Integrated fire and flight control system for controlling the angle of attack of a rotary wing aircraft

An integrated fire and flight control system of the type which controls aircraft flight dynamics to referenced values defined by a weapon launch solution to provide optimum aircraft to target orientation, further optimizes the aircraft's angle of attack (AOA) to the target by controlling the aircraft's vertical speed to modify the aircraft's rate of climb or descent as necessary to produce an actual pitch attitude that is within a range of AOA values corresponding to the range of permissive weapon launch vertical speeds recommended by the weapons manufacturer.

DESCRIPTION 
1. Technical Field 
This invention relates to flight control systems for rotary wing aircraft, 
and more particularly to an integrated fire and flight control system of 
the type in which the fire control portion supplements the authority of 
the flight control portion during pilot selectable intervals. 
2. Background Art 
Integrated fire control and flight control systems are known for use in 
weapon equipped rotary wing aircraft to coordinate the aircraft's flight 
attitude with that required for accurate launching or firing of the 
aircraft's weapons. These integrated systems embody both an automated 
flight control function, which controls the aircraft's response in its 
yaw, pitch, roll and collective axis to the sensed state of the aircraft's 
flight dynamics, to provide stable aircraft responsiveness to pilot 
commanded maneuvers, as well as a fire control function which modifies the 
flight control authority under certain circumstances to provide optimum 
aircraft-to-target orientation for weapon launch. One such integrated fire 
and flight control (IFFC) system is disclosed and claimed in commonly 
owned U.S Pat. No. 5,331,881 entitled Helicopter Integrated Fire and 
Flight Control Having Azimuth and Pitch Control, issued Jul. 26, 1994 to 
Fowler et al. 
The '881 referenced IFFC system provides an override of the flight control 
authority in the yaw and pitch axes by replacing the yaw attitude feedback 
error signal and the pitch attitude feedback error signal with an azimuth 
command signal and an elevation command signal provided by fire control 
circuitry. The purpose is to desensitize the fight control system to small 
pilot commanded stick inputs which may otherwise affect the desired 
azimuth and elevation required for accurate target sighting while the 
aircraft is in a weapons launch mode. 
While the '881 reference discloses an IFFC system with two axis fire 
control, commonly owned U.S. Pat. No. 5,263,662 entitled: Helicopter 
Integrated Fire and Flight Control System Having Turn Coordination Control 
issued Nov. 23, 1993 to Fowler et al, and U.S. Pat. No. 5,465,212 
entitled: Helicopter Integrated Fire and Flight Control Having a 
Pre-launch and Post-lazinch Maneuver Director, issued Nov. 7, 1995 to 
Fowler et al., each discloses TFFC systems with three axis (yaw, pitch and 
roll) fire control. The three axis system of the '662 reference provides 
yaw and pitch axis override by replacing the yaw and pitch rate error 
signals with the rate of change in the azimuth and elevation commands 
provided by the fire control system, and provides a bank angle signal to 
place the aircraft in a roll angle. The system's objective is to provide a 
substantially coordinated turn and optimum stabilization during aircraft 
maneuvers in the weapon launch period. Similarly the '212 reference 
discloses a three axis IFFC which provides the forward acceleration and 
velocity profile necessary to satisfy optimum aircraft to target weapon 
launch. 
Each of the referenced IFFC systems incorporates a fire control system 
logic which is functionally integrated with the aircraft's flight control 
system so as to provide seamless transition between flight control and 
fire control authority when commanded by the pilot. Each of these systems 
further includes a bi-functional flight control comprising a primary 
flight control system (PFCS) with primary authority in enacting pilot 
commanded inputs to the aircraft, and an automatic flight control system 
(AFCS) which supplements PFCS performance with additional trim functions 
to optimize the aircraft's dynamic response to the commanded inputs. The 
PFCS and AFCS arc functionally coordinated and their performance is 
characterized by a model following transfer function of the type disclosed 
in commonly owned U.S. Pat. No. 5,238,203 entitled: High Speed Turn 
Coordination for Rotary Wing Aircraft, issued Aug. 24, 1993 to Skonicczny 
et al. 
