Shape Memory Alloy Springs Used as Reduced Power/Weight Actuators

The paper presents a concept and realization of using shape memory alloy (SMA) spring actuators for deployment of flight control surfaces of small air vehicles. These actuators replace heavy servomotors resulting in increased endurance of the vehicle as well as reduced power consumption. The actuator represents a spiral wound tubular SMA helical spring that is extended in its martensitic phase prior to actuation. The actuation can be achieved by directing exhaust gas from the onboard engine, i.e. providing an influx of heat. When activated, the spring returns to its original (compressed) shape generating a force in the range of 10 to 25 lbf. The advantage of using SMA springs is related to the enhanced stiffness after activation, as the material transforms from the martensitic to austenitic phase. Such added rigidity is useful to deploy telescoping wing surfaces and implement extensive geometric airframe changes. Numerical examples conducted with a typical spring material and geometry illustrated that the required stroke can be achieved with the spring index of about 10, coil diameter of 2.5 inches and SMA diameter in the range from 0.14 to 0.24 inches. Experimental data confirms these numerical predictions. The present study has proven the feasibility of using SMA actuators for the deployment of wing surfaces of small air vehicles.