JOHANNESSCHWEIGER European Aeronautic Defense and Space Company
Military Aircraft Business Unit Muenchen, Germany
INTRODUCTION
Probably the most famous photograph in aviation, (Fig. 1) depicts the first successful manned powered flight on 17 December 1903 by the Wright brothers. It also shows the first successful airplane that had an active structure. The Wright brothers’ design must definitely be called smart in many aspects and design features. They were among the first pioneers in aviation who had realized that un- coupled control about all three vehicle axes was required.
They had done systematic experimental aerodynamic re- search to achieve the maximum possible aerodynamic per- formance. And they had learned how to design and manu- facture lightweight structures in their bicycle shop. Their solution for adequate roll control of the airplane, moreover, was more than one century ahead of the state of the art in aviation technology. As we are approaching the centennial celebrations of this remarkable event, no single airplane exists yet that uses a smart structural concept to control the flight of the vehicle.
Rather than fighting the low torsional stiffness of their braced biplane wing design, they used this characteristic positively. By the sideways motions of the pilot, who lay on a sliding cradle, the wires attached to the cradle twisted the wing tips in opposite directions, thus producing the desired aerodynamic loads to roll the airplane. Figure 2 from Orville Wright’s book (1) demonstrated this princi- ple, which is also a very good example of the importance of integrated or multidisciplinary design concepts, espe- cially in aeronautics. Unfortunately, this knowledge was lost and forgotten over the years, mainly because of more expert knowledge in single disciplines and more formal and
Figure 1. First flight of Wright Flyer I on 17 Dec. 1903 (available as a postcard).
AIRCRAFT CONTROL, APPLICATIONS OF SMART STRUCTURES 43
Cables attached to cradle-sliding cradle to left of machine pulls trailing edge of right wing downward Cable (not attached to cradle) is moved automatically by downward movement of right wing
The accompanying picture of the Wrights′ powered machine (with motor and propellers removed) shows the method of twisting the rear of the wings. A movement of only an inch or two, to the left or right, of the operators′ hips resting on the little cradle was enough to give greater lift to whichever wing needed it, and to restore sidewise balance.
Right Left
Cradle
Figure 2. Active structural concept of the Wright Flyer I for roll control [adopted from (1)].
bureaucratic processes in designing new airplanes. Only in recent years, some prophets in aerospace are trying to spread the news about this old idea again and develop some new ones. Weisshaar (2), for example, in 1986, cited the suc- cess of the Wright Flyer as a good example of the need for integrated design methods. The Wright Flyer also demon- strates that smart aircraft structures do not necessarily rely only on advanced active materials.
Even earlier than that, active structural concepts were studied. Alois Wolfm ¨uller (1864–1948), the producer of the world’s first motorcycle, bought the No. 2 model of the production glider “Normal-Segelapparat” (normal soaring apparatus) from Otto Lilienthal in 1894 (3). Both avia- tion pioneers were communicating about improvements in performance and maneuverability by controlling the air loads through flexible wing twist. Wolfm ¨uller tried to im- prove performance by introducing a flexible hinge in the wings to modulate aerodynamic control forces by flexible deformations.
Today, aircraft control is achieved by control surfaces at- tached to the main aerodynamic surfaces. These devices—
aileron, elevator, and rudder—create the required forces and moments to control the motion of the aircraft about all three axes in space. Depending on the size and speed of the aircraft, these surfaces are actuated manually or by hydraulic systems. If the Wright brothers had used sepa- rate ailerons to roll the airplane, the additional structural weight might have been too much for the available power from the engine.
The idea of active or smart structures to control air ve- hicles is as old as the earliest known attempts to fly in heavier-than-air machines. Early attempts by humankind to fly were usually based on efforts to understand and copy the flight of birds. Besides the difficulty in controlling an unstable flying vehicle, which requires to day’s high com- puting power or the complex neural network of animals to control their muscular systems, it is even more difficult to sense and actuate the dynamic motions of continuously deforming, flexible aerodynamic surfaces.
