Several problems which drive aircraft control technology are discussed. Highly unstable vehicles, flutter speed boundary expansion, and low level automated flight that follows terrain are discussed

Cunningham, T. B.

English

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OPPORTUNITIES FOR AIRCRAFT CONTROLS RESEARCH 
Thomas B. Cunningham 
Honeywell, Inc. 
Systems and Research Center 
Minneapolis, MN 
First Annual NASA Aircraft Controls Workshop 
NASA Langley Research Center 
Hampton, Virginia 
October 25-27, 1983 
571 
https://ntrs.nasa.gov/search.jsp?R=19840012531 2020-03-22T08:18:15+00:00Z
AIRCRAFT PROBLEMS THAT DRIVE 
CONTROL TECHNOLOGY 
In the course of this discussion, I'd like to deal with what I consider 
to be four categories of aircraft problems which drive our technology. 
First are highly unstable vehicles that are emerging. Second, control tech- 
nology is being driven by the current thrust and ultimate benefits that can 
be achieved by expanding the flutter boundary of combat aircraft. Third, we 
are witnessing, both in the fixed-wing and the rotary-wing world, a new 
emphasis on low-level penetration. This, as I'll discuss, will have some 
direct impacts on control technology. Fourth, in general, we along with 
other members of the avionics community have suffered some setbacks in our 
attempts to demonstrate "ilities" in the past. NASA and DOD have made great 
strides in this area; however, improved "ilities", if I can use the word as 
such, will continue to drive our control research technology. 
. HIGHLY UNSTABLE VEHICLES 
l FLUTTER SPEED BOUNDARY EXPANSION 
. LOW LEVEL AUTOMATED FLIGHT 
. IMPROVED “ILITIES” 
572 
L 
- 
CONTROL RESEARCH AREAS 
Breaking down our discussions into technology research areas, I'd like 
to deal with control from a system theoretic standpoint, particularly multi- 
variable control theory and adaptive control. Many of the aircraft techno- 
logy drivers I discussed earlier are going to affect what we do in the 
future of physical device research, particularly actuators, but also 
sensors, computers, and data transmissions. Some overall system concepts 
are going to be impacted by these directions, such as fault-tolerant archi- 
tectures and some continued work required in the analytical fault-tolerant 
areas. This latter category is technology that we have pretty well in hand 
in our controls area, but other avionics system people really need the bene- 
fit of our experiences. Two other areas I'll just touch on basically are 
what control technology will have as an impact for future flight research 
and some comments on higher order language software. 
SYSTEM THEORETIC 
l MULTIVARIABLE CONTROL THEORY 
. ADAPTIVE CONTROL 
PHYSICAL/DEVICE RESEARCH 
. ACTUATION 
l SENSORS 
l COMPUTERS 
l DATA TRANSMISSION 
SYSTEM CONCEPTS 
l FAULT-TOLERANT ARCHITECTURES 
. ANALYTICAL FAULT TOLERANCE 
OTHERS 
. FLIGHT RESEARCH 
. HOL SOFTWARE 
573 
BANDWIDTH INCREASES DUE TO 
RELAYED STATIC STABILITY 
Beginning with highly unstable aircraft, we may be witnessing a signi- 
ficant increase in bandwidth requirements for fighter aircraft, in particu- 
lar due to the desire to achieve the benefits of low drag and maneuvera- 
bility at supersonic speeds. 
This chart shows a simple single-input/single-output Bode relation- 
ship. For instance, we can think of instability being measured by the 
magnitude of the root that is farthest to the right in the right half 
plane. If we denote that by the letter "a" in a frequency response plot, 
we'd like to be 6 dB in gain above this position as a rule of thumb. 
Another rule of thumb for control design engineers is that we'd like to be 
in the area of 20 dB per decade roll-off in the region of crossover. This 
simple relationship would then put the crossover frequency at 2 a. Like- 
wise, for stability and robustness, we'd like to be at least 6 dB below zero 
gain at a frequency of 4 a. 
Two examples of the loop requirements to stabilize the vehicle are the 
X-29 and the F-16. The only one that is in production is the F-16 fighter. 
Its worst flight condition in terms of stability contains a root at +2, 
which means our crossover frequency is at 4 radians per second, and we must 
be rolled off to the tune of 6 dB at 8 radians per second. However, if we 
look at the X-29 Forward Swept Wing fighter, we see that at its most unsta- 
ble flight condition, it has a root at +7, which is approximately 3 l/2 
times what the F-16 represents. This impacts our control bandwidth because 
we need a crossover at 14 radians per second, and we must be rolled off 6 dB 
at 28 radians per second. 