While the prior art IFFC systems each address the primary aircraft flight 
control dynamics which affect weapon aiming accuracy, there are secondary 
considerations. One such consideration which is important to safe and 
accurate launch of weapons is the angle of attack (AOA) of the aircraft. 
Defined as the difference between pitch, i.e. the spatial orientation of 
the centerline of the aircraft, and the air-referenced flight path angle, 
i.e. the airspeed vector in the vertical plane, the AOA substantially 
contributes to the initial conditions of the weapon's launch state to the 
extent that if the AOA magnitude is too great the initial conditions 
required for safe weapon launch may be exceeded. 
As known, the fire control launch solution for the aircraft's on-board 
weapons prescribes the pitch attitude required for weapon launch. 
Therefore, if the AOA is to be controlled it must be done by changing the 
aircraft's vertical speed. This requires fire control authority in the 
lift, or collective axis, which is not known in the prior art IFFC 
systems. 
DISCLOSURE OF INVENTION 
One object of the present invention is to provide an integrated fire and 
flight control (IFFC) system which limits the angle of attack (AOA) of a 
rotary wing aircraft to minimum values that are consistent with the launch 
vertical speed envelope prescribed by the weapon manufacturer for accurate 
launch of weapons in a weapons launch mode. Another object of the present 
invention is to provide an IFFC system which is capable of coordinating 
fire control authority of AOA with other existing fire control authorities 
in a non-interference manner. 
According to the present invention, an IFFC of the type which controls 
aircraft flight dynamics to referenced values defined by a weapon launch 
solution to provide optimum aircraft to target orientation, further 
optimizes the aircraft's angle of attack (AOA) to the target by 
controlling the aircraft's vertical speed. In further accord with the 
present invention the IFFC system incorporates fire control logic in the 
aircraft's collective axis to modify the aircraft's rate of climb or 
descent as necessary to produce an actual pitch attitude that is within a 
range of AOA values corresponding to the range of permissive weapon launch 
vertical speeds recommended by the weapons manufacturer. In still further 
accord with the present invention the constraints on vertical speed 
provided by the present IFFC are enabled in response to enablement of an 
altitude hold state by the pilot. In yet still further accord with the 
present invention the vertical speed constraints supplant the authority of 
the flight control with respect to vertical speed, and can override the 
pilot commanded vertical speed. 
The present IFFC system control laws for AOA are integrated with the 
advanced flight control laws of the aircraft and generate body axis 
angular rate (or vertical rate) commands that drive command models in the 
AFCS and feedforward commands that sum with the AFCS output commands. When 
the weapon launch mode is selected by the pilot, the aircraft is first 
orientated to bring the aircraft's weapon within the "target window", i.e. 
the aircraft is positioned within the azimuth and elevation constraints 
commanded by the firing solution, then the AOA constraints are satisfied 
while the other IFFC control laws continue to maneuver the aircraft to 
point directly at the target's position, as defined by the launch 
solution. 
The AOA limiting algorithm commands a rate of climb/descent that is a 
function of commanded pitch angle, and it is specific to the selected 
weapon's launch constraints. The permissive weapon launch vertical speed 
envelope is defined for each of the on-board weapons. If the constraint is 
small the climb angle is commanded to be equal to the pitch attitude, thus 
driving the AOA to zero. This function is only phased in when the aircraft 
is positioned near the target azimuth solution so it will not interfere 
with the attack maneuver. At low speeds this function is disabled to 
prevent unnecessary collective activity. 
These constraint laws use pre-defined aircraft maneuver capabilities. The 
weapon and sensor constraint aiding functions couple the aircraft to the 
weapon or sensor directed line of sight (LOS) as in the basic coupled 
aiming except that there is an appropriate deadzone of operation around 
the LOS. Within the deadzone, normal aircraft control remains in effect. 
When the deadzone is exceeded the IFFC control laws provide coupling 
commands to maintain the aircraft at the edge of the constraint boundary. 
