Active Twist Wing Really Takes Off!

A novel concept for active wing twist.

Recently structural engineers have developed a healthy interest in active structures that can change their shape depending on different requirements during a mission. Development of composite and smart materials has created opportunities to divert from the old-fashioned perception of aircraft structures, it now becomes possible to innovate with structures. Nevertheless, the paradox of a strong and rigid structure and the possibility to change shape is the greatest challenge for the future. The purpose of the research summarized here is to study the concept of active twist structures and to demonstrate one specific configuration that appears to be a practical and feasible configuration; the Warp Induced Active Twist Wing, furtheron referred to as ATWing.

by: ir. Martin Nagelsmit, PhD Researcher Aerospace Structures and NLR

Working Principle

During the second year structural analysis course it is explained that twisted thin-walled tubes with closed cross-sections will develop shear stresses as the warping displacement is constrained. Closed cross-sections are very stiff and will thus not show large deformations. If the tube is slit along its length, then the twisting results in substantial warping displacement of the cross-section. This is represented by relative movement of the opposite sides of the slit with respect to one another. This principle also works the other way around: when the open section tube is warped, it will twist too. Open cross-sections however will show large warp-twist deformations but are inherently flexible in torsion. Warp displacement can be induced along the entire span of the wing, whereas twist displacement has to be induced at the tip, which makes actuation difficult. In the ATWing design a combination is found of an easy to warp open cross-section skin, see figure 1, and a stiff inside structure, see figure 2. Ribs rotate around a circular spar, while allowing the skin to slide over them in the spanwise direction via tracks. The warp displacement is induced by a screwthread located near the trailing edge which also closes the cross-section.

Figure 1 (left): Warping and twisting skin

Figure 2 (right): Inside Structure

Windtunnel Results
The first version of the wing was tested in the windtunnel in 2005. It achieved a tip twist of fifteen degrees both ways with only four milimeter of warp displacement. It also showed that a considerable increase in lift coefficient could be achieved together with a decrease in induced drag, increasing the lift to drag ratio. These results are shown in figure 3. Based on these promising results the research is continued.

Rotary Wing Aircraft
Rotary wing aircraft have a very large range of operating conditions as they both hover and fly forward. These two flight conditions ask for different characteristics of the rotor blades. To be able to tailor the rotor blades to the needs of the flight condition, would seriously increase the performance of the rotorcraft and this would require quasi-static actuation of the blade. Also within one or several revolutions per minute, active shape control can be very advantageous. During the great time spending my internship at Penn State University in the US, a new and more accurate model of the ATWing was built using rotorblade dimensions. Figure 4 shows the model with angle gauge. Actuation torque tests were performed for different loading conditions experienced during flight. Tests showed low and predictable results for the actuation torque. Still tolerances and flexibility issues have to be addressed to reduce the influence of applied loads and improve the reliability of the warp-twist behaviour.

Fixed Wing Aircraft
Fixed wing aircraft experience different flight conditions during a flight. Differences can be found in airspeed, pressure, altitude, load factor, weight etcetera and all of these differences ask for a different wing too. Currently aircraft wings are designed for a certain loading condition; typically cruise as they spend most of their flight time in this condition. For landing and takeoff the airfoil can be changed by using high-lift devices such as trailing or leading edge flaps, changing chord and camber.
Back in Delft two lightweight versions of the wing were adapted together with the MAVLab for use on a radio controlled airplane. A big challenge was to design a suitable actuator to which the screw thread could be attached. To achieve a proper response at least five degrees of twist deformation has to be reached within 0.5 seconds. A normal servo, which does only 180 degrees rotations, was rebuilt with a new potentiometer to do several full rotations in order to achieve the required twist deflection. After some mechanical modifications the screw thread could be attached to the servo and the wings were working.
The original aircraft only used its rudder for directional control, whereas the model modified with ATWing relied on the twisting deformation of the wings. During the test flight only the roll control of the wings was used to steer the aircraft. With some delay in the response it was indeed possible to control the aircraft with only twisting wings!

Drag Reduction
The main reason to implement a complicated wing like ATWing is drag reduction, which comprises of two separate possible improvements: reducing induced drag and reducing profile drag. Induced drag is mainly caused by the pressure difference at the wing tip, creating vortices. Changing wing tip washout, (negative) tip twist with respect to the root, the pressure difference can be changed in order to decrease the wing tip vortices and thus induced drag. Normal wings are optimised for only one flight condition, usually cruise, where ATWings can be adapted to every flight condition so an aircraft can always fly at the highest lift to drag ratio. This requires only small changes in twist angle at quasi-static actuation. Control surfaces induce a large amount of drag due to the non-continuous surfaces such as hinges. With the use of ATWings no separate control surfaces are needed and profile drag can be reduced. A combination of both is possible too, and also just parts of a wing or rotorblade can be fitted with variable twist.

Figure 3: Windtunnel results

Applications
Basically everywhere a finite wing shape is used the ATWing principle can be beneficial. Not only fixed and rotary wing aircraft, but also F1 car wings, wind turbines, sailboat keels, kites, and propellers. Less drag means less energy consumption, and with natural energy resources getting scarcer and global warming this has recently become a worldwide issue. There is no doubt reducing energy consumption is favourable for everybody and thus active twist wings have to be considered seriously.

Conclusion
Smart structures do not necessarily have to include smart materials for actuation. Smart design, such as warp induced twisting deformations used in ATWing, using traditional activation mechanisms can sometimes accomplish the desired result more efficiently. A considerable advantage of the ATWing concept over piezoelectrically actuated smart wings is the large deflection that can be achieved at low energy costs. Substantial improvements to the concept still have to be made on the actuation and manufacturing quality. But so far it is proven that an Active Twist Wing is feasible and can control an airplane. Actually seeing your own creation take-off is a sight and feeling I will never forget, and a great way to graduate in Aerospace Engineering!

Figure 4: RC Aircraft fitted with ATWings