Historically, flight control designers have had 6 dB to model systems 
that have the highest accuracy near crossover, i.e., 2 a and 2 a half of 
decade either side. In the case of the X-29 and potentially the emerging 
ATF class of fighters, this could easily mean that we need to know in 
greater detail the dynamics of the vehicle about 3 l/2 times higher in 
frequency than we've ever dealt with before. This would logically include 
more detailed knowledge of the bending/aeroelastic characteristics of the 
vehicle. 
GAIN 
LOOP 
1 
b 
. 21 44 FRE(IUENCV 
= UNSTABLE 
ROOT 
FREQUENCY 
EXAMPLES: 
F.16 FIGHTEA X.29 FORWARD SWEPT WING 
574 
FLIGHT RESEARCH 
One of the initial impacts that high bandwidth control poses is in the 
area of flight research, and the goal as stated earlier is to achieve model 
uncertainty reduction in the vicinity of control bandpass. In the case of 
highly unstable vehicles, this means that we have to deal with higher and 
higher frequencies, and one of the ways we achieve model uncertainty reduc- 
tion is through a series of flight examinations and parameter identifica- 
tion. The payoff, obviously, is to achieve an ultimate lowering of the 
eventual actuator rate excitations for high bandwidth control. 
GOAL -‘MODEL UNCERTAINTY REOUCTION IN 
CONTROL PASSBAND 
CHALLENGE -‘PARAMETER ID AT HIGHER 
FREQUENCIES 
PAYOFF + LOWER ACTUATOR RATE EXCITATIONS 
FOR HIGH BANDWIDTH CONTROLS 
575 
SENSORS 
Let 'us look at how this nature of highly unstable vehicles impacts 
various hardware components. Historically, flight control sensors have had 
ample bandwidth. We at Honeywell feel that if more bandwidth is needed, 
then expansion is already feasible within the state of the art. Another 
issue that is cropping up here more and more is the notion of shared flLght 
control/navigation/weapon delivery sensors. The placement issue is the one 
that we have to deal with in the flight control community, and the real goal 
here is to achieve a robust multivariable control for sensor fault tolerance 
and high bandwidth performance in the face of ill-placed and dispersed 
sensors. This technology development should continue. 
l FLIGHT CONTROL SENSORS - BANDWIDTH 
EXPANSION FEASIBLE 
l SHARED FLIGHT CONTROL/NAVIGATION/WEAPON 
DELIVERY SENSORS 
-PLACEMENT ISSUES: ROBUST MVC FOR 
SENSOR FAULT TOLERANCE 
HIGH BANDWIDTH PERFORMANCE 
576 
ACTUATION TECHNOLOGY 
The next technology area relating to controls is in the area of actua- 
tors. We have three issues that we must deal with here. 
The first issue is fault tolerance. There is an emerging class of 
technical developments dealing with digital servo electronics which will 
indeed make the actuators more fault tolerant. This is required, because if 
one looks at the reliability analysis of a current flight control system, we 
see that from a research standpoint we have been dealing effectively with 
computers and somewhat effectively with sensors, but the real reliability 
bottleneck in aircraft flight control is the actuator system. 
The next issue is this bandwidth expansion that we've been discussing. 
Although I'm not an expert in actuator technology, I'll pose a couple of 
issues and concerns that I have. One concern is whether we have the hydrau- 
lic technology available to achieve the high authority and rate/bandwidth we 
need for unstable vehicles. I'll also note that electromechanical actuators 
are emerging, which might give us high authority and expanded rate and band- 
width. This technology development should be pursued. 
Third, there is the rate expansion itself for highly unstable vehi- 
cles. One possible solution, within the system theoretic framework, is the 
capability to formalize an optimal blending, if you will, of multivariable 
control solutions which allows us to share the rate requirement among a num- 
ber of different surfaces. 
. FAULT TOLERANCE 
- DIGITAL SERVO ELECTRONICS 
. BANDWIDTH EXPANSION 
-HYDRAULIC +COMPRESSlBILITY LIMITS 
- ELECTRO-MECHANICAL + EMERGING 
. RATE EXPANSION 
- MVC SOLUTIONS FOR OPTIMUM BLENDING 
577 
COMPUTERS 
Another important element in our control loop is the onboard digital 
flight computer. Computer technology is being driven by a number of differ- 
ent applications, flight control being one. For instance, throughput is 
really being driven by signal processing requirements on various aircraft 
and spacecraft applications. The word length related to the accuracy of the 
computations is being driven more by other avionics functions, such as navi- 
gation. Memory, likewise, is being driven by other functions also. Some of 
the storage requirements for digital landmass data, etc., come to mind. 
