INVESTIGATING THE PIAGGIO AVANTI DESIGN USINGCEASIOM

Zhang, M.∗ , Cristofaro, M.∗∗ , Wang, Y.∗∗∗ , Da Ronch, A.∗∗ , Rizzi, A.∗∗Royal Institute of Technology, Sweden

∗∗University of Southampton, Southampton, S017 1BJ, United Kingdom,∗∗∗China Academy of Aerospace Aerodynamics, Beijing, 100074, China

Keywords: Computational fluid dynamics, aircraft design, stability and control

Abstract

In early steps of aircraft design the unificationof configuration definition is important to avoiduser–input errors. Also coupling with each othercan strengthen different tools with different spec-ifications simultaneously provided that the geom-etry definition is transferred with minimum dataloss. This is vitally useful especially when geom-etry data is transferred in order to perform high–fidelity analysis. This paper reports the analy-sis for the pitch control of a three–lifting–surfaceaircraft Piaggio Avanti using CEASIOM, a tool–chain software for aircraft preliminary design,with the baseline configuration coming from theconceptual design code AAA, linked by a com-mon name–space CPACS for the means of datacollaboration.

1 Introduction

Figure 1 spells out the details in the early stepsof aircraft design for the definition of the config-uration. The figure illustrates two design loops inthe conceptual design phase that follow the first–guess sizing (usually done by a spread–sheet) toobtain the initial layout of the configuration. Thefirst one, the pre–design loop, is aimed at estab-lishing a very quick (time–scale can be from oneto a few weeks) yet technically consistent sizedconfiguration with a predicted performance. Thesecond one, the concept–design loop, is a pro-tracted and requires intensive effort involvingmore advanced first–order trade studies to pro-

duce a refinement in defining the minimum goalsof a candidate project. At the end of the concep-tual design phase all the design layouts will havebeen analysed, and the “best” one, or possiblytwo, designs will be down–selected to the prelim-inary design phase. During the preliminary defi-nition, project design is still undergoing a some-what fluid process and indeed warrants some el-ement of generalist–type thinking, but the mini-mum goals of the project have already been es-tablished during the conceptual definition phaseand the aim is to meet these targets using methodswith higher order than those used during the con-ceptual definition phase. Furthermore, the partic-ipants in this working group are mostly genuinespecialists in each respective discipline. Figure 1indicates the way in which data, or information,is passed between specialist groups during the de-sign process. The specialist groups must considerthe level of advanced technology to be adoptedtogether with all of the other active constraintson the design. The data flow lines indicate howthe technology areas influence the aircraft config-uration through its performance. The specialistdepartments/offices provide the input data to theproject designers who then coordinate a system-atic search to find the “optimum” configurationand settle disputes between conflicting specialistopinions. There exists today a good deal of ineffi-ciencies in interactions between all these variousgroups.

This paper shows the application of the highfidelity aircraft design code CEASIOM [1], theComputerised Environment for Aircraft Synthe-

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ZHANG, CRISTOFARO, WANG, DA RONCH, RIZZI

sis and Integrated Optimization Methods for Pi-aggio Avanti configuration which comes fromAdvanced Aircraft Analysis AAA [2] by inves-tigating its longitudinal stability and control. Thegoal is to model the known three–channel con-trol surfaces and to show how the three–lifting–surface for pitch control gives lower trim dragthan conventional two–lifting–surface configura-tions.

Fig. 1 The two design loops in the conceptual de-sign phase process and the down–select to projectstudy in preliminary design

2 Conceptual Design Tool AAA

Advanced Aircraft Analysis (AAA) provides apowerful framework to support the iterative andnon–unique process of aircraft preliminary de-sign. The AAA program allows design engi-neers and preliminary design engineers to takean aircraft configuration from early weight siz-ing through open loop and closed loop dynamicstability and sensitivity analysis, while workingwithin regulatory and cost constraints.

The current version of AAA is based on themethods of Airplane Design Parts I–VIII by JanRoskam, Airplane Flight Dynamics Parts I–II byJan Roskam, Airplane Aerodynamics and Perfor-mance by Jan Roskam and Eddie Lan and meth-ods developed for airplane design by DARcorpo-ration engineers. Since 1991, when DARcorpo-ration acquired the rights for AAA and contin-ued the development as a commercial venture,

AAA has been improved and upgraded severaltimes.

AAA enables a fully functioning three–dimensional aircraft drafting tool Shark/AP [3].More information about AAA geometry formatand description can be found in [2] or on the web-site 1.

