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NASA ADVANCED CONTROL TECHNOLOGY: 
AN OVERVIEW 



Peter R. Kurzhals 
NASA Headquarters 



ABSTRACT 

NASA's current and projected advanced control 
technology programs for future transport aircraft include 
the design and verification of full-flight envelope 
autopilots, the development and flight test of all- 
digital fly-by-wire systems, the evolution of low-cost 
innovative avionics concepts such as split surface 
stability augmentation systems, the evaluation of 
integrated propulsion control and cooperative autopilot/ 
propulsion control systems, the application of active 
control systems to short-haul and long-haul transports, 
and the demonstration of reconfigured active-control 
aircraft. Key technical features and anticipated 
contributions of these technologies are outlined. 

INTRODUCTION 

With the advent of digital microelectronics, practical 
design of a new class of transport aircraft with 
significant performance gains and weight savings through 
active controls has become feasible. NASA is conducting 
several research and development programs specifically 
aimed at generation of the critical technology for such 



Preceding page blank 



advanced control aircraft and for the associated digital 
flight control systems . This paper provides an overview 
of these programs. 

Other ongoing NASA efforts, such as the development 
of highly-reliable, easily-maintainable computer systems 
or automatic landing systems - essential but not unique 
to active controls - are not treated here. NASA- sponsored 
aircraft design studies concerned primarily with the 
definition of advanced control technology requirements 
and benefits for future transports - but not including 
control research and development efforts - are also 
omitted . 

TECHNOLOGY OVERVIEW 

NASA advanced control programs can be naturally 
broken into the development of improved control systems 
and into the application of the resultant control concepts 
for the design of more efficient transport aircraft. 
Figure 1 introduces the related activities. Work on 
advanced control systems is addressed by programs on 
full flight envelope autopilots (FFEAP) , digital fly-by- 
wire systems, innovative avionics systems, and propulsion 
control systems. The extension of these control capabil- 
ities to the definition and validation of advanced aircraft 
designs will be addressed by the active control aircraft 
and the proposed active-control-configured transport 
programs . 



Major design and flight test milestones, indicated 
on the figure, for the NASA programs, will be outlined 
in the specific summary of each program. Completion of 
these design and verification tasks by the early 1980 's is 
planned to permit the incorporation of advanced control 
concepts in the next generation of transport aircraft - 
currently projected to enter service in the mid to late 
1980's. 

Full-Flight-Envelope Autopilots 

Under the first program on improved controls, 
the Ames Research Center is investigating the application 
of optimal control theory to the design of full flight 
envelope autopilots (reference 1) . The associated design 
approach is illustrated in figure 2. The aircraft is 
calibrated over the entire flight regime by trim maps 
which tabulate lift, drag, and moment coefficients versus 
critical aircraft variables such as angle of attack and 
flap angle. The control deflections required to obtain 
a commanded acceleration can then be calculated from 
these trim maps, with feedback used to compensate for 
mismatch between the trim maps and the actual aircraft 
characteristics. The result is a linear acceleration 
command system, and linear optimization theory can be 
applied to define desirable overall trajectory and attitude 
control algorithms. 



Detailed design of this full-flight-envelope auto- 
pilot (FFEAP) is presently underway and is expected to 
be completed in mid 1975. Following FFEAP validation 
during six-degree-of-freedom simulations in 1976, the 
first implementation of the FFEAP is planned as an 
experiment, the FFEAP algorithms will be programmed 
on the onboard STOLAND system, and evaluated during 
representative flight operations. If proven successful, 
follow-on flight tests of the FFEAP system will be 
conducted on short-haul powered-lift aircraft, such as 
a tilt-rotor configuration, in late 1978. 

Preliminary FFEAP results indicate that the 
optimized controller mechanization could significantly 
increase transport aircraft performance over the entire 
flight envelope, and could minimize delays and fuel 
consumption during terminal area operations. 

