Man has been trying to find ways and means to conquer the universe. By putting challenges to the limits of his thinking, he has brought about a revolution in every field.
A century ago the Wright brothers astonished the whole world with their new invention. Every single person was in awe of what they did. The first aircraft and the first human flight opened new horizons for humanity. Since then, there has been continuous progress in the field of aerospace. The designs were improved upon every year by different companies catering to both military and commercial clients.
Aerospace engineers have always been striving to make aircrafts fly higher, faster and further. Today, we are able to fly 10 to 12 times the speed of sound and move around the world without refuelling and can fly above 350000 feet. But the problem is that not all aircrafts can manage all the three. Some that can fly faster than the speed of sound are not able to fly for a very long time without refuelling. Or some of these are not able to fly high enough at a fast speed.
These limitations have posed several problems and strategic needs. Both the B-52 and B-2 stealth (aircrafts that can't be detected easily by ordinary radars) bombers have a relatively longer wingspan or reference area which allows for the optimum cruise for long-range missions where an aircraft has to travel at longer distances from one continent to another. These aircrafts can't manoeuvre like a combat fighter. As compared to them, aircrafts like F-18 Hornet and F-14 Tomcat have relatively small wingspan and can speed up above sonic range and can manoeuvre quickly. That's why it's been a common observation that bombers like B-52 and B-2 are escorted by these combat aircrafts for safety.
This is the basic motivation for a morphing wing structure. The basic idea is inherited from birds that cruise in atmosphere with variable wing positions. The birds sweep their wings in the backward direction when they are flying at a relatively high speed to reduce the air drag and push their wings to higher open span position when they need to increase the effective lift. That is, they morph their wings in accordance with the flight conditions. A morphing wing is defined as a wing which can undergo large changes in geometry to optimise its whole wing configuration for different flight conditions.
Aerospace engineers are working on the morphing wing structures for aircrafts to build a machine that can perform optimally well in different flight regimes and conditions. The optimal flight conditions are partially determined from the shape of the wing used in an aircraft. Usually, aircrafts deploy different structures like flaps and ailerons along wings to change flight conditions. These deployable structures are attached, usually, at the end of the structures. But these structures increase the drag acting on the aircraft, hence, reducing an aircraft performance and efficiency. Morphing wing aircrafts have an advantage as they improve the efficiency of the machine while changing optimal flight conditions in a single mission.
The X-5 by Bell aircrafts became the first flying machine to integrate this technology in 1951. This led to its first military integration as Grummaan aircraft F-10-F, which demonstrated sweeps at different angles. The US Navy fighter F-14 Tomcat is utilising this capability nowadays.
The current projects by the big companies in this field involve wing structures that utilise this technology in an entirely different and dynamic fashion. One of the leading aircrafts manufacturing companies is working on a wing that can fold like a seagull towards the fuselage and could recover again in accordance with the mission. A similar kind of research is carried out by NexGen Aeronautics to utilise the idea of sliding structures that could effectively change the area of the wing, hence changing the pressure distribution altogether making the structure even lighter and stronger.
But the question is, what materials could be used and what actuation mechanism can be performed to help this technology. One choice of material is “smart materials” like shape memory materials (SMM). These behave like rigid hard materials in normal flight conditions but change their nature to rubber-like and deformable form at different temperature (temperature above their respective glass transition point).
Above this transition temperature, SMM acts like deformable materials and can change their shape accordingly with the help of proper actuation mechanism. SMAs are further divided into three categories which are shape memory polymers (SMP), shape memory alloys (SMA) and liquid crystalline elastomers (LCE).
One other choice is polyurethanes, invented in 1930s, and was thought to be used as an alternative to rubber. Polyurethanes have the elasticity of rubber while having the toughness and durability of metal. It has shown resistance to abrasions and to other reagents like oils, fats and gasoline.
However, experimental tests conducted by Michael Thomas Kikuta from Virginia Polytechnic Institute and State University proved that none of these materials could be used as “skin material” for morphing wing. But his work has directed engineers towards an entirely new direction and has provided with certain results that help to evaluate the criteria for the candidate materials that can be used for “skin”.
Overall, a material should possess characteristics like elasticity, flexibility, high recovery rate, resistance to different weather conditions and resistance to abrasions, and should have hardness number high enough to withstand any kind of aerodynamic loads during the flight.
More experiments are being carried out in this field, new materials are being tested and new actuation mechanisms are being developed. Morphing wing technology, when properly integrated to the aircraft, can bring about a wind of change and could prove to be a breakthrough in aerospace industry. Many tests have not been successful but experiments are still being carried out.