3 Preliminary Design Toolset CEASIOM

The Computerized Environment for AircraftSynthesis and Integrated Optimization Meth-ods, CEASIOM, developed within the European6th Framework Programme SimSAC (Simulat-ing Aircraft Stability And Control Characteris-tics for Use in Conceptual Design), is a frame-work tool for conceptual aircraft design that in-tegrates discipline–specific tools like: CAD andmesh generation, computational fluid dynamics(CFD), structures, stability and control analysis,etc., all for the purpose of early preliminary de-sign [1]. It is an ad hoc framework that offerspossible ways to increase the concurrency andagility of the classical conceptual–preliminaryprocess outlined in Fig. 1. CEASIOMsoftware hasfour core functions: geometry and mesh genera-tion, CFD, aeroelastic analysis, and stability andcontrol (flight dynamics). Significant features de-veloped and integrated in CEASIOM as modulesare:

• Geometry module CPACScreator–sumo [4, 5]. A customized geometryconstruction system coupled to automatedsurface and volume grid generators,resulting model exported to ComputerAided Design (CAD) via Initial GraphicsExchange Specification (IGES) standard.

• Aerodynamic Model Builder AMB–CFD [6]. A complete toolbox of aerody-namic analysis methods ranging from theempirically based DATCOM to physics-based linear and non–linear CFD (Euler &RANS) offering broad choice in fidelity:

– Digital DATCOM.

1http://www.darcorp.com/Software/AAA/[retrieved 14 July, 2014]

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Investigating the Piaggio Avanti Design using CEASIOM

– Steady/unsteady vortex–lattice code(VLM) TORNADO for low–speed(linear) aerodynamics and aeroelas-ticity.

– CFD solvers in EDGE code. Eulersolver (EDGE code in Euler mode)for inviscid flow cases where totalpressure and vorticity fields are toocomplex to model with isentropicequations e.g. at high speed orswirling flow. Examples of theseare shock waves and propeller slip-streams. RANS (Reynolds–AveragedNavier–Stokes) flow simulator (e.g.EDGE CFD code) for high fidelityviscous flow analysis at extremeflight conditions.

• Stability and Control module S & C (e.g.SDSA [7]). A simulation and dynamicstability and control analyser and flying–quality assessor. Includes:

– Performance prediction.

– Test flights by six Degrees of Free-dom flight simulation.

– Stability Augmentation System(SAS).

• Aero–elastic module NeoCASS [8].Quasi–analytical structural analysis meth-ods that support aero–elastic problemformulation and solution.

CEASIOM is intended to support engineers inthe conceptual/preliminary design process of theaircraft, with emphasis on the improved predic-tion of stability and control properties of elas-tic aircraft achieved by higher–fidelity methodsthan found in contemporary aircraft design tools.Moreover CEASIOM integrates into one appli-cation the main design disciplines, e.g. aero-dynamics, structures, and flight dynamics, im-pacting on the aircraft performance. It is thusa multi–disciplinary analysis toolbox brought tobear on the design of the aero–servo–elastic air-craft [9, 10]. CEASIOM however does not carryout the initial sizing of a baseline configuration,

and thus needs to collaborate with a tool likeAAA, which was described in Section 2.

4 Interfaces and Wrappers

If an analysis module is not developed to explic-itly serve a central data model it is unlikely thatthe module and the central model share the sameparameterization. Hence conversions need to bemade. The first step in such a conversion is thefiltering of data. By applying mapping rules onlythe data relevant for the analysis module is trans-ferred. In a second step the tool wrappers do theconversion of the data.

Figure 2 shows that all the related softwaretools for aircraft concept–design are linked to thecentral model approach CPACS [11] (visualizedvia CPACScreator), then the data are sent to thehigher order physics-based analysis tools CEA-SIOM. The baseline geometry studied in this pa-per is obtained from AAA–CPACS interface [3].

Fig. 2 Different conceptual design tools linkedto preliminary design tool–chain CEASIOM byCPACS

5 CEASIOM Down–Select Configuration &Pitch Control Study

Figure 3 shows the AAA 3–view drawing of thefinal conceptual design for the Piaggio Avanti.This is the configuration that is down–selectedfrom conceptual design and is now ready for pre-liminary design, as illustrated in Fig. 1.

A primary goal of preliminary design is to ob-tain the final wing design with optimized perfor-mance: e.g. maximized aerodynamic efficiency

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Table 1 Structure of the aerodynamic database constructed in CEASIOM for use in the flight simulationSDSA module.

