development of an innovative configuration for

DEVELOPMENT OF AN INNOVATIVE CONFIGURATION FOR TRANSPORT AIRCRAFT; A PROJECT OF FIVE ITALIAN UNIVERSITIES. A. Frediani*, L. Balis Crema**, G. Chiocchia***, G.L. Ghiringhelli****, L. Morino***** * Department of Aerospace Engineering “Lucio Lazzarino”, Pisa University, Italy ** Department of Aerospace Engineering, Università di Roma, “La Sapienza, Italy *** Department of Aerospace Engineering, Technical Universit,y of Turin, Italy **** Department of Aerospace Engineering, Technical University of Milan, Italy ***** Department of Mechanical Engineering and Automation, University of Roma Tre, Italy

Abstract: In the next twenty years, civil transport aircraft are requested to reduce the Direct Operative Costs by 30% and to cut noise and noxious emissions. It is generally accepted that these results can be hardly obtained by improving the efficiency of the to-day conventional aircraft, because their efficiency, as a result of decades of progresses, is close to the top. So, a great interest is now devoted towards new non conventional aircraft for the civil aviation of the future. In Italy, five Universities carried out a preliminary aerodynamic project of a non conventional transport aircraft, in which the aerodynamic efficiency can improved through a strong reduction of the induced drag. The starting point of the project is a theoretical result by Prandtl, published in 1924, showing that the lifting system with the minimum induced drag, under certain conditions, is a wing box in the front view and, in honour of Prandtl, the aircraft configuration has been named as "PrandtlPlane". The project, titled “Development of an innovative configuration for transport aircraft”, allowed us to develop the computational tools for design and optimisation of the PrandtlPlane configuration, together with the preliminary aerodynamic design of a very large, 600 seat category, transport aircraft. The total budget was
485.468, 70% of which was financed by the Italian Ministry of University, for a project of two years (2000 - 2001) The research units involved were: University of Pisa, University of Roma “La Sapienza”, University of Roma Tre, Technical University of Turin and Technical University of Milan. Alenia Aeronautica in Turin carried out subsonic wind tunnel tests on a PrandtlPlane aircraft model. The present paper shows the overall organisation of the project, the main results obtained by the single research units and the final configuration of the very large aircraft. The results of the project were also applied to the aerodynamic and flight mechanics design of an ultra-light PrandtlPlane aircraft.

1. Introduction The aeronautical passenger and cargo traffic are supposed to double in the next two decades, especially along medium and long range routes worldwide. In order to be competitive on long routes, airline companies fleets should include a new large aircraft (i.e.bigger than the Boeing 747-400) with the following requirements: reduction of Direct Operative Costs by 30%, cutting of noise and noxious emissions, more available space and comfort, time reduction (more than 10%) for boarding and disembarkation of passengers and luggage, improvement of cargo capacity, possibility of operating from present runways and airports, cruise speed of 0.85 Mach, approach and landing wake vortex turbulence separations smaller than present. The increase of the aircraft capacity and the technology advancements reduce Direct Operative Costs and noise and emissions per passenger or per unit weight. Unfortunately, the process of increasing the aircraft dimensions is limited by compatibility with the existing airports (any aircraft must be included in an 80x80m horizontal square) and, then, the advantage of increasing the aircraft dimensions will be at its end when the A380 aircraft will enter into service. So, the next generation aircraft bigger than the A380 will not be a scale enlargement of the A380 and, then, it is time to think about non-conventional solutions for the next generation transport aircraft, because (according to Philippe Busquin, European Commissioner for Research) “The aeronautics sector needs a long term vision to tackle future challenges”. The present project moves into this long term vision, by proposing a new non conventional transport aircraft. The main property of this aircraft configuration is a possible reduction of the aircraft drag. The improvement of the aerodynamic design against drag is essential for the commercial success of any transport aircraft programme and, also, for reducing pollution and noise (by improving the aerodynamic efficiency at low speed). Other challenges to be competitive in the world market are higher levels of survivability of accidents in take off and landing and a better comfort of passengers and reduction of the internal noise in cabin. In a large transport aircraft during cruise flight in still air, drag is mainly due to friction drag (about 47%) and induced drag (about 43%, but higher during gust). Different ways for reducing friction drag of conventional aircraft are now under examinations (hybrid laminar flow design, suction integration, hybrid laminar flow tests, transition prediction, realisation of adaptive wings, turbulent drag reduction, separation control); the overall benefits have to be fully evaluated. The induced drag depends on the lift distribution along the wing span; the lift distribution of conventional large transport aircraft is so optimised that no significant reduction seems to be possible in the future. The present project is based on the concept that a possible jump forward in air transportation will come from the introduction of completely new, non-conventional, aircraft. In particular, the project will be focused on a reduction of the induced drag; in this way, both the benefits from friction drag and induced drag reductions will be summed up. The aircraft of the present project is based on an aerodynamic intuition of Prandtl, published in NACA TN 182, 1924 [1]. According to Prandtl, the lifting system with minimum induced drag is a box-like wing (named as ''Best Wing System'' by Prandtl himself), in which the following conditions are satisfied: same lift distribution and same total lift on each of the horizontal wings, and butterfly shaped lift distribution on the vertical tip wings. When this condition of minimum occurs, the velocity induced by the free vortices is constant along the two horizontal wings and identically zero on the vertical side wings. The efficiency increases with the gap. In his paper, Prandtl used an approximate procedure; a closed form solution of the Prandtl problem was given by Frediani and Montanari, in 1999 [2], confirming that the Prandtl results, at least in the range of the wing gaps of interest for applications, were correct. The lift distribution on the horizontal wings results from the superposition of a constant and an elliptical part (figure 1). The ratio between the induced drag of the best wing system and the optimum monoplane is shown in figure 2. It shows that, in the range of

