I I-

0 AL-AERO-PROP-TM-458 AR-005-583

DEPARTMENT OF DEFENCE
DEFENCE SCIENCE AND TECHNOLOGY ORGANISATION
AERONAUTICAL RESEARCH LABORATORY
MELBOURNE, VICTORIA

Aero Propulsion Technical Memorandum e458

AERODYNAMIC MODEL TESTS OF EXHAUST AUGMENTORS

FOR F/A-18 ENGINE RUN-UP FACILITY AT
RAAF WILLIAMTOWN - SUMMARY REPORT (U)

by

S.A. Fisher and A.M. Abdel-Fattah

Approved for Public Release

DTIC
_S ErE' TF

MY231989
(C) COMMONWEALTH OF AUSTRALIA I 8
DECEMBER 1988

C':- 099
 AR-005-583

DEPARTMENT OF DEFENCE
DEFENCE SCIENCE AND TECHNOLOGY ORGANISATION
AERONAUTICAL RESEARCH LABORATORY

Aero Propulsion Technical Memorandum 458

AERODYNAMIC MODEL TESTS OF EXHAUST AUGMENTORS FOR

F/A-18 ENGINE RUN-UP FACITY AT RAAF WILLIAMTOWN -
SUMMARY REPORT [UI

by

S.A. Fisher and A.M. Abdel-Fattah

SUMMARY

Model tests of the air cooled exhaust augmentors proposed for the
F/A-18 engine ground run-up facilities at RAAF Williamtown were
undertaken, to confirm satisfactory aerodynamic behaviour of the augmentor
designs and to provide data for optimising certain aspects of the designs. The
tests were carried out on 1145 scale models, using an unheated air jet to
represent the engine exhaust. Geometric features were identified which had
important influence on augmentor duct flow symmetry and the cooling flow
augmentation ratio.

DSTO
MELBOURNE

POSTAL ADDRESS: Director, Aeronautical Research Laboratory,

P.O. Box 4331, Melbourne, Victoria, 3001, Australia
 CONTENTS

1. Introduction .................................................. 1

2. Test Models and Instrumentation ................................ 1

3. Interpretation of Test Results ................................... 2

4. 'Installed' Model Tests ......................................... 2

4.1 Jet Alignment

4.2 Effect of Augmentor Duct Roof Cutback

4.3 Primary Augmentor Tube

4.3.1 Lateral Flow Distribution

4.3.2 Adopted Configuration

4.3.3 Augmentation Characteristics

4.3.4 Sensitivity to Vertical Jet Misalignment

4.4 Effect of Inlet Fairings

5. 'Uninstalled' Model Tests ...................................... 6

5.1 Flow Symmetry

5.2 Effect of Augmentor Duct Roof Cutback

5.3 Inlet Fairings I For

ed
6. Conclusions ................................................. 7 L101

r st ri t I on/

A.Vallabiy Codes'
Dist AvaSpcal
- an /or
 (1)

1. Introduction

This report summarises the results of model tests of the proposed exhaust
augmentors for both the 'installed' and 'uninstalled' engine run-up facilities being
designed for the F/A-18 aircraft at RAAF Williamtown. The purpose of the tests
was to pro,,idc data for optim!n" detailed aspects of the aerodynamic design o the
two facilities. The scope of the tests was agreed with Australian Construction
Services (ACS), and the work was undertaken in response to ACS Purchase Order No
S21/89PD dated 23 November 1988.

2. Test Models and Instrumentation

The basic configuration and dimensions of the 'installed' model were based on
drawings tabled at the project review meeting on 15 September 1988, reproduced in
outline together with leading full scale dimensions in Figure 1. The secondary inlet
dimensions were supplied by Challis and Associates on 6 October 1988, and primary
augmentor tube dimensions were supplied on 11 November 1988.

The basic form of the 'uninstalled' model, which was constructed earlier, was
based on a drawing dated 26 July 1988, reproduced in outline in Figure 2. Primary
tube dimensions were not available, and were scaled from the drawing. The shape of
the secondary inlet ducts was different from that which appeared on subsequent
drawings, in that 'kinks' which were later added to the aft walls of the ducts,
adjacent to the acoustic splitter trailing edges, were not included in the model.

