Sources of discrepancies noted between propulsion aerodynamic characteristics as predicted from wind tunnel tests and as measured in flight are discussed. The variable Reynolds capability of the National Transonic Facility will provide a tool for understanding and quantifying uncertainties involved in extrapolating the corrected wind tunnel data from tunnel conditions to full scale flight conditions

Bowditch, D.

English

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REPORTOFTHEPANELONPROPULSIONAERODYNAMICS
David Bowditch
NASALewis Research Center
INTRODUCTION
Effort allocated to wind-tunnel investigations of propulsion-system in-
stallations has increased in recent years because of the dramatically increas-
ing impact of these installations on overall airplane performance and on the
cost and duration of flight-test programs. Unfortunately, the effectiveness of
this effort has been seriously limited by a lack of knowledge of the degree of
accuracy with which wind-tunnel data can be used to predict propulsi0n-system
installation performance under actual flight conditions. Experience to date has
been spotty, the degree of agreement between wind-tunnel-derived results and
flight results ranging from excellent to very poor.
The discrepancies noted between propulsion aerodynamic characteristics as
predicted from wind-tunnel tests and as measured in flight appear to arise from
four basic sources:
(i) Difficulties involved in obtaining accurate wind-tunnel data:
The models used tend to be muchmore complex than the usual external aerodyna-
mics models because of the necessity for simulating and modulating engine and
auxiliary airflows, for representing inlet and exit geometries in extensive
detail, for providing an unusually large amount of instrumentation (sometimes
including separate force measurementson inlet and nozzle components) and for
using nonstandard types of model support systems
(2) Uncertainties in the corrections applied to the wind-tunnel data to
allow for sting tares, blockage, wall effects, etc.
(3) Uncertainties involved in extrapolating the corrected wind-tunnel
data from tunnel conditions to full-scale flight conditions
(4) Difficulties involved in obtaining comparable and accurate flight
data.
Propulsion aerodynamic data obtained in the National Transonic Facility (NTF)
at subflight Reynolds numberswill be subject to all these problems. The
variable Reynolds number capability of this facility, however, will for the
first time provide the analyst a tool for understanding and quantifying the
factors involved in item (3) - the process of extrapolating the model data to
full-scale conditions. This capability is believed to be a very important
contribution.
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FOCUS
There was a consensus that the NTFwould be of outstanding value as a
propulsion aerodynamics research facility. The discussion of the panel was
focused in the following areas related to such usage:
(i) Identification of research emphasisand primary objectives
(2) Identification of special provisions, equipment, instrumentation,
etc. considered either necessary or desirable
(3) Identification and prloritization of specific propulsion aerodynamics
problems believed to merit investigation in the facility at Reynolds numbers
extending beyond present facility capabilities
(4) Identification of precursor research and studies which can and should
be undertaken prior to utilization of the facility for this type of research.
PROPULSIONAERODYNAMICSRESEARCHEMPHASISANDPRIMARYOBJECTIVES
The panel was in general agreement that emphasis in propulsion aerodyna-
mics investigations in the NTFshould be placed on the study of Reynolds num-
ber sensitive phenomena. It was felt that in addition to helping to clarify
data extrapolation problems, the data obtained would be of direct use to the
designer. Further, by permitting checks of theory against experiment at realis-
tic conditlons, the data would lead to the development of greatly improved ana-
lytical and theoretical methods. An important consensus of the group was that
one of the primary services of the tunnel will be that of providing a standard
for judging the capabilities and limitations of other propulsion aerodynamics
research facilities. There is no question but that facilities other than NTF
will have to carry the large bulk of propulsion aerodynamics research for the
foreseeable future; hence, it was emphasized that the capabilities and limita-
tions of these other facilities must be established reliably.
FACILITYANDEQUIPMENTCONSIDERATIONS
The panel was in agreement with the other panels and with the NTFprogram
personnel in the belief that the first order of business in the NTF is research
on the tunnel itself. In addition to the establishment of the operating enve-
lope, considerable effort must be devoted to achieving a high-quality test-
section flow (flow uniformity, turbulence and noise levels, etc.), to insuring
that the various tunnel interference and blockage effects have been minimized
adequately in the design and are predictable, and to achieving avery precise
tunnel calibration. In addition, the boundary-layer development at the cryo-
genic condition must be comparedwith boundary-layer development in flight at
similar Reynolds numbers. Transition, turbulence spectrum, separation,
166
reattachment, etc., all need to be studied in order to verify that the viscous
flows are similar at wind-tunnel and flight conditions. The panel was emphatic
on the need for this effort inasmuchas the forces, pressures, and viscous flow
development for somepropulsion models (especially Jet nozzle-afterbody models)
seemto be very sensitive to these factors.