In most of these efforts, the pilot was supposed to actu- ate birdlike aerodynamic surfaces to produce the required
lift force, create the forward thrust, stabilize the vehicle, and apply the necessary control inputs in time. For a mul- titude of reasons, all of these early efforts were doomed to failure from the beginning:
very limited knowledge of the laws of aerodynamics and flight mechanics;
insufficient load capacity of the available materials, only primitive manufacturing techniques, and only a rudi- mentary understanding of structural mechanics;
not enough sustained power output from the human body to produce the required thrust or lack of engines that had sufficient power density;
efforts in trying to copy structural design principles from nature did not take into account the scaling laws of physics; the resulting designs were too heavy, too fragile, or too flexible; and
and, in most cases, also the very complex efforts re- quired to stabilize and control a flying vehicle with- out natural stability.
Only in recent years, after almost one century since the first successful powered flight, was it possible to design, build, and fly vehicles powered by the human body for the sole purpose of winning trophies.
For these reasons, the first successes in aviation were possible only by using design concepts for almost “rigid”
surfaces and natural stability of the vehicles. Nevertheless, a major contribution to the success of the Wright broth- ers was their “Smart Structures” flight control system for the roll axis. They were among the first pioneers in avi- ation who had realized that uncoupled control about all three vehicle axes was required. They had done systematic experimental aerodynamic research to achieve maximum possible aerodynamic performance. They had a combustion engine that had sufficient energy density available “just in time,” and they had learned how to design and manufac- ture lightweight structures in their bicycle shop.
SMART STRUCTURES FOR FLIGHT IN NATURE
Although complete plants do not fly, they have to withstand aerodynamic loads by using proper aeroelastic reactions, and sometimes their existence relies on their aerodynamic performance. The seeds of some plants are optimized in shape for long distance flight and mass distribution by mil- lions of generations in a genetic optimization process. The interest in micro air vehicles in recent years has increased aerodynamic research efforts in this area (4). Although not for free flight, the leaves of trees and their stems are built to withstand strong winds. The joints between leaves and branches must have the right amount of flexibility in bend- ing and torsion to reduce aerodynamic loads and at the same time avoid excessive unsteady loads from flutter.
More interesting for aircraft applications of smart struc- tures is the flight of animals. As mentioned before, early aviators tried to learn from birds. However, the required knowledge about the physics of flight was missing in these early attempts. The complex interactions of steady
44 AIRCRAFT CONTROL, APPLICATIONS OF SMART STRUCTURES
Figure 3. “Structural” design concept for dragonflies.
and unsteady aerodynamic forces and the active motions and passive deformations of wings and feathers are still not completely understood today. We are only now begin- ning to understand the functions and importance of the individual components for the efficiency of animal flight.
Pendleton (5) gives some good examples from the prehis- toric flying saurian to modern birds that have feathers.
But even more astonishing are the achievements in the art of flying for another species. Insects show by far the widest variety and most advanced structural concepts of active and passive control. An ordinary fly can land and take off from the tip of your nose or from the ceiling of a room. Dragonflies like that shown in Fig. 3 use a combi- nation of passive mass balance to prevent flutter at the wing tips and an advanced structural design that has stiff chitin “spar” elements to support the membrane skins and flexible hinges of resilium to adjust the shape for all flight conditions.
The variety and large number flying members in the family of insects has been attributed to their ability to fly, which offers advantages of reaching and conquering new territories more easily.
In the context of formal optimization methods in aero- nautics, genetic algorithms became fashionable in recent years. The question in the context of technical products is, can we really wait as long as in nature to get better products ? Or should we continue to rely on gradient based methods, which can be seen as targeted “artificial genetic manipulations” similar to the “biological engineering” ap- proaches of today?
