35% chord
47% chord
Side force effectiveness
Stiffness
Rotational axis locations
Stiffness
Figure 18. Achievable aeroelastic effectiveness using variable attachment stiffness for different locations of the spigot axis.
component. This can, for example, be a mechanical spring that has variable stiffness and a conventional hydraulic ac- tuator or, as a more advanced system, a hybrid actuator us- ing smart material elements, such as magnetorheological fluids. The objective of the current DARPA program “Com- pact Hybrid Actuators” is to develop such components at high energy density and 10 times the stroke of current systems.
Of course, such systems can also be used for horizon- tal stabilizers or outboard wing sections. Compared to a horizontal tailplane, where the fuselage flexibility causes losses of aeroelastic effectiveness, a forward surface can exploit additional benefits from the fuselage flexibility.
QUALITY OF THE DEFORMATIONS
The amount of internal energy required for the desired de- formations depends strongly on the static aeroelastic effec- tiveness involved. As depicted in Fig. 19, the aerodynamic loads can either deform the structure in the wrong direc- tion and require additional efforts to compensate for the deformations caused by external loads, or the internal, ini- tial deformation is used so that the desired deformations are only triggered and the major amount of energy required is supplied by the air at no cost. In the first case, the re- quired deformation generated by internal forces already creates a high level of internal strain in the structure, re- sulting in reinforcement and extra structural weight. In the second case, the required internal actuation forces and the strain levels are much smaller. For a favorable solu- tion, the design process must reduce the total load level of the structure in the “design case,” thus reducing the to- tal weight required for the structure and actuation system
AIRCRAFT CONTROL, APPLICATIONS OF SMART STRUCTURES 55
Forced deformation and aeroelastic response - Case 1 -
Forced deformation and aeroelastic response - Case 2 -
Applied force without external load
Energy for actuation Deformation x Force
Deformation without external load Energy
loss
Aeroelastic deformation Required
additional energy for initial deformation
Applied force without external load
Deformation x Force
Deformation without external load
Aeroelastic deformation Additional
energy supplied from air Energy
for actuation
Figure 19. Forced structural deformation and aeroelastic response of different design approaches.
compared to a conventional design, as indicated in Fig. 20, and achieving better performance.
The effort required and the results achievable for a spe- cific type of deformation depend strongly on the typical properties of the wing structure, which in most cases can be described as a beam. First of all, lift forces create bend- ing deformations in the direction of the lift force. Because drag forces are much smaller (1/10) and because of the shape of the airfoil, in-plane bending deformations can be neglected. Depending on the chordwise location of the re- sulting lift force relative to the beam (torque box) shear center location, it is possible to twist the wing. This can, for example, be used to reduce the bending deformation. Be- cause of the high stiffness of a modern wing in a chordwise section and because of the resulting aerodynamic pressure distribution that usually acts in one direction, a chordwise bending deformation (camber) is very difficult to achieve by internal or external forces. This is also true for a reduced thickness rear section of a wing, for example, to replace a deflected control surface.
Concepts performance
Required weight (structure + actuation system) Achievable deformation
(for desired performance)
(Passive) baseline design
Active static deformation 2 Aeroelastic deformation 2
Aeroelastic deformation 1 Active static deformation 1 (good)
(bad) Favourable solution 3
Figure 20. Performance of active structural concepts in weight and performance.
ACHIEVABLE AMOUNT OF DEFORMATION AND EFFECTIVENESS OF DIFFERENT ACTIVE AEROELASTIC CONCEPTS
Classical active aeroelastic concepts rely on the adaptive use of aerodynamic control surfaces and their aeroelastic effectiveness under various flight conditions. In conven- tional designs, the aeroelastic effect is more pronounced as airspeed increases, as demonstrated in Fig. 21 for potential losses and gains.
Conventional active aeroelastic concepts exploit the in- creasing effectiveness in the upper half of this figure, as well as the recovering effectiveness of a conventional aileron beyond the reversal speed. A combined operation of leading and trailing edge surface results in an achievable roll rate, as indicated in Fig. 22.
To exploit aeroelastic effects more beneficially, increas- ing the aeroelastic sensitivity of the design in a wider range of the flight envelope is required. This can be achieved,
Aeroelastic effectiveness
Dynamic pressure Aileron
reversal
Performance index Divergence
1.0 Rigid aircraftRigid aircraft
Figure 21. Typical range of aeroelastic effectiveness.