Up to the present time, only limited attention has been given to utiliza-
tion of the NTFas a propulsion aerodynamics research facility. The only per-
tinent feature noted s_ecifically in the workshop presentations was the alloca-
tion of 50 cm2 (7.8 in z) of flow area in the standard sting for the piping of
internal flow gases at a pressure of 41.4 MN/m2 (6000 psi). This and many
other features require detailed study. Concernwas expressed, for example,
relative to the time required for model changes in the tunnel. Propulsion
aerodynamics studies, especially jet-exit-afterbody investigations, characteris-
tically require muchmore frequent tunnel entries than do more straightforward
aerodynamic tests. Unless tunnel entry time can be decreased to a major ex-
tent or the models can be automated to an as yet unprecedented degree (or both)
only very basic propulsion tests with very simple models will be practicable.
Certain equipment, in addition to that currently planned, was identified
by the panel as necessary to permit utilization of the NTF for propulsion re-
search. This equipment as an initial minimumincluded:
(1) Jet and secondary-flow gas supply systems
(2) Special support systems (for example, a "double hockey, flow-through
sting")
(3) "Flow-through" and other special propulsion balances
(4) Boundary-layer measurementinstrumentation
(5) Surface flow visualization equipment
Other equipment such as high-angle-of-attack stings or sting knuckles (up to an
angle of attack of 70° for fighter models), flow-fleld visualization equipment,
and dynamlc-flow measurementinstrumentation obviously is desirable and should
not be forgotten in equipment planning.
Provision of someof the listed items will necessitate rather extensive
research and development activities in their own right. Inherent in the gas
supply systems, for example, is an extensive research program needed to deter-
mine and validate jet-simulatlo n techniques for the special conditions en-
countered in the NTF. A similar effort may also he needed to establish techni-
ques for controlling and measuring internal model flows. It is understood that
research is either under way or planned to examine model surface finish require-
ments (hopefully, including effects of roughness, gaps, and steps) and to
establish pressure-orifice size requirements. Such information is necessary to
the efforts of the propulsion model designer.
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INITIAL PROPULSIONAERODYNAMICSSTUDIESPROPOSEDFORTHENTF
The three propulsion aerodynamics studies considered by the panel to be of
greatest interest for early implementation in the NTFare now discussed in
descending order of priority.
Effects of Reynolds Numberon Drag of Simple Afterbody Models
Incorporating Simulated Jets
As illustrated by sketch (a), current data from various facilities on the
effects of Reynolds numberon the drag of a given afterbody shape are often in-
consistent.
Pressure
drag
M=K
Reynolds number
Sketch (a)
The objective of the research would be to clarify the situation with considera-
tion given to such factors as body shape, exhaust-plume characteristics, and
effects of adjacent airframe components. The model proposed for investigation
is illustrated in the upper left corner of figure i.
The single jet model would be a simple body of revolution with an ogival
nose and with several afterbody shapes, including boattails with and without
flow separation. Provision would be made for the addition of tail surfaces.
Instrumentation would be provided to determine afterbody forces and surface
pressure distributions, forebody pressure distributions (to detect forebody
drag changes which might offset observed afterbody drag changes), and boundary-
layer separation and reattachment locations. Subsonic and transonic testing,
first as a pressure model and then as a force model, would be conducted jet on
jet off, and with solid jet simulators. Test results would be compared with
theory and with test results for the same models in other facilities. Such
comparisons would be of assistance in studying the wall-interference and
168
blockage-correction problems of the NTFand would provide someinsight regard-
ing flow quality effects. A further objective of the tunnel-to-tunnel compari-
son testing would be the establishment of criteria for facility selection for
propulsion aerodynamics testing. It is anticipated that one of the boattailed
body configurations eventually would be chosen as a reference standard calibra-
tion model similar to the current Supersonic Tunnel Association standard nozzle.