GENERAL REMARKS ON ASPECTS OF AIRCRAFT DESIGN One reason that we have not seen more progress in the application of smart structures in aeronautics may be the lack of understanding of the interactions between the dif- ferent classical disciplines in aircraft design and between these disciplines and the specialists from the smart mate- rials area for each others’ needs and capabilities.
To assess the possibilities of smart structure applica- tions for aircraft control, it is advisable to look at some aspects of aircraft design first. The structural engineer is
usually concerned with the strength of a design for critical conditions and may be surprised in some cases, how rela- tively small deformations of the structure can have severe impacts on the aerodynamic performance or on maneu- verability. The aerodynamic and flight control engineers want the structure “as rigid as possible,” but at the same time want different shapes for different flight conditions.
These requirements may attract the smart structure spe- cialists to make the design deformable and at the same time meet tight requirements for the external shape in changing flight conditions. This requires a look at the stiff- ness of the structure from the viewpoint of strength, as well as at the additional internal loads created by active deformations. And finally, aeroelastic aspects like flutter stability and effectiveness of deformed aerodynamic sur- faces have to be considered.
An aircraft design is always a compromise for the aero- dynamic engineer between different flight conditions: a small flat wing for cruising with low drag at high speed and a large, cambered wing for take-off and landing at low speed. This can be met partially by extendable control surfaces attached to a fixed surface by complex kinematic systems. On the other hand, the complexity of these sys- tems increases the structural weight. Therefore, it looks at- tractive to replace these mechanisms and integrate their functions into one actively deformable structure. To con- sider such options, it is necessary to look at basic struc- tural design requirements for aircraft wings. They have to carry loads from 2.5 to 9.0 times the total weight of the airplane. And they must be strong enough in torsion to keep the aerodynamic shape and transmit the loads from control surfaces. To achieve this at an acceptable structural weight, sophisticated lightweight design concepts were de- veloped during the first half century of manned aviation.
Besides improvements in materials and manufacturing, lightweight design by shape —within the prescribed aero- dynamic shape is the main principle.
Simplified, this principle leads to the placement of ma- terial on the external shape of the airfoil and to closed cross sections of maximum area. This provides maximum strength for a fixed amount of material. At the same time, this structure also has maximum stiffness. This fact must be kept in mind when active deformable structural con- cepts are considered for airframe components.
TRADITIONAL ACTIVE OR ADAPTIVE AIRCRAFT CONTROL CONCEPTS
One kind of active aircraft control concept was developed and demonstrated in the early 1980s: artificial stabiliza- tion of an airplane’s flight path by fast motions of the con- trol surfaces, commanded by a digital flight control system.
Today, this system helps to reduce trim drag and increase the agility of all modern fighter aircraft by avoiding nega- tive contributions from a stabilizer surface to the total lift force to establish natural static stability.
Most airplanes have wings adaptive by additional de- ployable surfaces like slats at the leading edge or Fowler flaps at the trailing edge to provide additional lift for take-off and landing or to provide clean flow conditions
AIRCRAFT CONTROL, APPLICATIONS OF SMART STRUCTURES 45 for extreme maneuvers. The resulting effects are usually a
combination of increased wing surface, increased camber, and accelerated air flow. These concepts are well described in most aerodynamic text books (6), where more sophis- ticated and exotic concepts like telescope wings can also be found. Variable wing sweep is rather common for com- bat aircraft, among them the American F-111. The Mis- sion Adaptive Wing (MAW) demonstrator version of this aircraft used a complex mechanical system to adjust the wing camber for changing flight conditions (5). However, this system proved too complex and heavy for applications on production aircraft.
Enhancement of active flutter stability by control sur- face deflections to create unsteady aerodynamic damping forces was also demonstrated in the 1970s (7). There are two main reasons that we do not see them on today’s air- planes:
flutter within the flight envelope in failure of the system is considered too critical;
the performance of these systems was not very good be- cause of the loss of static aeroelastic effectiveness of the control surfaces as airspeed increased; it is typi- cally close to zero for ailerons mounted near the wing tip at the trailing edge.
