Earth-referenced wind adjustment for hovering aircraft

In a rotary wing aircraft, longitudinal groundspeed error and lateral groundspeed error are converted to earth coordinates, and the result scaled and integrated, and retransformed into aircraft coordinates for application to a pitch attitude control system and a roll attitude control system. The invention thus instantly transfers wind trim between the longitudinal and lateral channels, whenever a heading change is undertaken.

TECHNICAL FIELD 
This invention relates to providing constantly-updated integrations of 
longitudinal and lateral groundspeed errors as inputs to an automatic 
flight control system, thereby to instantly compensate for wind and 
variations in the direction and magnitude of wind. 
BACKGROUND ART 
Methods for implementing automatic hover hold control systems known to the 
prior art rotary wing aircraft utilize completely independent control 
channels for the pitch and roll axes. To effect a heading change, the 
aircraft will rotate in the yaw axis. Since there is no advance 
information concerning what wind correction will be required for various 
heading orientations, the pitch and roll control channels do not respond 
until there are either velocity or position errors sufficient to cause a 
response. The lag between the occurrence of a change in heading, and 
therefore an accompanying change in the relative direction of wind 
impinging on the aircraft, together with the delay in the aircraft 
responding to the change in aircraft velocity and position, as a 
consequence of the pitch and lateral wind trims not changing when the 
relative direction of the wind to the aircraft changes as a consequence of 
a change in aircraft heading, all accumulate to introduce significant 
errors in position whenever a heading change is made during a hover 
maneuver. As a result, the aircraft may drift significantly from the 
desired hover position during heading changes, and typically will require 
repositioning, either manually or automatically. 
DISCLOSURE OF INVENTION 
Objects of the invention include providing for improved automated heading 
changes during hover of a rotary wing aircraft, reducing the effect of 
wind when changing heading of an aircraft in flight, and providing for 
minimal positional error resulting from heading changes executed by a 
rotary ring aircraft during hover in wind. 
According to the present invention, groundspeed counters in both the 
longitudinal, pitch attitude command channel and the lateral, roll 
attitude command channel of an automatic flight control system are 
resolved into earth coordinates and integrated with a scaling factor to 
provide a north attitude wind adjustment signal and an east attitude wind 
adjustment signal (or the like), which are indicative of the accumulated 
trim required to neutralize the wind, and these signals are retransformed 
into vehicle coordinates (pitch and roll) and summed into the longitudinal 
and lateral, respectively, control channels. In accordance with the 
invention, as a vehicle yaws, the components of required wind adjustment 
are transferred from the roll channel to the pitch channel, and vice 
versa, as necessary, to compensate for the wind. 
The present invention may be implemented with analog circuitry, and in fact 
is described in functional block form, but the invention is preferably 
implemented in a suitably programmed flight control computer, having 
functional capabilities similar to any suitable popular personal computer, 
utilizing mathematical algorithms and other subroutines which are well 
known in the art. The invention has the advantage of automatically 
adjusting for heading, the components of wind-compensating trim. The 
invention has the additional advantage of benefiting a position hover hold 
control function, which generates velocity commands to the velocity hold 
control laws in order to control aircraft position. The invention is 
effective without any independent knowledge about actual wind conditions 
(that is, earth-referenced, sensed wind information), since it simply 
matches the commanded pitch or roll to the actual pitch or roll, which can 
only be caused by wind. 
With the invention, an aircraft which is trimmed with respect to wind in a 
given heading will remain trimmed with respect to the same wind, degree by 
degree, as the heading is changed. 
Other objects, features and advantages of the present invention will become 
more apparent in the light of the following detailed description of 
exemplary embodiments thereof, as illustrated in the accompanying drawing.

