Past diagnostics studies revealed the primary causes of performance deterioration of high by-pass turbofan engines to be flight loads, erosion, and thermal distortion. The various types of airplane loads that are imposed on the engine throughout the lifetime of an airplane are examined. These include flight loads from gusts and maneuvers and ground loads from takeoff, landing, and taxi conditions. Clarification is made in definitions of the airframer's limit and ultimate design loads and the engine manufacturer's operating design loads. Finally, the influence of these loads on the propulsion system structures is discussed

Dickenson, K. H.

Martin, R. L.

English

NASA Technical Reports Server

THE NATURE OF OPERATING FLIGHT LOADS AND
THEIR EFFECT ON PROPULSION SYSTEM STRUCTURES
Kenneth H. Dickenson
Richard L. Martin
Boeing Commercial Airplane Company
ABSTRACT
Past diagnostics studies revealed the primary causes of performance
deterioration of high by-pass turbofan engines to be flight loads, erosion and
thermal distortion. This paper examines the various types of airplane loads that
are imposed on the engine throughout the lifetime of an airplane. These include
flight loads from gusts and maneuvers and ground loads from take-off, landing and
taxi conditions. Clarification is made in definitions of the airframer's limit
and ultimate design loads and the engine manufacturer's operating design loads.
Finally, the influence of these loads on the propulsion system structures is
discussed.
INTRODUCTION
The traditional transport airplane structures analyst's treatment of an engine
is very simple: The engine is a "concrete block" whose properties are entirely
inertial, and the main concerns are that it should not fall off the strut and
that it is located properly from a wing flutter viewpoint (figure 1). In the era
of the turbojet and the low bypass ratio turbofan, such treatment was acceptable
because it was nearly correct. Engines and their inlets were so compact and
rigid (figure 2) that the internal structural problems could be left to the
engine manufacturers who needed only to be advised of the accelerations to be
applied at the mount locations.
In the late 1960's the advent of the high bypass ratio turbofan engine with it's
large fan case relative to its core brought change. Early in the Boeing 747
program, for example, it was found that thrust forces caused "ovalization" of the
engine case because the engine's combination of large diameter (figure 3) and
high thrust imposed a substantial couple at the engine mounts. This problem was
alleviated by adding a "thrust yoke" that transferred thrust directly to the
strut and reduced distortion of the case due to thrust.
The high bypass ratio turbofan engines have large inlet airflows relative to the
engine core size. Thus, a large momentum change is required to align the airflow
with the engine at high angles of attack. Since inlets are usually bolted to the
front flange of the fan case, the inlet aerodynamic loads associated with this
momentum change induce bending and distortion into the smaller diameter engine
core case. These case distortions may cause rubbing between the rotors and the
static case structure while the desire for higher overall pressure ratio requires
better control of tip clearances. The aerodynamically induced operating loads
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that act on the inlet are modest in comparison to overall airframe design loads.
It remained for the 1973 oil embargoand the ensuing dramatic rise in fuel prices
to motivate a deeper investigation of airframe and engine structural
interaction.
Attention was focused on the causes of engine fuel consumption deterioration in
the NASAsponsored Pratt & Whitney Aircraft JT9D Diagnostics Program. In this
effort, several different probable deterioration mechanismswere identified and
evaluated analytically. Prominent amongthemwas rubbing between the rotor tips
(of the fan, compressors, and turbines) and the engine case caused by flexing of
the engine under operating loads. The result of rubbing was increased clearance
between the rotor and case since material was worn from the "rub strips" and the
blade tips. Increased clearance reduced component efficiency and increased
specific fuel consumption.
AIRPLANE LOADS
Design Loads
Due to the overriding importance of safety, airplane design loads have been
studied intensively for many years and are the subject of a large body of
doctrine and practice developed by airframe manufacturers and enforced by the
Federal Aviation Administration (FAA). A "limit design load" is determined for
airframe structure as the maximum load that the structure can be expected to
encounter during the entire life of the airplane fleet. At the limit design
load, the structure is not permitted to suffer permanent deformation; i.e., the
maximum stress may not exceed the elastic limit. To provide an added degree of
safety, an "ultimate design load" is also specified, usually as 1.5 times the
limit load value. Up to the ultimate load, the structure is permitted to suffer
"permanent set", but it must not fail.
The design loads are determined by analyzing the airplane in a variety of load
conditions that are contained within the envelope of the "V - n diagram" (a plot
of the accelerations that the airplane must withstand versus airspeed). Three
main types of load conditions are considered. The first is maneuver. Transport
category airplane limit loads are determined in 2.5 g turns or pull-ups with
flaps retracted and in 2.0 g turns with flaps down. The second type of load
condition is due to atmospheric turbulence. It is assumed that gusts of a
defined shape and velocity will be encountered by the airplane at speeds
specified in relation to the design operating speed limits chosen by the
manufacturer. The airplane's response is determined by analyzing the
aerodynamic, elastic, and inertial characteristics in detail. The third loading
