BILL READ reports on how aircraft designers are looking at ways to improve aircraft performance by changing their shape to adapt to different flying conditions.
In 2001 NASA conducted research on the 21st Century Aerospace Vehicle, also called the Morphing Airplane. The aim of the Morphing Project was not to design a flying prototype (although it did produce the above artist’s impression of how it might look) but to develop and assess smart technologies (including materials, adaptive structures and micro-flow control) that could enable flight configuration changes for optimum flight characteristics. (NASA)
Birds and aircraft both fly. But they fly in very different ways. Birds can bend, twist or deform their wings and bodies to turn or change their speed. Birds can assume different shapes for taking off hovering, diving, landing or adapt to different flying conditions, such as wind gusts. A conventional aircraft, on the other hand, is designed to be rigid. It can only manoeuvre by moving parts of its rigid structure, such as the flaps and ailerons on the wings and the the elevators and rudder at the tail.
As any aerodynamics student will tell you, conventional aircraft wings are a compromise in terms of performance. While they allow the aircraft to fly in a range of different flight conditions, the performance at each condition is less than optimal. A conventional aircraft can change the geometry of its wings to adapt to different circumstances, such as increasing lift for take-off and landing but only to a limited extent.
Ever since the dawn of aviation, designers have been looking at ways of learning from nature and adapting some of the techniques adopted by birds to use on aircraft. The aim is to produce a ‘morphing’ aircraft which can change its shape, in particular its wings, to adapt to different flying conditions. A morphing aircraft capable of adapting to different flying conditions, such as take-off, landing, accelerating and cruising, could result in a design which is aerodynamically efficient, has more lift and less drag, be more agile in flight and has the environmental benefits of both reduced noise and fuel consumption. An aircraft could also adjust its shape depending on the weight and distribution of fuel being carried.
Another advantage of morphing aircraft could be multiple missions. At present, the shape of an aircraft is determined by what it is used for. Surveillance UAVs, for example, are fitted with long, thin wings designed for low-speed, long-endurance flight. Fighter aircraft, on the other hand, have shorter swept back wings to reduce drag and enable them to manoeuvre at high speeds. A morphing aircraft would no longer have to be mission-specific as it could adapt its shape to perform a variety of different tasks. For example, a maritime patrol aircraft could fly quickly to a particular area and then change its wing shape to one most suited to performing low-speed surveillance.
The time is Wright
Changing the shape of an aircraft wing is not a new idea. Otto Lilienthal used morphing in his glider flights at the end of the 19th century while the Wright brothers fitted wing-warping mechanisms in the first Wright Flyer which controlled the roll of the aircraft by twisting the wings using cables actuated directly by the pilot. Moveable variable geometry swing wings have also been fitted to a number of military aircraft, including the Grumman F10F Jaguar; General Dynamics F-111, B-1B Lancer bomber and the F-14 Tomcat, the Panavia Tornado, as well as a number of Russian designs, such as the Sukhoi Su-17.
Until recently, efforts towards creating morphing aircraft has proved to be an expensive and complex business with many technical problems to be overcome. The materials needed to create a flexible structure have not been available while the actuation mechanisms needed to change the shape make the aircraft heavier. However, in recent years, the drive to develop more-fuel efficient aircraft designs, together with the development of ‘smart materials’ and new technology which allows designers to distribute actuation forces and power more optimally and efficiently, has led to a renewal of interest in morphing designs.
Researchers into morphing aircraft need to take into account a wide range of factors, including the structure of the aircraft, its aerodynamics, actuation requirements, weight, and likely missions. Flight control also represents another big challenge, as there are more variables to consider which may require complex control systems.
In theory, a morphing aircraft could change the shape of any of its parts — including the fuselage, wing, engines or tail. However, altering the shape of a fuselage, particularly one with people inside, is not a practical proposition, although the idea could be used in UAVs. In practice, most morphing aircraft research has focused on the wings, as they control aircraft performance in different flight conditions. Wing shapes can be changed in a number of ways, including planform alteration (span, sweep, and chord), out-of-plane transformation (twist, dihedral/gull and span-wise bending) and aerofoil adjustment (camber and thickness).
When looking at how to change the shape of a wing, two main factors have to considered. First, a morphing wing needs to be flexible and/or adjustable to allow configuration changes while, at the same time, remaining rigid and strong enough to withstand the strains of flight. Secondly, the weight of the internal actuation system required to operate the wing morphing mechanism must be kept to a minimum. Recent research has focused on ‘smart structures’ which save weight by combining the wing structure with the actuation system. The aim is to produce a structure which can be deformed and continuously adapted during flight to suit different conditions of flight and mission requirements.