Photographs of the two models are shown in Figure 3. The 'installed' model is
seen in partly modified form, with bellmouth fairings on the secondary air inlets,
while the 'uninstalled' model appears in original, unmodified form. Both models
were 1/45 full scale, constructed mainly from timber and glass. In the vicinity of the
air inlets, only those features thought to be directly relevant to the augmentor duct
internal flow were represented on the models; the test bay structures, for example,
were not included.

The engine exhaust jet was simulated with unheated air issuing from a
convergent-divergent nozzle of correct geometrical scale. The nozzle blowing
pressure was 340 kPa, providing a pressure ratio equal to that of the exhaust nozzle
of the F404 engine operating in the full afterburner mode, so that the Mach number
at the nozzle exit plane and the scaled jet momentum were both similar to the full
scale jet. All tests were carried out with an arrangement representing single engine
operation in full afterburner. The jet nozzle was positioned with its exit plane
approximately 1.4 m (full scale)* upstream of the commencement of the parallel
section of the primary augmentor tube.

Air velocity measurements were made in the exit planes of the main ducts
using a traversing pitot-static tube aligned with the duct axis. The pressures were
measured with strain gauge transducers and recorded on an X-Y plotter. In some
cases measurements were also made further upstream in the ducts, but the results
are not reported .here. Likewise, wall static pressure distributions measured with

* All dimensions quoted in this document relate to the full scale facilities
 (2)

flush tappings in the duct rooves are not included in this report, Flow visualisation
with optical schlieren apparatus and wool tufts was also used in the investigation.

3. Interpretation of Test Results

Notwithstanding the correct geometric scale of the models, only loose

simulation of the mixing flows in the full scale augmentor ducts was possible with
the unheated air jet. The duct exit velocity distributions presented below therefore
give only a qualitative indication of the behaviour of the flow in the full scale
facilities.

A primary measure of augmentor effectiveness is the cooling flow

augmentation ratio, defined as the ratio of the total entrained air mass flow to the
jet mass flow. The values presented in this report are as calculated for the model
tests with the unheated air jet. According to theory backed by empirical experience,
taking account of differences in gas properties as well as temperature, the 'cold'
augmentation ratios may be translated to the full scale, hot jet situation by
multiplying by a factor of approximately two. This factor takes no account of
differences, between model and full scale, of such effects as internal skin friction,
but these would be expected to be of second order importance.

4. 'Installed' Model Tests

4.1 Jet Alignment

In the 'normal' test configuration for the 'installed' model, the jet was aligned
to represent single installed engine operation. This is shown schematically in Figure
4. For investigative purposes some tests were also conducted with different jet
alignments.

4.2 Effect of Augmentor Duct Roof Cutback

Initial tests indicated that the exhaust deflector ramp which appears in Figure
1 adversely affected the velocity distribution at the duct exit plane, and probably
imposed unnecessarily high back-pressure on the duct. Figure 5 shows the nature of
the modification which was investigated, in this case with the jet central and coaxial
with the augmentor duct. Figure 6 shows the effect on the distribution of flow
velocity at the duct exit plane indicated in Figure 5 (which was effectively moved
1.75 m upstream in terms of full scale dimensions) calculated from measurements
with the traversing pitot-static pressure probe. In taking these measurements, as
with all of the velocity measurements at the duct exit, it was recognised that there
may have been a degree of misalignment of the flow relative to the probe, due to the
influence of the deflector ramp. However, the designed tolerance of the probe to
misalignment was such that this should not have had a substantial effect on the
results.

Figure 6 shows that there was a marked improvement to the exit plane velocity
distribution when the roof was shortened, as well as to the augmentation ratio
calculated from the measured velocities which is also shown in the Figure. All
subsequent results apply to the modified roof configuration.
 (3)

4.3 Primary Augmentor Tube

Numerous tests were carried out to investigate the effect of the primary
augmentor tube and to identify the optimum configuration. The results are
presented here in essence only.

4.3.1 Lateral Flow Distribution

Figure 7 shows velocity profiles measured on the mid-height horizontal plane at

the exit of the main augmentor duct, with 'normal' jet alignment, with a primary
tube of 1.5 m x 2.4 m obround cross section and four different lengths. For all but
the very shortest 'tube' (of length equal to the thickness of the wall in which the
tube was mounted) the lateral location of the peak velocity at the main duct exit was
displaced a significant distance from the projection of the nozzle centreline. This
apparently resulted from lateral angular deflection of the jet in the primary tube,
caused by interaction between the tube wall and the asymmetrically disposed jet.
This effect is revealed in the schlieren photograph in Figure 8, showing the jet being
deflected in its passage through an obround tube in isolation.