56 AIRCRAFT CONTROL, APPLICATIONS OF SMART STRUCTURES
Combined effectiveness
Leading edge
Trailing edge Dynamic pressure
Active aeroelastic wing: Blending of leading and trailing edge effectiveness
Performance index
Figure 22. Achievable roll performance by combining leading and trailing edges.
for example, by an all-movable aerodynamic surface that has adaptive rotational attachment stiffness. This also pro- vides high effectiveness at low speeds, and excessive loads from diverging components or flutter instabilities at high speeds can be avoided.
The usable aeroelastic effectiveness for conventional concepts is rather limited between take-off and cruise speed. Aileron reversal usually occurs between the cruise speed and limit speed, and too high an effectiveness of lead- ing edge surfaces must be avoided at the limit speed. On the other hand, adaptive all-movable concepts can provide high effectiveness at all speeds and avoid excessive loads at the high end of the speed envelope, as indicated in Figs. 23 and 24. This means, for example, that a stabilizer surface can be built smaller than would be required by “rigid” aero- dynamic low-speed performance.
NEED FOR ANALYZING AND OPTIMIZING THE DESIGN OF ACTIVE STRUCTURAL CONCEPTS
Of course, active materials and structural components, together with the stimulating forces, need a correct
Active aeroelastic concepts Range of aeroelastic
effectiveness on conventional designs
Dynamic pressure Dynamic pressure
Effectiveness Effectiveness
Rigid aircraft
Range of effectiveness for advanced active aeroelastic concept
1.0
Rigid aircraft
Figure 23. Aeroelastic effectiveness of conventional and adaptive-all-movable active aeroelastic concepts.
For conventional active aeroelastic concepts
Usable range of aeroelastic effectiveness by flight envelope
For advanced active aeroelastic concept
Dynamic pressure Dynamic pressure
Rigid aircraft Vmin
VC VD
VD
Effectiveness Effectiveness
1.0
Vmin VC
Rigid aircraft
Figure 24. Usable range of aeroelastic effectiveness for conven- tional and advanced active aeroelastic concepts.
description in theoretical structural or multidisciplinary analysis and optimization (MDAO) models and methods.
Once this is provided, the actively deforming structure needs another approach for static aeroelastic analysis. The deflections of selected control surfaces of an aircraft that has conventional control surfaces can be predescribed for aeroelastic analysis. For an actively deformed structure, initial deformations without external loads first need to be determined, for example, by static analysis.
As described before, the deformations achievable in con- junction with the distribution of external aerodynamic loads are essential for the effectiveness of active structural concepts for aircraft control. This requires efficient tools and methods for simultaneous, multidisciplinary analyti- cal design. The best design involves optimizing
rexternal shape,
rarranging the passive structure (topology), rsizing the passive structure,
rplacing and sizing the active elements, and ra control concept for the active components.
The aims of this approach are the optimum result for the objective function (minimum weight, aerodynamic perfor- mance), fulfillment of all constraints like strength, and also optimization of additional objectives, such as minimum en- ergy. As depicted in Fig. 25 for the optimization of a passive structure that has different constraints for the required rolling moment effectiveness, the energy required to actu- ate the control surface can be considerably reduced, even if the required (low) roll rate is already met.
MDO does not mean combining single discipline ana- lytic tools by formal computing processes. It means first a good understanding of what is going on. This is essential for a conventional design. Only from this understanding can the creative design of an active concept start.
It is also very important to choose the proper analytic methods for individual disciplines. Usually, not the high- est level of accuracy is suitable for the simulation of impor- tant effects for other disciplines. This also refers to refin- ing the analytic models, where local details are usually not
AIRCRAFT CONTROL, APPLICATIONS OF SMART STRUCTURES 57
Structural weight Rolling
moment effectiveness
Baseline static design 1.0
Aileron hinge moment [kNm]
Rigid 50.0
10.0
Figure 25. Optimization of the rolling moment and hinge mo- ment of a trailing edge aileron of a low aspect ratio fighter wing.
interesting for interactions. It is more important to keep the models as versatile as possible for changes in the de- sign concepts and to allow the simulation as many variants as possible. This also means an efficient process for gener- ating models, including the knowledge of the user for this process. Fully automated model generators can create ter- rible results, if the user cannot interpret or understand the modeling process.
Any improvement in a technical system is often referred to as an optimization. In structural design today, this ex- pression is mainly used for formal analytic and numerical methods. Some years after the introduction of finite ele- ment methods (FEM) for analyzing aircraft structures, the first attempts were made to use these tools in an automated design process. Although the structural weight is usually used as the objective function for optimization, the major advantage of these tools is the fulfillment of aeroelastic con- straints, not the weight saving. Other than static strength requirements, which can be met by adjusting the dimen- sions of individual finite elements, the sensitivities of the elements to aeroelastic constraints cannot be expressed so easily.