interest of h/b in the present application (10-15%), the induced drag is reduces of about 20% - 30%, equivalent to a reduction of total drag, in still air, ranging from 8 to 13% (and higher during gust). Owing to the Munk theorems, the induced drag is independent of the sweep angles of the wings and, therefore, the Prandtl concept can be applied also to transonic transport aircraft. In honour of Prandtl, the configuration was named as “PrandtlPlane”. 01 0

h

b

Fig. 1. Basic lift distribution in the Prandtl’s BWS problem solution

Db/Dm

1 0.9

Prandtl

0.8

Actual

0.7 0.6 0.5 0.4 0

0.1

0.2

0.3 h/b

0.4

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Fig. 2. Prandtl results vs results in [2]

The PrandtlPlane configuration can be used to design a complete family of aircraft, ranging from small aircraft to wide bodies, larger than the A380. All the aircraft of the family are compatible with the present airports. In fact, in the case of aircraft larger than e.g. A380, the high efficiency of the configuration can be used to reduce the wingspan inside 80m, without drag penalty with respect to a conventional aircraft. The PrandtlPlane configuration can be used also to design small aircraft or UAVs (unmanned aerial vehicles), combining high efficiency and safety. In Italy, five universities, (Universities of Pisa, Roma Tre, Roma “La Sapienza” and the Technical Universities of Torino and Milano) carried out a national project, titled “Development of an innovative configuration for transport aircraft”. The project was partially financed (70%) by the Italian Ministry of University. The aim of the project was to set up the mathematical tools for developing and optimising the aerodynamic configuration of a large PrandtlPlane aircraft (600 seat category) and, also, for carrying out the preliminary design of this aircraft, taking aerodynamics, static stability of flight and structural design of the lifting system into account. In this paper, a brief summary of this joint activity is given. After two years of joint activity, the project was completed successfully. In parallel with the activities on the very large aircraft, a parallel activity was performed on a small two-seat aircraft because, contrary to the very large aircraft, this aircraft could be designed and manufactured quickly. The main results of the project can be summarised as follows. A geometrical and grid generator was set up allowing us to obtain a parametric generation of a PrandtlPlane configuration, without any limitation in the number of parameters; of course, the code is valid for any kind of configuration. A computational code was set for the multidisciplinary optimisation of a PrandlPlane configuration, A computational code was set up to optimise the structures of the lifting system Wind tunnel tests were carried out by Alenia Aeronautica for obtaining a preliminary experimental evaluation of the efficiency of the configuration, An innovative experimental procedure was set up for obtaining wind tunnel model in composite, Wind tunnel tests were carried out at the Technical University of Torino on a model of an ultra light two-seat aircraft. A flying model of the same two seat aircraft was tested in flight. The group of the five Italian universities is ready to apply the results obtained to develop PrandtlPlane aircraft of interest for industry; in particular, a 250 seat PrandtlPlane aircraft is of great interest to-day. 2. The PrandtlPlane configuration main characteristics. In a PrandtlPlane aircraft, the aerodynamic efficiency is strictly connected to the static stability of flight. In a configuration, in which (in order to improve the flutter stiffness) both the wings are connected to the fuselage, the central part of rear wing was discovered to have a low aerodynamic efficiency in the transonic range, due to the

connection to the fuselage and to the aerodynamic interference with fin; on the contrary, the central part of the front wing was proved to be more efficient and the result is a potential instability or little stability of flight. An example of this condition is shown in figure 3 ([3], [4]); it is the starting solution of the Italian project mentioned before, relevant to a very large aircraft (600 seat category), with the same fuselage of Airbus A380 (two decks for passengers and one for goods and luggage). The aerodynamic analysis showed that the stability during cruise flight is possible only when the position of the centre of gravity is closer to the front wing; so, the front wing is more loaded than the rear one and, consequently, the Prandtl’s conditions on minimum induced drag are no longer satisfied.