The last point also applies to the effectiveness of maneuver load alleviation systems by symmetrical deflected outboard aileron surfaces.
THE RANGE OF ACTIVE STRUCTURES AND MATERIALS APPLICATIONS IN AERONAUTICS
Besides aircraft control, including load alleviation, aero- dynamic performance improvement by shape, and static aeroelastic applications for maneuverability and static sta- bility enhancement, which will be the main points of the following sections, the following main additional applica- tions in aeronautics are being addressed by theoretical and experimental research:
Health monitoring. Because piezoelectric materials can serve as actuators and sensors as well, these materials can be used to monitor changing static or dynamic internal loading conditions that result from airplane maneuvers or from failures in the structure.
Although these functions have been widely discussed in the literature for two decades, the author knows of no application to a production airplane to date.
Vibration control. This application can be subdivided into two categories:
to treat local or global vibrations of structural components. This was also the first flight test demonstration of piezo materials applied to a skin panel affected by engine noise of the Rock- well B-1B bomber (8).
to reduce vibrational levels on equipment that is mounted internally or externally on the air- craft structure. This can be done to support the integrity of the equipment or to improve
the performance of equipment acting as sen- sor systems. Because rather small deforma- tions and forces have to be handled here, the most promising and near term applications of smart materials on airplanes in this area were already predicted in the survey paper by Crowe and Sater (9).
Active flutter suppression. A lot of research has been dedicated to this task. The NASA wind tunnel test program “Piezoelectric Aeroelastic Response Tailor- ing Investigation”(PARTI) (10) demonstrated this technology. Applications to real aircraft are rather unlikely because of the previously mentioned safety aspects. Panel flutter as a special case for the insta- bility of individual skin panels at supersonic speed is also addressed in many publications. The required effort for installing and controlling active devices is inappropriate compared to a simple structural rein- forcement of critical panels.
Acoustic noise reduction. Several research projects are dealing with cabin noise reduction, especially for turboprop airplanes.
AIRCRAFT STRUCTURES Definition of a Structure
To design active structural systems for airplane control, it is necessary to understand the functions of the structure, the requirements that define its properties, and the rea- sons that existing aircraft structures are built in a specific way.
The main functions of the airframe are
to bear and carry the loads acting on the vehicle;
to provide space and support for engines, equipment, payload, and fuel; and
to satisfy the tight rigidity and smoothness required by aerodynamics for the external shape.
In other words, the external shape of the vehicle is defined by aerodynamic performance requirements, and the struc- ture has to meet these requirements within this shape at minimum weight. This requires carefully combining the shape and load-carrying purpose of the individual struc- tural elements with the best available material for this purpose. Early designs used fabric to collect the aerody- namic pressure loads on a wing surface and transfer them to the fuselage through a wooden framework supported by thin wires. As soon as it was discovered that the aero- dynamic drag of these wires is 100 times higher than that of a carefully designed airfoil of the same thickness, design efforts concentrated on cantilevered wings where the loads could be completely transferred internally. Alu- minum skins paved the way for modern monocoque air- frames, where the main loads are carried by a monolithic shell structure which needs mainly only the internal struc- ture to maintain the shape. The shape of this shell provides highest resistance against bending and torsion loads at minimum weight.
46 AIRCRAFT CONTROL, APPLICATIONS OF SMART STRUCTURES
Rigidity of Wing Structures
Considering active deformations of a wing structure, it is useful to look at the differences in the possible passive deformations under different loading conditions. The lift force is the largest component of the aerodynamic forces. It corresponds to multiples of the total weight of the airplane, 2.5 times for transport aircraft and currently up to 9 times for fighters. At the same time, the external shape of a wing has the smallest dimension in its height and the largest in a spanwise direction. This means that the structure needs the largest cross sections of its skins on the upper and lower surfaces. Therefore, to deform a wing in bending actively would be difficult but not impossible. However, the bending deformation of an unswept wing has no impact on the aero- dynamic characteristics or loading conditions. Only swept wings are sensitive in this respect. Swept forward, bend- ing increases the local aerodynamic angle of attack in the streamwise direction, as indicated in Fig. 4. This increases the bending moment and causes structural divergence at a flight speed, called the divergence speed, which depends on the bending stiffness and geometric properties of the wing.
