Method and linkage for positioning a convergent flap and coaxial arc valve

A convergent/divergent gas turbine engine exhaust nozzle having reverse thrust capability includes a concentric flap (22) and arc valve (36) pivotable about a common axis (34). A method and linkage (48) schedule the opening of the valve (36) in response to rotation of the flap (22) into a blocking position with respect to the normally aftward flow of the exhaust gas (12). The arc valve (36) is accelerated by the linkage (48) to full opening speed prior to unsealing an alternate, thrust reversing flow passage (38).

FIELD OF THE INVENTION 
The present invention relates to a method for scheduling and coordinating 
the movement of a convergent flap in a gas turbine engine exhaust nozzle 
and the opening of an arc valve therein. 
BACKGROUND 
It is common in thrust vectoring exhaust nozzles for gas turbine engines to 
use flaps or other movable surfaces to direct the exhaust gases in order 
to produce the desired thrust angle and/or exhaust nozzle flow area. In 
order to achieve reverse thrust, certain thrust vectoring nozzles use a 
combined valve-flap arrangement wherein the flow diverting flap or flaps 
is positioned in a blocking arrangement with regard to aftward flow of the 
exhaust gas while a valving surface or other structure is opened to admit 
the exhaust gases into an alternate, generally forwardly directed flow 
passage. 
Such reversing systems utilizing an alternate flow path and corresponding 
valve structure require careful coordination with the blocking flap in 
order to avoid reducing the total nozzle outlet flow area below a minimum 
area required to maintain operational stability in the gas turbine engine. 
If total nozzle outlet area is reduced below such minimum during a 
reversing maneuver, elevated back pressure at the engine exhaust could 
induce an engine stall or other undesirable engine instability. 
In addition to the timing of the opening of the valve structure, it is also 
desirable that the rate at which the valve opens meet or exceed the rate 
at which the blocking flap closes off the aftward nozzle flow area. This 
is especially difficult to achieve in those nozzle arrangements wherein 
the valve structure remains normally at rest while the diverter flap moves 
through a range of orientations for achieving normal forward thrust 
vectoring and aftward outlet area control. 
One further desirable feature for such nozzles is the provision for a safe 
failure mode wherein the nozzle flaps and valve structure automatically 
revert to an unvectored, forward thrust orientation upon failure of the 
nozzle actuators or associated linkages. An aircraft with a failed linkage 
would thus be operable in a forward thrust mode which in turn enhances the 
likelihood of recovery from this type of failure. 
As with all aircraft components and especially for thrust vectoring 
nozzles, the weight and complexity of the physical hardware is of prime 
importance to the designer, with the lightest and simplest arrangement 
being favored from cost, weight, and reliability standpoints. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a method and 
means for coordinating the positioning of a flow directing flap and a flow 
regulating valve in a thrust vectoring exhaust nozzle for a gas turbine 
engine. 
It is further an object of the present invention to coordinate the opening 
of the valve responsive to the movement of the flow directing flap. 
It is further an object of the present invention to schedule the opening 
rate of the valve with respect to the current rate of change of the 
aftward nozzle outlet flow area in order to avoid inducing instability in 
the operation of the gas turbine engine. 
It is still further an object of the present invention to accelerate the 
valve between a static dwell state, in which the valve remains closed and 
motionless in response to movement of the flap within a normal operating 
range, and an opening state wherein the valve opens at a rate in excess of 
the flap closing rate. 
It is still further an object of the present invention to accelerate the 
valve to its full opening speed prior to admitting any exhaust gases into 
the reversing flow passage. 
According to the present invention, an exhaust nozzle having a pivotable 
semi-cylindrical arc valve for regulating the flow of exhaust gases into a 
reverser flow passage or the like is held in a closed, sealed state during 
movement of a coaxial convergent flap structure through a range of motion 
corresponding to normal, forward nozzle thrust operation. The arc valve is 
accelerated from rest to an opening angular speed which is in excess of 
the rate of movement of the convergent flap as the flap is moved into a 
blocking orientation with respect to the aftward flow of the engine 
exhaust. Such acceleration is accomplished while maintaining the arc valve 
in a closed state with respect to the admission of exhaust gas into the 
reverser flow passage controlled by the valve. 
