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MSC Developments in Aeroelasticity

Erwin H. Johnson

The MacNeal-Schwendler Corporation

815 Colorado Blvd.Los Angeles, California 90041

Abstract

The MacNeal-Schwendler Corporation has a long h istory of involvemen t withaeroelasticity. This pap er briefly reviews past development efforts and the current

capabilities in th is area. Recent developments tha t have not been incorpora ted

into stand ard d ocum entation are given somew hat m ore emp hasis. This paperconclud es with a discussion of ongoing d evelopment activity.

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1.0 Introduction

Aeroelasticity is a branch of aeromechanics that deals with the interactionamong inertial, aerodynam ic and structural stiffness effects in air vehicle

design. It is an area that requ ires soph isticated engineering d esign, analysisand testing. MSC/ NASTRAN has had the ability to p erform aeroelastic

analyses since the early 1970s and has had a design (optimization)

capability since the release of Version 68 in 1994. This capability has beendocum ented in a nu mber of MSC pu blications, most notably the

MSC/NA STRA N Aeroelastic Analysis Users Guide of Reference 1. This p aperreports on th e current state of aeroelasticity at MSC. As such, it can be

considered an up date of a similar paper that w as presented in 1990

(Reference 2). The cur rent paper emp hasizes developm ents that haveoccurred since the pu blication of these documents and discusses a nu mber

of ongoing activities in the aeroelastic area.

2.0 Current Capability

2.1 Solution Sequences

A br ief review of the aeroelastic capability cur rently residing inMSC/ NASTRAN is given to provide a basis for the remainder of the p aper.

Table 1 identifies the four MSC/ NASTRAN solution sequences used in

aeroelastic ana lysis. SOL 144 add resses static aeroelasticity and , as such, isuseful for making a p reliminary assessment of the aircraft design loads and

prov ides estimates for rigid and elastic stability and control derivatives.

Reference 6 is a comp anion p aper in this conference that presents w orkdone to includ e a higher order panel method to comp ute the aerodynam ic

forces used in th is solution sequ ence. The Ongoing Developm ent sectionof this paper p reviews a nu mber of enhancements that are underway in this

area.

Table 1: Solution Sequences Related to Aeroelasticity.

SOLUTION Description

144 Static Aeroelasticity

145 Aerodynamic Flutter

146 Dynamic Aeroelastic Response

200 Design Sensitivity and Optimization

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The aerodyn amic flutter of SOL 145 represents the most m ature of the four

solution sequ ences. The Recent Enh ancements section of this paperdiscusses some changes that have been made in this area in Version 69 and

V70 of MSC/ NASTRAN.

The Dynamic Aeroelastic Response capability of SOL 146 provides the

capability to analyze the transient or frequen cy structural response in the

presence of either an aerod ynam ic (gust) or other dynam ic (e.g., land ing)loading. Reference 4 is a recent publication th at goes into som e detail as to

how this solution sequence was ap plied extensively in the d evelopm ent ofthe B-2 Bomber.

The Static Aeroelastic and Flutter solution sequences were incorpora ted in

MSC/ NASTAN s design optimiza tion capability of SOL 200 in Version 68.Material in the User s Guides of Reference 1 and Reference 5 is the best

source for learning abou t the capability.

2.2 Support Activity

The aeroelastic capability is suppor ted in a nu mber of ways. The precedingsubsection has identified key reference material. Num erous other

publications are listed at the Internet Web Site for MSCs aerospacebusiness: www.macsch.com/aerospace. First line technical sup por t is provided

by Technical Representatives distributed arou nd the wor ld, and a ded icated

second level sup por t staff is located in Los Angeles. The MSC/ NASTRANDevelopm ent organization is available to answer detailed questions and to

investigate potential code errors.Seminars tha t explain the Aeroelastic capability are given period ically as

part of MSCs training course offerings. In addition, the training m aterialsthat go w ith this course are an ad ditional resource and includ e the briefing

charts used in the seminar and a volume of reference material that contains

pu blications relevant to the aeroelastic capability.

3.0 Recent Enhancements

The Reference 1 Users Gu ide is based on Version 68 of MSC/ NASTRAN.

Subsequen t enhan cements to the aeroelastic capability are not included in

this manu al. The majority of the aeroelastic development activity for theserecent versions can be classified as maintenan ce, includ ing error corrections.