A morphing wing requires three specific elements: a movable substructure with an array of linkages, a flexible wing skin to cover the joints and actuators that push, pull or rotate to change the wing from one position to another. Sensors may also be fitted to detect when the correct shape has been achieved as well as locking mechanisms to hold the wing in the new configuration.
US manufacturer Cornerstone Research Group (CRG) has produced a reinforced carbon fibre laminate smooth aerodynamic surface wing skin embedded with small, stretchy, electrode heaters. When heated, the skin softens so that it can move with the underlying substructure joint as it morphs to a new desired shape. When cooled, the polymer ‘rigidises’ in the new shape. (CRG)
Research into morphing wings has been looking at solutions to each of these elements.
Active aeroelastic structures
One area of interest is in the potential of active aeroelastic structures (AASs) which can manipulate the aerodynamic shape of a lifting surface by modifying its internal structure, without the need for complex and heavy mechanisms. Such adaptive structures can improve drag performance as well as roll and loads control and could create significant performance and control improvements.
Designing a skin to cover a morphing wing is challenging and has many conflicting requirements. The skin has to be soft enough to allow shape changes made by the underlying morphing structure but also stiff enough to withstand aerodynamic pressure loads and maintain the required shape/profile. Stretchy elastomers, such as rubber, while flexible, are too soft to handle high-pressure air loads without additional reinforcement to prevent puckering. One solution has been corrugated skins while another is skins made from stiffened elastomer matrix composites. A particular area of interest has been the potential of reinforced shape memory polymer (SMP). Made up from ‘multiphase’ thermoset polymer networks, SMPs can be stretched by up to 200% into a new shape when subjected to heat. If restrained while cooling, SMPs can retain the elongated shape and, when reheated above a specific ‘trigger temperature,’ the material relaxes to its original shape, due to the elastic energy stored during the temporary deformation.
Another challenge is how to move the substructure without the need for large and heavy hydraulic systems. Researchers are currently concentrating on small, lightweight piezoelectric or electro-active polymer (EAP) actuators which expand when heated with an electrical current, a phenomenon that can be exploited to create small movements throughout a structure. Piezoelectric materials offer relatively high force output in a wide frequency bandwidth.
US research projects
The latest morphing aircraft research in the US has seen a joint NASA/AFRL project for a seamless 'flexible flap' called the Adaptive Compliant Trailing Edge (ACTE). (NASA).
In the US, research into shape-changing designs have been initiated by NASA, the Air Force Research Laboratory (AFRL) and the Defense Advanced Research Projects Agency (DARPA).
Advanced Fighter Technology Integration
In the 1980s, NASA, the USAF and Boeing worked on a project called Advanced Fighter Technology Integration (AFTI)/F-111. Flight tests were conducted using a modified F-111 fighter fitted with a device which controlled the wing curvature by means of an automated control system to optimise performance according to the external wing pressure loads. The wings of the F-111 were fitted with three independent sections for the trailing edge in each wing with sliding panels for the lower surface and glass fibre flexible panels for the upper one together with two flexible composite panels for the leading edge — all controlled by electro-hydraulic actuators. The results of the tests showed a 20-30% increase in range, a 20% growth in aerodynamic efficiency and 15% increase of wing airload at a constant bending moment.
In the 1990s, DARPA, AFRL, NASA, and Northrop Grumman collaborated on the Smart Wing programme to see how standard hinged control surfaces could be replaced with adaptive wing structures with integrated actuation mechanisms to provide variable, optimal aerodynamic shapes for different flight regimes. The programme used shape memory alloy (SMA)-based actuation to contour the trailing-edge control surfaces and SMA torque tubes to vary the wing twist. Wind-tunnel tests were conducted on a 30% scale Northrop Grumman UCAV fitted with hingeless contoured flexible leading and trailing edge control surfaces. Wind-tunnel tests showed a 8%-12% improvement in lift and roll control over a traditional wing.
Structural loads testing on the Active Aeroelastic Wing (AAW) F-18. (NASA).
Another NASA project was the Active Aeroelastic Wing (AAW) conducted at the Dryden Flight Research Center at Edwards Air Force Base in California in 2005. Jointly sponsored by NASA, the US Air Force Research Laboratory (AFRL), Wright-Patterson Air Force Base, Ohio; and Boeing’s Phantom Works, St Louis, the project aimed to test a derivative of the Wright Brothers’ concept of wing-warping to control aircraft turns up to supersonic speeds. Instead of stiffening the wings and making them heavier.
AAW reduced the structure and weight and then actively controlled wing flexibility using computerised flight controls. The project used a US Navy F/A-18A Hornet fitted with instrumentation to measure the twisting and bending of the wing during flight. Although not a true ‘morphing’ wing project, AAW was intended to benefit aircraft that operate in the transonic speed range from 80 to 120% of the speed of sound, where traditional control surfaces become minimally effective or ineffective.