In other words, the requirement of static stability of flight produces a potential reduction of the aerodynamic efficiency. The challenge of the present project was to obtain a stable aircraft (with a given margin of stability) with equal lift on the two wings. After having studied many configurations at cruise speed, a new PrandtlPlane configuration was found [5]; figure 4 is the example in the case of ultra-light aircraft and figure 5 refers to the modifies 600 seat category aircraft; this last figure is the evolution of the aircraft shown in figure 3. The rear wing is not connected to the rear fuselage but it is positioned over the fuselage itself, connected to it by means of two fins. This aircraft was proved to be stable in cruise flight, with a proper margin of stability and, at the same time, the lift is equally distributed on the front and rear wings. figure 3. Starting configuration of the project This result is the consequence of the high aerodynamic efficiency of the central wing trunk of the rear wing, and depends on the characteristics of the aerodynamical “channel”, defined by top fuselage, bottom rear wing and lateral fins. So, the main characteristics of this last PrandtlPlane configuration can be summarised as follows.

Fig. 4. ULM PrandtlPlane aircraft configuration of the project

Fig. 5. Modified configuration of the very large PrandtlPlane aircraft

Figure 5. Final

Fuselage. The fuselage is enlarged horizontally, with a constant width along most of the aircraft longitudinal, axis up to the end (which is edge-shaped in the lateral view). The front wing crosses the fuselage under the cargo floor and, then, the cargo compartment is much wider than conventional aircraft. Passengers occupy one single deck; a second upper deck for passengers, limited to front fuselage, could be designed for aircraft bigger than A380 (say, 700-900 passengers). The main landing gear is positioned inside cargo bay and, also, inside lateral fairings between front and rear wings; the large width of fuselage reduces the fairing dimensions. The fuselage is equivalent to a doubly supported beam, the supports being the front and rear wings. The low frequency bending eigen-modes of the front fuselage (and also of the rear one) in the horizontal plane, typical of conventional large aircraft, are not present in the PrandtlPlane Lifting system. The lifting system is over-constrained to fuselage; on the other end, the local stiffness along the wing span is lower than conventional aircraft and, then, the aeroelastic efficiency is an open question. Preliminary results show that the maximum static bending deflection of the wing system is lower than conventional aircraft and, also, that the static aeroelastic phenomena are completely different from conventional aircraft, because front wing, positive swept, produces an high stability against divergence on the rear wing, negative swept. The lifting system provides an intrinsic structural safety as far as Damage Tolerance is concerned, due to the over-constrained solution. Two engines are positioned under the front wing and the others, preferably, on the fuselage sides; so, out of the engines, the wings are clean from the aerodynamic point of view. Given the lack of concentrated loads due to the engines and the possibilities of tailoring the primary structures, the lifting system could be made of composites or hybrid materials; this is an important subject of research. Fuel is contained into both the wing boxes and could be consumed in the same way during cruise; so, only small variations of the centre of gravity can occur during cruise flight. Friction drag of the wings depends on the Reynolds number; in the case of a PrandtlPlane configuration with the same wing surface of a conventional aircraft, the local Reynolds number is nearly the half of a cantilever wing and, then, the stream could be more laminar than in a conventional wing (but wind tunnel tests will be fundamental for checking this property). Preliminary wind tunnel tests showed that the stall of the lifting system is very smooth and that the rear wing is stable up to very high angles of attack. Flight Mechanics and Control. The pitch control could be obtained by means of two elevators, one on the front and the other one on the rear wing, moved in phase opposition; this control is a pure couple in pitch. Another strategy of pitch control is that of using the elevators on the front wing only; in the second case, the behaviour of the aircraft is the same of a canard. In any case, pitch manoeuvre is much safer than conventional aircraft, especially close to the ground. Trimming the PrandtlPlane is easier, because the distance between aft and rear control surfaces is much larger (nearly the double) than in the case of a conventional aircraft. The available results show that the PrandtlPlane configuration is very stable with respect to stall, due to the stall stability of the rear wing. The lateral control is unconventional due to the double rudder and, also, to the presence of the vertical tip wings (they could be also used for lateral control). The ailerons could be positioned on the rear, negative-swept, wing. High lift devices. With a proper design, the theoretical condition of “Best Wing System” could be valid with the high lift devices extended, contrary to conventional aircraft. The necessary thrust could be correspondingly lower and the same for noise and emissions. As said before, the best wing system concept could be valid in take off and landing, as well. In fact: in the Prandtl’s best wing system, the lift on the horizontal wings is made of an elliptical and a constant contributions (figure 1). The optimum occurs for a certain proportion of these contributions, but variations of this ratio are not so critical. A near-optimum configuration in take off and landing could be obtained by increasing the elliptical part of the lift, equally in both the wings; so, slats and flaps have to be positioned along the wing span on both the wings as wider as possible. The aerodynamic optimisation of the low speed configuration was not possible in the present national project, but only preliminary results were obtained. Freighter aircraft. The application of a twin-fin PrandtlPlane configuration as a freighter aircraft is straightforward. In fact, the fuselage width is large and constant up to the back. Besides, given the flat shape of the bottom end fuselage, cargo doors could be positioned there. With the cargo doors on back fuselage, the loading and disembarkation of goods and luggage could be simpler and quicker in any civil PrandtlPlane aircraft. Light aircraft. The configuration shown in figure 4 is an example of a two seat, ultra light aircraft (ULM). At the Technical University in Torino, wind tunnel tests have been carried out on a scaled model; the results shows that the aircraft has a small induced drag and a high degree of stability to stall. The configuration in figure 4 can easily modified into an amphibious aircraft. In conclusion, it seems that the PrandtlPlane configuration has the potential for achieving, all together, the top levels objectives, identified by the Strategic Agenda for European Aeronautics, that is: a) Strengthening the competitiveness of the manufacturing industry in the global market, b) Improving environmental impact with regard to noxious emissions and noise, c) Improving aircraft safety and d) increasing the operational capacity and safety of the air transport system. 3. The Italian project “Development of an innovative configuration for transport aircraft” In the last decade, the PrandtlPlane configuration has been studied at Pisa University and other Italian Universities. A list of references is reported in the Bibliography of the present paper. The present Italian national project was carried out in the period January 2000-January 2003 with a total budget of
485.468, 70% of which was financed by the Italian Ministry of University. The Italian Universities involved into the project were: University of Pisa, University of Roma

