Hearth, Donald P

Perchonok, Eugene

English

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NATIONALADVISORYCOMMI’ITEE
TECHNICAL NOTE 3834
EFFECT OF AMBIENT -TEMPERATURE VARIATION ON THE
MATCHING REQUIREMENTS OF INLET -ENGINE
COMBINATIONS AT SUPERSONIC SPEEDS
By Eugene Perchonok and Donald P. Hearth
Lewis Flight Propulsion Laboratory
Cleveland, Ohio
Washington
January 1957
.-
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NATIONAL ADVISORY COMMITTEE FOR
TECKNICAL NOTE 3834
AERONAUTICS 0Cl&L75L
EFFECT OF AMBIENT—TEMPERATUREVARIATION ON THE MATCHING REQUIREMENTS
OF INLET-ENGINE COMBINATIONS M! SUPERSONIC SPEEDS
By Eugene Perchonok and Donald
SUMMARY
The effect of ambient temperature on the
P. Hearth
matthing requirements of
inlet-engine combinations has been analyzed for two typical turbojet en-
gines up to a Mach number of 3.5. The changes in ambient temperature
ordinarily encountered in flight can msrkedly influence the performance
of matched inlet-engine combinations for engines operated at constant
mechanical speed. Up to 30 percent of the capture mass flow may be
spilled at the design Mach number because of typical variations in the
ambient temperature. Thus, to avoid subcritical operation, the use of
variable inlet features sre required even at the design Mach number.
If features such as a bypass or a movable compression surface are to
.
be used for efficient inlet-engine matching, the inlet should be sized
for the lowest ambient temperature to be encountered. The capacity of
such variable features must be appreciatively increased over that for a
* constant ambient temperature to prevent subcritical operation during off-
design temperature operation. Moreover, the capacity of certain matching
techniques may be inadequate if both variations in free-stream Mach num-
ber and ambient temperature are considered.
INTRODUCTION
When aircraft are operated over an altitude and climatic range, large
variations in ambient temperature are encountered (ref. 1). In addition,
a large variation in air temperature during a given season of the year is
not unusual, even at the same altitude and geographical location. This
variation may approach 80° F and can smount to a 25-percent change in ab-
solute temperature. Although the effect of ambient temperature on
turbojet-engine operation and performance is recognized, a “standard day”
is usually assumed in order to simplify the procedure for matching the
airflow characteristics of a supersonic inlet to those of a turbojet en-
gine (e.g., ref. 2}. If, as is present-day engine design practice, the
engine operates at constant mechanical speed, any variation in compressor-
.
inlet temperature causes a change in the corrected engine airflow rate,
sffects the engine performance, and upsets the match point of the engine
w’ and inlet.
2 NACA TN 3834
It might be expected that all turbojet aircraft would suffer or bene-
fit from ambient-temperaturechanges in a like manner and that no relative_
advantage exists for any particular aircraft. However, if positive inlet
design measures are tsken, the off-design-temperatureinlet-engine match-
ing can be improved. It is apparent that certain of these measures may be
more effective than others.
Considerable effort is being expended on the problem of efficient
inlet-engine matching at off-design Mach _numberconditions. Techniques
developed for matching at off-design Mach numbers include vsriable inlet _
geometry and bypass air systems. Since the problem of matching as Mach
number changes is similar to that due h ambient-temperaturechanges}
consideration should be given to utilizing the ssme techniques for both
matching problems. This report evaluates the magnitude of the effects
of ambient-temperaturechanges on the matching requirements for two typi-
cal turbojet-engine airflow characteristics and examines the possibility
of utilizing the usual,inlet-enginematching techniques when, in addition,
a change in smbient temperature is encountered.
WMBOLS
A
Ac
M
f(MJ
‘e/mo
%/%
P
P/P.
to
At.
w
area, sq ft
inlet cowl area, sq ft
Mach number
engine mass-flow ratio
spillage mass-flow ratio
total pressure, lb/sq ft
inlet total-pressure recovery
ambient temperature, OR
.
l -


U
—
—
.