Morphing Aircraft Structures
The NextGen Aeronautics Batwing UAV developed as part of DARPA’s MAS project. The aircraft used an aluminium articulated substructure, powered by hydraulic actuators to change wing sweep and chord length (NextGen)
In 2003, DARPA began the three-phase Next Generation Morphing Aircraft Structures (MAS) programme with the aim of developing technology for a new generation of military aircraft with multi-role capabilities. Two wing prototypes developed by Lockheed Martin and Hypercomp/NextGen were tested which could change their dimensions and configurations by up to 300%. Lockheed Martin developed a folding wing design which could convert from a flat, loiter-efficient shape into a smaller swept wing within 10 to 30 seconds. The hinge areas were covered with a flexible silicone skin reinforced with metal mesh that could stretch by up to 150%. NextGen worked on the Batwing which used an aluminium ‘kinematic articulated substructure’ with scissor-like joints moved by a distributed array of small hydraulic actuators that could sweep the wings and change chord length. Half-span scale models of both designs were tested at NASA Langley’s Transonic Wind Tunnel in September 2005 to speeds of over Mach 0.9.
The FishBone Active Camber (FishBAC) is a morphing structure capable of generating large changes in aerofoil camber. (University of Swansea)
The European Union has also been involved in a number of wing morphing research projects as part of its FP7 technology research programmes. One of these was NOVEMOR (NOvel Air VEhicle Configurations: From Fluttering Wings to MORphing Flight) whose task was to investigate novel air vehicle configurations with new lifting concepts and morphing wing solutions, such as the joined-wing configuration and the potential of morphing wings to enhance lift capabilities and manoeuvring. The project continued research started in an earlier EU project called Smart intelligent aircraft structures (SARISTU).
Another EU project was FOS3D (Fibre optic sensors for morphing wings) which looked at how fibre optic technology could be used simultaneously to aid deflection control and monitor structural integrity. Other European projects included SMYTE (Advanced concepts for trailing edge morphing wings), SMorph (Smart Aircraft Morphing Technologies) and the Active Aeroelastic Aircraft Structures (3AS) research project which involved a consortium of 15 European partners in the aerospace industry.
In the UK, Bristol and Swansea Universities have both been active in morphing aircraft research. The University of Bristol has taken a holistic view of morphing aircraft structures and the interaction between active winglets, multistable composite structures, aeroelastic tailoring; compliant mechanisms and the flight mechanics of flexible aircraft. Swansea University has worked on the Fish Bone Active Camber (FishBAC) — a biologically inspired concept consisting of a compliant skeletal core and elastomeric matrix composite skin fitted with a driving mechanism capable of generating large changes in aerofoil camber.
Researchers have also been looking at the potential of applying morphing technology to helicopter rotor blades which are exposed to different oncoming flow velocities both in hover and in flight. Several smart rotor concepts have been developed featuring leading and trailing-edge flaps actuated with smart materials, controllable camber/twist blades with embedded piezoelectric elements and active blade tips. The DARPA Mission Adaptive Rotor (MAR) project is working on a rotor that can change its configuration with every revolution before and during missions. By varying blade diameter, sweep, and chord; morphing tip shapes and variable-camber aerofoils; varying blade twist, anhedral/dihedral, tip speed, stiffness and damping, DARPA hopes to increase rotorcraft payload by 30% and range by 40%, while reducing acoustic detection range by 50% and vibration by 90%. The EU NOVEMOR project is also looking at improved lift capabilities of a helicopter rotor blade section fitted with an electromagnetic-actuated bistable trailing-edge flap. US manufacturer FlexSys has produced a rotor blade fitted with an embedded compliant mechanism which can vary the geometry of the leading edge giving it between 0 to 10° deflection capability.
The shape of things to come
The ultimate aim — a UAV that flies like a bird? (CRG)
Although much research has been conducted on different aspects of morphing technology (there are many more projects than space to list them in this article), much more work still needs to be done before morphing aircraft can become a practical and economic option for airframe designers. More research is needed in particular areas, such as the ability of flexible or stretchable skins to carry loads, the incorporation of actuators and other mechanisms within the airframe to reduce weight and a better understanding of how aircraft will behave in different configurations.
Although much has been learned and several new components and systems developed which can be used to improve the performance of conventional aircraft, few complete morphing aircraft designs have progressed to wind-tunnel testing and even fewer have actually flown.
At present, the development of a morphing passenger aircraft appears remote, as what works for a small model may not prove practical when ‘scaled up’ into a larger size. The most promising future morphing applications are most likely to be for UAVs or UCAVs which can more easily change their shape without safety implications.