The use of NASTRAN in propulsion system structural integration analysis is described. Computer support programs for modeling, substructuring, and plotting analysis results are discussed. Requirements on interface information and data exchange by participants in a NASTRAN substructure analysis are given. Static and normal modes vibration analysis results are given with comparison to test and other analytical results

Beste, D. L.

White, J. L.

English

NASA Technical Reports Server

NASTRAN APPLICATIONS TO AIRCRAFT PROPULSION SYSTEMS
John L. White, Boeing Commercial Airplane Company
and
David L. Beste, Boeing Computer Services, Inc.
SUMMARY
!he use of NASTRAN in propulsion system structural integration analysis is
[escribed. Computer support programs for modeling, substructuring and
ilotting analysis results are discussed. Requirements on interface informa-
tion and data exchange by participants in a NASTRAN substructure analysis are
liven. Static and normal modes vibration analysis results are given with
_omparison to test and other analytical results.
INTRODUCTION
_he versatility of NASTRAN makes it an ideal tool for the complex analysis
problems associated with aircraft propulsion systems. These systems experienc,
great variety of loads and environments requiring sophisticated analysis
Lools for accurate analysis.
particularly attractive advantage of NASTRAN is its low initial cost of
acquisition and its common availability. Thus, it can serve as a unifying
structural analysis language in joint engine and airframe company structural
integration efforts. This has been the major use of NASTRAN in the Boeing
Commercial Airplane Company, although some detailed analysis of engine
components has been carried out for risk evaluation. This paper is therefore
directed primarily at overall propulsion system structural analysis rather
than detailed component analysis.
This paper describes NASTRAN history at Boeing, the various pre and post
processors developed for enhanced utilization, propulsion system modeling,
substructuring procedures, and various analysis cases with some correlation
with test and other analyses.
I
NASTRAN BACKGROUND
NASTRAN has been operational on Boeing computers since release 8.0 was
available in 1969. Currently release 15.5 is running on the IBM 370/168's
under HASP and LASP, the IBM 360/65 under OS and the CDC 6600's under
KRONOS 2.1. During this time NASTRAN has been used in the analysis of a
large number of aerospace and other structures, e.g., MVM, LST, Minuteman,
Lunar Rover, Roland, turbine blades, HLH combustor, YC-14 propulsion system,
and a large cable-stayed bridge.
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https://ntrs.nasa.gov/search.jsp?R=19750023418 2020-03-19T21:23:08+00:00Z
l
To aid in the various analyses and to improve "ease of use" of NASTRA_,variou_
computer programs have been developed. SAIL, SPANand XFETCHare three such
computer programs. Also routines have been developed to provide an overall
equilibrium check, to generate multipoint constraints, and to recover multi- i
point constraint forces.
SAIL II (Structural Analysis Input Language II) (Reference i) is a language
for describing NASTRANbulk data. Basic finite-element input data such as
gridpoints, element connections, and loads are defined in an easy, straight-
forward manner, using the SAIL II statements. The SAIL II features include
looping, data block transformation, and external data generators plus all
the capabilities of FORTRAN.The NASTRANbulk data deck is generated from a
relatively small number of SAIL II statements, making it very convenient to
incorporate structural and geometry changes into a NASTRANmodel.
SPAN(Substructure Partition Automation for NASTRAN)(Reference 2) is a
NASTRANsupport program that automatically generates the partition vectors
required for assembling Phase I matrices during Phase II of a NASTRANsub-
structure analysis. The partition vectors generated can be based on identical
grid or scalar point identifications, identical XYZcoordinates, or specified
connection points. User labels can be retained during Phase II to allow loads,
constraints, and elements to reference the original grid points. The displace-
ment results are identified by the original gridpoint labels. Structural plots
can be produced during Phase II.
SPANuses the Phase I checkpoint tapes and a small amount of card input to
determine the substructure definition. The SPANoutput is the NASTRANPhase
II input deck that includes the required partition vectors. Extensive error
checks are madeto insure proper matrix ordering and consistency.
XFETCHis a subroutine that reads NASTRANdata files from a checkpoint/
restart tape. The NASTRANdata can be returned to an incore storage array,
or copied and reformatted to data sets easily read by FORTRAN.The copy
_nd reformat feature is useful in applications where the data sets are too
large for convenient incore processing. Both tables and matrices can be read.
PROPULSIONSYSTEMSTRUCTURESBACKGROUND
In the development of aircraft propulsion systems, major structural components
are provided by both the engine manufacturer and the airframe manufacture.
Typically the strut and nacelle, including inlet, nozzles, reversers, tail-
cone, cowling and systems equipment - or about 20 percent of the propulsion
