Experimental Flight Dynamics Testing for Highly Flexible Aircraft

为了不断提高飞机效率，航空航天业正在考虑采用更灵活、更精细的航空结构系统。其中一种方法是增加机翼的展弦比（AR），即增加翼展，使机翼更细长。这种高展弦比机翼为民用和军用运输机以及某些类型的无人驾驶飞机（如用于高空长航时传感、环境监测等）提供了改善空气动力学效率的前景。高增强现实机翼通常比传统设计更灵活，以尽量减少结构质量。这反过来又会增加机翼本身和整个飞机的动态响应的复杂性。这些响应包括不同的运动模式，与机体气动弹性（指机体气动、结构和惯性特性之间的相互作用）和所谓的“刚体”运动（代表飞行器独立于任何弹性/灵活性影响的行为）和飞行控制模式有关。在传统（更刚性）飞机的设计和分析中，气动弹性模式通常比飞行动力学和控制模式频率更高，并且通常能够使用线性方法很好地建模；在这种飞行器中，任何与飞行动力学行为的耦合的程度和复杂性都很低。然而，机身越灵活，所有这些模式之间可能的相互作用（耦合）就越强。此外，非线性的影响也在增加，尤其是高增阻翼的几何非线性，以及其他潜在的非线性特性，如空气动力学和控制系统。近年来，高柔性飞机的数值模拟方法得到了广泛的研究和发展，其中包括必要的耦合和非线性现象。然而，通过控制实验验证或校准这些预测方法是一项挑战——通常通过在风洞中测试机翼作为刚性支撑在其根部的悬臂来解决。在现有的测试平台中，扩展实验方法以适应捕获飞行动力学和控制模式以及气动弹性模式之间耦合所需的自由度的范围非常有限。这样的钻机确实存在，通常用于有限的运动幅度，以测试气动弹性不稳定性的开始，而不是针对大振幅的机翼弯曲，扭转和模型运动，以开发或探索非线性。本文提出了一种新的实验概念，允许在受控风洞环境中研究这种耦合行为。它需要对布里斯托尔大学新颖的5自由度动态测试平台的当前测试方法进行具有挑战性的扩展，并设计合适的柔性驱动和仪表模型。该程序将建立在之前的刚体测试成果的基础上，并将扩展早期的主动钻机控制工作，以确保风洞中看到的耦合动态现象尽可能与自由飞行的动态现象相匹配。这一探索性研究的成功结果可以推动这种新的测试技术在工业风洞中实施的发展。它还将评估扩展在柔性机翼中纳入减载控制能力的可行性。此外，它将产生增强类型的数据，以评估非线性计算建模技术的预测能力，并使其适应或校准测量行为。通过这种方式，提出的研究提供了大幅提高风洞能力的前景，以支持具有复杂动态相互作用的未来先进飞机机翼/机身的设计和分析。

The aerospace sector, in its ongoing quest to improve aircraft efficiencies, is considering more flexible and finely tuned aero-structural systems. One such approach is to increase the aspect ratio (AR) of the wings, i.e. increase the span such that the wings are more slender. Such higher aspect ratio wings offer the prospect of improved aerodynamic efficiency for civil and military transport aircraft and for certain types of unmanned aircraft, such as those used for high-altitude long-endurance sensing, environmental monitoring, etc.High AR wings are typically more flexible than conventional designs in order to minimise structural mass. This in turn can increase the complexity of the dynamic responses of the wings themselves and the aircraft as a whole. These responses comprise different modes of motion, associated with airframe aeroelasticity (which refers to the interaction between airframe aerodynamic, structural and inertial properties) and with the so-called 'rigid-body' motions (representing the behaviour of the air vehicle independent of any elastic/flexibility effects) and flight control modes.In design and analysis of conventional (more rigid) aircraft, the aeroelastic modes are typically at higher frequencies than the flight dynamics and control modes and are usually able to be well modeled using linear methods; in such air vehicles the extent and complexity of any coupling with the flight dynamics behaviour is low. However, the more flexible the airframe, the stronger the likely interaction (coupling) between all these modes. Furthermore, the influence of nonlinearity increases - in particular geometric nonlinearity in high AR wings, along with other potential nonlinear characteristics such as in the aerodynamics and control system.Methods for numerical modeling of highly flexible aircraft, incorporating the necessary coupling and nonlinear phenomena, have been extensively researched and developed in recent years. Validating or calibrating these predictive methods via controlled experiments is, however, a challenge - usually addressed by testing a wing as a cantilever supported rigidly at its root in a wind tunnel. There is very limited scope in existing test rigs for extending the experimental approaches to accommodate the degrees of freedom needed to capture the coupling between the flight dynamics and control modes and the aeroelastic modes. Such rigs that do exist are usually intended for limited motion amplitudes in order to test for onset of aeroelastic instability, rather than being aimed at large-amplitude wing bending, torsion and model motions to exploit or explore nonlinearity.This proposal introduces a new experimental concept that allows this coupled behaviour to be investigated in a controlled wind tunnel environment. It entails a challenging extension to the current testing approach for the University of Bristol's novel 5-degree-of-freedom dynamic test rig and the design of suitable flexible actuated and instrumented models. The procedure will build on previous rigid-body test accomplishments and will extend earlier work on active rig control to ensure that coupled dynamic phenomena seen in the wind tunnel match those of free flight as closely as possible.A successful outcome of this exploratory research could launch the development of this new test technique towards implementation in industrial wind tunnels. It will also assess the feasibility of extending the capability to incorporate load alleviation control in the flexible wings. Furthermore, it will generate enhanced types of data to evaluate the predictive ability of nonlinear computational modelling techniques and to adapt or calibrate them to measured behaviours. In this way, the proposed research offers the prospect of substantially improved wind tunnel capability to support design and analysis of future advanced aircraft wings/airframes featuring complex dynamic interactions.