With the longer tubes the degree of jet core displacement observed in the main
augmentor duct would pose an unacceptable risk of wall overheating in the full scale
facility. On the other hand, jet/primary tube interaction is arguably favourable in
the case of the 1.3 m length tube, since the jet core is deflected a small degree from
the projection of the nozzle axis, so as to become approximately centralised at the
main duct exit. Selection of this primary tube geometry for the full scale facility on
the basis of this evidence would involve an implied assumption that the effect of
jet/primary tube interaction is the same for the hot, full scale jet as with the cold,
small scale jet. This is uncertain, although probably not a dangerous assumption.

The effect of increasing the cross-sectional width of an obround primary tube

of length 3.1 m is shown in Figure 9. Some benefit is apparent, but jet/tube
interaction remains substantial.

Lateral velocity profiles are shown in Figure 10 for two geometrically similar,
relatively short primary tubes of different cross-sectional size. As might be
expected, the degree of jet deflection increased when the cross-sectional size was
reduced so as to bring the jet into closer proximity with the primary tube wall.

4.3.2 Adopted Configuration

If a primary augmentor tube is to be used, tube dimensions chosen on the basis

of the above results could be as shown in Figure 11. Filling some of the airspace
around the outside of the tube within the inlet enclosure, as shown in the Figure,
should favourably reduce the scale of the recirculation associated with the separated
flow in that region. This configuration was adopted for further investigation of the
merits of using a primary tube, outlined below.

4.3.3 Augmentation Characteristics

Figure 12 compares the main duct exit velocity distribution measured in the
presence of the adopted primary tube arrangement with that obtained when no tube
was in place. The results shown actually apply to a configuration with secondary
inlet bellmouth fairings added (see below), since it was only with this arrangement
 (4)

that full sets of data were available to make this comparison. The curves reproduce
the benefit in lateral flow symmetry due to the short primary tube which was
apparent in Figure 7 and mass flows calculated from the velocity distributions
indicate a marginally superior augmentation ratio with the tube in place.

4.3.4 Sensitivity to Vertical Jet Misalignment

Further tests were carried out to determine whether the adopted primary tube
arrangement would unreasonably exacerbate the effect of inadvertent vertical
misalignment of the engine nozzle, due to jet/tube interaction similar to that
observed in the horizontal plane.

Figure 13 compares velocity profiles on the vertical centreline at the main

duct exit, measured with and without the primary tube in place. It should be noted
that, for the 'no tube' configuration in particular, the centrel'ije traverse plane did
not coincide with the lateral location of maximum exit velocity (see Figure 12).
Flow profiles are shown:

(a) for 'normal' jet alignment,

(b) with the jet horizontal but displaced downwards by 100 mm (full scale),

(c) with the nozzle displaced downwards by 100 mm and also tilted downwards
by an amount (approximately 1 degree) which would apply had the 100 mm
displacement been caused by rotation of the aircraft about its main
undercarriage.

The effect of each level of misalignment on the centreline velocity profile was
not significantly affected by having the short primary tube in place, and in each
case approximated the effect which might have been qualitatively estimated on the
basis of simple geometric projection of the jet nozzle axis.

4.4 Effect of Inlet Fairings

On the basic model it was observed by the use of tufts that gross flow
separations occurred at the upstream edges of the secondary inlets, and also at the
vertical corners where the secondary inlet ducts were integrated with the main
augmentor duct. These flow features, shown diagrammatically in Figure 14(a), could
be expected to increase the aerodynamic losses and reduce the augmentation ratio
and, arguably, increase the tendency to internal flow asymmetry. Modifications
investigated to improve these aspects were bellmouth fairings on the secondary
inlets (Figure 14(b)) and fairing of the internal vertical corners (Figure 14(c)).