system below the wing weight - are produced by the airframe manufacturer.
The engine manufacturer ordinarily develops and tests the bare engine on a
rigid test stand, exclusive of flight hardware. The airframe manufacturer
provides design envelope loads to the engine manufacturer who then determines
if the engine will function properly under such loads.
92
P
cause the propulsion system is usually not tested on the wing as an inte-
ated structure until flight testing begins, it is important that at an
irly stage of the engine/airframe development program an integrated structural
!alysis of the total propulsion system be carried out. A structural integration
ol such as NASTRAN can provide this analysis by accurately simulating the
!al propulsion installation and providing detailed knowledge of internal loads,
inning clearances, and total system vibration response. The needs for propul-
_on system and airframe structural integration are discussed further in Refer-
Lce 3.
PROPULSION SYSTEM MODELING
r enhanced utilization of NASTRAN, comprehensive modeling procedures have
en developed. Because of the axisymmetry or cyclic symmetry of most of the
_opulsion structure, automation of the modeling is particularly easy.
gure 1 illustrates how the engine structure is modeled utilizing the SAIL II
neral purpose input language described earlier. An engine fan frame is shown
_ereby half of one strut is idealized, then by SAIL II built-in transformation
broutines, it is reflected, rotated and joined with other structure to generate
e entire fan frame substructure. Where practicable, geometry is digitized
Irectly from engineering drawings including grid point coordinates, plate sec-
_on properties, beam offsets, section properties and orientation grid points.
_e of the most beneficial aids for model checkout has been large size CALCOMP
ots identifying grid points and the different element types and their numbers.
wever, model checkout is never assumed complete until a successful loads case
s been fully executed and results plotted.
figure 2 is a NASTRAN plot of a high bypass ratio, fan jet propulsion system.
he models shown are symmetric halves made up essentially of quadrilateral
late and beam elements and include a beam-lumped mass representation of the
[otors. Multipoint constraints simulate the bearing housings as rigid rings
_ich are coupled to the rotor by scalar spring and damper elements. Direct
Latrix input is used to input rotor spin stiffening and Coriolis terms when
Lpplicable.
he vibration and dynamic response models are obtained from the static models
ihown in Figure 2 by the standard methods of Guyan reduction or possibly,
_re economically by mass lumping. In either case considerable non-structural
_ss lumping is required due to the many propulsion system accessories. The
_ASTRAN generated gross mass matrix has been found useful for manually re-
|istributing the mass for vibration analysis.
93
MULTICOMPANY INTEGRATION ANALYSIS
Since NASTRAN is in the public domain and is available to everyone, it is !
quite straightforward for many companies to join in performing a substructure
analysis of the complete structure. Phase I for a particular component is
done by the company responsible for the component at its computing facility; I
Phase II is done at a mutually acceptable computing facility; and Phase III is
run at the company responsible for the component. Before a joint effort is
undertaken, however, the participating companies should establish the followin_
ground rules to aid the effort: t
i) A basic XYZ coordinate system for the entire structure.
2) Unique grid point numbers for the entire structure so that Phase I grid
points instead of scalar points can be used in Phase II.
3) Compatible displacement coordinate system at interface points.
4) Grid point numbers at an interface increasing in the same direction for
all substructures.
5) Unique coordinate system numbers for each substructure.
6) Agree on a common buffer size.
7) Compatible user and checkpoint tapes.
8) Unique substructure plot element ID's for Phase II plots.
If SPAN is used, then the ASET degrees of freedom at an interface grid point
are required to be the same for all substructures that connect to the inter-
face point.
ANALYTICAL RESULTS - STATICS
NASTRAN models of the type illustrated in Figure 2 have been used to analyze
a variety of static loads cases, including thrust, inlet lift, inertia, and
gyroscopic moments. Typical loadings and the manner in which they are intro-
duced onto the model are illustrated in Figure 3. A best guess distribution
and force balance provided thrust loads which were distributed circumferential
at various axial locations. The distributed gravity load is calculated within
NASTRAN using the GRAV card. Inlet lift was distributed over the fan case/
inlet attach flange in this early study prior to the availability of an inlet
model. Gyroscopic moments, a case requiring antisymmetric boundary conditions
were applied at the major inertia locations of the rotor and were based on
overall rigid body pitch velocity of the aircraft.
The deflections of the engine structure under load are shown in Figure 4
which were plotted using the standard built in NASTRAN plot capability.
94
