Torsion Coupling
7.8 Role of Aeroelasticity
7.8.1 Reputation of Aeroelasticity
Some years aft er the Wright brothers’ success with their active wing, designers began to fear the fl exibility of the structure. Th e famous MIT Lester B. Gardner Lecture “History of Aeroelasticity”
from Raymond L. Bisplinghoff [13] quotes many of the early inci- dents involving aeroelastic phenomena, and the famous comment from Th eodore von Kármán: “Some fear fl utter because they do not understand it, and some fear it because they do.” Also quoted from a review paper on Aeroelastic Tailoring by T. A. Weisshaar [2]: “As a result, aeroelasticity helped the phrase ‘stiff ness penalty’
to enter into the design engineer’s language. Aeroelasticity became, in a manner of speaking, a four-letter-word.…it deserves substan- tial credit for the widespread belief that the only good structure is a rigid structure.” Th e role of aeroelasticity in aviation is depicted in Figure 7.6. It shows the impact on aircraft performance over the years, mainly caused by increasing speed. But the upper dot in 1903 also indicates that aeroelasticity can also act in a positive way, if properly used and understood, also today, and on faster airplanes. Smart structures concepts will help to reverse this nega- tive trend of aeroelastic impacts on aircraft performance. Similar to Figure 7.5, the progress in aeronautics can also be connected to the progress in aeroelasticity and related external stimuli and events, as draft ed in Figure 7.7.
7.8.2 Aeroelastic Effects
Because of the diffi culty to describe aeroelastic eff ects by proper theoretical models, involving a good description of the structure with its fl exibility and structural dynamic characteristics as well as steady and unsteady aerodynamic properties, solutions were limited in the early years of aviation to selected cases with only a few degrees of freedom. More general solutions required modern computers with respect to storage space and computing time.
Th e aeroelastic triangle (Figure 7.8), for the fi rst time quoted by Collar [14] in 1946, describes the involved types of forces in the diff erent aeroelastic phenomena. Looking at these forces and
Year Performance
Wright flyer I
Rigid A/C performance Aeroelastic impacts
Active aeroelastic concepts
Aeroelastic degradation
1903 2000
Langley aerodrome
FIGURE 7.6 Impact of aeroelasticity on aircraft performance.
interactions, it becomes obvious that smart structures for aero- nautical applications will have a close relationship with aero- elasticity in most cases.
7.8.2.1 Static Aeroelasticity
In static aeroelastic eff ects, no inertia forces are involved by defi - nition. Th is is true for aileron reversal, an eff ect, where the roll- ing moment due to a control surface defl ection changes the sign at a certain fl ight speed due to opposite deformation of the fi xed surface in front of the control surface. Th is eff ect has to be avoided within the fl ight envelope of the aircraft in order to avoid deterioration of the pilot when he moves the stick to roll the airplane.
If the Wright brothers had used conventional ailerons on their fi rst airplanes, they might have experienced aileron reversal because of the low torsional stiff ness of their wings, even at very low speeds. On the other hand, the Wright brothers’ main com- petitor, Samuel P. Langley, was very likely less fortunate with his Aerodrome designs because of insuffi cient aeroelastic stability [13]
aft er scaling the successful smaller unmanned vehicle to larger dimensions.
For fi ghter airplanes, it is not suffi cient to avoid aileron rever- sal. Even at the worst fl ight condition, a high roll rate must be achieved to provide high agility. Th is is usually done by reinforc- ing the wing structure because the basic static design of a fi ghter wing yields rolling moment eff ectiveness slightly above or even below zero at the worst fl ight condition. In the case of the U.S.
aircraft F-18, the basic design had to be revised aft er delivery of the fi rst batch of production aircraft . An additional weight of 200 lbs per wing side was reported to the author for the Israel Lavi lightweight fi ghter aircraft project to provide suffi cient roll power. In addition to the loss of roll power, the adverse deformation of the control surface required larger control surface defl ections, which resulted in higher hinge moments and required stronger actuators. Th e diffi culty in predicting the most eff ective distri- bution of additional stiff ness for improved roll eff ectiveness, especially in conjunction with the introduction of modern composite materials with highly anisotropic stiff ness properties for airframe design, inspired the development of formal mathematical structural optimization methods [15].
Aileron reversal has usually the most severe static aeroelastic impact on aerodynamic forces and moments. But all other aero- dynamic performance or control characteristics of an airplane are aff ected as well by static aeroelastic deformations and aero- dynamic load redistributions to a more or less severe degree.
Weisshaar [16], for example, mentions the excessive trim drag due to aeroelastic wing deformations on the delta wing of a supersonic transport aircraft .