In each case, the control axis coupling articulates the constraint 
boundary applicable to that weapons system and the constraint is in effect 
only while approaching the constraint envelope limit. The constraint 
aiding laws are authority limited so that the pilot can override them if 
necessary. 
The collective axis authority of the fire control portion of the present 
IFFC system is interfaced with the model following Altitude Hold mode of 
the flight control portion. The collective axis interface is similar to 
the implementation in the other IFFC axes and comprises a vertical rate 
command and a proportional feedforward command. The fire control 
collective authority is enacted when the Altitude Hold mode is engaged by 
the pilot, and is otherwise inactive. The pilot can override the function 
by moving the displacement collective stick on the left side of the 
cockpit. The pilot can move the collective against trim while leaving the 
collective AFCS engaged, or the pilot can disable stick trim and the 
vertical AFCS by pressing the collective trim release switch (the trigger 
switch under the collective stick grip). 
These and other objects, features, and advantages of the present invention 
will become more apparent in light of the following detailed description 
of a best mode embodiment thereof, as illustrated in the accompanying 
Drawing.

BEST MODE FOR CARRYING OUT THE INVENTION 
Referring to FIG. 1, the integrated fire and flight control (IFFC) system 
10 of the present invention includes a primary flight control system 
(PFCS) 12, an automatic flight control system (AFCS) 14, and an IFFC 16. 
The PFCS 12 and AFCS 14 each receive displacement command signals from the 
pilot operated displacement collective stick 18 and force command signals 
from a four axis side arm controller 20. The displacement stick is 
typically located to the left of the pilot's seat and pivots about a point 
21. The position of the displacement stick is sensed by a linear variable 
differential transformer (LVDT) 22 which provides an electrical signal 
indicative of stick position on a line 23 to the PFCS and AFCS. To give 
the pilot tactile feel of the collective load the PFCS provides a drive 
signal on a line 24 to a servo 25, which in turn drives the displacement 
collective stick 18 so that it tracks the command signal on line 24. 
The displacement collective stick also includes several pilot switched 
discrete signals, including altitude hold mode, IFFC engaged mode, and 
weapon launch mode signals, which are provided through line 26 and, 
together with the signals on lines 23 and 24, are presented through trunk 
lines 28 to the PFCS and AFCS, which also receive the output force signals 
from the sidearm controller on trunk lines 30. The PFCS, AFCS, and IFFC 16 
each receive sensed signals from aircraft sensors 32 on lines 34. The 
sensed signals arc representative of the instantaneous actual value of 
selected aircraft parameters including: vertical acceleration, vertical 
rate, radar altitude, barometric altitude, angular rate, attitude, and 
velocity. 
It should be understood that the reference herein to the term trunk lines 
is a figurative term for the collective signal pathways between the 
various elements of the system 10. As known to those skilled in the art, 
while the individual signal paths may be conductive wires which are 
bundled in trunks for purposes of utility and/or protection (the trunk 
lines may be shielded against both mechanical and electro-magnetic 
interference), the trunks may equally well be digital signal busses, such 
as MIL-STD-1553, ARINC 429, 629, or RS422. As described in U.S. Pat. No. 
5,263,662 cited hereinbefore, the PFCS and AFCS each have separate yaw, 
pitch, roll and lift control logic for providing four axis control of the 
aircraft. The logic is included in PFCS control modules 35-38 and AFCS 
control modules 39-42, respectively. In the present system 10, however, 
the IFFC 16, while including yaw, pitch and roll control modules 44-46 
similar to those modules described in the '662 reference, it further 
includes lift control module 47 to permit the IFFC to also provide 
vertical axis control as well as yaw, pitch and roll axis flight control 
of the aircraft during target acquisition in the presence of the pilot 
initiated coupled aiming flight mode. Within the PFCS the control modules 
35-38 receive the pilot command signals from the displacement collective 
stick 18 and sidearm controller 20 on lines 28, 30, as well as the sensed 
parameter signals on lines 34, through trunk lines 48. Similarly, these 
signals are distributed within the AFCS on lines 50 to control modules 
39-42. 