P
—
(t. - 392), OR
airflow, lb/see
NACA TN 3834 3
.
r ratio of specific heats
F 5 ratio of total pressure to NACA standard sea-level pressure of
2116 lb/sq ft
e ratio of totsl temperature to NACA standard sea-level temperature
of 519° R
Subscripts:
e engine
o free strem
Superscript:
* conditions at Mach number of 1
RESULTS AND DISCUSSION
Anibient-temperatureconditions are generally represented by one or
more of the many “standsrd days.” These typical temperature variations
wtth altitude have been determined statistically from weather observa-
.
tions such as in reference 1. A frequently wed reference day is the
NACA standard day (fig. 1), which has the temperature decreasing linearly
. from sea level to 35,000 feet and then constant at 392° R from altitudes
between approximately 35,000 to 100,000 feet. The two additional stand-
ard days shown on figure 1 reasonably represent the extremes which may be
expected (ref. 1).
Changes in ambient temperature occur not only as altitude and geo-
graphical location change but &Lso vsxy from day to day even during the.
ssme season of the yesr. The maximum and minimum summer temperatures ob-
served at the ssme altitude (52,600 ft) at seversl localities axe shown
in figure 2. At San Antonio, Texas, for example, temperature variations
up to 600 F would be encountered. Obviously, larger temperature changes
will be encountered if, in addition, altitude, season, and location sre
varied.
The effect of inlet-air temperature on turbojet-engine performance
is well lmown. If, for any psrt of its operation, the engine must be
maintained at a constant mechanical speed, the corrected engine airflow
till v~y fith c-es in ambient temperature. ‘Thus, even without a
change in airspeed, verying tiflow demands are made on the supersonic&
inlet supplying engine air. Without variable geometry the inlet operat-
ing condition can, therefore, deviate greatly from the design point.
f
4 NACA TN 3834
The variation of the corrected engine speed psrameter l/@ Wltll “ –
free-stream Mach number is presented in figure 3. This parameter repre-
sents the percent of corrected engine speed at rated mechanical speed u
and determines the corrected airflow the inlet must deliver. A family
of ambient temperatures is shown in terms of the deviation in degrees
Fahrenheit from the standard NACA temperature above 35,000 feet (392° R).
It is quite appsrent that if a turbojet engine is limited to a constant
mechanical speed, corrected-engine-speedchanges approaching 10 percent
can often be encountered, even at a constant flight Mach number. With a
constant-corrected-speeddesign this problem is eliminated and the inlet ~
and engine airflows remain properly matched.
The severity of the change in corrected engine speed due to ambient-
temperature variation on the inlet airflow required is a function of the
particular engine airflow characteristics. Two different engine airflow
curves (for hypothetical engines designated as A and B) have been assumed
and since only the relative slope of the curve is important for the prob-
lem being considered, the curves are shown in nondimensional form in fig-
ure 4. These assumed curves are representative of current engine designs
with most of the current engines falling close to the curve for engine B.
—
Figure 5 shows the effect of ambient temperature on the corrected
engine airflow yi.ththe engines of figure.4 operated at constant mechani-
cal speed. For temperatures below 392° R (negative Ato} the engine would
require more corrected airflow than at anambient temperature of 392° R
since the corrected speed is increased. On the other hand, positive At.
would result in a decrease in the airflow required. Because the airflow l
curve for engine B has a greater slope than that for engine A, engine B
is more sensitive to changes in the smbiegt temperature. Nevertheless,
with either engine an inlet must be capable of supplying a large range of
corrected airflow at constant Mach number. Unless vsriable geometry is
utilized to provide this required airflow range, serious deviation from
the inlet design point will result.
It can be shown that
P/P. .