The results are shown in Figure 15. These tests were conducted with no
primary tube in place and with 'normal' jet alignment, so all of the illustrated exit
flow distributions feature the residual lateral asymmetry which was characteristic of
this arrangement. The bellmouth fairings, which from visual observation clearly
improved the qualitative nature of the flow in the inlet ducts, also significantly
increased the maximum velocity registered near the centre of the main duct exit.
There was also a compensating tendency towards reduced velocity near the side walls
so that, on balance, there was only a modest improvement in augmentation ratio.
 (5)

Adding the internal corner fairings further increased the augmentation ratio,
presumably be reducing or eliminating the flow separation at those corners. It
appears likely that the effect of the bellmouth fairings on the internal flow was such
as to increase the tendency towards flow separation at the internal corners, so that
the need for corner fairings went hand in hand with addition of the bellmouths. This
apparent aerodynamic coupling between the effects of the two features may be
worthy of further investigation.

5. •'Uninstalled' Model Tests

5.1 Flow Symmetry

In the 'uninstalled' facility the jet would be expected to be always aligned

coaxially with both the primary tube and main augmentor duct. Notwithstanding
this, early model tests with a primary augmentor tube 1.3 m (full scale) in diameter
and 2.3 m long showed that:

(a) angular misalignment of up to 0.50 caused flow asymmetry at the main

duct exit similar to that which might be predicted by linear projection of
the nozzle axis,

(b) the symmetry of the exit flow was much more sensitive to srmall lateral
displacements of the nozzle from its correct coaxial location.

These observations were consistent with the more detailed quantitative

measurements of similar effects in the 'installed' model described in Section 4.3
above. In view of this, and in order to minimise sensitivity to any inadvertent
misalignment of the engine in the 'uninstalled' facility, it was thought to be logical
to adopt a primary tube of corresponding proportions to that described in Section
4.3.2 above. A tube 1.3 m in diameter cut back to 1.2 m in length was used for most
subsequent tests, the smaller diameter (relative to the 1.5 m vertical dimension
adopted for the 'installed' model primary tube) being:

(a) preferred in order to reduce the entrained air flow through the primary
tube and minimise the air velocities over the engine test stand

(b) probably tolerable in this basically coaxial facility.

The main duct exit flow distribution and augmentation ratio with this
arrangement are shown in Figure 16(a).

5.2 Effect of Augmentor Duct Roof Cutback

Figures 16(a) and (b) show that, as with the 'installed' facility, the
augmentation ratio in this model increased significantly when the exit plane was
effectively moved 1.75 m (full scale) upstream by shortening the duct roof. The exit
flow distribution also improved, although there remained a noticeable degree of flow
distortion in the vertical plane. Flow surveys taken further upstream (not shown
here) indicated that this residual distortion was still caused mainly by upstream
influence of the exhaust deflector ramp, so there may be scope for even further
reduction of the roof length if this is acceptable from the acoustic viewpoint.
 (6)

5.3 Inlet Fairings

In this model, inlet bellmouths and fairing of the vertical corners at the entry
to the square augmentor duct were added simultaneously. It was assumed that there
would be a degree of aerodynamic coupling between the effects of these features, as
was observed in the 'installed' model, althougi; the presence of acoustic splitters in
the secondary inlet ducts of this model may well have weakened this effect. The
geometric nature of the modifications was similar to those applied to the 'installed'
model, shown in Figure 14. Comparison of Figures 16(b) and 16(c) shows that the
bellmouths and fairings further improved the augmentation ratio although, as might
have been expected because of the acoustic splitters, by a lesser amount than was
observed in the 'installed' model.

6. Conclusions

(a) The primary augmentor tube proposed in the original design of the
'installed' facility interacted with the asymmetrically positioned jet to
cause substantial lateral flow asymmetry in the main augmentor duct.

(b) Increasing the width of the primary tube improved but did not correct the
flow asymmetry.

(c) The flow asymmetry increased with reduced primary tube cross-sectional
dimension and with increased primary tube length.

(d) A configuration with relatively short primary tube length was identified
which, at least with an unheated jet at model scale, interacted favourably
with the jet to yield a symmetrical main duct flow distribution, and which
was tolerant of modest degrees of angular, lateral and vertical jet
misalignment.

(e) A design with no primary augmentor tube at ail would aibo bIc cpable
but marginally inferior aerodynamically.

(f) A proportionate reduction in length of the primary augmentor tube

initially proposed in the 'uninstalled' facility should also provide improved
tolerance to inadvertent jet misalignment in that facility.

