GAS TURBINE ENGINE CONTROL

DISPOSITIF DE COMMANDE-REGULATION POUR TURBOMOTEUR A GAZ

English Abstract

GAS TURBINE ENGINE CONTROL
ABSTRACT OF THE DISCLOSURE
In one form of the present invention, the
thrust of a gas turbine engine is inferred. If a droop
in thrust is predicted, components of the engine are
modulated in response to restore thrust.

Claims

- 8 - 13LN 01692
The embodiments of the invention in which an
exclusive property or privilege is claimed are defined
as follows:
1. A gas turbine engine control comprising:
(a) temperature error means for deriving an
error signal, T5 ERROR, indicative of the deviation of
the temperature of an engine component from a reference
temperature;
(b) variable exhaust nozzle (VEN) means for
modifying the area of the exhaust nozzle of the engine
as a function of T5 ERROR; and
(c) T5 ERROR compensation means for modifying
T5 ERROR such that T5 ERROR first increases then
decreases to thereby induce the VEN means to first
decrease and then increase the nozzle area.
2. In a gas turbine engine control which
modulates a variable exhaust nozzle as a function of
parameters which include a temperature error signal,
T5 ERROR, derived from comparison of the temperature of a
component of the engine with a reference schedule of
temperatures, the improvement comprising:
(a) first modification means for modifying
T5 ERROR;
(b) second modification means for modifying
T5 ERROR such that the combined effect of the first and
second modification means is to initially increase T5
ERROR according to a predetermined time constant and then
to decrease T5 ERROR according to a predetermined
second time constant.
3. In a control according to claim 2 in which
(a) the first modification means modifies
T5 ERROR substantially according to the following LaPlace
transform:
<IMG>
(b) the second modification means modifies

- 9 - 13LN 01692
T5 ERROR substantially according to the following LaPlace
transform:
<IMG>
in which T2 > T1.
4. A method of compensating for the droop in
thrust which occurs in a gas turbine engine upon
acceleration, comprising the following steps:
(a) inferring the occurrence of droop from
a change in the temperature of the low pressure turbine
exhaust T5;
(b) increasing and then decreasing the
difference between T5 and a reference to produce a compen-
sated temperature signal, T5 COMP; and
(c) modulating the exhaust nozzle area of the
engine as a function of T5 COMP.

Description

13LN-169Z
GAS TURBINE ~NGI~E CONTRO~
The present invention relates to gas turbine
engine controls and, more specifically, to a type of
control termed a thrust droop compensator. Such a
compensator reduces the droop in thrust which sometimes
occurs because o~ di~ferent rates of thermal growth of
different engine components during engine acceleration.
BACKGROUND OF THE INYENTION
Pigure 1 illustrates a gas turbine aircraft
engine as known in the art. Ho~ gases 3 provided by a
combustor 6 impart energy to a high pressure turbine 9
which is surrounded by a shroud 12. A tip clearance 15
exists between the turbine 9 and the shroud 12. When
the engine is idling, say at 11770 rpm, a given tip
clearance will exist, such as 0.047 inches. However,
during a sudden acceleration, such as to 16140 rpm
within 4 seconds, increases in centrifugal forces due
to the greater rotational speed causes the turbine to
expand) thus reducing the tip clearance to .016
inches. Temperature in the region of the high pressure
turbine 9 will increase, thus causing the turbine 9 and
the shroud 12 to expand thermally. However, since the
thermal mass of the shroud 12 is much less than that o~
the turbine 9, the shroud 12 heats up faster, and thus
expands faster. Accordingly, the tip clearance 15
initially increases, say to .028 inches, and then
decreases to the steady-state level of 0.016 inches
when the temperature o~ the turbine 9 reaches its
steady-state value.
This initial increased tip clearance 15 is
undesirable because it imposes a penalty in engine