Digital Fly-By-Wire Systems 
A companion NASA control system program, conducted 
jointly by the Flight Research Center and the Langley 
Research Center, involves the development and flight 
verification of digital fly-by-wire (DFBW) systems. 
The basic phases of this program are represented in 
figure 3. Phase I (references 2-4) has demonstrated 
the feasibility and performance of DFBW control using 



Apollo hardware in a single-channel primary system with 
a triplex analog backup system (reference 5) installed in 
an F-8 aircraft. Direct, stability augmentation, and 
command augmentation system modes were successfully 
evaluated during approximately 18 months of flight testing. 
For Phase II, the Phase I fly-by-wire systems will be 
replaced by a triplex all DFBW system using aircraft- 
compatible computers and sensors. The all-DFBW system 
will then serve as a test bed for early verification 
of critical Space Shuttle software concepts and for 
flight implementation of several advanced control law 
concepts. The first of these, summarized in figure 4, 
will investigate performance improvements obtainable 
through synthesis of selected control configured 
vehicle (CCV) concepts. Specific CCV systems considered 
include static stability augmentation, maneuver and gust 
load control, and envelope limiting. The associated 
control algorithms were designed through an iterative 
quadratic optimization process (reference 6) , and are 
being validated during laboratory simulations at the 
Langley Research Center. After software coding and 
iron-bird checkout of the CCV algorithms, first flight 
tests of the CCV system are scheduled for mid 1976. 

The second advanced control study, illustrated in 
figure 5, addresses the mechanization of an adaptive 
control system compatible with potential transport 

7 



applications. Candidate concepts under investigation 
are an implicit identification scheme (reference 7) 
involving multiple-model hypothesis testing; and two 
explicit identification schemes based on different 
techniques for parameter identification and control 
optimization. The first uses a recursive, weighted- 
least-squares identifier, and an algebraic equation to 
determine the control changes from the previous coiranands 
(reference 8) . The second uses a modified Newton- 
Raphson technique for identification. Other potential 
advanced control approaches being considered for flight 
test include self-organizing systems (references 9-10) , 
which can automatically restructure themselves to 
accommodate sensor and actuator failures with considerable 
attendant reliability increases; and learning control 
systems with the capability to evolve improved aircraft 
modeling and estimation techniques during flight. The 
most promising of these control concepts will be selected 
for flight implementation in 1976. In-flight tests on 
the F-8 will then be conducted in 1977, following mechan- 
ization and ground verification of the resultant flight 
control systems. 

Current Flight Research Center plans for the total 
redundant DFBW systems tests call for a 30 month flight 

test program beginning in 1976. Approximately six 
months will be devoted to validation of the basic system. 



configuration and to inflight verification of Space 
Shuttle software designs. The remainder of the program 
will be available for the advanced control law tests. 

Innovative Avionics Systems 

Besides these efforts on the exploitation of digital 
control, work at the Flight and Ames Research Center is 
concerned with the design and mechanization of innovative 
avionics systems which could reduce avionics cost 
through simplification and modularization. While the 
primary users of such concepts will be general aviation 
aircraft, many of the associated design philosophies 
may be applicable to transports as well. 

One of these concepts , depicted in figure 6 , 
involves the development and flight demonstration of a 
separate surface stability augmentation system (SSSAS) 
on a Beech 99 commuter airlines under a contract managed 
by the Flight Research Center. With this approach 
(reference 11) , the aircraft control surfaces are split 
into primary and secondary segments, and the separate 
secondary surfaces are incorporated in a limited- 
authority ride smoothing and gust alleviation system. 
Since the primary control system can overide the 
secondary system in case of a hard-over failure, the 
SSSAS may be mechanized with single-string, low-cost 
components with considerable associated system cost 



savings. Major improvements in ride quality are expected 
through this approach, which will be validated during 
extensive flight tests in 1975. 

Another low-cost avionics program, conducted by 
the Ames Research Center, focuses on the design of 
integrated avionics systems which take maximxim advantage 
of recent advances in microelectronics and digital 
circuit technology. The design philosophy for this 
system, illustrated in figure 7, will be initiated with 
subsystem concept studies and 1980 technology an^ air 
traffic projections. The resultant specifications and 
requirements will be used to define candidate modular 
avionics systems. The most cost-effective of these 
systems will be carried through subsystem development 
and final design by 1979; and will be evaluated through 
piloted flight simulations in 198 0. 