As shown and described in detail in the hereinbefore cited references to 
U.S. Pat. No. 5,238,203 to Skonieczny et al; U.S. Pat. No. 5,263,662 to 
Fowler et al; U.S. Pat. No. 5,310,135 to Fowler et al; and U.S. Pat. No. 
5,331,881 to Fowler et al, each of which are hereby incorporated by 
reference herein, the PCFS and AFCS each use a model following algorithm 
in each of the four control axes to shape the pilot's side arm controller 
and displacement stick commands to produce the desired aircraft response. 
The control modules of both systems, which are connected together through 
lines 51, collectively provide rotor command signals on PFCS output lines 
52 to the aircraft's rotor mixing function 54. In response, the mixing 
function 54 positions the aircraft's controlled surfaces by commanding the 
displacement of mechanical servos 56 and linkages 58 to control the tip 
path plane of main rotor 60, as well as commanding displacement of tail 
rotor servos 62 and linkages 64 to control the thrust of the aircraft's 
tail rotor 66. 
The IFFC receives, in addition to the sensed aircraft parameter signals on 
the lines 34, fire control command signals from a fire control system 68 
on lines 70. These signals are presented through trunk lines 71 to the 
IFFC control modules 44-47. The fire control system 60 provides the 
azimuth and elevation fire control command signals in response to target 
position data presented to it from target position/angle data inputs 72 on 
lines 74. The target position/angle data may be provided from a number of 
alternate, well known sources, such as line-of-sight angle sensors, map 
and position data, infrared sensors, laser sensors, and radar sensors. 
The IFFC control modules 44-47 are also connected through lines 51 to the 
four axis control modules in the PFCS and AFCS. As described hereinbefore 
each of the different axis control modules incorporate a model following 
algorithm. In the yaw, pitch and roll axes the model following algorithm 
is incorporated in the PFCS, which has primary authority in these three 
axes. In the collective axis, however, the mechanical stick has primary 
authority so the model following algorithm is located within the AFCS, as 
shown in FIG. 2. 
Referring now to FIG. 2, which illustrates the functional interconnection 
of the lift axis control modules 38, 42 and 47 of the PFCS, AFCS, and IFFC 
respectively. In the best mode embodiment of the IFFC system it has been 
deemed preferable to have the PFCS and AFCS functional roles altered for 
the collective axis control modules. The PFCS in the collective axis 
control is essentially a full mechanical control in consideration of the 
pilot having a displacement collective stick, and the AFCS embodies the 
electronic content of the flight control, including the model following 
algorithm. The PFCS control module 38 (FIG. 1) receives the pilot command 
signal entered through the displacement collective stick 18 on line 28 
where it is presented directly to PFCS summing junction 76. The pilot 
commanded collective is there summed with a modifying collective command 
signal presented on line 78 to the summing junction from the AFCS, which 
is described in detail hereinafter. The summed resultant signal becomes 
the actual collective command signal which is presented through output 
lines 52 to the mixing function 54. 
The AFCS lift control module 42 receives the vertical command signals from 
the pilot through the sidearm controller 20 on lines 30, and the sensed 
aircraft parameter signals (from sensors 32, FIG. 1) on lines 34. The AFCS 
also receives the pilot discrete signal inputs on the lines 26, which 
include an "Alt Hold" discrete input signal (i.e. altitude hold mode 
signal--labeled A in FIG. 2) which enables (when Alt Hold is selected) or 
disables (when Alt Hold is not selected) the AFCS modifying collective 
command signal from reaching the summing junction 76; the result being 
that when Alt Hold is not selected the AFCS and, therefore, the AOA 
constraint algorithms from the IFFC control module 47 do not modify the 
plot commanded collective signal, which is presented directly to the rotor 
mixing function (45, FIG. 1). 