One notable fix in this regard is that Version 69 has restru ctured theaeroelastic calculations in order to significantly red uce the d atabase

requirements. With the correction, the V69 database for an aeroelastic task

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may be half the size of the V68 database. Four other enhancements are

worthy of specific mention h ere: 1.) the PKNL flutter m ethod , 2.) mu ltiplebound ary conditions in flutter analyses 3.) enhanced sorting of flutter roots

and 4.) splining of aerodynam ics to u pstream superelements.

3.1 The PKNL Flutter Method

A new flutter method, designated PKNL (PK No Looping), was m adeavailable in Version 69 of MSC/ NASTRAN. The new method, available as

an op tion on th e FLUTTER bulk data entry, is a variation on th e PK method

that d oes not loop on all combinations of user input density, Mach n um berand velocity.

With the PK meth od, all combinations of these param eters are used so thatthe input of two densities, two Mach numbers and eight velocities results in

32 flutter analyses. The new method requires inpu t of the completespecification of density, Mach num ber and velocity for each point tha t is to

be analyzed and is designed to add ress the requirement of performing only

match p oint flutter analyses. It is suggested that this method be app lied bymoving through the atmosphere at a constant Mach nu mber in order to

avoid the expensive generation of aerodyn amic matrices at a large nu mberof Mach num bers.

Through an oversight, the PKNL method was not m ade available for d esign

in Version 69. This was corrected in Version 70 so that flu tter constraints canbe specified in SOL 200 based on a PKNL meth od of flutter analysis.

3.2 Multiple Flutter Boundary Conditions

The flutter analysis capability w as enhanced in Version 69 to allow for the

simultaneous consideration of multiple bound ary conditions. This is mostuseful in Solution 200 in that these conditions can now be treated in a

design task. It should be emph asized that there is an MSC/ NASTRANrestriction in aeroelastic analysis that only one aerodynam ic symm etry is

allowed per ru n so that symm etric and an tisymm etric aerodynam ics cannot

be run simultaneously.

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3.3 Enhanced Sorting of Flutter Roots

A minor enhancement for flutter analysis in Version 70 has been animp roved sorting of flutter roots extracted using the PK or PKNL flutter

method s. Previously the roots were sorted on th e frequency of the complexmodes so that frequency crossings, which are often present in a flutter

analysis, were not prop erly d isplayed. Figure 1 shows a before and after

d isplay of a flutter ana lysis and it is seen that the roots are correctly sortedand tracked in Version 70.

3.4 Splining to Upstream Superelements

The standard aeroelastic capability requires that the user connect the

aerodynam ic mod el to the structural mod el at points that are in the residualsuperelement. For structural mod els that have been developed using

superelements, this may requ ire extensive remodeling in order to move theapp ropriate grids from the u pstream superelements to the residual. A

DMAP alter has been d eveloped that allows d irect splining of

aerodynam ics to the up stream superelements. Information on the alter,includ ing examples, can be found in the SSALTER library. It shou ld be

cautioned that the method is most suitable in SOLut ions 145 and 146 and isnot ap prop riate for SOLution 144.

4.0 Ongoing Development

This section d iscusses cur rent d evelopment efforts at MSC in the aeroelastic

area, both to publicize these efforts and to solicit feedback. The m ajority ofthis work is preceding und er the aegis of the Flight Loads System d iscussed

in some detail in Reference 3, presented at th is conference. This pap er givesan overview of this project and emphasize some comp onents not discussed

in Reference 3.

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Figure1.

Enhanced

Sorting

ofFlutterR

oots.

b.

Version

70

a.

Priorto

Version

70

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4.1 Flight Loads System

4.1.1 Generic Control Law

This aspect of the Flight Loads System acknowledges the increasingimp ortance and complexity of control systems in the pred iction of flightloads. It is recognized that th e control system complexity makes it unlikely

that the full sophistication of control/ structures interactions could be easilycaptured in MSC/ NASTRAN. Instead, this development effort is intended

to address some of the key features of the control system and hop efully aid

in the p reliminary d esign of an air veh icle. Among the concepts beingadd ressed are:

1. Control surface hinge moments

2. Position and hinge mom ent limits on the control surface travel.

(The p osition limits can be a function of dynam ic pressu re).

3. Gain schedu ling of the control surface as a function of Mach nu m-

ber and angle of attack.

4. Ability to consider redu nd ant control surfaces when performing a

trim analysis.