13L~ 2
--2--
efficiency: the hot gases 3 can bypass the turbine 5
by leakin~ through the tip clearance region 15, and
thus the leaking gases do little or no work upon the
turbine 9.
OBJECTS OF THE INVENTION
It is an object of the present inYention to
provide a new and improved gas turbine engine control.
It is a further object of the present
invention to provide a new and improved gas turbine
engine control which compensates for thrust droop
occurring after engine acceleration.
SUMMARY OF THE INV~NTION
In one form of the present invention, the
~hrust of a gas turbine engine is inferred. If a droop
in thrust is predicted, components of the engine are
modulated in response to restore thrust.
BRIEF DESCRIPTION OF THE DRAWING
FIGVRE 1 illustrates a gas turbine engine.
FIGURES 2-5 illustrate the changes experienced
by various engine components with time during
acceleration.
FIGU~E 6 illustrates one form of the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
Figure 2 illustrates a plot of the transient
16 of the temperature (termed T5) of the exhaust of a
low pressure turbine 18 during acceleration. T5 is
measured at point 21 at Figure 1, downs~ream of the low
pressure turbine 18. The vertical axis in Figure 2
displays T5 in degrees F, but referenced to an
arbitrary zero which is actually the maximum scheduled
temperature for T5 which, in practice, is about
1500 F. During the 60 second interval 24 and earlier,
the engine is operating at idle, and at time T = 0, the
engine is accelerated, causing T5 to rapidly increase
by about 800~ F within the 4-second interval 27.

13LN-1692
-3-
Figures 3-5 illustrate the effect of this
rapid increase in T5 upon the temperatures of the
turbine 9 and the shroud 12 of Figure 1. Figure 3
illustrates the transient 30 of the shroud diameter,
and Figure 4 illustrates the transient 33 of the
turbine diameter, both as functions of time. The
vertical axes in both these Figures are plotted in
arbitrary units of diameter Measure. However, even
though both the turbine 9 and the shroud 12 expand upon
being heated, the shroud transient is given a positive
slope while the turbine transient 33 is given a
negative slope. This is because the plots are
referenced to the tip clearance 15 in Figure 1, and not
to absolute diameter. Since e~pansion o~ the shroud 12
in Figure 1 causes tip clearance 15 to increase, the
shroud transient 30 in Figure 3 is ascribed a positive
slope. However, since the expansion of the turbine 9
causes tip clearance 15 to decrease, the turbine
transient 33 is ascribed a negative slope.
The transients of Figures 3 and 4 are
algebraically added to produce transient 36 in Figure
5. Transient 36 illustrates the net tip clearance 15
in Figure 1 as a function of time during the
acceleration. Again, the vertical axis in Figure 5 is
in arbitrary units indicative of clearance size. The
reader will note that there is initially a peak
clearance at point 39 which gradually decreases toward
the steady-state value 42. As the Figure shows, it
takes approximately 4 or 5 minutes for the tip
clearance 15 to reach its steady-state value 42.
Figure 6 illustrates one form of the present
invention which compensates for the effect of the tip
clearance transient 36 in Figure 5 upon thrust. Block
50 in Figure 6 generates a digital output versus time
signal which is similar to the transient 30 of Figure

~ZZ~; 1 3 LN -1~ g 2
3, and block 53 generates a signal similar to the
transient 33 of Figure 4. The summation in summer 56
in Figure 6 produces a net output on line 59 similar to
the transient 36 of Figure 5. The net output is
subtracted from summer 62 in Figure 6 in which the
actual, measured, T5 is added by block 64 on line 65.
Also subtracted in summer 62 is a T5 reference derived
from a schedule indicated by block 68. The latter
block 68 is a schedule, or listing, of various Yalues
of T5 which are calculated in advance for various
engine operating conditions. Thus, the value
subtracted at summer 62 by block 68 will change,
depending upon the operating condition of the engine.
Blocks 64 and 68 (ignoring the effect of the
net ou~put on line 59) pro~ide on line 72 a T5 ERROR
signal, in the sense that the error signal indicates a
deviation of actual T5 from scheduled T5. The addition
of the net output (from summer 56) on line 59 modifies
the T5 ERROR signal into a T5 FRROR signal, but droop
compensated (T5E-DC). TSE-DC is integrated in block 75
designated by the symbol l/S (l/S being a/LaPlace
transform term) and having a gain indicated by the term
K. The output, on line 78, is a Yariable exhaust
nozzle (VEN) area trim signal.
The VEN refers to a nozzle 95 in ~igure 1
which is modulated in area as shown by dashed nozzle
95A to modify the pressure at poin~ 21 in the engine.
This pTeSSUre modification, in a manner which need not
be understood here, alters the thrust produced by the
engine. Control of the modulation is accomplished by
apparatus which is known in the art and outlined
below. The signal provided by block 101, a VEN area
demand signal on line 103, is added in summer 105.
Subtracted from summer 105 is the output of summer 108
which receives as inputs a feedback signal along line