α M β δele δrud δail p q r CL CD Cm CY C` Cnx x x – – – – – – x x x x x xx x – x – – – – – x x x x x xx x – – x – – – – x x x x x xx x – – – x – – – x x x x x xx x – – – – x – – x x x x x xx x – – – – – x – x x x x x xx x – – – – – – x x x x x x x

or minimized drag coefficient (CD), usually start-ing with the cruise point.

The wing aerfoils are chosen by a skilful en-gineer as the initial design, which may not be op-timum, but can be used as a good starting point.Then it should be put in an optimization loop todetermine the optimized airfoil shapes (thicknessand cambers), twist distributions according to thelimits of lift coefficient (CL), pitching momentcoefficient (Cm), bending moment, span loading,fuel tank volumes, etc. for corresponding flightconditions. Take the Piaggio Avanti for exam-ple, the aerfoils of the wing are well–designedwith cambers and twists to give sufficient lift inorder to balance the weight during cruise. Addi-tionally to get a well designed wing a multi–leveloptimization [12] should be considered, namely,optimizations for cruise, take–off and landing.

Fig. 3 3–view drawing of Piaggio–Avanti in AAA

Another goal is to determine a database ofaerodynamic forces and moments that cover suf-ficiently the flight envelope so that it is appro-priate input to a flight simulator for the study of

the vehicle performances and handling qualities.Table 1 indicate how the aerodynamic databasecomputed in CEASIOM is organized. It showsthe static and quasi–static stability coefficientsand the control coefficients. In Table 1 are pre-sented the lift, drag and lateral force coefficients(CL,CD,CY respectively), pitching, rolling andyawing moment coefficients (Cm,C`,Cn respec-tively). The angle of attack is presented as α,M is the Mach number and β the side slip an-gle while q, p and r are the three rotations inpitch, roll and yaw. The three control surfacesthat can be deflected are the elevator (δe), therudder (δr) and the aileron (δa). The dynamicderivatives (Cmα̇

,CZα̇,CXα̇

,CYβ̇,C`

β̇,Cn

β̇) are in-

stead computed only for different Mach numbers.The coefficients must be obtained for each of

these parameters throughout the flight envelope,hence the data is voluminous. In this paper weonly focus on longitudinal control analysis in or-der to validate the advantage of this three–lifting–surface configuration. Thus only the second andsixth rows in Table 1 are filled with CFD Eulercomputations.

6 Piaggio Avanti

Piaggio Avanti is a three–lifting–surface twin–engine turboprop aircraft that has a small forwardwing (canard) to produce extra lift and a T–tail forlongitudinal and lateral control. It is claimed thatit can save up to around 30% fuel compared withsimilar aircraft due to the non–traditional config-uration. The main wing is designed to have lami-nar flow over a very high percentage of the wingchord, and the fixed canard is designed to stall

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Investigating the Piaggio Avanti Design using CEASIOM

before the main wing, resulting in a nose–downeffect improving the airplane good performanceat high angles of attack. The Piaggio Avanti isdesigned to cruise at Mach number of 0.62 atan altitude of 39,000 ft (economy cruise). Moreabout the cruise speed and altitude can be re-ferred in [13] and summarized in Table 2 accord-ing to Instrument Flight Rules IFR Range & Pay-load graph.

Table 2 Cruise speed at maximum cruise power [13]Description Speed [kts] Altitude [ft]

Service ceiling at OEI – 24,000Maximum speed2 395 30,000

Cruise 370 37,000∗Economy cruise 356 39,000Service ceiling 320 41,000

The typical mid–cruise weight is estimatedby:

W = Operating weight+4PAX+1/2Full fuel (1)

at ISA condition and IFR reserves. The lift isproduced to balance the weight, which is around450 kN estimated by taking maximum payloadand half full fuel [13]. The CFD solvers operateinside CEASIOM and then all the data are sent toStability and Control Analysis module SDSA inCEASIOM to model/simulate the pitch controls.

Figure 4 shows its 3–channel standard controlsurfaces illustrated on the 3–view drawing. Thecanard is a fixed lifting surface, and elevators onthe horizontal tail control the pitch.

Tornado VLM and Edge Euler computationsare carried out at Mach number 0.62 (economycruise 356 kts) in order to build a complete aero–database to verify the advertised the superiorflight qualities. The VLM method is fast but withlower fidelity. It is used to quickly generate acomplete data–set as Table 1 is shown that is sentinto SDSA for stability and control analysis.