“’w (1)
Based upon this relation, the effect of ambient temperature on the inlet
operating condition has been computed and is presented in figure 6. For
convenience the subcritical diffuser efficiency was assumed equal to the
critical and design value. If the inl?t does not include features de-
signed to compensate for ambient-temperaturevariations, temperatures be- ‘ b -
low design will result in supercritical.operation (fig. 6(a)), while tem-
peratures above design will.result in subcriticaloperation (fig. 6(b)). ~
NACA TN 3834 5
Since at 60)000 feet the ambient temperature may be as much as 58° F (see
fig. 1) less than the reference value (392° R), the inlet msy be forced
r to operate supercritically and, for engine B, with as much as a 20-percent
reduction in pressure recovery. ‘At the same time the velocity distortion
at the compressor face would be increased considerably over the value at
critical inlet operation. On the other hand, at this altitude the ambient
temperature msy reach 33° F higher than the reference value. If a con-
stant subcritical inlet pressure recovery were assumed, mass-flow spillage
reaching 8 to 12 percent of the supercritical mass flow would result. If,
as is usually the case, the pressure recovery decreased subcritically, the
spillage would be even greater, increasing the same percentage aE the de-
crease in recovery (see eq. (l)). This spillage causes a large increase
in the external drag and may put a fixed-geometry ifiet into buzz. Be-
cause of these serious departures from design-point inlet operation} it
would appear that, in addition to variable geometry for Mach number match-
ing, inlets must be provided with features designed to compensate for
ambient-temperature changes at a given free-stream Mach number.
In order to illustrate the effect of anibient-temperaturevariations
on inlet design, the capture area and spillage requirements have been
computed for the two assumed engines up to a free-stresm Mach number of
3.5. A constant inlet kinetic energy efficiency of 0.92 at design w%
assumed. Three values of ambient temperature were considered, 334°, 392°,
.
and 425° R. These conditions correspond to the range in temperatures en-
countered at an altitude of 60,CX)0feet (fig. 1).
. The inlet capture srea required for critical inlet operation is pre-
sented in figure 7. The lowest ambient temperature requires the largest
inlet. For example, at a Mach number of 3.5 the inlet for engine A would
have to be 20 percent larger on an Air Force cold day than on an Air Force
hot day. For engine B the inlet would have to be 44 percent larger. It
is interesting to note that engine B, which has the smaller change in
capture area with flight Mach number, has the larger change with ambient
temperature. Both of these effects are due to the steeper slope of the
engine airflow curve.
If the inlet does not have a variable capture area, it is usually
sized for the flight condition requiring maximum area. At off-design
conditions the excess inlet flow mJ~ is spilled subcritically or by
means of a translating compression surface or a bypass.
With the inlet sized at a free-stream Mach number of 3.5 for the
NACA standard day above 35)~ feet (ambient temperature, 392° R), the
spillage at the three temperatures would be as shown in figures 8(a) and
. (b). For ambient temperatures above 3920 R the off-design spillage is
increaaed. For temperatures below 392° R, the off-design spillage is
reduced, and unless additional airflow were supplied, the inlet would be
J forced to operate supercritically over at least a part of the speed spec-
trum. Since supercritical operation is generkll.yundesirable because of
6 NACA TN 3834
.
reduced pressure recovery and increased velocity distortions at the com-
pressor face, it is concluded that the inlet should be sized for the cold
day (figs. 8(c) and (d)). When so sized, the hot-day spillage is in-
creased. However, calculations indicated that the thrust penalty due to
the increase in estimated bypass spillege drag is less than that asso-
ciated with cold-day supercritical operation of a standard-day inlet
design.
Thus, when ambient-temperaturevariations are considered in the de-
sign of an inlet employing variable geometry for inlet-enginematching,
the inlet should be sized for the lowest temperature to be regularly en-
countered. The capacity of the spillage system employed should then be
selected for the maximum ambient temperature anticipated. These addi-
tional design considerations significantly influence inlet sizing and
operation. For exsmple, capture-area increases approaching 25 percent
(fig. 7) and spillage capacity increases between50 and 100 percent (fig.
