DARPA's Smart Wing Program

The success of the smart wing program naturally outgrew in to the morphing concept which DARPA/NASA are presently after. The Smart Wing program, being the foundation for the Morphing Program is extremely important from an evolutionary stand point as it represents an important milestone in the development and application of smart materials and structures technologies to active control. This program consisted of two phases in which the first phase involves the development of an SMA based, hinge-less, smoothly contoured trailing edge and actively variable wing twists with SMA torque tubes (Jardine, et al., will be discussed shortly)

The First Phase consisted of the development of a hingeless actuation, that proved to result in 9.7 and 10.2% improvement of Lift and Roll for flap only configuration, 17.6, 17.1% improvement for flap and aileron combined configuration and 11.5 - 15.6% improvement for a 5 degree wing twist alone, with a 15.3 and 17.3% improvement for a combined aileron and wing twist. This essentially lead to the efforts for circumvention of the bottlenecks of actuation bandwidths and improvements of the actuation range, caused by the Shape Memory Alloy based actuation used in the first phase.

(Source: Overview of the DARPA/AFRL/NASA Smart Wing Program, J.N.Kudva, et al., Industrial and Commercial applications of Smart Structure technologies, SPIE Smart Structures and Materials, Vol.3674,pp 230-236, March 1999).

SMA torque tubes were designed, fabricated and tested by Northrup Grumman for DARPA. Their design has a solid SMA ingot that powers the connector to the wing box, producing the required torque. The torque tube was modelled as a concentric shell for computation of the analytic solution. The SMAs are trained to behave in a torsional mode (So, the Ti Ni Ingot is trained to deform in torsional mode). The mechanism which transmits the load from the SMA ingot to the structure is a tube which houses the ingot and in-turn is connected to the airframe, transmitting the load to structure.
The stiffness of the structure is allowed to act as a bias spring.
They were the first to have done SMA training for twisting purposes. Application of this mechanism (torque tubes) to twist a 1/6th scale model of the FA/18 wing, achieved a 2 degree twist with a 2000 in.lbs torque at the mid rib section. This design incorporates a method of heating the SMAs with Nichrome as direct heating may require a lot of power and apparently is unwieldy. These values were observed in the pre - assembly stage.
After assembly on to the airframe, the same 2000 in-lb at the mid rib could produce only 1.18 degree of twist and at the tip of the wing, a torque of 500lb.in. would give a twist of approximately 0.3 degree. From the twist results the roll rate improvement was found to be around 8%. So there is a significant improvement in the aircraft performance due to the twisting of the wing.

Source : ("Improved Design and Performance of the SMA Torque Tube for the DARPA Smart wing program - A.Peter Jardine, et al.,Industrial and Commercial applications of Smart Structure technologies, SPIE Smart Structures and Materials, Vol.3674,pp 260-269, March 1999.; "Smart Wing Shape Memory Alloy Actuator Design and Performance", Peter Jardine, et al., SPIE Vol.3044, pp 48-55, 1997).

The phase 2 of the program involves the demonstration of larger scale wings with high band windth actuation, being tested at fight Mach numbers and dynamic pressures (of a subsonic UCAV, for imminent applications), and consequently, we see a switch over from SMAs to more sophisticated actuation schemes. A 30 percent scale model of the NGC UCAV was used in  the wind tunnel tests. One of the tests demonstrated a hingeless SMA based LE control surface to counter the loss of aileron effectiveness and the other demonstrated the high rate actuation system which had the requirement of 60-75 degrees of deflection per second with a maximum deflection of 20 degrees. Hence, this aspect of the program addresses not only the design and development of such an actuation system which meets the requirements, but also the related issues of its transmission and integration in to the structure. The actuation mechanism uses ultrasonic piezo electric motors from the Sensei Corporation called SPL-801. Transmission system essentially converts this rotary motion from the motor to a translating motion on the planes in which the trailing edges is attached. The continuity in the edges is achieved by a silicone skin supported by a honey comb structure which permits flexibility. The eccentuator is a bent tube which rides on a bearing surface. So there is a resultant motion of this bearing surface in the upward or in the downward direction depending upon the direction of rotation of the bent eccentuator.
(Sources: "Design, fabrication and testing of scaled wind tunnel model for the Smart Wing Phase 2 program", Martin, C.A., et al., Industrial and commercial applications of Smart Structures Technologies, SPIE Smart Structures and Materials Vol.4698,pp 44-52, March 2002; "Development, control and test results of high-rate, hingeless, trailing-edge control surface for the smart wing phase 2 wind tunnel model", Bartley-Cho, D.P., et al., Industrial and commercial applications of Smart Structures Technologies, SPIE Smart Structures and Materials Vol.4698,pp 53-63, March 2002; "DARPA/AFRL Smart Wing Phase 2 wind tunnel test results", Scherer, L.B., et al., Industrial and commercial applications of Smart Structures Technologies, SPIE Smart Structures and Materials Vol.4698,pp 64-73, March 2002.)
All in all, the Smart Wing program had a hingeless trailing edge which demonstrated greater controllability with respect to conventional wings. The conventional wing and the hingelss wing actuation were mounted on either sides of the NGC UCAV and tested in the wind tunnel. The results were compared which favored the smart wing. Hence, there was a general feeling of commanding greater control surfaces for continuous shape change, leading to the ideas of morphing involving large shape changes. A more quantitative documentation of why a uniform variation in configuration is advantageous, how the rigid and elastic structural theory with linear aerodynamics can be used to show the improvement the aerodynamic coefficients in this particular design is given by "Aerodynamic and Aeroelastic Characteristics of wings with conformal control surfaces for Morphing Aircraft", Brian Sanders, US Airforce Research Laboratory et al., Journal of Aircraft, Vol.40, No.1, January -February, 2003. The article concentrates on the results from phase 1 of the project where the 16% scale model of an F-18 wing was fitted with the smart wing, design of which we saw a while ago.


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