At the next higher level of detail, the graph-ical surface modelling tool sumo can be usedto define a more detailed geometry based on a

2At ISA conditions

Fig. 4 Control surface illustrated on 3–viewdrawing [13]

moderate number (often less than 30) spline sur-faces. This description is used to generate inputfor CFD solutions based on the Euler equations.Horizontal trim is studied for both full and canardoff configurations.

6.1 Aerodynamics using Vortex LatticeMethod

In VLM method the compressibility Prandtl–Glauert correction was applied for high subsonicMach number (0.62). Figure 5 are the VLM meshand solution for economy cruise at Mach numberof 0.62.

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(a) Tornado VLM panels

(b) Tornado VLM pressure coefficient distribution forsteady flight at M=0.62 and CL=0.65

Fig. 5 Tornado VLM solutions from CEASIOM

6.2 Euler solutions

Figure 6(a) shows an Euler mesh used in this pa-per generated in sumo with 9.4 million nodes.The results for movable control surfaces are com-puted not by physically deflected, but by transpi-ration boundary condition in Edge. Figure 6(b)shows the Mach contour on the same conditionpredicted by CEASIOM Euler solver. We see thata weak shock is formed at the mid–chord of thewing due to high lift. Note that the VLM modelonly includes lifting surfaces such as wing, verti-cal tail and stabilizer, while the sumo geomet-rical representation also includes aerfoil thick-ness and non–lifting surfaces such as the fuse-lage. Again the VLM only treats the flow fieldsaround the lifting surfaces linearly, which has

poor fidelity on non–linear aerodynamics such asthe wing in transonic flow. All of these abovemotivate us to turn to Euler solver to get moreaccurate solutions.

(a) Euler mesh generated by sumo and TetGen [14]

(b) Euler computation for cruise at CL = 0.565 andMach=0.62 at 39,000 ft

Fig. 6 Euler solutions from CEASIOM

Figure 7 shows the forces and moments pre-dicted for both the full and the canard–off con-figurations from Euler solver in CEASIOM. Thecanard is very small that only produces slightlift, so the total lift from both configurations arequite close. We see significant differences on CDand Cm for both configurations. Note that the er-ror bars showing maximum deviations of the last500 iterations indicated very poor convergencefor steady flow computed for drag coefficient andpitching moment for negative angles of attack.Hence, the simulations below zero degree angleof attack is computed in unsteady model. Thismeans that the deviation bars for the results ofnegative angle of attack cases indicates the actualunsteadiness predicted by simulation.

Figure 9 shows the pressure distributions Cpfrom Euler solutions at trimmed flight conditionsfor both full & canard–off configurations. For the

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Investigating the Piaggio Avanti Design using CEASIOM

(a) Lift coefficient

(b) Drag coefficient

(c) Pitching moment coefficient, reference pointat 7.41m from the nose

Fig. 7 Euler solutions at Mach number of 0.62,canard ON & OFF cases; error bars representmaximum deviations during the last 500 CFD it-erations

full configuration, the horizontal trim is achievedby deflecting the elevator up (negative) 3.33 degat α=0.56 deg to maintain the desired lift. For thealternative geometry without canard, the horizon-tal trim can be achieved at α=0.62 deg with ele-vator deflection at δe=-4.25 deg. The elevator de-flection angles are small for both configurationsat trimmed flight, however the canard–off aircraftis too stable compared with its fully configuredcounterpart. During the presented study the cen-ter of gravity position was assumed unchangedbetween canard ON & OFF cases at 23.41 ftfrom the nose. As figure 7(c) shows, the staticmargin for the full configuration is around 7.5%MAC, while for the canard–off one is around42% MAC. Adding the canard moves the aerody-namic center forward that reduces the static mar-gin accordingly.

Fig. 8 Cp from Euler solutions for trimmed flightat Mach number of 0.62 (U = 356 kts at 39,000ft), full & canard–off configurations

Then we would like to ask why both configu-rations have very close trim angles. If we go backto figure 7(c), we could see that, round α = 0(computed cruise lift coefficient CL ≈ 0.56), thepitching moments Cm for both configurations arequite close. This means to get the pitching mo-ments balanced for both configurations requiresimilar efforts, namely, similar nose–up momentsprovided from the horizontal stabilizer. How-ever, if we expand the speed and altitude fromthe economy cruise point according to Table 2,we found that although the differences of thetrimmed angles of attack between the full andcanard–off configurations are within 0.5 deg forconcerned speeds and altitudes, the elevator de-

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flections vary significantly for both configura-tions. From figures 9 and 10 we can see thefull canard configuration superiors in the hori-zontal trim flight: the elevator deflected anglesare almost kept constantly at small negative val-ues; while for the canard–off one the behavior ison a common level, the elevator deflects less (inabsolute value) as the speed increases.