In normal operation the pilot may enter vertical rate command information 
to the AFCS by pulling up or pushing down on the sidearm controller to 
command a climb or descent. These command signals are presented within the 
AFCS to a summing junction 80, which also receives the modifying vertical 
rate command signal on a line 81 from the IFFC vertical axis module. The 
modifying vertical rate command signal will be described in detail 
hereinafter with respect to FIG. 3. The summed result from the junction 80 
is presented on line 82 as the unfiltered altitude rate command d(ALT)/dt 
to a command model 84, which is a first order lag filter that provides the 
filtered command on lines 86 to the inverse vehicle model transform 88, to 
an integrator 90, and to summing junction 91. The inverse model 88 is 
typically a Z-model transform which may be embodied as a first order lead 
filter, as described more fully in the hereinbefore referenced U.S. Pat. 
No. 5,238,203, and which provides a proportional signal representation of 
a modified commanded vertical displacement signal on line 92 to AFCS 
output summing junction 94. 
The integrator 90 integrates the vertical rate of change signal on lines 86 
to provide an altitude command on lines 96 to summing junction 98, where 
the integral is summed with the aircraft's sensed actual altitude on line 
100 to provide the summed result as an altitude error signal on line 102. 
This error signal is multiplied by a proportional gain factor K1 104, and 
the proportional resultant signal is presented to summing junction 106. 
The error signal is also multiplied by gain K2 108 and integrated through 
integrator 110 to provide an integral altitude signal to summing junction 
112. Finally, the summing junction 91 sums the altitude rate of change 
signal on line 86 with the aircraft's sensed actual vertical rate on line 
114 to provide an altitude rate of change error signal on line 116. This 
rate error signal is multiplied by gain K3 118 and presented to sum 
junction 112. 
The net summed signal from junction 106 and 112 is a proportional plus 
integral plus derivative feedback signal which is presented on lines 120 
to junction 94. The summed result of junction 94 is presented through 
lines 51 to the PFCS module 38 as the modifying collective command signal, 
which is presented on the line 78 to the summing junction 76 via the 
limiting circuitry 122. The limiting circuitry splits the modifying 
collective signal into low frequency and high frequency components. The 
low frequency component is rate limited and presented through tracking 
logic 124 which then backdrives the displacement collective stick through 
servo 25 to provide the pilot with the necessary tactile feel of the full 
collective load. The high frequency component is amplitude limited and 
passes without limit to the line 78 input of the junction 76, the summed 
output of which is the modified collective command signal presented to the 
rotor mixing function 54. 
Referring now to FIG. 3, the IFFC control module 47 receives a weapons 
select status signal on lines 70 from the fire control 68 (FIG. 1) 
identifying an onboard weapon system selected by the pilot. The weapon 
identifying information is presented to a vertical speed constraint 
database 126, typically a look-up table which correlates the 
manufacturer's specified vertical speed value by weapon system 
identification. The output of the look-up table is presented on lines 128 
as a signal representative of the manufacturer's specified vertical speed 
constraint value for the selected weapon. The specification signal is 
presented to limiter circuitry 130 which reduces the specified value of 
the vertical speed to provide an added "safe margin" tolerance. The 
limiter gain is selectable, and may be established on a case by case 
basis. In the best mode embodiment the limiter gain is set at 0.8 v/v to 
provide a 20% attenuation factor. 
The AOA threshold limit (+/-.phi. for assent/descent) is calculated as the 
arc tangent value .phi.=Tan.sup.-1 N/D in module 132, where the numerator 
N is the "as limited" specified vertical speed and the denominator D is 
the aircraft's sensed actual airspeed. The actual airspeed signal is 
presented through lines 34 (FIG. 1) to the IFFC and is converted from 
knots-per-hour to feet-per-second in converter 134. The converted quantity 
is presented through lines 135 to limiter 136 which provides unity gain 
(1.0 v/v) to the sensed airspeed values between lower 138 and upper 140 
airspeed limits established for the aircraft. The output airspeed signal 
from the limiter is then presented through lines 142 to the module 132. 
The calculated AOA value .phi. represents the maximum (+/-)desired 
(assent/descent) angle of attack magnitude, and it is presented through 
lines 144 to limiter 146 . The limiter establishes a unity gain transfer 
(1.0 v/v) between the lower limit -.phi. 148 and the upper limit +.phi. 
