Skip to main content

The Potential Uses of Smart Materials and Stuctures on Aircraft

Claire Miller is an engineering graduate who is now wading through the chaos of trying to be a responsible adult.

“I think nature’s imagination is so much greater than man’s, she is never going to let us relax.” - Richard P. Feynman

One of the ultimate objectives for aeronautical design and engineering is to create an aircraft that can adapt to its environment and the flying situations that it is subjected to, whilst maintaining optimal performance. For centuries, nature has inspired the pursuit, conceptualisation, and evolution of human flight − and continues to do so.

Fibre composites have been introduced more and more in the manufacturing of aircraft due to their high strength-to-weight ratios. The Boeing 787 Dreamliner contains approximately 35 metric tons of carbon fibre reinforced polymer (CFRP), and is the first major commercial airplane to use composites in the fuselage, wings, and most other airframe components (Toray Industries, 2005).

Currently, a conventional aeroplane comprises of a series of primary and secondary control surfaces that can be used to direct its flight and increase or decrease its aerodynamicity. However, this can be further improved upon using smart structures.

The development of smart structures has been a point of interest for the past two decades, and appear to be the next logical step in the evolution of aircraft design. They refer to structures that have the ability to adapt to the environment which they are subjected to in order to maintain a particular requirement.

For example, smart structures could be used in order to effectively adjust the sweep angle and shape of the wings in order to maintain optimal performance and efficiency throughout the duration of the aircraft’s flight.

A smart structural system would contain the following to be able meet the capabilities of this project:-

  • Sensor: to pick up on the velocity of the aircraft (e.g. pitot tube, lasers, etc.);
  • Actuator: to provide force to change the wing shape (e.g. piezoelectric transducers, shape memory alloys, SMART layers, etc.);
  • Processor: to monitor and process data from the sensor and actuators (e.g. microcomputer, built-in response, etc.).

    (Baker, Dutton, & Kelly, 2004)

Piezoelectric Actuators

An actuator is a mechanical component that can be used to convert a form of electrical or fluidic power into motion. They can be used for a number of different purposes, such as opening and closing valves, activating and deactivating motors and pumps, and triggering switches.

Actuators have been used in the flight systems of aircraft for decades, one of the biggest examples being in the movement of flight control surfaces.

Piezoelectric transducers are able to generate charge when subjected to mechanical stress, known as the piezoelectric effect, and can equally change geometry when subjected to an electric field. This makes them ideal for use as both sensors and actuators simultaneously in the same structure, unlike other actuators (Baker, Dutton, & Kelly, 2004). The most common piezoelectric materials consist of ceramics such as lead zirconate titanate (Pb(ZrTi)O3) – more commonly known as PZT.

Piezoelectric transducers are lightweight and can be cheaply manufactured on very small scaled in the form of thin sheets or fibres to be embedded into the aircraft skin or structure to result in a highly conformable smart material.

In order to be used for our example of altering the sweep of the wings, the piezoelectric materials could be attached at each corner of an assembly of smaller structures. This setup would act as a ‘muscle’ for the sweep motion, guided by an additional four-bar linkage assembly and slider mechanism at the root of the wing. By having this ‘muscle’ assembly, the loads at the wing’s root are distributed along the rib as opposed to them acting on the wing’s main pivot point.

Morphing wing structure

Morphing wing structure

The piezoelectric actuators would then be subjected to voltage signals from the fly-by-wire computer. The amount that the actuators move would depend on the voltage received from the computer, thus resulting in different sweep angles.

Scroll to Continue

Shape Memory Alloys (SMAs)

Shape memory alloys were accidentally discovered in the 1960s when the company Nitinol (an acronym for Nickel Titanium Naval Ordnance Laboratories) invented a nickel-titanium alloy for use in missile nose cones. A sample that had been bent out of shape many times stretched back to its original shape when it was subjected to heat (Otsuka & Wayman, 1998).

The reason behind the remarkable property of shape memory alloys is because their crystal transformation is fully reversible. In most crystal transformations, the atoms in the structure will travel through the metal and dislocate, changing the composition locally. Meanwhile, during a reversible transformation, all of the atoms shift at the same time to form a new structure while still maintaining the same bonds.

Before and after deformation in SMAs (top) and general alloys (bottom). The crystal bonds in the SMA deform with the atoms in order to remain connected, whereas the atoms in the general alloy dislocate and bond again in a different structure.

Before and after deformation in SMAs (top) and general alloys (bottom). The crystal bonds in the SMA deform with the atoms in order to remain connected, whereas the atoms in the general alloy dislocate and bond again in a different structure.

In order to achieve morphing, potential materials would need to be flexible to minimise the amount of energy required of the actuators to morph the wings, while still being able to be rigid and strong in order to maintain the desired shape. While it would be possible to change the shape of the structure by heating the shape memory alloy up, shape memory alloys are slow to react. The response time is also asymmetrical, which means that the deactivation time is typically much greater than the initial reaction time.

Shape memory alloys are also very expensive to produce, and they are subject to functional fatigue, meaning that they could start to lose their shape over time. The switch threshold of the temperature at which it will change shape can be difficult to control, which can decrease the efficiency of the aircraft as opposed to enhancing it (TEDx Talks, 2017).

In terms of the feasibility of using smart structures for changing the sweep angle of an aircraft’s wings, piezoelectric transducers appear to be the more superior method of wing morphing until the limitations of shape memory alloys are resolved. However, smart structures should not just be limited to changing the shape and angle of the wings, but also to enhance, monitor and maintain the entire aircraft.

NASA demonstrated that smart structures have a number of different uses in 1999, when they invented the Macro-Fibre Composite (MFC) actuator, an encapsulated, high-performance, in-plane, piezoelectric fibre composite strain device. The MFC actuator consists of rectangular piezoelectric fibres sandwiched between layers of adhesive and electrode polyimide film.

Due to the interlocking assembly of the electrodes and the nature of piezoelectric materials, the MFC actuator can be used with or without voltage. If voltage is applied, it will bend or distort materials, counteract vibrations, or even generate them. Without voltage, the device can act as a highly sensitive strain gauge, sensing deformations, noise, and vibrations. The piezoelectric effect of the MFC also makes it possible for the structure to harvest energy from vibrations it picks up on (Smart Material, 2003). The technology behind the piezoelectric effect could potentially be used to create fuel-electricity hybrid aircraft that run on less fuel than conventional aircraft.

Works Cited

Andersen, G., & Cowan, D. (2007). Aeroelastic modeling, analysis and testing of a morphing wing structure. Honolulu: 48th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, Structures, Structural Dynamics, and Materials and Co-located Conferences.

Baker, A., Dutton, S., & Kelly, D. (2004). Composite Materials for Aircraft Structures (2nd edition). Reston: American Institute of Aeronautics and Astronautics.

Otsuka, K., & Wayman, C. (1998). Shape Memory Materials. Cambridge: Cambridge University Press.

Smart Material. (2003). Smart Material - Home of the MFC. Retrieved from Smart Material: https://www.smart-material.com/MFC-product-main.html

TEDx Talks. (2017, November 20). Smart Materials | Anna Ploszajski | TEDxYouth@Manchester. Retrieved from YouTube: https://www.youtube.com/watch?v=py5tPlOaJVY

Toray Industries. (2005). Market Research Report: Strategic Business Expansion of Carbon Fiber. Tokyo: Toray Industries.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2021 Claire Miller

Related Articles