Roll control improvement by means of active concepts was and still is the most oft en studied application of active concepts for static aeroelastic phenomena on aircraft . Although active structures or materials are not involved, the Active Aeroelastic Wing project [17] or Active Flexible Wing project, as it was called before, has undergone fl ight test trials in 2001 on a modifi ed Year
Aircraft performance
increase
Theoretical models
Active aeroelastic
concepts Computers
1900 2000
Discover phenomena
Analytical methods
Aeroelastic tailoring
Description of phenomena
Finite element methods
Composite materials
Active materials
FIGURE 7.7 Relationship between aircraft performance, advances in aeroelasticity, and external stimuli.
Static aeroelasticity
Elastic forces Inertia forces Aerodynamic forces
Dynamic aeroelasticity
Structural dynamics Flight mechanics
FIGURE 7.8 Aeroelastic triangle.
Front spar Rear spar
FIGURE 7.10 Fokker D-VIII monoplane, where aeroelastic divergence caused several fatal accidents aft er reinforcement of the rear spar.
(Modifi ed by author, photo from Internet.)
F-18. Th is concept originated in several theoretical studies and wind tunnel demonstrations in the 1980s. A summary of these activities is presented in a special edition of the Journal of Aircraft in 1995 [18]. Figure 7.9 depicts the wind tunnel model installed in the transonic dynamics tunnel at the NASA-Langley Research Center (NASA-LARC).
Static aeroelastic eff ectiveness losses for the lateral stability and rudder yawing moment are a well-known design driver for vertical tails. Surprisingly, almost nobody has looked so far for smart structures concepts to obtain better designs. Sensburg [19]
suggested a smart passive solution, called the Diverging Tail, by means of aeroelastic tailoring the composite skins and modify- ing the fi n root attachment to a single point aft position, to achieve higher yawing moments compared to a rigid structure.
Aeroelastic divergence was the most severe instability for early monowing airplanes. If the wing main spar is located too far behind the local aerodynamic center of pressure (at 25%
chord), a lack of torsional stiff ness will cause the wing structure to diverge and break at a certain speed. As Anthony Fokker describes in his book [20], suffi cient strength of the design had already been demonstrated by proof load and fl ight tests for his D-VIII (Figure 7.10) when regulations called for a reinforced rear spar with proportional strength capacity to the front spar.
Th is redistribution of stiff ness caused torsional divergence under fl ight loads. Th is example also demonstrates the potential eff ects and impact from smart structures applications to an airplane structure.
Th e introduction of high-strength composite materials, with the possibility to create bending–torsion coupling eff ects from the anisotropic material properties, caused a renaissance of the forward swept wing in the late 1970s [2], which was ruled out
before for higher sweep angles because of the bending–torsion divergence, as depicted in Figure 7.4.
Static aeroelasticity also includes all eff ects on aerodynamic load distributions, the eff ectiveness of active load alleviation systems by control surfaces, and fl exibility eff ects on the aero- dynamic performance. In this case, the variable inertia loads from payload or fuel on the structural deformations have to be considered simultaneously.
7.8.2.2 Dynamic Aeroelasticity
Flutter is the best known dynamic aeroelastic stability problem.
It belongs to the category of self-excited oscillation systems. In this case, any small external disturbance from a control surface command or atmospheric turbulence, which excites the eigen- modes of the structure, will at the same time create additional unsteady aerodynamic forces. Depending on the mass and stiff - ness distribution, and on the phase angles between the involved vibration modes, the aerodynamic forces will dampen the oscil- lations, or enforce them in the case of fl utter.
Active control for fl utter stability enhancement by means of aerodynamic control surfaces was fashionable in the late 1970s [21]. In this case, the eff ectiveness of the system depended on the static aeroelastic eff ectiveness of the activated control surfaces.
Mainly because of safety aspects, but also because of limited eff ectiveness, none of these systems has entered service so far.
Active control by active structures devices was a popular research topic in recent years [10], but it is doubtful if we will ever see applications because of the same reasons.
Panel fl utter is a special case, where only individual skin elements of the structure (panels) are aff ected. Th is usually hap- pens at low supersonic speeds, and only structural elements with low static load levels, like fairings, can usually be aff ected. Active control by smart materials is possible, but there are no considerable impacts on aircraft eff ectiveness.
FIGURE 7.9 Active Aeroelastic Wing model mounted in the NASA tran- sonic dynamics wind tunnel at the NASA Langley Research Center. http://
www.dfrc.nasa.gov/Newsroom/.X-Press/special-editions/AAW/images/
121703/windtunnel-500.jpg (July 6, 2008).
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].