150 to the commanded pitch attitude signal on line 152. The commanded 
pitch altitude comes from the AFCS and equals the integral of commanded 
pitch rate signal. The commanded pitch attitude signal is presented 
through converter 154 which provides a scale factor of .pi. to convert the 
signal from pi-radians to radians and is presented on lines 152 to the 
signal input 156 of the limiter 146. For commanded pitch attitude values 
between +/-.phi. the commanded pitch attitude is passed through the 
limiter onto output lines 158 to summing junction 160. Values of commanded 
pitch attitude outside of the lower or upper limits is blocked by the 
limiter resulting in a zero output on line 158 to the junction 160. The 
result is that the summed output of the junction 160 on lines 162 
(referred to as the commanded climb angle) is zero for values of commanded 
pitch attitude between +/-.phi., i.e. within the +/- AOA range, or 
"deadzone", and is equal to the actual commanded pitch attitude for all 
values outside of the AOA range. 
Module 164 determines the tangent of the commanded pitch attitude angular 
value and the tangent value is multiplied in multiplier 166 with the 
actual airspeed signal on lines 135 to provide on lines 168 the desired 
aircraft vertical speed in feet-per-second. The desired vertical speed is 
presented to summing junction 170, which also receives the pilot commanded 
vertical speed on lines 30, which is presented through limiter circuitry 
172. The limit values for the limiter 172 are typically .+-.10 ft./sec. 
(i.e. the limiter passes pilot commands up to .+-.10 ft) and the limiter 
output signal is presented on lines 174 to the junction 170. The junction 
sums the AOA commanded vertical speed with the negative value of the pilot 
commanded vertical speed (with the effect of allowing the AOA to cancel up 
to +/-10 ft/sec of pilot commanded vertical speed) and provides the sum 
difference on lines 176 to multiplier 178. 
The other input to multiplier 178 is a gain factor which is a function of 
earth referenced target direction. It is derived from the aircraft's bank 
angle and the body axis azimuth and elevation to the target. The purpose 
of this gain function is to phase in the AOA algorithm only when the 
aircraft is pointing near the target. In this way the algorithm will not 
interfere with the aircraft maneuvering state when weapon launch is not 
imminent. 
This is accomplished in module 180, which calculates the earth-referenced 
target direction angle (the angle between the aircraft heading and the 
target). Module 180 receives body-referenced azimuth and elevation target 
direction angles (AZB and ELB) and aircraft bank angle (.phi.B) on lines 
70 from the fire control 68 and from the AFCS respectively. The target 
direction angle AZB is multiplied by the cosine of the bank angle (.phi.B) 
and target direction angle ELB is multiplied by the sine of the bank angle 
(.phi.B), and the two products are summed to provide AZE, the 
earth-referenced target directions angle. AZE is passed to the look-up 
table in module 182 via line 181. The module 182 provides a variable value 
multiplicand which is used to multiply the AOA vertical commanded signal 
on lines 176. If the value of AZE is between +/- 5.degree. the multiplier 
is unity (1.0). The multiplier value decreases substantially linearly to 
0.0 when the absolute value of AZE is greater plus or minus 
15.degree..vertline.AZE.vertline.&gt;+/- 150.degree.). The multiplicand 
output of 182 is presented through line 183 to multiplier 178, where it is 
multiplied with the vertical command signal, and the resulting product is 
presented on line 185 to the TRUE input of switch 186. The switch 186 is 
responsive to AOA engage logic (LANGENG) which is a function of Altitude 
Hold engaged, IFFC engaged, and airspeed greater than 30 knots. When 
LANGENG goes from "FALSE" to "TRUE," the output of block 186 switches in a 
smooth, transient-free manner from the FALSE state input, nominally 0.0, 
to the TRUE state input. The maximum rate of change of the output signal 
is defined by the DC input and is typically 10 feet/second/second. The 
output of switch 186 is presented on line 187 as the modified vertical 
rate command signal on line 81 to summing junction 80 (FIG. 2). 
