Experimental Flight Dynamics Testing for Highly Flexible Aircraft

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.

As demonstrated by the content of the project partner letters of support, the research is a low technology-readiness level (TRL) activity that offers the prospect of maturing into much improved techniques for designers of flexible aircraft. The ability to explore coupled aeroelastic/flight dynamics behaviour in a controlled wind tunnel environment, including actuated control surfaces/devices intended to provide load alleviation, will expand on existing capabilities to validate or calibrate predictive numerical models and to confirm model predictions and control laws in multi-degree-of-freedom 'virtual flight' testing.

The research will therefore impact on those working in wing design (including structural optimisation, aeroelastic tailoring and incorporation of load alleviation methods for high aspect ratio flexible wings), overall aircraft architecture, flight dynamics and control and aircraft systems. It applies most directly to those working on very flexible configurations such as high-altitude sensor platforms (e.g. the BAE Systems PHASA35 and Airbus Zephyr 'High Altitude Pseudo-Satellite') however, the testing technique itself, once refined and validated in the context of rig characteristics (opportunities and limitations), will be applicable also to more conventional designs in which coupling between aeroelastic and flight dynamics modes are relevant, such as T-tailed transport aircraft.

The interest shown by industry (Airbus and BAE Systems), Dstl and the Aircraft Research Association (ARA) suggests that the research has the potential to enhance activities in all these sectors. The fact that ARA - which operates a strategically-recognised UK transonic wind tunnel facility - has assessed the technical research objectives as a potential contributor to their own future advanced dynamic testing concepts gives weight to the possible benefits to be realised by taking this research forward.

It is also likely that the concepts entailed in this research may impact other engineering disciplines in which fluid-structure coupling leads to complex dynamics behaviours - in particular the wind turbine sector. Modern turbine blades have extremely high aspect ratios and enough flexibility to introduce significant aeroelastic phenomena that can interact with the support structure (both aerodynamically and structurally). Hence similar structural optimisation and aeroelastic tailoring approaches to those in aircraft are being developed in the renewal energy sector, with associated challenges in experimental validation.