Aerodynamic performance is an essential consideration in unmanned aerial vehicle (UAV) design. It is necessary that UAVs cruise close to the best lift-to-drag ratio (L/D), which means flying at a constant angle of attack.
Finding a balance between weight, altitude, speed, and/or wing area is crucial because a failure to do so may cause the L/D to be lower than optimum and the range will be correspondingly less. A variable camber wing may provide a solution.
SMA as actuator
Smart material is a suitable candidate for adaptive airfoil design since it can be customized to generate a specific response to a combination of inputs. These materials include piezoelectrics and electrostrictives, as well as shape memory alloy (SMA), which is lightweight and produces high force and large deflection.
In the case of an adaptive airfoil, the ideal material should respond quickly to external stimuli, be capable of large and recoverable free strains, transform effectively the input energy to mechanical energy, and not be affected by fatigue issues. The benefits of using smart material compared to pneumatic or hydraulic actuators are reduced complexity and improved reliability of the system.
Among all the smart materials, SMAs appear to have superior capability in producing large plastic deformations. In recent years, interest in SMA applications for adaptive structures has increased not only due to this unique quality but also because of their high power-to-weight ratio and low driving voltages. SMAs are thermomechanical materials typically composed of a mixture of nickel and titanium, which changes shape when heated or cooled.
When they are cooled to below a critical temperature, their crystal structure enters the martensitic phase, where alloy is plastic and can easily be manipulated through very large strain ranges with little change in the material stress. When heated above the critical temperature, the phase changes to the austenitic phase, where the alloy resumes the shape that it formally had at the higher temperature.
Nickel titanium is the most commonly used SMA, to which copper is sometimes added to aid in the strain recovery process. The process of shape change or creating movement is composed of a five-step procedure that occurs within the material in which the shape memory effect is developed.
The first step is the parent austenitic phase, which occurs at a high temperature with zero stress and strain. To create twinned martensite, the parent austenitic structure is cooled in the absence of both stress and strain. Next, the twinning process is reversed by stressing the material, which causes the now detwinned martensite to develop inelastic strains. While still maintaining its detwinned form with the elastic strain, the load is then released. Finally, the material returns to its original shape and composition when all inelastic strains are recovered by heating the SMA to its parent austenitic start temperature.
There are some drawbacks in using SMAs, such as nonlinear response of the strain to input current and hysteresis characteristic, as a result of which their control is inaccurate and complicated. The accuracy of the mathematical model is critical as the efficiency of an SMA actuator depends on the preciseness of its control.
Conceptual wing design
The initial prototype was a wing panel that was 200 mm wide with the spar extended to 300 mm. Its chord was 500 mm. (Wing span was not considered since it was a 2-D study.) Initially a symmetrical airfoil NACA 0012 was chosen as the base airfoil. Two-dimensional FEM (finite element method) analysis and aerodynamic modeling were carried out with promising results.
Due to limitation during fabrication, the airfoil was changed to Clark Y, which has a flat base. The wing consists of a rigid foam section in the leading edge as well as ribs, and a base and spar each made of wood. The solid leading edge will be attached to the base using a tape hinge to allow movement during deformation. Plywood is used as the wing skin. The wing is actuated by two SMA wires, one at the leading edge and another at the trailing edge.
The SMA wires will act as actuators, shrinking when heated and pulling the wing skin causing it to deform. The structural elasticity will force the SMA wires to return to their original length when they are cooled down.
The design of the prototype was simplified so that the configuration could be easily changed in later stages.
FEM analyses were used to predict the effectiveness of the SMA actuator. Different configurations were analyzed by changing the skin material, the position of the SMA actuators within the wing, and forces exerted by it on the skin. In the simplified 3-D FEM model, a structural static simulation of the wing-panel deformation was considered with SMA actuator action incorporated by means of concentrated forces.
In the FEM model, the wing panel was represented as wing skin with the spar, ribs, base, and solid leading edge built in as boundary conditions. The airfoil chord was 1 m, the span was 0.5 m, and the thickness of the skin was 5 mm. Plywood, aluminum, and ABS were analyzed for the wing skin.
Placement of the actuator is critical in obtaining the desired change of the airfoil camber. Nine cases were analyzed with different combinations of applied forces by the SMA actuators. The actuators are attached to two points on the upper side of the airfoil and two points on the lower side.
The FEM analysis of the adaptive airfoil predicted a maximum trailing-edge deflection of 1.52 mm for a wing panel with aluminum skin, 8.48 mm for plywood skin, and 45.69 mm for ABS skin when both SMA actuators were fully actuated. The deformation effectively created a camber across the airfoil.
The SMA actuator near the trailing edge has a bigger effect on deforming the panel compared to the actuator near the leading edge.
The design prototype of the wing panel with plywood skin was expected to produce a significant change in camber when the SMA wires are actuated to produce an increase of L/D.
Also based on the numerical results, ABS would be a better choice for the wing skin of the full-scale wing model.
The next step was to conduct experimental testing on the prototype to investigate the deformation when the SMA actuators are fully deployed. Some problems were encountered with this initial prototype during wind-tunnel testing, so a new prototype was fabricated and has undergone wind-tunnel tests with favorable results.
The updated results are expected to be presented as a conference paper later this year.
This article is based on SAE International technical paper 2009-01-3272 by Ermira J. Abdullah, Cees Bil, and Simon Watkins, School of Aerospace, Mechanical, and Manufacturing Engineering, Royal Melbourne Institute of Technology.