As the convergent flap reaches a position wherein further movement thereof 
would otherwise result in an unacceptably small nozzle outlet flow area, 
the now accelerated arc valve, moving at the full desired opening speed, 
opens to admit at least a portion of the engine exhaust gas into the 
alternate flow passage, maintaining the total nozzle outlet flow area as 
the diverter flap continues to move into a final position substantially 
blocking all aftward exhaust flow. 
The present invention also provides a simple linkage for accomplishing the 
foregoing operation, including a cam race movable with the flow diverting 
flap and a four bar linkage driven by a roller engaged with the cam race. 
The linkage drives a crank secured to the arc valve causing the movement 
of said valve to be wholly responsive to movement of the convergent flap. 
The linkage according to the present invention accomplishes the desired 
operation of the arc valve without a separate actuator or other driving 
means, thereby simplifying the overall nozzle mechanism and increasing the 
reliability thereof. 
Another advantage of the linkage and cam arrangement of the present 
invention is the continuing "fail open" bias maintained by the internal 
nozzle pressure on the convergent flap. Failure of the flap driving 
actuator, for example, causes the convergent flap to open, simultaneously 
closing the arc valve and unblocking the aftward flow path. The nozzle is 
thus restored to normal, forward thrust, or maintained in this state if 
the failure occurs during normal flight. 
These advantages as well as others will be apparent to those skilled in the 
relevent art upon review of the following specification and the appended 
claims and drawing figures.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows a two dimensional thrust vectoring exhaust nozzle 10 receiving 
a flow of exhaust gases 12 from a gas turbine engine 14 for propelling an 
aircraft (not shown). The nozzle 10 is termed "two dimensional" by virtue 
of its ability to direct the exhaust gases 12 over a range of outlet 
vectors lying in a single plane. The nozzle 10 of FIG. 1 thus, by movement 
of the various flaps in a fashion well known in the prior art, may direct 
the exhaust gases generally upward, downward, or in a reverse thrust 
direction. 
Not only does the nozzle shown in FIG. 1 provide for vectoring of the 
exhaust gas flow 12, but it additionally provides for the control of the 
aftward nozzle outlet flow area as measured by the throat distance 18 
between a pair of upper and lower convergent flaps 20, 22. The throat 18 
is thus varied to match the current engine and/or afterburner power level 
to provide an optimum outlet flow area and exhaust velocity for engine 
efficiency and thrust. 
Diversion of the exhaust flow 12 as well as variation of the rate of 
expansion of the exhaust gases are achieved by the aftward divergent flaps 
24, 26 which are hinged to the trailing edges of the convergent flaps 20, 
22, respectively. 
In the nozzle as shown in FIG. 1, the convergent flaps 20, 22 are secured 
at opposite ends thereof to disks 28, 30 which are in turn pivotable about 
transverse axes 32, 34. The disks and hence the convergent flaps are 
positioned by means of hydraulic or other actuators (not shown) which 
selectably position the disks and flaps for control of the exhaust gas 
flow 12. 
With further reference to FIG. 1, the nozzle 10 also includes at least one 
arc valve 36 having a semi-cylindrical configuration and concentrically 
positioned and pivotable about the corresponding convergent flap 22. The 
arc valve 36 regulates the flow of exhaust gas 12 into the corresponding 
alternate reverser flow passage 38 whence it may be directed forwardly or 
otherwise by a vane cascade 40 or other flow directing means. 
During operation of the nozzle 10 in the reverse flow operating mode, the 
convergent flaps 20, 22 are pivoted into a full blocking arrangement 
whereby the aftward nozzle throat 18 is substantially eliminated, thereby 
blocking off the aftward flow of exhaust gas 12. During such operation the 
arc valve 36 is likewise pivoted to admit the exhaust gases 12 into the 
flow passage 38, thereby achieving the desired reverse thrust. 