The same effect reduces the bending moment on a swept- back wing under load, which acts as a passive load alle- viation system. But to control these deformations actively by internal forces would mean stretching and compressing the skins in the spanwise direction—a rather difficult task.
The aerodynamic drag forces that act in the streamwise direction are smaller than the lift forces by a factor of 10.
At the same time, the shape of the airfoil creates a high static moment of resistance in this direction. For these rea- sons, the loads in this direction need no special attention in the structural design. An active deformation would be both very difficult and meaningless.
Torsional loads on a wing can be very high, depend- ing on the chordwise center of pressure locations and on additional forces from deflected control surfaces. A center of pressure in front of the fictive elastic axis through the wing cross sections causes torsional divergence at a cer- tain flight speed, and a center of pressure too far behind the elastic axis twists the wing against the desired angle
Swept back wing V
Deformations along elastic axis
Streamwise deformation
Forward swept wing V
Deformations along elastic axis Streamwise deformation
Figure 4. Bending deformation of swept wings and impacts on the aerodynamic angle-of-attack.
of attack or control surface deflection. Therefore, the wing torsional deformation is very sensitive to the loads acting on the wing, to the spanwise lift distribution which is im- portant in the aerodynamic drag, and to the effectiveness of the control surfaces. As mentioned before, a closed torque box that has a maximum cross section is desirable for the structural designer. But the possibility of adjusting tor- sional flexibility would also allow several options for active control of aerodynamic performance, load distribution, and control effectiveness. This active control by internal forces could theoretically be achieved by active materials in the skins or by an internal torque device fixed at the wing root and attached to the wing tip. Such a device, based on a shape memory alloy, has already been demonstrated on a wind-tunnel model (4). For practical applications, the pa- rameters that define the torsional stiffness of a structure that has a closed cross section, should be kept in mind.
Torsional stiffness is proportional to the square of the com- plete cross-sectional area and linearly proportional to the average thickness of the skin. This demonstrates how dif- ficult it would be to modify the stiffness by changes in the skins or by an internal torque tube that has a smaller cross section.
The most often mentioned application of active struc- tures for aircraft application is camber control and the in- tegration of control surface functions into the main sur- face by camber control. This would mean a high chordwise bending deformation of the wing box. As mentioned before for spanwise bending, the skins would have to be stretched and compressed considerably, but in this case based on a smaller reference length and a smaller moment arm. For this reason, we do not see chordwise bending deforma- tions on conventional wings under load. Aeroelastic tailor- ing, addressed later, by adjusting the carbon fiber plies in thickness and direction to meet desired deformation char- acteristics, was also addressing camber control in the 1970s and 1980s as one specific option. Because of the previously mentioned constraints, there has been no application of this passive aeroelastic control feature on a realistic wing design.
Structures and Mechanisms
To a certain extent, the main functions of structures and mechanisms are opposite: a structure must provide rigi- dity, and a mechanism must provide large defined motions between parts. If active structures are considered, both functions must be integrated into the structure, the perfor- mance of this system should be better, and the total weight should be lower compared to a conventional design. This shows the difficulty of developing active structures for air- craft control.
Therefore, the intention of making the structure more flexible to allow deformations is a contradiction. Hinges are required to allow deformations without producing internal forces. If this function is desired within the structure, the structure has to become more flexible in distinct small re- gions. But this attempt will create high internal loads in regions that have small cross sections, and the desired de- formations for aircraft control functions will produce a high number of load cycles.