“La Sapienza”, University of Roma Tre, Technical University of Turin, and Technical University of Milan. As said before, the project aimed at setting up tools and methods for the design and the multidisciplinary optimisation of the PrandtlPlane configuration and, in parallel, to develop the aerodynamic design of a very large PrandtlPlane aircraft. After two years of a coordinated activity of the five Universities, the project was completed successfully. In addition, it emerged that the PrandtlPlane configuration could be applicable to small aircraft and a second research activity was carried out on this respect. Starting from an initial configuration, a very large PrandtlPlane aircraft was modified by changing the general architecture, up to a typical configuration shown in figure 5. This configuration is not optimised; for example, the design at low speed and the optimisation of the high lift devices and controls could produce even significant modifications to the showed configuration. The final design will derive from an optimisation process taking all the flight anf ground conditions into account. Wind tunnel tests were carried out in Torino, both from Alenia Aeronautica (a not scaled model) and Technical University (the ultra light aircraft scaled model); the main mathematical tools for the development of the aircraft were the Multidisciplinary Optimisation code and the structural optimisation of the wing system; the first code was set up at the University of Roma “La Sapienza” and the University of Roma Tre and the second was developed at the Technical University of Milan. A brief summary of the contributions of the single research units is given in the following. University of Pisa Pisa University was responsible of the development of the configuration of the PrandtlPlane and, also, of the coordination of the whole project. The development of the configuration was conducted with reference, mainly, to the cruise attitude, and resulted from CFD aerodynamic computations in the transonic range (0.85Mach); the results obtained were the starting point of local or global shape modifications of the aircraft. In this process, the wing span was nearly the same to that of A380 (78m) and the optimisation process of the configuration was carried out on view to fulfil the Best Wing System concepts, namely: same lift and same lift distribution on the two horizontal wings and butterfly shaped lift on vertical wings. The condition of optimum ratio between constant and elliptical parts of lift was not considered as fundamental. In fact, this concept could produce high tip cord lengths (and, consequently, intolerable wing weight). So, it is evident that the structural optimisation of the wing system is fundamental for the final success of the configuration and, then, the cooperation of all the other research units plays the key role in the project. The coordination activity of Pisa University aimed at obtaining such a cooperation and, in this respect, the activity was successful. In particular, two kind of optimisation procedures were provided; the first is a structural optimisation of the lifting system, taking static stability of flight and flutter into account; the second is a multidisciplinary optimisation, taking also into account costs, investments, fuel consumption etc. The structural optimisation code was carried out at Technical University of Milan and the MDO optimisation at Roma Tre and Roma “La Sapienza”. In order to develop the configuration, a parametric geometry generator and surface aerodynamic grid generator were needed. The configuration is so complex that the existing CAD codes were proved to be unsatisfactory, because they are not completely parametric, we are not allowed to control the shapes of rounds and connections between fuselage and wings, etc.. So, a geometry generation code was developed [6] and, then, modified many times along with the development of the aircraft configuration. In the final version of the code, all the generation is based on Nurbs functions. This code was used to generate geometry and surface aerodynamic grids. The configuration was modified according to the results of the CFD computation in Aerodynamics and Flight Mechanics, starting from the initial design shown in figure 3. For any configuration, the aerodynamic analysis allowed us to obtain a distribution of lift between the front and the rear wings, together with the margin of static stability of flight. A typical result is shown in figure 6; a scaled geometry wing lay-out is shown, together with twist angles along the span, positions of the centres of pressure and pressure variations, the margin of stability of the aircraft, etc.. As said before, the final goal of the project was to have equal lifts on the wings and static stability of flight, with a given margin of stability in all the flight conditions, at high speed (0.85Mach) as well as at low speeds (with high lift devices extended and in the presence of the ground effects). The parameters used were the sweep angles (one for any wing trunk), chord distributions, twist angles along the span, airfoils, etc. Other parameters were the geometry of the high lift devices and the control surfaces, the positions of the rotation axes, etc. The aerodynamic computations were carried out by means of FLUENT, a CFD (Computational Fluid Dynamics) code. As a further product of the Italian national project, the PrandtlPlane concept was used to design also the small aircraft shown in figure 4. The Technical University of Torino carried out a wind tunnel test campaign in order to determine the main flight mechanics derivatives on a scaled model of the aircraft [8]. Alenia Aeronautica in Torino joined to the project; they designed and manufactured a wind tunnel test programme on the basic Prandtlplane configuration, shown in this paper, and the results obtained were presented in [9]. The research unit of Pisa collaborated with the other units; in particular, with the unit of Torino, for the definition of the wind tunnel tests of the ultra-light aircraft, with Alenia for the wind tunnel tests, with the unit of Milano, for the definition of the lifting system configuration for the structural design and optimisation.