For later testing, after the highest priority tests have been completed,
the single-jet model could be modified into twin-jet and nonaxisymmetric models,
as illustrated in the center and bottom right of figure i. The conventional
twin-jet model would be used to study base and interfairing problems and jet-
to-jet interference problems. The nonaxisymmetric twin jet, in addition to
providing jet-shape-effects data, also would be used to study thrust-vectoring
and induced-lift effects.
Correlation of Propulsion Aerodynamics Test Data From Wind Tunnel
and Flight Tests At or Near Flight Reynolds NumberConditions
The preceding recommendedprogram would be expected to clarify the basic
effects of test Reynolds numberon the wind-tunnel to wind-tunnel propulsion
aerodynamics data correlation problem. It still will be necessary, however, to
close the loop by extending the study to a comparison of wind-tunnel and flight
data. This extension will require investigation in the NTF of propulsion
models of complete aircraft. As an initial step, it is proposed that some
]980-era fighter be selected because, by then it will already have been subject-
ed to extensive propulsive aerodynamics tests in other wind tunnels and in
flight. A fighter configuration is believed to be a better choice for the
study than either a bomberor a commercial transport because the model size of
its propulsion system for a given permissible model frontal area is muchgreat-
er than those for the other two classes of aircraft.
The first objective of the model tests in the NTFwould be to obtain
pressure-distribution data in model regions near the inlets and exits for the
exact configurations and the exact subsonic and transonic operating conditions
that have been explored previously in flight. Boundary-layer profiles at cri-
tical points on the configuration should also be compared. It is believed that
it would be satisfactory to delay very high angle-of-attack studies and the
procurement of overall drag correlation data to later phases of the program.
Pressure-distribution data would be obtained at lower than flight Reynolds num-
bers for wind-tunnel to wind-tunnel data correlation purposes.
Effects of Reynolds Numberon Inlet Transonic Drag
No data concerning the effects of Reynolds numberon inlet spillage drag
exist for either conventional or supercritical lip shapes. This
is an important deficiency inasmuchas all supersonic aircraft are being design-
ed currently with rounded rather than with sharp inlet lips. This information
deficiency will extend to the case of subsonic aircraft as progressive thinning
of the inlet lips accompaniesextension of the subsonic design cruise Machnum-
ber to values beyond about 0.9. The possibility exists that important perfor-
mancegains can be attained by optimizing the inlet-lip and afterbody shapes.
Such optimization requires experimental design data over a wide range of
169
fllght-level Reynolds numbers. An isolated drag model of a rectangular super-
sonic inlet is proposed for the initial study. Drag tests would be conducted
over the Machnumberrange from 0.7 to 1.2, over the Reynolds numberrange from
current wind-tunnel levels to NTFmaximumvalues, and over appropriate ranges
of mass-flow ratio and angle of attack. Pressure-distrlbutlon and flow-
visualization measurementswould be conducted separately at appropriate stages
of the investigation to stud_ flow phenomenaand to guide the inlet-lip deve-
lopment effort. A circular inlet also would be studied in a later stage of the
investigation. Special requirements for the investigation are: the develop-
ment of accurate flow through balances to function in the cryogenic high-
dynamic-pressure environment, definition of model surface finish requirements,
and accommodationof a large number (approximately 200) of pressure measure-
ments.
Additional Propulsion Aerodynamics Problems
Additional propulsion aerodynamics problems considered by the panel to
have merit for future investigation in the NTFwere:
(I) Effects of Reynolds numberon inlet-flow distortion (consideration
given to forebody flow fields, inlet lip shapes, internal contours, etc.)
(2) Exploration of propulsive lift concepts (cruise and maneuvercases
with wing in influence of jet)
(3) Study of effects of Reynolds dumber on jet-flow-field interactions
(4) Study of close-coupled inlet-exit systems.
PRECURSOREFFORT
It was the strong opinion of the panel that a large amount of precursor
research and engineering development work needs to be undertaken now so that
the NTFcan be utilized for propulsion aerodynamics research within a reason-
ably short time aster it is placed in research operation. A good example is
the need for studies to determine the requirements for achieving valid jet
simulation. Other examples are mentioned in the foregoing discussion. The
consensus of the panel was that if such work cannot be done in-house within a
suitable time frame, outside assistance should be enlisted through the medium
of suitable contracts or grants.
170
f i le
Figure i.- Basic research models.
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The panel on propulsion aerodynamics

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