category is associated with the ground. These loading conditions include take-
off, landing and taxi. Analytically determined design loads are corroborated by
extensive flight load surveys using accelerometers, strain gages and pressure
transducers.
The oustanding structural safety records of today's commercial air fleets
demonstrate that the design loads issues are very well understood. However, the
loads that cause day-to-day TSFC deterioration are less severe than design loads
176
and are not so well understood. They may be termed "operating" loads, and a
different approach is needed to understand them.
Operating Loads
The parameters for determining operating loads are the same as those for design
loads; i.e., airplane aerodynamic, elastic, and inertial properties on the one
hand, and flight conditions (maneuvers, turbulence, etc.) on the other. To a
considerable degree, the problem resembles that of analyzing structural fatigue.
Service life, maintenance, and economy are the main considerations, with
statistical descriptions of the operating environment being the scenario, as
opposed to a set of extreme conditions.
Fatigue damage is assessed from the cumulative occurences of different stress
levels. The part of TSFC degradation due to clearance change, on the other hand,
depends on the probable time period (or number of flights) until any given load
level is exceeded once. Figure 4 shows the analysis sequence. The starting
point is the set of mission profiles that typify the airplane's utilization
(upper left corner). Mission length is important because it determines how many
"ground-air-ground" (GAG) cycles are flown per hour of operation. The altitude
and speed profiles determine the frequency and severity of gusts.
Load exceedance probability per flight can be inferred from airplane character-
istics and the mission profile. The sketch at the upper right of figure 4 refers
to inertia load exceedances, but a similar plot can relate to airloads. When the
probable loads are known, the probable tip clearance changes can be inferred from
the elastic properties of the engine itself. These may be obtained by analyses
varying from simple beam representations to finite-element models containing
thousands of elements. Recent experience supports the need for the more complex
finite-element approach. When tip clearances become negative, rubs are
indicated, and TSFC deterioration can be expected.
In addition to revenue service missions, other flight profiles must be
considered, such as crew training. A significant mission that occurs only once
on each airplane is the "acceptance flight" (figure 5). All transport airplanes
are checked for satisfactory flight characteristics and functioning of warning
systems before delivery to the customer airline. In such flights, the airplane
is not tested to the limit loads of the flight design envelope but to more normal
operating conditions, such as maximum airspeed (dynamic pressure), maximum Mach
number, and minimum airspeed (stall warning) where warning devices such as stick
shakers automatically alert the pilot to the situation. Since such a flight
always occurs first in an airplane's history, it establishes a starting set of
rubs and clearances for subsequent exceedance studies.
Statistical descriptions of the inertia load environment have been obtained by
accumulation of a great many speed/acceleration/altitude ("VGH") recordings made
in actual airline service (figure 6). Airspeed (V), normal acceleration (g), and
altitude (H) are recorded continuously. The recordings are later analyzed by
177
counting acceleration peaks. The number of "occurrences" of a particular
acceleration level is defined as the number of peaks found over some time period
that fall between the upper and lower bounds of that level. The number of
"exceedances" of that level is the sum of the occurrences of that level and all
higher levels as shown in figure 7.
Histograms (figure 8) can then be constructed which show occurrences and
exceedances per flight hour or per flight versus load level, and plots such as
shown in figure 9, depicting probable nacelle inertia load exceedances, can be
drawn. This figure, incidentally, shows a characteristic feature of the environ-
ment of wing-mounted engines. The motions and accelerations of the nacelles are
larger than those at the airplane center of gravity because of the wing
aeroelastic response to dynamic loads such as gusts and landing impacts. In
additon to accelerations, gyroscopic loads caused by airplane angular motions
must also be considered.
Under normal conditions, the most severe engine aerodynamic loading occurs at
takeoff when the maximum engine thrust produces a high mass flow rate through the
inlet combined with a high angle of attack. These effects are illustrated in
figure 10 which shows low pressure caused by suction on the lower inside lip of
the inlet. This is associated with high local velocity as the flow turns
sharply. Supersonic flow usually occurs in this region, followed by shocks and
sometimes by local flow separation.
Of lesser importance, but significant because it is a condition that creates a
load reversal, is the maximum dynamic pressure condition shown in figure 11.
This condition involves a negative local inlet angle of attack and an inlet
pitching moment acting downward. This moment is, however, of much smaller
magnitude than the nose-up moment at takeoff.
Neither the maximum dynamic pressure condition nor the one shown in figure 12 --
stall warning at 10 o flaps -- are normal revenue service flight conditions. Both
of these, however, are currently flown in the flight acceptance test of every new
airplane.
One of the more uncertain assumptions regarding inlet pressures has been the
circumferential distribution. A simple, one wave cosine distribution,
illustrated in figure 13, rotated to account for non-symmetric effects has been
customary. This assumption awaits validation by the results of the Nacelle
