drag, and yawing moment solutions compare favorably with the analytic solutions developed in the text. The solutions presented here can be used to inform the design of morphing aircraft.

Flexible Composite Wing with Internal Actuation for Roll Maneuver

Abstract : This paper is concerned with designing an optimum composite flexible wing structure to enhance roll maneuver capability at high dynamic pressures using an embedded actuating system without external control surfaces. A minimum weight design, with constraints on strength for three different flight conditions, on the frequency distribution and lift effectiveness was used for this study. The elastic twist and camber is achieved by providing a system of actuating elements distributed within the internal substructure of the wing to provide control forces. The modal approach is used to develop the dynamic equilibrium equations which culminates in the steady roll maneuver of a wing subjected to aerodynamic loads and the actuating forces. The distribution of actuating forces to achieve the specified steady roll rate and roll angle of the flexible vehicle within a specified time was determined by using Independent Modal-Space Control (IMSC) design approach. Here, a full-scale realistic wing is considered for the assessment of the strain energy and distribution of actuator forces required to produce the antisymmetric twist and camber deformation to achieve the specified roll performance.

Optimization of Flexible Wing Without Ailerons for Rolling Maneuver

An optimum flexible wing structure with enhanced roll maneuver capability at high dynamic pressures is designed. A flexible wing optimized for weight with constraints on strength for 9-g symmetric pull-up maneuver at M=0.85 and constraints on the frequency distribution were used. Elastic twist and camber are achieved by providing a system of actuating elements distributed within the internal substructure of the wing to provide control forces. The modal approach was used to develop equilibrium equations for the steady roll maneuver of a wing subjected to acrodynamic loads and actuating forces. The distribution of actuating forces to achieve a specified flexible roll rate was determined by using an optimal control design approach. Here, a full-scale realistic wing was considered for the assessment of strain energy as a measure of the necessary power required to produce the antisymmetric twist and camber deformation to achieve the required roll performance

Composite wing optimization for enhancement of the rolling maneuver in subsonic flow

This paper is concerned with designing an optimum composite wing structure with enhanced roll maneuver capability at high dynamic pressures using a control system to retwist and recamber the wing. Antisymmetric twist and camber distribution on a composite flexible wing, weight optimized with constraints on strength and aileron efficiencies also satisfying flutter requirements, were determined to counter act the detrimental twisting moment of the aileron at high dynamic pressure. The retwisting and recambering of the wing is achieved by providing control forces obtained from a technique referred to as “Fictitious Control Surfaces” (FCS). Two different optimally designed wings with unacceptable roll performance were selected for roll performance improvement investigation. The control energy necessary to retwist the wing to improve the unacceptable roll performance for subsonic design condition was calculated to provide the structural designer an alternative, to the traditional increase of stiffness to satisfy roll performance.

Method for Enhancement of the Rolling Maneuver of a Flexible Wing

A technique for augmenting the aileron to increase the roll rates of a high-performance aircraft at high dynamic pressures for enhancement of the rolling maneuver is examined. Antisymmetric twist and camber distribution on a realistic flexible wing is determined to counteract the detrimental twisting moment of the aileron rotation to achieve recovery of the lost roll rates at high dynamic pressures. A method for prescribing the antisymmetric wing twist and camber distribution by reversing the twist and camber resulting from use of the aileron only is described. The retwisting and recambering of the wing is achieved by providing control forces obtained from a technique referred to as fictitious control surfaces. The technique of retwisting and recambering a flexible wing demonstrates a full recovery of roll rate at all dynamic pressures. Here, a full-scale realistic wing is considered for the assessment of strain energy as a measure of necessary control energy required to produce the antisymmetric twist and camber deformation on the aileron-twisted wing to recover the lost roll performance.