8) are indicated.
Figure 8 represents the total mass-flow spillage required, and in-
cludes any spillage ahead of the inlet lip behind either a normal.shock
or behind one or more oblique shocks. When designing for a constant em=
.-
bient temperature only, it is possible that with certain inlet-engine
combinations sufficient oblique-shock spillage may occur as the flight
speed is varied to achieve essentially critical inlet operation continu-
ously without the need of variable-geometryfeatures. However, this in-
.-
herent spillage from a fixed compression surface at below-design Mach
numbers would not in itself be adequate when variations in ambient tem- b
peratures are considered. As can be seen in figure 8, up to 30 percent
of the capture mass flow may be spilled at off-design ambient tempera-
tures even at the design Mach number of 3.5.
Figure 9 demonstrates this seineproblem at design Mach numbers be-
low 3.5. The curves represent on-design spillage of inlets sized for the
cold-day condition and operated on both the NACA standard day and the Air
Force hot day. It is appsrent that considerable spillage is required at
design Mach numbers as low as even 1.5. Clearly, variable geometry to
accommodate this spillage is required to obtain efficient operation at
even the design free-stream Mach number. Without it, variation in the am-
bient teqerature can drive the inlet into buzz.
SUMMARY OF RESULTS
The following results were obtained from an analysis of the effect
of ambient-temperature changes on turbojet-engine - supersonic-inlet
matching requirements. .
1. With engines designed for constant mechanical speed operation,
changes in ambient temperature ordinarily encountered in flight can msrked-
L
ly influence the performance of the matched inlet-engine combination.
NACA TN 3834 7
.
At the design free-stream Mach number) “tibienttemperatures below the
design value force a fixed inlet to operate supercritically, whereas tem-
+ peratures above design would cause subcritical operation. Changes of aa
much as 20 to 30 percent in either pressure recovery or mass flow can
result.
2. In order to make use of translating spikes or bypass systems in-
stalled for off-design free-stream Mach number operation the inlet should
be sized for the lowest ambient temperature to be encountered at the de-
sign free-stream Mach number. The capacity of such spillage systems will.
probably be considerably larger than that required for only standard-day
operation.
3. Those ftied-geometry inlet systems which rely on oblique-shock
spillage for efficient inlet-engine airflow matching at below-design free-
stream Mach number are heavily penalized when ambient-air temperature
variations are considered.
Lewis Flight Propulsion Lab+matory
National Advisory Committee for Aeronautics
Cleveland, Ohio, October 30, 1956
,
REFERENCES
l 1. Ratner, Benjamin: Temperature Frequencies in the Upper Air. Weather
Bur., U.S. Dept. Counuerce,Jan. 1946.
2. Wyatt, DeMarquis D.: An Analysis of Turbojet-Engine-Inlet Matching.
NACATN 3012, 1953.
NACA TN 3834
.
.
120
\
80 \
\
40 \
\
\
o \
\
l/kLl”id \
Air Force hot day
\ /
/
\
-40
x I I I
\ NA.c Aatandmiday
-80 \
\
-120 \\&r ForCa Col(.&y
-MO
o 10 20 30 40 50 60 70 8cacP
Iutxtude,ft
Figure 1. - Ambient-teqeratura variation for several wtandexd daye.
.
‘.
NACA TN 3834 9
-50
-
-70
+0
.
-130
.
Il!l!l
Washington, San Antonio, Oakland, Fairbanks,
D. C. Tex. (!alif. Alaska
Figure 2. - Example of summer temperature variations at alti-
tude of 52,600 feet. (Data from ref. 1.)
10 NACA TN 3834
Mee-stream hkch number, ~
Figure 3. - Effect of ambient temperature on yercent corrected en-
gine speed at rated mechanical speed.
.
b
.P
.
NACA TN 3834
.
I
Eugine
140 B
120 /
A
100
.