As will be appreciated by those familiar with the operation of gas turbine 
engines, reduction of the total nozzle outlet gas flow area below a 
certain minimum results in excessive back pressure at the engine outlet 
and possible stalling of the engine compressor or other undesirable and 
unstable engine operating conditions. During the transition between 
forward and reverse thrust, it is thus clear that the nozzle 10 must 
accurately and positively schedule and coordinate the movement of the 
closing divergent flaps 20, 22 with the opening of the arc valve or valves 
36 to avoid reducing the total nozzle outlet area below the required 
minimum while simultaneously avoiding the loss of thrust efficiency which 
results from operation of the engine-nozzle combination with too large a 
total nozzle outlet area for the current engine power level. 
One such area schedule which achieves the desired total nozzle outlet area 
is shown graphically in FIG. 2 wherein the flow areas 42, 44 of the 
aftward nozzle outlet and the alternate passage regulated by the arc valve 
are shown along with the total nozzle outlet area 46 indicated by the 
broken line. The relative area of each parameter is represented by 
vertical displacement on the graph of FIG. 2 while the horizontal 
displacement is representative of the position of the convergent flap 22 
in the range defined between the fully open angular position .theta..sub.O 
and the fully blocked angular position .theta..sub.B. 
The aftward nozzle outlet area 42 is thus seen decreasing directly as the 
convergent flap 22 is pivoted between the open position, .theta..sub.O and 
the closed position .theta..sub.B. The aftward nozzle area decreases, 
reaching the minimum allowed area A.sub.min at the corresponding angular 
position .theta..sub.min. Continued pivoting of the convergent flap 22 
past .theta..sub.min without providing an alternative flow path for the 
exhaust gases results in an increased likelihood, if not certainty, of 
stalling the gas turbine engine. 
As can be seen from the area schedule according to the method of the 
present invention, the alternate passage flow area 44 is opened at 
.theta..sub.min and increases at a rate in excess of the closing of the 
aftward nozzle flow area as the convergent flap 22 is rotated into the 
full blocking position .theta..sub.B. Thus at no point in the operational 
range of the convergent flap 22 does the total nozzle outlet area fall 
below A.sub.min. 
By opening the arc valve 36 at an angular speed greater than the closing 
rate of the convergent flap 22, the method according to the present 
invention provides an increasing flow area to the reverser exhaust passage 
38 at a rate greater than the closing of the convergent flap throat 18 
thereby increasing the total nozzle outlet area 46 as the convergent flap 
22 approaches the full blocking position .theta..sub.B. This schedule of 
area is particularly adapted to provide the nozzle 10 with the capability 
of smoothly switching between forward and reverse thrust operation. In 
transitioning between forward and reverse thrust, it is typical to reduce 
the power level of the gas turbine engine to a minimum with a consonant 
reduction in the aftward nozzle outlet flow area and then to increase 
engine power upon opening the reverser exhaust passage in order to achieve 
the desired reverse thrust. 
The total nozzle outlet area 46 as provided by the method according to the 
present invention and shown clearly in FIG. 2 is well adapted to provide 
the required area for the operation of the nozzle in a forward to reverse 
thrust transition, providing the proper total nozzle outlet area for the 
exhaust gases over the expected engine operating range. The closely 
coordinated scheduling between the nozzle outlet area and the engine power 
level provides the optimum nozzle outlet area without increasing the 
likelihood of inducing engine stall or similar operating instabilities. 
FIG. 3 shows a schematic of a linkage disposed between the convergent flap 
22 and the arc valve 36 for effecting the method according to the present 
invention. The linkage, designated generally by reference numeral 48, 
moves generally in a single plane and is supported by the nozzle static 
structure, such as the sidewalls 16, as shown in FIG. 3. A cam race 50 is 
disposed within a portion of the convergent flap structure 22a and 
rotatable therewith about the pivot axis 34 as discussed hereinabove. The 
cam race 50 includes a first dwell portion 52, a second acceleration 
portion 54 and a third full speed or action portion 56. The dwell portion 
52 is generally equidistant from the pivot 34 while the action portion 56 
runs generally inwardly toward the pivot axis 34. The acceleration portion 
54 forms a curved transition between the dwell and action portions 52, 56. 