Fig. 6. A typical result of aerodynamic computation

University of Roma “La Sapienza” In the first stage of the research activity, a non-commercial multidisciplinary optimisation code, called MAGIC (Multidisciplinary Aircraft desiGn of InnovativeConfigurations), has been developed in collaboration with Roma Tre University. In this code, a sequential unconstrained minimization problem (SUMT, Sequential Unconstrained Minimization Technique), using the extended internal quadratic penalty function is solved. Specifically, a multiobjective function (depending on the operative empty weight and range) has been minimized taking into account both structural, aerodynamic, aeroelastic, performance, and flight mechanics (for the longitudinal stability) design constraints. All the state variables needed to built the objective function and constraints were evaluated separately in the different analysis modules, the leading characteristics of them is that accurate results are obtained and computational time are reduced. The box wing resulting from a preliminary analysis is not the final solution so far (for example, no limitation is assumed for the unit wing load, the sweep angles are fixed, etc.). The wing span, structural weight and overall stiffness are reduced and increase vertical and horizontal gaps between the wings are increased (Figure 7). The importance of the optimisation process is indicated by the following indicative) results: a reduction of the Maximum Take Off of 2.65%, a reduction on the structural wing weight of 9.86% and an increase of the range of 22.25%.

Fig. 7. Initial Prandtl-Plane layout (left), final optimised layout (right)

In the subsequent phase, a landing gear system has been proposed together with a multibody analysis aimed at evaluating the resulting dynamic loads on the airplane during landing. For this analysis, the commercial code MSCADAMS V.12 was used; the mechanical behaviour of the aircraft is considered as a multibody chain, i.e., a chain of rigid and/or elastic bodies, each one with proper translations and rotations degrees of freedom. The governing dynamic equations are strongly non linear and are solved numerically via time marching schemes. The configuration is supposed to be formed by “dual twin tandem” four main landing gears, with six wheels each, to withstand the load factors during the ground impacts; a dual twin, with four wheels, were adopted for the front landing gear. The structural dimensioning has been obtained in the hypothesis that the fuselage is a rigid body of 100.000 Kg, and the internal libraries of MSC.ADAMS have been used for the tyre characterization. The ground reactions (Figure 8) and the acceleration at the axle position (Figure 9), allowed us to achieve useful information on landing gear and fuselage structural design.