One area that is currently being driven by flight control is fault 
tolerance. In the emerging set of research issues from a hardware stand- 
point, one needs to look at integrated-functions architectures such as 
integrated flight/navigation and integrated flightlpropulsions. 
Software on the other hand is being driven to higher order languages 
such as Ada. We have to be careful in the controls community not to accept 
massive, unreliable compilers that aren't necessary to do control work. One 
possible solution that is being explored by the software community is to 
allow defined subsets of higher order languages to be used by highly flight 
crucial functions like flight control. 
In summary, control problems in general do not drive computer technol- 
ogy* Certainly the first three issues I've listed here are being driven by 
other requirements. Fault tolerance is currently being driven by flight 
control, but as we'll see when we get into the impact of low-level penetra- 
tion, we will find that other avionics systems indeed need the same kind of 
fault tolerance that flight control currently does. 
. THROUGHPUT + DRIVEN BY SIGNAL PROCESSING 
. WORD LENGTH + DRIVEN BY NAVIGATION 
. MEMORY + DRIVEN BY OTHER FUNCTIONS 
. FAULT TOLERANCE 
HARDWARE: INTEGRATED FUNCTIONS 
ARCHITECTURES 
SOFTWARE: HOL COMPILER SUBSETS 
BOTTOM LINE + CONTROL PROBLEMS 00 NOT DRIVE 
COMPUTER TECHNOLOGY 
578 
MULTIVARIABLE CONTROL THEORY 
I would like to comment on aircraft research problems that drive the 
control theoretical developments. Two areas I'll talk about are the actual 
basic mathematical theory itself and additionally how we might look at a 
research area for applying these techniques. First, in my discussion of 
unstable vehicles, I've alluded to uncertainty reductions and new regions in 
frequency that have yet to be dealt with. Likewise, in our development of 
mathematical theory, we have to formalize ways of dealing with uncertain- 
ties. For instance, if we can generate a mathematical description of 
uncertainty in a given frequency range, we should seek to formalize our 
control design problem to deal directly with that level of uncertainty. 
There is a body of research being conducted at Honeywell and other places 
actually dealing with what we call structured uncertainties. 
The other side of the structured uncertainty issue is the design 
itself. One design technique we're looking at is direct frequency domain 
optimization that attempts to deal with a formalized approach to achieve 
performance goals in the face of uncertainty constraints. 
Uncertainty representations for practical aircraft control design prob- 
lems have not been developed beyond simple examples. Therefore, formalized 
uncertainty boundaries for practical examples should be examined. 
The next item concerns the state of the art in analysis and design 
itself for the whole hierarchy of preliminary design to flight control 
development, to actual production of flight control systems. We in the 
control theoretic community have been remiss for decades in not providing 
the end-user, i.e., the guy who's actually going to design a flight control 
system, with the proper CAD tools to do his job. And even though we've 
taken a useful step back towards the frequency domain, the ultimate flight 
control designer will never use the tools if all he has is a published paper 
out of the Transactions on Automatic Control. 
Third, more directly to the flight control problem itself, I alluded 
earlier to some sharing of surfaces to achieve some overall higher bandwidth 
goals. Coupling this idea with the proliferation of control effecters on 
board modern aircraft, one should look at ways to formalize the control law 
design. Once again, a driving goal is to achieve higher bandwidth control 
for highly unstable vehicles. In addition, emerging classes of work include 
the areas of integrated flight and propulsion control and also some rather 
higher risk areas dealing with robust reconfiguration of control surfaces in 
combat situations, which could have high payoffs. 
. DEVELOPMENT: 
-STRUCTURED UNCERTAINTIES 
- DIRECT FREQUENCY DOMAIN OPTIMIZATION 
. APPLIED: 
- FORMALIZE0 UNCERTAINTY BOUNDARIES 
-ANALYSIS AND DESIGN TOOLS - CA0 
-MULTIPLE EFFECTOR BLENDING FOR COMMON 
GOALS 
HIGHLY UNSTABLE VEHICLES 
INTEGRATED FLIGHT/PROPULSION 
ROBUST RECONFIGURATION 
579 
I 
ADAPTIVE CONTROL RESULTS 
There have been some discussions about the usefulness of the adaptive 
control theory that has been developed over the last couple of decades. I'd 
like to go on record as saying that despite these emerging reservations, we - 
at Honeywell are pretty proud of the adaptive control work that we've per- 
formed in the past, particularly under NASA sponsorship. This chart shows 
some of the more recent efforts at the Honeywell Systems and Research Cen- 
ter. This figure shows some successful adaptive work that we've done with 
NASA and also a very effective technology spinoff. Without going into 
detail, we essentially used the techniques we developed for the F-8 aircraft 
and the CH47 helicopter (which were demonstrated at Dryden in 1977) to 
design an off-shore drilling ship positioning concept that is now in produc- 
tion. 