The arc valve 36 is positioned bY the arc valve crank 36a which rotates 
about the common axis 34. The crank 36a is pivotably joined to a drive 
link 66, which is in turn connected to a slave link 60. The slave link 
rotates about a static pivot point 62 in the nozzle static structure 16. A 
roller 58 runs in the cam race 50 and is secured to the four bar linkage 
formed by the static structure 16, slave link 60, drive link 66, and crank 
36a at the pivot joint between the drive and slave links 66, 60. 
The roller 58 and cam race 50 thus drive the four bar linkage 16, 60, 66, 
36a in response to the movement of the convergent flap 22. As will be 
appreciated from FIG. 3 and the following description, the roller 58 and 
hence the drive link 66 and arc valve crank 36a remain essentially static 
as the cam race 50 and the convergent flap structure 22, 22a rotates 
through the dwell portion 52 of the cam race 50 which is disposed 
equidistant from the transverse pivot axis 34. As the convergent flap 
position approaches .theta..sub.min, the roller 58 enters the acceleration 
portion 54 of the cam race 50 initiating movement of the drive link 66 and 
arc valve crank 36a. 
As the convergent flap 22 achieves an angular position equivalent to 
.theta..sub.min, the roller 58 according to the present invention has 
traversed the acceleration portion 54 of the cam race 50 whereby the arc 
valve 36 is now moving at its full angular speed about the pivot axis 34 
which is higher than the angular velocity of the rotating convergent flap 
22, 22a. 
It will be apparent from the preceding discussion that, as the convergent 
flap has not yet achieved .theta..sub.min at the time the roller 58 enters 
the acceleration portion 54 of the cam race 50, that some movement of the 
arc valve 36 takes place prior to the opening thereof for admitting 
exhaust gases 12 into the reverser flow passage 38. This "head start" 
motion allows the arc valve to achieve the desired angular velocity prior 
to opening and admitting exhaust gas into the passage 38 thus ensuring 
that the desired rate of increase of flow area in the reverser passage is 
achieved at the initiation of flow into said passage 38. Without providing 
for such preopening movement of the arc valve 36, the linkage according to 
the present invention would fail to achieve the area schedule shown in 
FIG. 2 as the arc valve, lightweight but still massive, cannot be 
instantaneously accelerated to full speed angular velocity as the position 
of the convergent flap reaches .theta..sub.min. 
This preopening range of motion is permitted by a forward portion of the 
arc valve 36 extending past the linear seal 68 between the forward nozzle 
static structure 70 and the arc valve 36 for sealing against flow of 
exhaust gases 12 into the reverser passage 38 when the valve 36 is in the 
closed orientation. This extended portion 72 as shown in FIG. 4 of the arc 
valve 36 allows the arc valve 36 to be pivoted aftward while the valve 36 
is being accelerated to its full opening angular velocity. As the 
convergent flap 22 passes the .theta..sub.min angular position, the arc 
valve 36, having been pivoted rearwardly by the drive link 66 and crank 
36a, disengages from the seal 68 and opens the flow area into the 
alternate reverse thrust flow passage 38 at the desired rate and time. 
It will further be apparent that such timing and rate may vary as to the 
particular nozzle design, operating environment, gas tubine engine, etc. 
and that the relative movements of the convergent flap 22 and the 
corresponding arc valve 36 may be rescheduled by merely altering the shape 
of the cam race 50. Such modifications may thus be achieved without 
extensive redesign or remanufacture of the various components of the 
exhaust nozzle 10 as future engine developments and operating experience 
accumulate. 
Although the method and linkage according to the present invention is shown 
as being operative in a symmetric 2-D nozzle arrangement wherein 
oppositely oriented convergent flaps 20, 22 and corresponding structures 
control and divert the exhaust gases 12, it should be appreciated that the 
flap and valve arrangement 22, 36 may be used singly to achieve thrust 
vectoring and area control without the need for a corresponding symmetric 
flap and valve arrangement. The method and linkage according to the 
present invention are thus not limited to use in the illustrated nozzle 
arrangement but rather may be PG,16 used advantageously in a variety of 
configurations and applications without departing from the spirit and 
scope of the invention as defined by the hereinafter presented claims.