Fig. 8. Reaction on Ground at the nose landing gear

Fig. 9. Acceleration at the main landing gear axle

In the last phase of the research activity, techniques for experimental modal analysis were introduced for updating the dynamic numerical model of the present wing structures. Making reference to the previous optimisation phase, and taking an appropriate geometrical scaling of the structure into account, a test article was manufactured for carrying out dynamical tests. A 3D parametric digital mock-up was simulated using the commercial code DASSAULT CATIA V5R9. This numerical model allowed us to evaluate, by an automated procedure, the influence of the geometrical properties of the model on its dynamic characteristics. The Finite Element model showed a good agreement with the experimental eigenvalues and eigenfrequencies. With this positive initial check, a more extensive research was conducted in order to update the finite element model by considering the correlation with the frequency response functions. This approach is based on the calculation of a sensitivity matrix of the correlation functions, for variations of the properties of the structure. The procedure was tested on a simple beam-like structure ( a part is shown in figure 10), representative of the Box-Wing test article. The design parameters were the elemental stiffness of the numerical model. The developed updating procedure was successful for the present wing configuration:

. Fig. 10. A view of the test article

University of Roma Tre In the Italian Project mentioned above, Roma Tre University was responsible of a computational code on MDO/PD (Multi-Disciplinary Optimization for Preliminary Design) methodology, for the analysis of standard as well as innovative configurations (in particular, the PrandtlPlane). The final result of the activity was the already mentioned MAGIC code. The starting point of MAGIC was code FLOPS, developed by Mc Cullers, which is essentially based on elementary and/or empirical algorithms with emphasis on the techniques for the integrated modelling of structures, aerodynamics, aeroelasticity and flight mechanics. In the case of a PrandtlPlane, new major difficulties emerged, because the approach of FLOPS is not possible for innovative configurations, for which the designer cannot rely upon past experience. Therefore, in MAGIC, the main attention was devoted primarily on the mathematical tools and the organisation of the code. The code is structured so that any single discipline is seen as a separate module that accepts the necessary input data and yields the values of the terms influencing the objective functions and/or in the constraint functions. But past experience and theoretical and experimental knowledge on the present configuration are missing, as well as an extensive data base on the aircraft. So, final conclusions on the overall performances of a PrandtlPlane

aircraft with respect to conventional configurations are not completely reliable. This situation was clear at the beginning of the project and, owing to that, an optimisation code devoted solely to the structural design of the wing system was included. The practical applications of code MAGIC will increase when more basic knowledge on the PrandtlPlane configuration will become available. Optimisation: in Magic, we limit our attention to preliminary design, where the integration of the various disciplines is the strongest. The optimisation procedure is a Sequential Unconstrained Minimization Technique (SUMT) and the BFGS (Broyden-Fletcher-Goldfarb-Shanno method), along with quadratic extended interior penalty function have been adopted for the optimisation process. Structures: the module for wing structures is a linear elastic beam-like model (by finite elements, for the static analysis and mode evaluation); this optimisation technique is really sensitive to the models used in the component analysis (e.g., in the constraint algorithms). Aerodynamics: the aerodynamical boundary element model is based on incompressible quasi-potential flows; the extension of the formulation which includes the effects of compressibility is also available in MAGIC. In a quasi potential flow, the flow are potential everywhere except for a zero-thickness wake surface emanating from the trailing edge. The effects of viscosity are included through an elementary boundary-layer model (based on the Blasius flat plate solution), which gives an adequate estimate for the viscous drag. Aeroelasticity: in order to perform the aeroelastic analysis in the framework of the optimisation procedure, a finite-state approximation for the aerodynamic matrix is considered. Specifically, the system has been reduced to the standard state-variable format, x
= A(U
) x . This approach allows one to reduce the aeroelastic stability analysis to a root locus for the matrix A, thereby avoiding standard methods, which would unnecessarily complicate the optimisation process. Static stability and performance: static stability is satisfied by imposing that the derivative with respect to the angle of attack of pitch moment coefficient (evaluated with respect to the center of mass G) be less than zero. In order to evaluate fuel consumption, the mission profile considered in this work consists of: take-off, climb, cruise, descent, and landing. The range is computed according to the Breguet equation.

