Torsion Coupling
7.9 Overview of Smart Structures Concept for Aircraft Control
Buff eting is a forced vibration where turbulent fl ow generated by one aerodynamic surface excites this surface itself or another surface located in the region of the turbulent fl ow. Also here, aerodynamic control surfaces located on the aff ected part can be used to counteract the vibrations. Compared to fl utter, the aerodynamic eff ectiveness of these surfaces is additionally reduced because of the turbulent fl ow conditions. Active struc- tures systems are more eff ective in this case. For this reason, and because the required active deformations are small, the fi rst large-scale active structures application on aircraft dealt with the buff eting problem of fi ghter aircraft vertical tails at extreme maneuver conditions. Aft er several theoretical [22] and small- scale experimental studies [23], full-scale ground tests were per- formed in a joint Australian–Canadian–U.S. research program [24] on an F-18 and in a German program for a simplifi ed fi n structure of the Eurofi ghter [25]. In both cases, piezoelectric material was used.
7.8.3 Aeroelastic Tailoring and Structural Optimization
Weisshaar [26] was one of the fi rst researchers who tried to give aeroelasticity a better reputation when modern fi ber reinforced composite materials with highly anisotropic directional stiff - ness properties were considered for primary aircraft structures.
Th ey provided the possibility to tailor the materials’ directional stiff ness within the composite lay-up in order to meet desired deformation characteristics for improved aeroelastic perfor- mance. Together with formal mathematical optimization meth- ods for the structural design, this approach allowed for the minimization of the impact from aeroelasticity.
Any improvement of a technical system is oft en referred to as an optimization. In structural design, this expression is today mainly used for formal analytical and numerical methods. Some years aft er the introduction of fi nite element methods (FEM) for the analysis of aircraft structures, the fi rst attempts were made to use these tools in an automated design process. Although the structural weight is usually used as the objective function for the optimization, the major advantage of these tools is not the weight saving, but the fulfi llment of aeroelastic constraints. Other than static strength requirements, which can be met by adjusting the dimensions of the individual fi nite elements, the sensitivities for the elements with respect to aeroelastic constraints cannot be expressed so easily. Th e option to tailor the composite material’s properties by individual ply orientations and diff erent layer thickness for the individual orientations required and inspired the development of numerical methods [27].
7.9 Overview of Smart Structures
surface or a deformation of the structure by internal forces results in diff erent rolling moments and requires diff erent eff orts to create the defl ection or deformation.
7.9.3 Variable Shear Stiffness Spar Concept In a similar way to the fi ctitious control surface, a study by Griffi n and Hopkins [29] used a “fi ctitious” variable stiff ness spar concept to modulate the rolling moment eff ectiveness of a generic F-16 wing model. Th ey assumed a small outboard trail- ing edge control surface on an analytical F-16 wing model for roll control, which would operate in a conventional mode at low dynamic pressures and the negative “postreversal” eff ectiveness could be enhanced by turning the spar web shear stiff ness off at high dynamic pressures. Th is concept was explained in a sim- plifi ed way by “link elements,” attached to the upper and lower spar caps by bolts and removable pins. Th e basic principle of this concept was also experimentally verifi ed on an aeroelastic wind tunnel model for an unswept, rectangular wing with removable spars [30]. Unfortunately, no references were found, for more technical smart structures solutions if they were ever investigated for this concept.
7.9.4 Innovative Control Effector Program In the Innovative Control Eff ector (ICE) program from NASA, Langley [31,32], the positions and required amount of small,
“fi ctitious control surfaces” were determined by means of a genetic optimization process for an advanced “blended wing- body” confi guration. Th ese “control eff ectors” were elements of the surface grid in the analytical aerodynamic model that created the “virtual” shape change. Ref. [31] gives an excellent overview on all research activities within the NASA’s Morphing Program.
7.9.5 Active Flow Control Actuators
Also as a part of the NASA Morphing Program, “synthetic jet actuators were developed and tested” [31]. Th is device is based on a piezoelectrically driven diaphragm, which sucks and blows air through a small orifi ce. It was originally developed for cavity noise control. To use it for aircraft control, where much higher forces are required, the power output needs to be multiplied.
7.9.6 Innovative Aerodynamic Control Surface Concepts
Although there are no active structural components involved, these concepts can also be considered as “smart structures.” In this case, the active deformation of the structure is actuated by aerodynamic control surfaces. Th e January/February 1995 edi- tion of the Journal of Aircraft [18] was a special issue, dedicated to the U.S. Active Flexible Wing Program, which started in 1985 and turned later into the Active Aeroelastic Wing Program. Th is basic idea was the improvement of roll eff ectiveness for a fi ghter
aircraft wing by the combination of two leading edge and two trailing edge control surfaces, which could also be operated beyond reversal speed. Th is concept was demonstrated on an aeroelastic wind tunnel model, with tests started in 1986.
Aft er theoretical studies on F-16 and F-18 wings, reported by Pendleton [5,17,33], the F-18, depicted in Figure 7.11, was selected as the candidate for fl ight test demonstrations, which are expected to started in 2001. For this purpose, the wing structure was returned into the original stiff ness version, which had shown aileron reversal in early fl ight tests.