(g) Reducing the length of the main augmentor duct roof by 1.75 m (full
scale), thus effectively moving the duct exit plane upstream by that
distance, significantly improved the exit flow distributions and
augmentation ratios in both the 'installed' and 'uninstalled' models. It
appeared that further reduction in the roof length may well have been
beneficial aerodynamically, at least in the 'uninstalled' model.

(h) Adding bellmouth fairings to the secondary inlets, and fairings to the
vertical corners at the main augmentor duct entry, improved the quality of
the inlet flow and the augmentation ratio in both models. Aerodynamic
coupling between these features made the latter modification an
important adjunct to the former, especially in the 'installed' model.
 (7)

(i) The cooling flow augmentation ratio was measured to be in the range 5.5 -
6.0 in the 'installed' model and 4.5 - 5.0 in the 'uninstalled' model. These
figures theoretically translate to approximately double these values for
the full scale facilities, with a single engine operating in the full
afterburner mode.
Prim~jry Augmentor T,,b,

I:EVAT1( t4

?"N V 1 nI U 'j~
 Primary Augme-tor Tube Main Duct

3.5 m

. . . . -- -. - -

ELEVATION

Engine On Test Stand

24. 2 m

3.5 m

tAN
F0:Oil
\ I I LtE

I(. tUIN S TAlI I EN(.INE PUN-[J|l t.ACILITY

 (a) 'INSTALLED' FACILITY

(b) 'UNINSTALLED' FACILITY

FIG. 3 PHOTOGRAPHS OF TEST MODELS

 Duct
Centreline

0.454 m

Nozzle
Centreline
I

FIG. 4 JET ALIGNMENT IN 'INSTALLED' MODEL

Modified 1.75 m | O ~Original

g a Dsg

Plane of
Measurement DelcoI~m

FIG. 5 MODIFICATION TO AUGMENTOR DUCT ROOF

 ORIGINAL DESIGN
00 85
L8
'Cold' augmentation

ratio =5.04

Local Velocity at
Exit Plane rn/s

MODIFIED ROOP

'Cold' Augmentation
0 90 Ratio = 5.67

FIG. 6 EFFECT OF ROOF MODIFICATION ON EXIT FLOW DISTRIBUTION - UNINSTALLED

FACILITY
 W 2.4 m H = 1.5 m (Full Scale)

O L = 3.1 m
o L = 2.2 m
AL 1. 3 mn
* L 0.5 m

Location of Jet
00 Nozzle Axis Projection

9 90

80

0 70
4)

46

o50

40

LL

Main Duct Width

Primary
Tube W
Dimensions

FIG.7 EFFECT OF PRIMARY TUBE LENGTH ON LATERAL VELOCITY PROFILES -

'INSTALLED' FACILITY
 Jet Nozzle
Axis

Augmentor
Tube

Jet Nozzle

FIG. 8 SCHLIEREN PHOTOGRAPH OF JET WITH PRIMARY TUBE IN ISOLATION-

'INSTALLED' rACILITY
 Nozzle Axis

0. 90

- 80
4j

4 70
U
0
41

, 50
40-

2?
Duct Width

L 3.1 m (See Fig. 7)

IA 1.5 m

O W = 2.4 m
El W = 2.8 m

WIDTH - 'INSTALLED' FACILITY

FIG. 9 EFFECT OF PRIMARY TUBE
 S100

Nozzle Axis

-~90

41

4280

a)
-4

,c 60
17,

50

40

Duct Width

W L
0 2.2 m 1.3 m 1.1 m (See Fig. 7)
~J2.4 m 1.5 m 1.3 m

FIG. 10 EFFECT OF PRIMARY TUBE SCALE - 'INSTALLED' FACILITY

 4.1 -

2. 4

Filled Space

1.3 in\\ \

FIG. 11 ADOPTED PRIMARY TUBE ARRANGEMENT ' INSTALLED' FACILITY

 ADOPTED PRIMARY TUBE
L = 1.3 m H = 1.S m
W = 2.4 m
(See Fig. 7)
100

0'Cold' augentation
8 7ratio = 5.77

Local Velocity at
Exit Plane m/s

NO PRIMARY TUBE
L = 0.5 m H = 1.5 m

W = 2.4 m

'Cold' Augmentation
Ratio = 5.64

60

FIG. 12 EFFECT OF ADOPTED PRIMARY TUBE 'INSTALLED' FACILITY

 No Primary Tube

---- 'Adopted' Tube

Arrangement
(a) 'Normal' Jet
Alignment

1l I I I I I

Height of
Nozzle Axis '