Technical University of Torino The Research Unit of Torino has conducted an experimental aerodynamic activity and, also, an analytical and numerical activity aimed at investigating the aerodynamic and structural issues related to the PrandtlPlane configuration. The experimental activities has considered two different PrandtlPlane models, shown in figure 11 and 12. The wind tunnel model in figure 11 has been designed and manufactured with the fundamental support of Alenia Aeronautica, Torino; the wind tunnel tests were conducte at the Alenia plants. The model is made of aluminium and has a 1.59m wing span and a 1.78m fuselage length. Both vertical wings connecting the horizontal wing tips can be removed, thus allowing us to compare the induced drag of the box wing and the traditional biplane. It was assumed as basic model for a preliminary study on closed wings and, for cost reasons, it has not been provided with pressure probes neither with movable surfaces.

Fig. 11. Alenia wind tunnel model

Fig. 12. ULM wind tunnel model .

The model in figure 12 is an ultra-light aircraft (ULM) aircraft. It has been designed at the Dept. of Aeronautical and Space Engineering in Torino, and built in composite material on the basis of an aerodynamic design defined in [10]. The design procedure of the model and the technology for manufacturing it are reported in [8]. The technology of composites allowed us to cut the costs of about 80% with respect to conventional metallic models and an easier mounting of the instrumentation inside. Unfortunately, this model is equipped with a limited number of pressure holes and, moreover, the wing gap is fixed. Both models can be connected to the wind tunnel into different ways, namely: by means of a rear sting and through an external balance from the bottom model to the tunnel floor. Supporting the model from below is more interesting for wake measurements, whereas supporting it from behind reduces the interference

effects during force and moment measurements. Furthermore, the double connections of the model allows testing in different wind tunnels. The experimental activities include polar curve measurements on the complete configuration (with and without yaw angles and with different deflections of the aerodynamic surfaces), flow visualisations and total pressure measurements on the model, and visualisation of the wake. These experimental investigations have shown some interesting characteristics. The stall of the model is nearly flat, the induced drag is low, the configuration is stable as far as flight mechanics is concerned in a wide range of angles of attack. The unit of Torino carried out also a research activity on the structural design of the PrandtlPlane configuration. Both static and dynamic structural analyses, using PATRAN/NASTRAN, have been conducted by means of detailed FEM models ([11]), including stringers, cover skins, transverse frames and ribs built in Aluminium alloys, composite materials or hybrid materials like Glare (figure 8). Evaluations of static and dynamic responses for different fuel allocations has also been given. A preliminary structural design of the different components has been obtained by an MDO procedure, named NAPAO; this procedure joins the optimisation procedure in NASTRAN with the code PANDA, which gives the optimum panels under buckling loading conditions. A further activity connected to the PrandtlPlane project is connected to static and dynamic Aeroelasticity. The results obtained showed the importance of the aeroelastic tailoring on the divergence and flutter speeds. On these subjects, an important collaboration was activated with people of the Department of Aeronautics and Astronautics of the University in Seattle (USA).

1.

Fig. 13. FEM stress analysis on a Prandtlplane Finally, analytical activities in Aerodynamics have been directed to provide simplified solutions of closed lifting systems. Investigations on annular wings, circular as well as elliptical, have been performed according to the classical lifting line assumption. First results show that increasing the ellipse eccentricity at constant angle of attack leads to a marked induced drag reduction a lift increase, with a consequent improvement of the aerodynamic efficiency. Technical University of Milan As said before, the success of PrandtlPlane configuration depends on the efficiency of the lifting system structural design. In the case of the very large transport aircraft into examination, an initial design requirement was that the wing surface could be not larger than the wing plus horizontal tail of the A380, in order to maintain nearly the same wetted surface. It follows that the local chord of a PrandtlPlane wing is nearly the half of that of A380 (or a little more); so the local bending, shear and torsional stiffness are much lower. But, contrary to conventional aircraft, the lifting system is over constrained to the fuselage, with many consequences for the structural design. A comparison between the two solution is not immediate, and structural tools for design and optimisation of the box wing system were not available at the beginning of the project. The problem is so important that it seemed necessary to have a proper structural optimiser before having a multidisciplinary optimisation. In the framework of the Italian project, the Technical University of Milano was responsible of the design and optimisation of the wing structures owing to their previous experience on optimisation of wing-box structures. At the end of the project, a structural optimisation code was completed. A description of the basic characteristics of the structural optimiser was presented in a workshop titled “Design challenges and mathematical methods ia aircraft and spacecraft”, held at the Mathematical School of the E. Majorana Centre in Erice, July 01-10, 2003; a presentation is also given in [12]. The optimisation code takes into account constraints like fatigue, flutter, buckling, divergence, aileron efficiency, etc. The objective function of the optimisation process is the structural weight, in the presence of manoeuvres, gusts, engines, landing loads. The structural optimisation is obtained through three stages; in particular, through two level analysis and a final check between performances and requirements. There is a specific structural model together with an optimisation algorithm for each level. The first level is a one-dimensional model (figure 14a), together with a gradient base optimisation algorithm); the second is a thin walled beam cross section model (figure 14b), together with a genetic optimiser, and the third optimisation level is a three-dimensional Finite Element model (figure 14c), together with a gradient base optimiser.