BEST MODE FOR CARRYING OUT THE INVENTION 
The present invention is described as it would apply to an unmanned aerial 
vehicle of the type generally referred to in commonly owned U.S. Pat. Nos. 
5,058,824, 5,150,857, 5,429,089, 5,552,983, and 5,676,334, all of which 
are incorporated herein by reference. That vehicle is generally toroidal 
in shape, having counterrotating, variable pitch blade rotors, coaxially 
driven within the center of the toroid. This vehicle has a very 
significant airspeed vs. attitude relationship, which is on the order of 
5.degree. vehicle attitude for every 10 knots of airspeed. Due to the 
essentially symmetrical fuselage, the moments generated by the shroud of 
the vehicle will be the same in response to lateral air speed as it is in 
response to a longitudinal air speed. 
Referring to FIG. 1, an exemplary vehicle 12 has a generally toroidal 
fuselage 13 which also comprises the shroud for the rotors 14. In FIG. 1, 
the vehicle 12 is in hover with its heading due north in a ten knot 
easterly wind. In order to remain stationary, the vehicle requires a ten 
knot easterly air speed. This is accomplished with a right lateral (or 
roll) attitude correction of 5.degree., and a longitudinal attitude 
correction of 0.degree.. In FIG. 2, the vehicle is shown to have changed 
its heading from north to west. For illustrative purposes, it is assumed 
that the vehicle completed the heading change substantially 
instantaneously, although in practice that will not be the case. In any 
event, initially, the attitude correction for wind on the aircraft has not 
changed, remaining 0.degree. forward and 5.degree. rightward, which now 
causes 10 knots of northward flight (to the right of the vehicle) and 10 
knots of forward flight induced by the wind. A combination of wind and 
rightward trim causes the vehicle 12 to accelerate northwesterly as shown 
by the vector 16. In this condition, there is no aft attitude correction 
trim to compensate for the wind, and there continues to be essentially a 
rightward trim, which is undesired. In FIG. 3, the independent roll and 
pitch trim channels begin to compensate for the heading change into the 
westerly wind. Assuming that each has changed by 2.5.degree., including a 
-2.5.degree. forward trim, which is an aft trim to compensate for the 
wind, and a 2.5.degree. rightward trim which is a reduction from the 
original 5.degree. rightward trim, the aircraft still drifts in the 
direction of the arrow 16, although it is slowing down. In FIG. 4, all of 
the trim has been transferred from the roll channel to the pitch channel, 
so that there is a 5.degree. aft trim and no lateral trim. However, in the 
time it takes to accomplish the transfer of the wind trim from one channel 
to the other, the drift in position may be quite large. 
Referring to FIG. 5, the functions shown above the dashed line 21 
illustrate a conventional pitch attitude control system which, when a 
switch 22 is in the position opposite to that shown, will control the 
pitch (longitudinal) attitude of the vehicle in response to an operator 
pitch attitude command on a line 23. The pitch attitude command is summed 
in a summer 26 with a center of gravity induced (CG) pitch attitude 
command on a line 27 and the sensed pitch attitude on a line 28. The CG 
pitch attitude command compensates for variations in the actual position 
of the vehicle center of gravity, which is always forward for stability; 
the variation results mainly from fuel burn. The CG pitch attitude command 
is relatively small, and over any reasonably short period of time can be 
considered to be a constant. The output of the summer 26 on a line 31 
comprises a pitch attitude error which is passed through proportional and 
integral paths 33, 34, respectively, the outputs of which are summed with 
a sensed pitch rate on a line 35 in a summer 36 to provide the pitch 
channel control output on a line 37. 
During automated flight, and particularly during automated hover control, 
the switch 22 will be in the position shown in FIG. 5 thereby providing a 
pitch attitude command from a velocity hold control system which comprises 
the functions disclosed between the dash line 21 and the dash line 41 in 
FIG. 5. A longitudinal groundspeed command is provided on a line 42 to a 
summer 43, the other input to which is the sensed longitudinal groundspeed 
on a line 44, such as may be provided by a global-positioning-system-based 
inertial system. The output of the summer 43 comprises a longitudinal 
groundspeed error on a line 47. The longitudinal speed command and the 
longitudinal groundspeed error are scaled in proportional channels 48 and 
49 and applied to a summer 50. 
All of the apparatus thus described above the dash line 41 is also provided 
in a roll channel illustrated below the dash line 51. Thus the roll 
channel provides a lateral groundspeed error signal on a line 52. 
According to the invention, the lateral and longitudinal groundspeed errors 
are transformed from aircraft coordinates to earth coordinates and 
integrated, the integration continuing until the desired groundspeed is 
reached, which in hover is zero velocity. In FIG. 5, an aircraft/earth 
reference transformation 55 is accomplished by providing a north velocity 
error on a line 56 which is equal to the longitudinal velocity error times 
the cosine of the true heading of the vehicle, as indicated on a line 58, 
minus the lateral velocity acceleration times the sine of the true heading 
of the vehicle. And, an east velocity error on a line 60 is equal to the 
longitudinal velocity error times the sine of the true heading plus the 
lateral velocity error times the cosine of the true heading. The earth 
referenced errors are scaled and integrated in paths 62, 63, so as to 
provide signals on lines 64 and 65 which represent complete north and east 
wind compensation, in earth referenced coordinates. Then, these trim 
values are returned to aircraft coordinates by earth/aircraft reference 
transformation 67, which provides a pitch attitude wind adjustment signal 
on a line 68 equal to the negative of the sum of: the north attitude wind 
adjustment times the cosine of the true heading with the east attitude 
wind adjustment times the sine of the true heading. The earth/vehicle 
transformation 67 provides a roll attitude wind adjustment signal on a 
line 69 as the east attitude wind adjustment times the cosine of the true 
heading minus the north attitude wind adjustment times the sine of the 
true heading. 
The pitch attitude wind adjustment on the line 68 is provided to the summer 
50 along the with scaled longitudinal groundspeed command and the scaled 
longitudinal groundspeed error on lines 70, 71. In hover, on target, the 
longitudinal groundspeed command will be zero velocity, and the sensed 
longitudinal groundspeed should normally be zero velocity except during 
wind gusts (changes in intensity or direction) or during a heading change 
maneuver. Thus, in steady state, the output of the summer 50 will simply 
be the longitudinal pitch attitude wind adjustment on the line 68. The 
roll channel is identical to the pitch channel, except that it responds 
to, and provides signals relating to the roll axis. 
An aircraft employing the invention of FIG. 5 which undergoes any change in 
heading will immediately cause a corresponding transfer between the pitch 
attitude wind adjustment and the roll attitude wind adjustment, which 
includes aircraft/earth reference transformation at the new heading, 
integration of the differences resulting from the new heading, and 
transformation back into aircraft coordinates. Thus, there is no need for 
the system to await a speed variation, which will integrate into a 
positional error, before adjusting for the change in heading. 
Thus, although the invention has been shown and described with respect to 
exemplary embodiments thereof, it should be understood by those skilled in 
the art that the foregoing and various other changes, omissions and 
additions may be made therein and thereto, without departing from the 
spirit and scope of the invention.