Flick and Love [34] performed a study on wing geometry sen- sitivities with respect to the potential improvements from active aeroelastic concepts based on the combination of leading and trail edge surfaces. Th e results, as shown in Figure 7.12 from Ref.
[5], indicates only very small advantages for low-aspect ratio wings. Th e theoretical studies for a generic wing model of the Eurofi ghter wing by the Flick, however, resulted in large improve- ments also for this confi guration, as can be seen in Figure 7.6.
Active aeroelastic concepts research by TsAGI in Russia has already demonstrated impressive improvements in fl ight test. In addition to also using leading edge control surfaces to improve roll performance, a small control surface was mounted at the tip launcher. Figure 7.13 from Ref. [35] shows the achievable improvement compared to the trailing edge aileron only. Note the size of the special surface in comparison with the conven- tional aileron.
For high aspect ratio transport aircraft wings, especially in combination with a winglet, similar devices could be used not only for roll control, but also for adaptive induced drag reduction, or load alleviation, as indicated in Figure 7.14 for a concept, called active wing tip control (AWTC) by Schweiger and Sensburg [36]. In this case, the winglet root provides suffi cient space and FIGURE 7.11 F-18 Active Aeroelastic Wing demonstrator aircraft . (From Sanders, B., Flick, P., and Sensburg, O. International Forum on Aeroelasticity and Structural Dynamics, Madrid, Spain, 2001.)
structural rigidity to integrate the control device and its actua- tion system. With respect to fl utter stability, the forward posi- tion of the masses increases the fl utter stability, which is reduced by the aft position of the winglet.
Of course, the static aeroelastic eff ectiveness of a control sur- face is also important for dynamic applications like fl utter sup- pression or load alleviation for buff eting of vertical tails. Th is fact is very oft en forgotten in favor of optimizing the control laws.
7.9.7 Active Structures and Materials Concepts
Dynamic applications for fl utter suppression [10] or buff etting load alleviation [37,38] by means of piezoelectric material were
demonstrated on wind tunnel models or in full scale ground tests. Th e involved mass and complexity, mainly for the electric amplifi ers, precludes practical applications at the moment. For dynamic applications however, a semiactive solution with shunted piezomaterials [39] with very little energy demand is an interesting option.
Th e use of piezoelectric materials for static deformations is limited by the small strain capacity, as well as by the stiff ness of the basic structure. Because of these facts, some researchers real- ized rather early that it is not advisable to integrate the active material directly into the load-carrying skins. In order to achieve large defl ections, it is necessary to amplify the active material’s stroke and to uncouple the to-be-deformed (soft ) part of the pas- sive structure from the (rigid) main load-carrying part. Because this will usually cause a severe “strength penalty” for the main structure of conventional airplanes, practical applications are
3.0
5.0 4.5
Aspect ratio4.0
Aspect ratio
AAW concept
Conventional
Conventional Taper ratio = 0.2
Taper ratio = 0.2
AAW concept
3.5 3.0
Wing skin weight
0.03 3 2 1 0.04 0.05
Thickness
0.06 3.5
4.0 4.5
5.0 0.03 0.040.05
t/c 0.06
0.9 Normalized TOGW
1.0 1.1 1.2
FIGURE 7.12 AAW technology advantages and wing geometry sensitivities for light weight fi ghter wings. (Results from Flick, P.M. and Love, M.H. RTA Meeting on Design Issues, Ottawa, Canada, October 1999; fi gures from Sanders, B., Flick, P., and Sensburg, O. International Forum on Aeroelasticity and Structural Dynamics, Madrid, Spain, 2001.)
Computations Flight tests
Aileron+
wdx
special aileron (δs.a. =δa.)
Aileron 0.2
0.1
0 700 900 Ve (km/h)
FIGURE 7.13 AAW technology in Russia: fl ight test results and com- parison with analysis for a special wing tip aileron. (Results adapted from Kuzmina, S.I., Amiryants, G.A., Ishmuratov, F.Z., Mosunov, V.A., and Chedrik, V.V. International Forum on Aeroelasticity and Structural Dynamics, , Madrid, Spain, 2001.)
for:
Increased roll effectiveness Drag reduction
Load alleviation
- By increased spanwise moment arm
Increased aeroelastic effectiveness
- By torsion Reduced aeroelatic effectiveness
Wing box Elastic
axis
FIGURE 7.14 Advantages of an AWTC device on a transport aircraft wing.
limited to unusual confi gurations like small Unmanned aerial vehicles (UAVs) or missiles [40]. As an example, Barrett [41]
developed such a device, where the external shell of a missile fi n is twisted by a PZT bender element.
Compared to piezoelectric materials, which responds very fast, SMAs are rather slow but they can produce high forces. Th is precludes applications which require speeds for the adaptation which are equal to or higher than the speed of fl ight or the speed of aircraft control device motions.