a)

The aerodynamic loads applied on the wings are calculated by a Boundary Element Method based on the Morino’s method in conditions of cruise or high load factors. Preliminary checks of the code were carried out on well known conventional wings, with satisfactory results. The optimisation procedure was then applied to very large PrandtlPlane lifting system configurations, defined by the Pisa research unit. A first configuration was the result of an intermediate design, in which the rear wing was connected to the fuselage. The second configuration studied was a double fin PrandtlPlane configuration, with a total wing surface of about 800 square meter. As shown before, the configuration is interesting from the aerodynamic point of view, but the structural efficiency is an open problem. In this case, in particular, the solution was intermediate; the total wing surface is about 200 square meter smaller than that of the equivalent conventional aircraft of reference (Airbus A380). The characteristics of the twin fin aircraft are reported in table 1; Max take-off mass Fuel Wing surface Wing span LE sweep front wing LE sweep rear wing Dihedral front wing (from root to 15m) Dihedral rear wing (constant) Fuselage Diameter

600000 kg 220000 kg 811 m2 78 m 38° -23° 10° -2° 8m

another font of errors could be the stiffness of the fuselage, which is considered perfectly rigid in this computation. Proper modifications could be applied easily in the future, while, at present, the main scope of the activity was setting up the structural optimisation of the wing system. The aircraft is provided of 4 engines; in the present analysis, two engines are positioned under the aft wing and two under the rear wing. Figure 15 and 16 show the structural model relevant to the first stage optimisation process. Tab. 1 Twin 1 main data A second configuration (Twin2) has been analysed, having the same wing surface but with a reduction of the wing span to 70m (with a better compliance with actual airports). The solution, shown in figure 16, is not necessarily the final one, because some modifications on the sweep angles or the relative positions of the two wings could be necessary. Anyway, the results show that the aerodynamic efficiency is reduced, but a great benefit in term of structural weight was obtained (the reduction of the structural weight could be of the order of 20%, with a total weight saving saving of about 26.000Kg). A further weight reduction could be obtained by improving the wing chords, in order to obtain a total wing surface of about 1000 square meter. The structural optimisation of the wing system is not completed; in fact, it needs to define the final wing design in the framework of a multidisciplinary scale. The partial results obtained show that the code appears as reliable and all the possible solutions can be examined; conventional and non-conventional materials can also be taken into examination.

Legend: Twin 1 Twin 2

Fig. 15 Twin1 Structural model

Fig. 16. Twin1 and 2 Wing Plant (m)

4. Conclusions The Universitis of Pisa, Roma “La Sapienza” and Roma Tre, and the Technical Universities of Torino and Milano, together with Alenia Aeronautica in Torino, carried out a coordinated project named “Development of an innovative configuration for transport aircraft”; the project was 70% financed by the Italian Ministry of University. The main results can be summarised as follows. - A computer code for multidisciplinary optimisation of a PrandtlPlane configuration was completed, even though more activity is needed owing to the lack of data. - A computer code for the structural optimisation of the lifting system of the aircraft was produced; the utilisation of new materials for the lifting system, like composites, can be simulated.

- A parametric geometry and surface grid generation code was completed; it can be applied to conventional and non conventional aircraft. - A PrandtlPlane very large aircraft has been modified until up to the Prandtl conditions on best wing system (in particular, the equality of total lift on the two wings), together with static stability of flight were obtained. More activities are needed in order to analyse the low speed configurations of the very large aircraft, with the high lift devices extended both in take off and landing. This aspect will be critical for reducing significantly the noise and noxious emissions close to the airport areas. - A ULM aircraft was designed, wind tunnel tests were carried out on a scaled model and a flying models have been tested. The aircraft is suitable for an amphibious version, which appears so appealing from the safety and design points of view. Finally, the group of five Universities has the necessary tools and integrations to carry out the preliminary design of any PrandtlPlane transport aircraft. A closer integration with aeronautical Industry is highly welcome for the future, with mutual benefits for Universities and Industry. Acknowledgment: The authors gratefully acknowledge the fundamental financial support of the Ministry of University and Research; a financial support to the University of Pisa was also provided by ASI (Italian Space Agency).

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