Projection at
Exit i Plane
P(b)
Nozzle Displaced
Downwards 100 mm
Duct Height /

I iI I I

(c) Nozzle Displaced

Downwards
and Tilted 100
10 mm

40 50 60 70 80 90 100

Centreline Velocity at Exit Plane m/s

FIG. 13 EFFECT OF VERTICAL MISALIGNMENT - 'INSTALLED' FACILITY

 AA
q

(a) FLOW SEPARATIONS IN UNMODIFIED DESIGN

B 0. 56 r ad

Section A-A Section i-B

(b) BELLMOUTII FAI RINGS

(c) DUCT CORNER .A I RI NG,!;

IG. 14 INLET MODIFICATIONS - "INSTALLED' FACILITY

 NO FAIRINGS

80 'Cold' Augmentation Ratio 5.52

70

60

Local Velocity at
Exit Plane m/s

INLET B3ELLMOUT1IU; FI TTED

((a(( 'Cold' Aumentation Ratio - '.64

50
50

INLET BELLMOUTIH;; AM' FAIRI.D

DUCT CORNER;

,)) 'Cold' Augment~ition Ratio -. 83

80
7o

510

F 1G. 15 EFFECT OF INLET MODIFICATIONS (NO

( INNACY
TUBE) - 'INSTALLED' FACILITY
low

(a) SHORT PRIMARY TUBE

90

80 'Cold' Augmentation Ratio 4.40

Local Velocity at
Exit Plane rn/s

11)0)

~~
171(4 77'C ~ ~
F1101 ~
~ CTIo (, ORUNTENETA DUCT1) F1)L
SH
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A.M. Abdel-Fattah

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AR-005-583 ARL-AERO-PROP-TM-451 DECEMBER 1988 AIR 99/040

4. TITLE 5. SECURITYC.ASSIFICATION 6. NO. PAGES

AERODYNAMIC MODEL TESTS OF EXHAUST (PLACE APPRIATE aAS IFICATION
AUGMENTORS FOR F/A-18 ENGINE RUN-Up IN BOC(S) 1R.SEoaff (S), O0r.(C) 24
FACILITY AT RAAF WILLIAMTOWN -
SUMMARY REPORT
RESTRICTED (R), tUNCASIFIED (U) ),
[1 [_] 7. W NO. RES.

DOUEN TITL= ABSTRACTr

8. NI7XI(s) 9. DCWlmADIwG/DELLDUTaG nemicr1o~

S.A. Fisher and Not applicable.
A.M. Abdel-Fattah

10. CPATE AIHOR AND ADDRES 11. OFFICE/PCSITION R FOR:

ISPCARTLE
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AERONAUTICAL RESEARCH LABORATORY
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14. DESCRIPTIRS 15. DRDA SUBJECT

CATEGOIES

Exhaust systems 0081D

F/A-18 aircraft
General Electric .f404 engine
Aerodynamic characteristics

16. ABSTRACT
--" Model tests of the air cooled exhaust augmentors proposed for the F/A-
18 engine ground run-up facilities at RAAF Williamtown were
undertaken, to confirm satisfactory aerodynamic behavio~r of the
augmentor designs and to provide data for optimising certain aspects
of the designs. The tests were carried out on 1/45 scale models,
using an unheated air Jet to represent the engine exhaust. Geometric
features were identified which had important influence on augmentor
duct flow symmetry and the cooling flow augmentation ratio. /
 SPAGECLA IFI CATION

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THIS PAGE IS TO BE USED TO RECORD INFORMATION WHICH IS REQUIRED BY THE ESTABLISHMENT FOR
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16. ABSTRACT (OW.)

17. IMPRINT

AERONAUTICAL RESEARCH LABORATORY, MELBOURNE

18. DocuH9XI SEIES AND MN R 19. C OC)E 20. TYPE OF REPORT AM PERIOD

AERO PROPULSION TECHNICAL 42 4894

MEMORANDUM 458

21. COMPUTERPROGRAMS USED

22. ESTABLISHMENT FILE REF.(S)

23. ADDITIONAL INFORMATION (AS REQUIRED)