Engine performance depends heavily on maintaining tight running clearances
between the rotors and case, particularly in the fan and high pressure com-
pressor. Rubbing increases clearances and decreases engine efficiency
causing increased specific fuel consumption. Therefore, detailed knowledge
of clearance changes under load is desired. To exhibit change of clearance
contours between the engine case and rotors, a NASTRAN postprocessor was
written (Reference 4). This postprocessor utilizes XFETCH, previously
described, to read NASTRAN restart tapes then plots contour maps for the
entire engine under the various loads. Two such plots are shown in Figure 5.
Absolute values are not given because of engine proprietary information agree-
ments between the airframe and engine companies.
The accuracy to be expected from NASTRAN analyses for propulsion system type
structure has not been determined. The few known correlations with test
data indicate it should be within the realm of 5 to l0 percent on peak
deflections. The prevalence of bolted flanges in the engine case, slop in
the installation system, and non-linear seal interface stiffnesses probably
make the analysis less accurate than the 5 percent accuracy usually associated
with the finite element methods.
A comparison of test and analysis is shown in Figure 6 for an engine compressor
case. The model was produced independently by the airframe company from engine
drawings and the analysis run without any knowledge of the tests which were
run independently by the engine company. Under such circumstances the correla-
tion should not be considered too bad.
ANALYTICAL RESULTS - VIBRATION
The ability to predict dynamic behavior of the propulsion system is of utmost
importance. This includes the response to external gust loads, turbulence,
takeoff, maneuver and landing loads in addition to critical rotor speeds and
various off design conditions for safety and reliability. The interface
problem between the engine and airframe manufacturer is of particular importance
here since much of the airframe produced structure is hung at the extremities
of the engine, magnifying the dynamic effects.
NASTRAN's dynamics capability provides an excellent tool for propulsion system
vibration and dynamic response analysis. The normal modes vibration analysis
of a typical high bypass ratio turbine engine is shown in Figure 7. The model
shown had 51 lineal masses, 7 rotary inertias and retained 151 freedoms from
the static stiffness model. A total of 120 symmetric and 120 antisymmetric
modes were extracted, three of which are shown in Figure 7. Mass lumping was
used due to excessive computing cost of Guyan reduction which has been found
to be typically four times higher than the mass lumping approach.
The versatile plotting capability in NASTRAN is a great help in understanding
vibration behavior as illustrated in the mode plots of Figure 7. Correlation
has been attempted between NASTRAN normal modes analysis and test data, and
with other analyses, i.e., simpler beam-spring-mass simulations.
9_
Certain NASTRAN analysis and test frequencies are very close as indicated
in Table I but there is not enough available test data to be sure if the
mode shapes correlate. Much the same situation exists in comparing NASTRAN
results to the simpler beam-spring-mass simulations, also noted in Table I.
A revision of the NASTRAN plot elements and plot viewing angles will be help-
ful in this regard.
CONCLUSIONS
The success of an engine/airframe structural integration effort depends
greatly on the timely exchange of interface information between the engine
and airframe manufacturers including not only elementary geometry and loads
but also comprehensive finite element models. A good tool for doing this is
NASTRAN since it has common availability.
Application of the general purpose finite element programs such as NASTRAN
to propulsion system structure is more recent than to airframe structure.
More analysis and test correlation is needed to establish better standards
for propulsion system modeling and analysis. Experience to date indicates
that NASTRAN accuracy for overall engine structures may not be as good as
for other more intensively analyzed structures.
ACKNOWLEDGEMENTS
The support and encouragement of M. N. Aarnes and E. D. Herness and the
assistance of V. L. Iverson, T. F. Yantis, and F. L. Yen in modeling and
postprocessor development is gratefully acknowledged. Also the cooperation
of General Electric and Pratt and Whitney in providing engine data is grate-
fully acknowledged.
REFERENCES
l.
.
,
1
Ice, M. W., "SAIL II - Structural Analysis Input Language II", 10204-001
Functional Description, 10204-002 Users Manual, Boeing Computer Services,
Inc., May 1974.
Beste, D. L., "SPAN - Substructure Partition Automation for NASTRAN",
10204-003, Boeing Computer Services, Inc., May 1974.
Aarnes, M. N. and White, J.L., "Propulsion System and Airframe Structural
Integration Program", Journal of Aircraft, Vol. 12, No. 4, April 1975,
p. 234.
Yantis, T. F., "IFPLOT - Interference Contour Plotter", BCS-G0697, Boeing
Computer Services, Inc., July 1975.
96
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NASTRAN applications to aircraft propulsion systems

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