The Morphing Aircraft Project at Bristol (2005-2008)

The morphing aircraft project at the University of Bristol was funded by the European Commission through a Marie Curie Excellence Grant. The project took a systems view of morphing aircraft structures and considered the structural design, airflow, structural dynamics, flight control system, aeroservoelasticity, and sensors and actuators. All these areas interact extensively, for example designing how the structure changes shape is critically dependent on the aerodynamic loads and the required flight control. While each topic is a huge area in its own right, a systems approach is the only appropriate way forward. There were five major topics of interest:

Active winglets as multi-axis effectors and novel aerodynamic concepts

Morphing technology allows the design of novel control effectors, often as a result of biological inspiration. One such design,involves innovative wing-tip extensions featuring differentially-variable dihedral angles, referred to as active winglets. These effectors enable lateral, directional and longitudinal control for a flying wing (a tail-less, body-less airframe with sweptback planform). The initial studies considered a pair of active winglets, and this has been extended to include two pairs of active winglets that can replace all of the conventional control surfaces. The concept has been analysed using a vortex lattice method and compared to wind tunnel results. Further analysis using a more detailed RANS model investigated flow interactions between the winglets. Other work, inspired by the aerodynamics of a dragonfly, investigated the possibility of a low speed airfoil by using a virtual shape definition.

Multistable composite structures

For large deformations of morphing aircraft the orthotropic properties of composite materials may be used. This may, for example, enable the elimination of hinges, which reduces the stress concentration around the pivot points, and consequently reduces the weight penalty introduced with morphing. Residual stresses that occur during manufacture are able to produce a structure with multiple stable states of equilibrium. Finite element analysis procedures have been used to model both the cool-down during manufacture and the snap-through during morphing. These predictions have been validated quantitatively for rectangular plates. Morphing aircraft examples, namely wing-tip devices and a variable camber wing profile, have been designed, analysed using finite element analysis, and manufactured. The wing-tip device was successfully tested in the wind tunnel.

Aeroelastic tailoring

Laminated composite materials are commonly employed in the aerospace industry due to their high strength and stiffness ratios. The elastic coupling properties of composite materials may used to morph the structure. Thus, for example, a wing may be designed to passively control the wing twist subject to the lift on the wing. The challenge is to develop an efficient optimisation strategy, based on a suitable choice of objective function and constraints. A novel two level strategy has been developed and applied to panels with T shaped stiffeners. Effects such as the skin-stiffener flange interaction, and variable thickness plates have been included. Approximate closed form solutions for buckling in stiffened panels have been developed that could be included in the optimisation. The extension to global optimisation of a wing structure has been developed, with the objective of minimum wing weight and structural and aerodynamic constraints were imposed. The two level optimisation strategy was used and a reduction in induced drag of 1.4% was achieved, albeit with an increased weight. Current activity will determine the best objective function to use, for example the use of specific aircraft range to optimise the fuel efficiency of the vehicle.

Compliant mechanisms

A compliant mechanism provides the desired shape alterations by elastic deformation as opposed to a mechanised approach. Such methods provide designs that are capable of smooth, conformal deflections together with the prospect of reduced inspection and maintenance and zero backlash. The research has concentrated on airfoil camber change, and the topology optimisation and actuator location problem is formulated based on a truss structure with beam elements, and includes the aerodynamic loads. The optimum location of a series of actuators has been determined for a target deformation with minimum actuator force. As an alternative the dimensions of the truss members have been optimised so that only a single actuator is required to obtain the target deformation. One clear requirement is the use of structural elements which have in-plane flexibility, but are stiff in bending. Elements composed of a cellular substructure and corrugated composite skins with silicone rubber surfaces have been analysed statically, and have been experimentally verified for the skin.

Flight mechanics and control of morphing and flexible aircraft

The flight control of morphing aircraft provides significant challenges such a highly coupled flight dynamics and the optimisation of the control allocation problem. The dynamics and response of a small-scale manned flying wing has been investigated based on an active winglets concept. The flying wing is modelled as a set of hinged rigid bodies, with actuators to change the winglet dihedral angles and provide the aerodynamic moments for maneuvering. The effect of the variation in the dihedral angles on the inertia properties, the centre of gravity location and modal parameters have been assessed. Issues related to the control of such a vehicle are still under investigation. The research has been extended to wings that morph continuously and seamlessly using a distribution of actuators. This requires an understanding of the interaction between the aerodynamics, structures and controls. An aeroelastic tool has been developed using an equivalent plate model of the structure and a vortex lattice model for the aerodynamics. This allows the tailoring of the structural flexibility and optimization of the actuators.