Two typical applications of SMAs were investigated in the DARPA/AFRL/NASA Smart Wing Program [5,31], an SMA torque tube to twist the wing of 16% scale wind tunnel model of a generic fi ghter aircraft , and SMA wires to actuate the hinge- less trailing edge control surfaces. Concerning the torque tube, the ratio between the torque tube cross section and the wing torque box cross section should be kept in mind. For the replace- ment of conventional control surfaces, the eff orts to create the deformation on a realistic structure still need to be addressed.
And, what is even more important than the limitations with respect to the actuation speed, the aeroelastic aspects should be kept in mind from the beginning, in order to evaluate and opti- mize the eff ectiveness of such concepts. As depicted in Figure 7.15 from Ref. [5], the eff ectiveness of the conformal trailing edge control surface is better than the conventional control sur- face at low speed, but becomes worse with dynamic pressure. As mentioned in this reference, such concepts are not developed to replace existing systems, but to demonstrate capabilities of active materials. If this is the case, realistic applications still need to be discovered.
Smart materials applications on small remote piloted vehicles (RPVs) are currently being investigated at the Smart Materials Laboratory of the Portugese Air Force together with the Instituto Superior Técnico in Lisbon [42].
7.9.8 Other Innovative Structure Concepts Because of the limited stroke of active materials and the inherent stiff ness of a minimum weight aircraft structure, some research- ers try to amplify the stroke by sophisticated kinematic systems and enable larger deformations more easily by “artifi cially”
reducing the structural stiff ness. So far, all these concepts show the following disadvantages:
High complexity for the actuation system
•
Higher energy demand compared with the actuation of
•
conventional control surfaces
Additional internal loads in the structure from the forced
•
deformation
Additional structural weight from the reduced strength
•
capacity
Reduced static aeroelastic eff ectiveness because of addi-
•
tional fl exibility in the rear wing area
Reduced aeroelastic stability (fl utter) from the reduced
•
stiff ness
As one example, such a system was described by Monner et al.
[43]. Th is paper summarizes the active structure research activi- ties by the German aerospace research establishment DLR applied to an Airbus type transport aircraft wing.
An old idea, the pneumatic airplane, as depicted in Figure 7.16, may be useful, if applied to small UAVs (for storage), or, on larger airplanes to selected structural elements, like spar webs, in order to adjust the shape by a variable pneumatic stiff ness to control the aeroelastic load redistribution.
7.9.9 Adaptive All-Movable Aerodynamic Surfaces
Adaptive rotational attachment or actuation stiff ness for all- movable aerodynamic surfaces can be seen as a special class of active aeroelastic structures concepts. If properly designed, this concept will provide superior eff ectiveness compared to a rigid structure also at low speeds. Other active aeroelastic concepts show their advantages only with increasing speed, in the same way as negative aeroelastic eff ects are increasing.
As an example, a fi xed root vertical tail can be made more eff ective if the structure is tailored in such a way that the elastic axis is located behind the aerodynamic center of pressure. Th is wash-in eff ect will, for example, increase the lateral stability compared to a conventional design on a swept back vertical tail, as depicted in Figure 7.17. Th e so-called diverging tail [19] has an improved eff ectiveness, but also experiences higher bending moments.
Instead of tailoring the structure, which essentially will always create a (minimized) weight increase, the tail can be designed as
0 100 200 300 400 500q
−0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Conformal Conventional CLβ
FIGURE 7.15 Comparison of rolling moment effectiveness for conventional and conformal trailing edge control surface. (From Sanders, B., Flick, P., and Sensburg, O., in: Pendleton, E. W. Ed., International Forum on Aeroelasticity and Structural Dynamics, Madrid, Spain, 2001.)
FIGURE 7.16 Goodyear Infl atoplane (1950s). http://upload.wikimedia.org/wikipedia/en/C/C6/Goodyear-infl atoplane.jpg (July 6, 2008).
a reduced size all-movable surface, and the location of the spigot axis is used to tailor the wash-in eff ect, while the attachment stiff ness is adjusted to the desired eff ectiveness. Th is would allow us to obtain the required eff ectiveness also at low speeds with a smaller tail. As described in Ref. [44], the proper shape of the surface in conjunction with the spigot axis location will also enhance the fl utter stability. Figure 7.18 depicts the resultant eff ectiveness for diff erent spigot axis locations at diff erent Mach numbers (and dynamic pressure) with variable stiff ness.
Th e crucial element of the all-movable surface with adaptive attachment stiff ness is the attachment or actuation component.
Th is can, for example, be a mechanical spring with variable stiff - ness and a conventional hydraulic actuator. Or as a more advanced system, a hybrid actuator with smart material ele- ments like magnetorheological fl uids. Th e objective of the cur- rent DARPA program “Compact Hybrid Actuators” is aimed at the development of such components with high energy density and 10 times the stroke of current systems.
Of course, such systems can also be used for horizontal stabiliz- ers or outboard wing sections. Compared to a horizontal tailplane, where the fuselage fl exibility causes aeroelastic eff ectiveness
losses, a forward surface can exploit additional benefi ts from the fuselage fl exibility.