The description heretofore of the present IFFC system functions has been 
disclosed in terms of functional block diagrams using frequency domain 
notation. It is understood by those skilled in the art the these functions 
as disclosed may be enacted in either dedicated hardware circuitry, or 
preferably in programmed software routines capable of execution in a 
microprocessor based electronics control embodiment. Referring to FIG. 4, 
in an exemplary embodiment of a preferred embodiment of a microprocessor 
based control 200 suitable for implementing each of the IFFC system 
control modules functions, the input signal lines 28, 30, 34 and 70 from 
the displacement collective stick 18, the side arm controller 20, the 
sensors 32 and the fire control 68 are received at an input port 202 which 
may include analog-to-digital converters (not shown), frequency-to-digital 
converter (not shown), and such other signal conditioning functions, as 
may be deemed necessary by those skilled in the art to convert the actual 
IFFC signal format to digital signal format. 
The input port is connected through buss 204 to a central processing unit 
(CPU) 206, to memory 208, and to an output port 210. As known, the buss 
provides signal transfer between the elements of the microprocessor 
control. The CPU may be any type of known microprocessor having 
performance characteristics such as may be deemed suitable by those 
skilled in the art for performing the invention in the manner described. 
Similarly the memory is a known functional type, such as RAM, UVPROM, or 
EEPROM, as may be deemed suitable for the application by those skilled in 
the art. The outport port 210 provides: the PFCS output signals to the 
rotor mixing function on lines 52, the return signal from the PFCS to the 
Displacement collective stick on lines 28, the return signals from the 
AFCS to the sidearm controller on lines 30, and the exchanged 
communications between the IFFC and the fire control over lines 70. The 
output port may comprise digital to analog converters (not shown), 
parallel to serial converter (not shown) and a discrete output driver (not 
shown). 
The IFFC of the present invention provides four axis (pitch, roll, yaw and 
collective, or lift) control. The IFFC control laws are integrated with 
the advanced flight control laws of the baseline rotorcraft. These control 
laws are implemented in the pitch, roll, yaw, and collective control axes. 
In general the IFFC control laws generate body axis angular rate (or 
vertical rate) commands that drive command models in the AFCS and 
feedforward commands that sum with the AFCS output commands. 
The weapon and sensor constraint aiding functions couple the aircraft to 
the weapon or sensor directed line of sight (LOS) as in the basic coupled 
aiming except that there is an appropriate deadzone of operation around 
the LOS. Within the deadzone, normal aircraft control remains in effect. 
When the deadzone is exceeded the IFFC control laws provide coupling 
commands to maintain the aircraft at the edge of the constraint boundary. 
The constraint aiding laws are authority limited so that the pilot can 
override them if necessary. The control laws for pilot command canceling 
are now phase sensitive so that the pilot is free to fly back into the 
deadzone. 
Generic IFFC control laws arc provided for each weapon and sensor type, 
including turreted weapons and fixed weapons. These constraint laws use 
pre-defined aircraft maneuver capabilities. In each case , the coupling 
articulates the constraint boundary applicable to that system and is in 
effect only while approaching the constraint envelope limit. Depending 
weapon which weapon is active, the IFFC function selects the appropriate 
limit signals for maintaining the aircraft within constraints. 
The collective axis of the IFFC control laws supports launch of both guided 
and unguided weapons by providing AOA constraint limiting and engine 
torque limiting. It is interfaced with the model following Altitude Hold 
mode of the baseline AFCS. If Altitude Hold is not engaged this mode is 
inactive. The torque limiting function reduces collective when the 
required engine torque exceeds available torque. The pilot can override 
these functions by moving the displacement collective stick on the left 
side of the cockpit. The pilot can move the collective against trim while 
leaving the collective AFCS engaged, or the pilot can disable stick trim 
and the vertical AFCS by pressing the collective trim release switch (the 
trigger switch under the collective stick grip). 
Although the invention has been shown and described with respect to a best 
mode embodiment thereof, it should be understood by those skilled in the 
art that various changes, omissions, and additions may be made to the form 
and detail of the disclosed embodiment without departing from the spirit 
and scope of the invention, as recited in the following claims.