ADAPTIVE CONTRACTS WITH HONEYWELL 
EXXON/HONEYWELL DEVELOPMENT (1979) 
F-8C DESIGN (1974 - LRC) 
FLIGHT TEST (1977 - DFRC) 
VALT DESIGN (1977 - LRC) 
DEEP WATER OFFSHORE 
DRILLING SHIP 
ADAPTIVE CONTROL 
580 
ACTIVE FLUTTER CONTROL 
Active flutter control has some obvious payoffs that have been recog- 
nized for a number of years. The technology necessary to perform this is 
not mature and also the notion of active flutter mode control, particularly 
the frequencies we're dealing with, is a risky business. It is one problem 
to stabilize an airfoil flutter condition on a wing with known characteris- 
tics, however, from a practical standpoint, future fighter aircraft flutter 
mode control needs to deal with wings that,have numerous dynamic configura- 
tions due to the different kinds and placements of stores. 
581 
ADAPTIVE CONTROL 
This leads us to a body of work that exists in adaptive control which 
has some demonstrated potential.in dealing with this. The issue is to iden- 
tify very rapidly the onset of a dynamic change in a configuration and to 
modify the control structure accordingly. A concept, based upon maximum 
likelihood estimation, has been demonstrated in wind tunnel tests of active 
flutter control with wing store changes. 
Another payoff area for adaptive control deals with the prospect of 
doing onboard real-time reconfigurations due to battle damages for future 
military aircraft. 
RAPID PARAMETER IDENTIFICATION: 
. ACTIVE FLUTTER CONTROL WITH CHANGING 
CONDITIONS 
. BATTLE DAMAGE RECONFIGURATION 
582 
- 
ADVANCED AUTOMATIC TERRAIN FOLLOWING/TERRAIN AVOIDANCE 
The final two areas are combined treatment of low-level penetration and 
improved "ilities." This figure points out one area of great interest 
within DOD; this is the notion of combined terrain following/terrain avoid- 
ance and threat avoidance. As shown schematically, this involves flying 
very low altitudes automatically while incorporating high g maneuvers, 
appropriate pilot interface, and a high level of flight safety. All of the 
avionics subsystems which interface to attack this problem become flight 
safety crucial. This would certainly include the guidance portion of the 
flight control area, navigation, and portions of the mission management. 
&-j-$# ;;PAbz 
GROUNO PATH PROJECTION 
OFPRESELECTEDPATH 
. ALGORllHi”S FOR AUTOMATIC, SIMULTANEOUS 
TERRAIN FOLLOWING~ERRAIN AVOIDANCE 
-LOW ALTITUOES 
-HIGH, MANEUVERING 
0 PILOT INTERFACE 
. FLIGHT SAFETY 
l SENSITIVITY ANALYSIS 
0 COYPUTATI0NA.L REOUIREMENTS 
583 
LOW-LEVEL PENETRATION 
In addition to the fixed-wing terrain following/terrain avoidance and 
threat avoidance, there is an emerging set of concepts that the Army would 
like to develop for a single-seat light helicopter for nap-of-the-Earth 
flight. The challenge that I believe exists is to somehow integrate flight, 
navigation, propulsion, weapons, etc., in a fault-tolerant system to allow 
us to do nap-of-the-Earth flight with high performance and a high degree of 
safety. 
One research area is sensor blending. I think our estimation techno- 
logy applies here. We also need more creative failure management solu- 
tions. Again, I think estimation technology applies, such as expanding upon 
things like analytic redundancy, plus development of emerging AI concepts 
based on expert systems and advanced planning. Also there is this critical 
need to automate verification and validation of systems, both the hardware 
and the software, for future flight management systems. And finally in the 
area of flight path management, we need to somehow go down the road and 
actually install trajectory optimization capabilities that operate in 
real time for onboard applications. In addition to that, there are some 
front-end decision making functions that perhaps could be handled by AI 
planning concepts. 
EXAMPLES 
. AUTOMATIC TF/TA/OA 
. SINGLE-SEAT LIGHT HELICOPTER - NOE 
CHALLENGE 
0 FAULT-TOLERANT INTEGRATED FLIGHT/NAV/ 
PRDPULSIONMTEAPDNS 
RESEARCH AREAS 
0 SENSOR BLENDING - ESTIMATION 
. CREATIVE FAILURE MANAGEMENT 
- ESTIMATION, Al 
0 AUTOMATED VERIFICATION AND VALIDATION 
. FLIGHT PATH MANAGEMENT -TRAJECTORY 
OPTIMIZATION, Al 
584 


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