80 /
60
40 /
20
/
0
40 60 80 100 120
Correctedengine speed, l/~, percent rated
Figure 4.
- Assumed engine airflow characteristics.
12 NAC!ATN 3834
Ix
o
N
a
v)
120
II
100
-P
La
80
140
120
100
80
60.
Free-stresm
Mach number,
——— 2.0
—. 3.0
\
-
.
L
-
Y ,
-
\
-do -40 0 40 80
Temperature difference,AtO, %
Figure 5. - Effect of ambient temperature
engine airflow.
120
on corrected
.
#
.
NACATN
o
N
m
m
II
3834
20
%
o
EngineA
4’
0
0“ //
40
30 ‘
20
10 ~
EngineB
d
/
o -40 -80
Ill
H-t
)Wee-stream
Mach number,
%3
t--H 1.0——. 2.0—-— 3.0
.20 Engine A ,
/
/
0’
.10
A
J
/ “ 0 “0
;’
Eii
/ ‘ /
-.
so-u
o 40 80 120
Temperature difference,Ato, %
(a)Ambient tempera- (b)Ambient temperatureabove
ture below design; design; inlet subcritical.
inlet supercritical.
Figure 6. - Effect of ambient temperatureon fixed inlet
performancefor inlet-engine,combinationcritically
matched at ambienttemperatureof 392° R.
13
24@
Ambient
temperature,
to,
%
w 220
Om 334
m
w /{
I
*o 200
/‘x Y/
: )
0
.
4*
: 180
/, ~/
2 /
*
/
d
o 16 0 /
~ /
~ / /
%
; 14 0 //
a
/
Ambient
temperature,
_
:
n
toJ
o’
/’ //
%
z lm
E 5
$ / / Y / -
/ ‘
m
;1 oo~ J ./ “ \ / ‘
*
FJ L d ‘ 392-
0
~
/ -
-8 0
\ / ‘ 425
g
~ A ~
~ —
60
1.0 1.5 2.0 2.5 3.5 1.0 1.5 2.0 2.5
~~e-mtream Mach number, ~
3.0 5.5
(a) Engine A. (b) Engine B.
Figure 7. - Effeot of ambient temperature cm required inlet oap’mrcareaform.ltiaalinlet-engine ~tnhing.
,’* l ’ km ‘ ‘
II
4,’
ilq’
a.
i (a) 2ngineA; mi..d ?or amblmt tmperattu.e of W@ R. Ioht BUPar-
C
aritloml for ?rm-stream *oh nmbwa EI.Mtar than 3.2 and awhient
tmlwature of SS4° R.
A
Ambimt
tqoratlms ,—
ton
,%
/ - <25
/ - 592
(b) Fmglne B; sized for ambimt tqerature of 392° R. Imht mfmr-
oritioal at all fIWI-otrem MMh mubm and ambimt tamwrai=ure
of 534° R.
: .8
5
:
# .4
/ : -
426
392
01.0 1.4 1..9 2.2
/ ‘ 3S4
2,6 S.o 3.4 3.8 1.0 1.4 1.8 2.2 2.6 S.o S.4 3..9
Free-stream Hnoh tumber, Ho
(a) 2@ne AJ B1zed far amblant trnperaturm of SS4° R. (d) 2ngine B, sls~ f.? ambient tmpemtum of Y&” R.
FIwra 0. . 2ffeot of ambient tmpm.aturn m inlat nplllage requlrmmts. Inlet desm for IYee-ntream Hac.h number of S.5.
F
m
16 NACA TN 3834
.
1.0 1.4 1.8 2.2 2.6 3.0 3.4 3.8
Design free-stream Mach number, Mo
Figure 9. - Effect of design Mach number on ambient-temperature-
induced spillage. Inlet sized for ati~ent temperatirreof 334° R.
. . . . . . ;.-. -— ...-
.
.
NACA - Langiey Field, Vm.


Effect of ambient-temperature variation on the matching requirements of inlet-engine combinations at supersonic speeds

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