In the world of aerodynamics, the design of the required twist and camber distribution for a desired lift at minimum drag is also an optimization task. Assuming that minimum drag is achieved by an elliptical lift distribution along the wingspan, this task can be solved by a closed formal so- lution and potential flow theory. More sophisticated nu- merical methods are required for the 2-D airfoil design or for Euler and Navier–Stokes CFD methods, which are now maturing for practical use in aircraft design.
Formal optimization methods have been used for con- ceptual aircraft design for many years. Here, quantities such as direct operating costs (DOC) can be expressed by rather simple equations, and the structural weight can be derived from empirical data. Formal methods such as op- timum control theory are also available for designing the flight control system.
So, one might think that these individual optimization tasks could easily be combined into one global aircraft op- timization process. The reasons that this task is not so
simple is the different natures of the design variables of in- dividual disciplines and their cross sensitivities with other disciplines. The expression multidisciplinary optimization (MDO) summarizes all activities in this area, which have intensified in recent years. It must be admitted that today most existing tools and methods in this area are still single discipline optimization tasks that have multidisciplinary constraints.
To design and analyze active aeroelastic aircraft con- cepts, especially when they are based on active materials or other active structural members, new quantities are re- quired to describe their interaction with the structure, the flight control system, and the resulting aeroelastic effects.
SUMMARY, CONCLUSIONS, AND PREDICTIONS
In the same way as it was wrong in the past to demand that an aircraft design to be as rigid as possible, it’s wrong now to demand a design that is as flexible as possible.
It is sometimes said that smart structural concepts can completely replace conventional control surfaces. But this looks very unrealistic, at least at the moment. The major difficulties for successful application are the limited defor- mation capacity of active materials, as well as their strain allowables, which are usually below those of the passive structure. However, this can be resolved by proper design of the interface between the passive and active structures.
But the essential difficulties are the stiffness and strain limitations of the passive structure itself. It cannot be ex- pected that the material of the passive structure just needs to be replaced by more flexible materials without an exces- sive weight penalty. It is also not correct to believe that an active aeroelastic concept becomes more effective, if the flexibility of the structure is increased. Aeroelastic effec- tiveness depends on proper aeroelastic design, which needs certain rigidity of a structure to produce the desired loads.
A very flexible structure would also not be desirable from the standpoints of aerodynamic shape, stability of the flight control system, and transmission of static loads.
Because large control surface deflections are required at low speeds, where aeroelastic effects on a fixed surface are small, it is more realistic to use conventional control surfaces for this part of the flight envelope and use active aeroelastic deformations only at higher speeds. This would still save weight on the control surfaces and their actua- tion system due to the reduced loads and actuation power requirements.
To produce usable deformations of the structure also at low speeds, all-movable aerodynamic surfaces that have a variable attachment stiffness are an interesting option.
This concept relies on development efforts for active de- vices that have a wide range of adjustable stiffness.
The reasons that we have not seen more progress to date in successfully demonstrating smart structural concepts in aeronautics may be that
rspecialists in aircraft design do not know enough about the achievements in the area of smart mate- rials and structures, and
rsmart materials and actuation system specialists, who try to find and demonstrate applications in
58 AIRCRAFT CONTROL, APPLICATIONS OF SMART STRUCTURES aeronautics, do not know or care enough about real- world conditions for airplane structures.
What we need is more awareness on both sides, as well as stronger efforts to learn from each other and work together.
Although there are strong doubts about useful applica- tions of smart structures for aircraft control, it should al- ways be remembered how often leading experts have been wrong in the past in their predictions, in many cases even on their own inventions. Norman R. Augustine quotes some of them in his famous book “Augustine’s Laws” (46):
r“The [flying] machines will eventually be fast; they will be used in sport but they should not be thought of as commercial carriers.” – Octave Chanute, aviation pioneer, 1910.
r“The energy produced by the breaking down of the atom is a very poor kind of thing. Anyone who expects a source of power from the transformation of these atoms is talking moonshine. – Ernest Rutherford, physicist, ca. 1910.
r“Fooling around with alternative currents is just a waste of time. Nobody will use it, ever. It’s too dan- gerous . . .it could kill a man as quick as a bolt of lightning. Direct current is safe.” – Thomas Edison, inventor, ca. 1880.
Also quoted by Augustine (46), the eminent scientist Niels Bohr remarked: “Prediction is very difficult, especially about the future.”
At the moment it looks more realistic that new hybrid, concentrated active devices, positioned between a passive but properly aeroelastically tailored main aerodynamic surface and the corresponding control surfaces are showing the like Hopefully this article will inspire useful applica- tions of smart structures and prevent some unnecessary research.
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