(a) Trim angle of attack

(b) Trim elevator deflection

Fig. 9 Longitudinal trim calculated and interpo-lated/extrapolated from Euler solutions for full(solid line) & canard–off (dashed line) configu-rations

(a) Trim elevator deflection

(b) Trim angle of attack

(c) Trim drag

Fig. 10 Economy cruise trimmed conditions ataltitude of 39,000 ft, calculated from CEASIOM–Euler

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Investigating the Piaggio Avanti Design using CEASIOM

6.2.1 Innovative design

The forward canard contributes to lift, since it isa fixed surface, and the pitch angle of the forwardwing is configured so that it always stalls beforethe main wing. The resulting automatic nose–down effect assures excellent in flight behavior athigh angles of attack. These aerodynamic advan-tages resulting from the aircraft innovative designand construction, cause the airflow to be laminarover a very high percentage of the aircraft wingchord.

6.2.2 Steady pull–up

To judge the maneuverability of the PiaggioAvanti, the elevator per g is calculated using Eu-ler solver, by maintaining a steady pull–up witha constant angular velocity as if the aircraft wereattached to the end of a whirling arm providedvery far away. Figure 11 shows the Mach plot for

(a) Upper surface

(b) Lower surface

Fig. 11 Mach contour from CEASIOM–Euler so-lutions for Piaggio–avanti 3g steady pull–up ma-neuver

the full configuration aircraft with 3g pull–up ma-neuver predicted from CEASIOM–Euler, at econ-

omy cruise condition. The freestream velocityis theoretically zero (but set to very small value,e.g. 1 m/s to assure numerical convergence inCFD computation) and the aircraft is evoked bythe angular velocity. Canard provides additionallift, also notice that the flow starts to accelerateat the lower side of root of the canard. This ma-neuver can be achieved with incremental controlforce and elevator deflection, in order to climb toa higher altitude, for instance, to the service ceil-ing altitude.

Euler simulations for both full and canard–off configurations for 3g steady pull–up maneu-ver are obtained, for flight at 39,000 ft. The ele-vator per g for the full configuration is -1.58 deg,whereas the value for the canard–off configura-tion is around -4.8 deg, indicating that the fullconfiguration has better maneuverability.

7 Conclusions

The aircraft design stages, conceptual and pre-liminary, are necessarily collaborative by theirvery nature. An example design exercise wascarried out to illustrate the collaborative aspectsof design using the tools AAA and CEASIOM,working respectively on conceptual and prelim-inary design. The chosen example is the Piag-gio Avanti that has three lifting surfaces and hasan advantage in horizontal flight performance.The exercise brought out some of the details in-volved when exporting the configuration geome-try from conceptual design, where the model isusually not water–tight and meshable, to prelim-inary design where a meshed model is the nec-essary starting point for further design work. Inthe example a small computer routine was writ-ten to convert the configuration data from AAA toinput data for CEASIOM–sumo so that the con-figuration was water–tight and meshable [3]. Thecommon–language used to minimize the re–workis a standard for the data describing the aircraft,i.e., the CPACS standard proposed by DLR (seeRef [11]).

Euler simulations for both full and canard–off configurations at steady and level flight con-ditions were computed. The trim analysis is car-ried out a number of different cruise conditions,

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showing that the full configuration with a canardhas advantages with minimized elevators anglechanges while no (significant) drag was added.The elevator per g for the full configuration ismuch less than the configuration without a ca-nard, when the aircraft is under a steady pull–upmaneuver, provided that the neutral point is fixed.All of these above validates that the three–lifting–surface configuration has some aerodynamic ad-vantages than conventional configurations.

In the future a more realistic model with pro-pellers can be made in CEASIOM for Euler sim-ulations. The effects are well modelled for in-viscid flow and these should be considered in thedesign process, one good example is Lötstedt’swork [15].

8 Acknowledgments

The computations were performed on resourcesprovided by the Swedish National Infrastructurefor Computing (SNIC) at PDC Centre for HighPerformance Computing (PDC–HPC). MarcoCristofaro thanks Politecnico di Torino for theexchange period at the University of Southamp-ton. Yongzhi Wang acknowledges the financialsupport provided by China Scholarship Council(CSC).

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9 Contact Author Email Address

Zhang, M. mzha@kth.seCristofaro, M. mcristofaro89@gmail.comWang, Y. apple_jw@126.comDa Ronch, A. A.Da-Ronch@soton.ac.ukRizzi, Z. rizzi@kth.se

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