Aerodynamic and Inertia Loads (NAIL) flight test program.
To illustrate the joint efforts in the JT9D Diagnostics Program and,the interde-
pendence of the engine and airframe manufacturer in the propulsion interface,
Figure 14 shows the mathematical model used to analyze the 747 propulsion system.
Government and industry foresight several years ago provided the NASTRAN finite
element program giving wide availability to this technology. Air breathing
propulsion structures are a relatively late application of this technology, and
there is currently a large effort being made toward test and analysis
correlations to enhance this application.
178
Currently, Boeing and the three major jet engine manufacturers are utilizing the
type of models illustrated here to calculate deflections, clearance changes,
internal loads, and vibration behavior on the 767, 757 and 737-300 new airplane
programs and on future powerplant installations for the 747 airplane program.
The airframe and engine manufacturer must each conduct their own analyses for
their specific needs. An exchange of data files provides each with this
capability and the integrated model as illustrated in figure 15. Recent trends
in nacelle design have resulted in much closer structural coupling between the
engine, nacelle, and strut. For example, in the 767 design the front mounting
system has been placed to minimize thrust bending moment, and the thrust reverser
and fan exhaust cowling are hinged from the strut and clamp onto the engine
through circumferential V-grooves. This not only simplifies engine removal and
maintenance but also serves as a dual load path with the mounts which provides
the redundancy required for fail safe design in case of mount failure. An extra
benefit from this dual load path is a reduction of engine loading which enhances
engine performance through reduced clearance changes under flight operating
loads. The close coupling inherent in this type of design necessitates use
of detailed nacelle-engine-strut finite element models to define interface
loads accurately.
A characteristic of engine structure relative to conventional airframe structure
is its inherently greater stiffness and complexity. This obviously must be the
case in order to maintain the dimensional constraints so important to engine
performance. The maximum engine bending deformations are typically one to two
orders of magnitude less than maximum strut deflection as exhibited in figure 16
for a "g" loading condition. The attainment of accuracy in the engine and
nacelle math model comparable to conventional airframe structures therefore,
requires a great deal of experience and effort and should rely heavily on
accurately measured data when available.
To illustrate this point, figure 17 shows typical calculated clearance change
contour lines for a normal takeoff condition. This plot is for the inboard side
of the number three engine on the 747 airplane. Clearance closure is denoted by
the shaded regions. The information shown here is used by the engine
manufacturer in a separate post processor program that calculates blade rubs,
stage-by-stage clearance increases, and TSFC deterioration.
The ultimate goal in the diagnostics effort is to provide adequate data for
taking actions toward eliminating performance deterioration. Much of the
required data has been generated in the early tasks. The flight loads portion of
the JT9D Diagnostics Program in which engine clearance changes are measured in
flight and the concurrent NAIL flight loads program will complete the data. The
task ahead is the application of this data and the appropriate use of design
tools in concerted efforts between the engine and airframe manufacturers to
reduce performance deterioration. A considerable effort has evolved in the area
of integrated engine-nacelle design studies aimed at stiffening current power
plant installations. More important is the use of the diagnostics data in
systems currently under design and development that recognize and build
deterioration prevention into the initial designs.
179
DESIGN LOAD PHILOSOPHY
C CD
CONCRETEBLOCK
/ " / I//
FIGURE1
EARLY TURBOJET INSTALLATION
FIGURE2
180
MOOERN ENGINE INSTALLATION
FIGURE3
RELATION OF AIRPLANE FUGHT LOAD EXCEEDANCES
TO BLADE TIP CLEARANCE
TIMI_
A T,P c,-_*.A.c_- I
FI GI.JRE4
181
ACCEPTANCE FUGItT PROFILE
ALTITUDE
/ _._....--M^x,M,,,_,o.
/ _-P,_ssuRE
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// ..... w.,.o-_ \
LAND! REVERSETIME
FIGURE5
1HRUST
ILLUSTRATIVE VGH RECORDING
FLIGHTPHASE -- CLIMB ENROUTI[ DESCENT
,ZACCEL. ', ,, _.':Y_,-;_,'l:l
TYPE -- TAXI AND GUST.-/ _--- MANEUVER LANDING
TAKE-OFF IMPACT
FIGURE6
182
ANALYSIS OF LOAD FACTOR TIME HISTORY
FROM VGH RECORDER
o.41-
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o,I _,/ " I/" V "l-/_7-vv_]lVv__
iiii,  -v " vV ';"
-O.4 I,
UNIT TIME le.|., I HOURI
EXCEEOANCES -
OCCUHAENCE$ £ OCCURRENCES
' 'l3 4 POSITIVES e
8 IS
S 1S
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FIGURE1
HISTOGRAM AND PLOT OF OCCURRANCES
OCCURRENCES
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6n
FIGURE8
183
NACELLE LOAD EXCIEEDANCE DATA
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LOADFACTOR( G's I
FIGURE9
ONCEPER
FLIGHT
ONCEPER|000 FLIGHTS
AIRLOADS
TAKEOFFROTATION
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184
AIRLOADS
MAXIMUM DYNAMIC PRESSURE
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FIGURE 11
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STALL WARNING, lO° FLAPS
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RESULTANT
PRESSURE LOADS
DISTRIBUTION
FIGURE 12
185
INLET CIRCUMFERENTIAL PRESSURE DISTRIBUTION
OUTBOARD
FIGURE ]3
PROPULSION SYSTEM SUBSTRUCTURES AND RESPONSIBILmES
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186
FINITE ELEMENT MODEL
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188


The nature of operating flight loads and their effect on propulsion system structures

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