13LN-1692
--5--
110 which is provided by a transducer (not shown) which
measures the actual V~N area. The other input o
summer 10~ is the VFN trim signal on line 73 and it is
subtracted therein~ The output of summer 105, on line
115, is processed as known in the art by VEN orward
path dynamics block 120, which includes digital filters
to stabilize the VEN control loop and the necessary
analog electric and hydromechanical components to
modulate nozzle area 95 in Figures 1 and 6.
The operation of the circuitry shown in Figure
6 can be described as follows. During the T5 transient
16 of Figure 2, a signal resembling the net output plot
36 of Figure 5 is subtracted, in Figure 6, from summer
62 by means of line 59. This, in effect, raises the T5
reference of block 68, because both the signals from
summer 56 and block 68 are applied to summer 62 with
the same algebraic sign. Therefore, the T5 ERROR
signal on line 72 becomes larger, so that the VEN trim
signal on line 78 induces the VEN area to approach a
size which will reduce the T5 ERROR, and thus increase
thrust. (It has been found empirically that thrust is
a function of T5, and, further, a generally linear
function at intermediate thrusts, so that control of T5
is tantamount to control of thrust.)
Symbols 110, 112, and 114 indicate signal
limiters, known in the art, which are considered
self-explanatory. For example, limiter 110 limits the
signal allowed to appear on line 116 to a range
representing 0 and -123 F. _ _
The term in block 50, namely, 1rl , is a
LaPlace transform. Thus, the signal p rocess ng which
occurs between point 120 and point 122 can be described
by the following transfer function (ignoring the action
of the limiters such as limiter 110):
T5 COMP = [T5-T5RE~ ~ ~ + ~5-T5RE3 ~ ~
wherein TSCOMP is the signal generated on line 59.

13LN-16g2
--6--
An invention has been described wherein a
droop in thrust occurring during acceleration o a gas
turbine engine is compensated. The droop is caused by
the transient increase in tip clearance between the
high pressure turbine and the shroud surrounding it.
The clearance transient is caused by ~he differential
thermal growth of the turbine as opposed to the
shroud. The present invention increases engine thrust,
as by manipulating parameters such as exhaust nozzle
area and fuel flow, as a function of tip clearance in
order to compensate for the loss induced by the
clearance transient.
One important aspect of the invention is that
the general shapes of the signals produced by blocks 50
and 53 in Figure 6 are predeteremined. That is, the
signals decay with predetermined time constants as do
the signals shown in Figures 3 and 4. However, the
initial magnitudes of the signals produced by these
blocks is a function of the signal present on line 122
in Figure 6. In this sense, the signals produced by
blocks 50 and 53 are produced according to
predetermined functions: the decay times (dictated by
the time constants in blocks 50 and 53) are
predetermined, but the initial magnitudes are dictated
by T5 ERROR.
Numerous modifications and subs~itutions can
be undertaken without departing from the true spirit
and scope of the present invention. For example, one
embodiment has been tested in which the following
variables were given the following values:
rl = 10 *18583~0.5
~FMCJ
r2 = 85 *~8583l0.5
\WFMCJ

13L~ 162
--7--
wherein the term WFMC refers to the fuel flow (in
pounds per hour), but corrected for the sea level
static fuel re~uirement at intermediate power. MaXing
the time constants ~1 and ~2 variable in this
manner corrects for the effect of Reynolds number on
heat transfer to the turbine disk 9 in Figure 1 and
shroud 12 in Figure 1.
However, it is to be recognized that it will
be known to those skilled in the art that controls for
different engines using the concept of the present
invention will almost certainly require different
values for these variables. The Yalues for a different
engine can be calculated from the Figure 5 type of
transient which corresponds to that engine. The time
constants,~ and ~ , are calculated from the time
constants of the plots of Figures 3 and 4, and the
gains ~, are calculated, as known in the art, to give
proper magnitude to the signal appearing on line 59 in
Figure 6.

Representative Drawing