One exam ple of this last concept is a vehicle that has m ultiple surfaces (e.g.,

inboard and outboard wing flap s and an elevator) to control pitching. Asynthesis technique based on the w ork documented in Reference 7 is used

to address this case.

4.1.2 Enhanced Aerodynamics

A nu mber of aerodynamic method s are available in MSC/ NASTRAN: theDoublet Lattice Method is the workhorse for subson ic aerodynam ics, and

ZON A51 fills a comparable role for superson ic flow. These flat p latemethod s are ideal for flu tter ana lysis, but it is felt that a higher fidelity code

is required for the calculation of loads. MSC has initiated the installation of

the A502 code (Reference 8) as an add itional aerodynamic method thatallows complete modeling of the three d imensional surface of the vehicle.

4.1.3 3D Splines

The add ition of 3-D aerodynamics necessitates enhancement of the

techniques u sed to couple the aerodynam ic and structural models. Anu mber of techniques are un der investigation for this task. As an interim

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goal, the next release of MSC/ NASTRAN is sched uled to enhan ce the

existing m ethods w ith a thin p late spline and to allow the separate spliningof forces and d isplacemen ts.

4.1.4 Aerodynam ic and Aeroelastic Databases and Data Import

The Flight Loads System w ill be required to function within an existing

prod uction env ironment that is likely to have customized processes thatwill either drive the Flight Loads Systems (e.g., external aerodynam ics

might be impor ted) or postp rocess results (e.g., deformed shapes) from the

system. This is being planned for by the developm ent of a special pu rposedatabase that w ill allow the specification of mu ltiple aerodynam ic models,

aerodynam ic results and aeroelastic results. The u ser will be able to useMSC provid ed tools to insert external data into the database. External data

mu st conform to the p ublished descriptions of the data formats.

4.2 Graphical User Interface

Work is und erway on the d evelopment of a user interface for the aeroelasticcapability. This welcome activity is to be bu ilt arou nd MSC/ PATRAN and

will sup port not only the Flight Loads System that is un der d evelopm ent,

but the curren t aeroelastic capability as well. Model generation and d isplay,as well as postprocessing of aeroelastic results, is to be sup por ted.

5.0 Concluding Remarks

Aeroelasticity is an integra l part of MSC/ NASTRAN an d h as been

continuou sly developed and maintained for over 25 years. The capability isin line with MSCs vision of provid ing CAE solutions to m ajor

manufacturers.

It should be recognized th at the p resentation of this paper and the entire

Aerospace User s Conference is an outgrowth of the establishm ent of MSCsAerospace Business Unit. This reorganization has brough t a reemp hasis on

the aerospace segmen t of MSC client base in general and on the

aeroelasticity area in particular. Significant staff and resources are being putinto aeroelastic development. User interest and involvement in these

developments is welcome.

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6.0 References

1. Rodd en, William P. and Johnson, Erwin H.,MSC/NASTRAN

Aeroelastic Analysis Users Guide, V68, The MacNeal-Schwen dler

Corporation, 1994.2. Johnson, E.H. and Rodd en, W.P., Ongoing Developm ent of the

MSC/ NASTRAN Aeroelastic Capability, presented at the 17th

MSC European User s Conference, Paris, Fran ce, Sept. 1990.

3. Whiting, B., and Neill, D.J., Interfacing Externa l, High Ord erAerodynam ics into MSC/ NASTRAN for Aeroelastic Analyses,

MSC Aerospace Users Conference, November 1997.

4. Britt, R.T., Jacobson, S.B. and Arth urs, T.D., AeroservoelasticAnalysis of the B-2 Bomber, p resented at th e, Rome, Italy, July

1997.

5. Moore, G.J.,MSC/NASTRAN Design Sensitivity and OptimizationUsers Guide, V68, The MacNeal-Schwendler Corp oration, 1994.

6. Neill, D.J., and Sikes, G. The MSC Flight Loads System, MSCAerospace User s Conference, Novem ber 1997.

7. Ausman , J.D. and Volk, J.A., Integra tion of Control Sur face Load

Limitin g in to ASTROS, AIAA-97-1115, Ap ril 1997.

8. Epton, Michael A. and Magnu s, Alfred E., PANAIR, A Computer

Program for Predicting Subsonic and Supersonic Potential Flows About

Arbitrary Configurations Using a High Order Panel Method, Volume I,

Theoretical Manual, Version 3.0, NAS-CR-3251, Revision 1, January,1992.