Propulsion Control Systems 
The application of advanced control techniques to 
the optimization of aircraft propulsion systems perform- 
ance can also result in large improvement in engine 
thrust and fuel economy. Two related NASA programs, 
conducted jointly by the Flight and Lewis Research 
Centers, are concerned with the development of integrated 
propulsion control systems (IPCS) and with cooperative 
aircraft and propulsion control. 



10 



For the first of these efforts, the Air Force and 

NASA have undertaken a joint program (reference 12) to 

demonstrate inflight the benefits obtainable from an 

integrated propulsion control system in an F-111 

aircraft. The associated design philosophy, indicated 

in figure 8, utilizes a high-response control system 

which rapidly senses changes in flow conditions 

and uses a digital controller to command engine 
inlet geometry configurations needed for optimal 

propulsion performance. Such an IPCS can minimize 
stall margin throughout the flight environment, and 
could permit significant reduction in current engine 
safety margins, with attendant increases in range 
projected as large as 10 percent. The F-111 IPCS is 
slated for flight tests in 1975. 

A second NASA effort on propulsion^control involves 
the integration of the propulsion and aircraft control 
systems (reference 13) for the YF-12 research vehicle. 
The analysis of supersonic flight tests on the XB-70 
and yF-12 indicate that airframe/propulsion system 
interactions are the primary cause of altitude fluctua- 
tions in supersonic cruise, of poor lateral-directional 
characteristics, and of severe transients during inlet 
unstarts. It is clear from these flight results that 
the propulsion system cannot be treated independently 
from the aircraft control system. A proposed integrated 



11 



airframe/propulsion control system, shown in figure 9, thus 
incorporates a digital control system which combines the 
inlet, engine and airplane flight controls. The longitu- 
dinal phase of this cooperative control system will be 
flight tested on a yF-12 in 1975, followed by YF-12 
flight tests of the lateral directional phase in 1976. 
Design specifications for a total cooperative control 
system, based on these interim test results, are expected 
to be available by late 1977. 

The most significant payoff of the advanced control 
approaches discussed so far requires consideration of 
their capabilities in the selection of the initial 
aircraft configuration through a new aircraft design 
approach which permits full tradeoffs between aerody- 
namics, structures, and control for the designated 
mission requirements. With this active control 
design approach, reductions in the aircraft natural 
aerodynamic stability and structural loads could be 
obtained through reliance on the damping and load 
control capabilities of a flight-critical automatic 
control system. These reductions in turn can permit large 
savings in aircraft gross weight and fuel. NASA is 
conducting two programs to provide and verify the 
critical technology required for early application 
of such active control designs in future civil transports. 



12 



Active Control Aircraft 
The Active Control Aircraft (ACA) program, 
carried out by the Langley, Flight and Ames Research 
Centers, concentrates on development of the integrated 
active control system and aircraft design technology 
to meet the needs of new short-haul and long-haul 
transport designs in the early 1980 's. Initial work 
will focus on the formulation of an adequate modeling 
and analysis base for ACA design. Specific associated 
tasks include the generation and validation of transonic 
aerodynamic pressure distributions for deflected and 
oscillating control surfaces, of aeroelastic design 
programs for flutter suppression, of prediction tech- 
niques for aircraft structural dynamics and static 
deformations , and of insensitive control techniques 
which allow for uncertainties in the aircraft aerodynamic 
and structural parameters. An integrated conceptual 
design program incorporating these modeling and analytical 
procedures for ACA will be derived to permit incorporation 
of all the active control functions into a workable 
system, and selection of the most cost-effective aircraft 
configuration for a given mission. One of the approaches 
under consideration for the conceptual design process is 
outlined in figure 10. After specification of general 
configuration guidelines and mission requirements, this 



13 



computer-aided design program defines the initial 
aircraft geometry and uses a quadratic optimization 
procedure to converge on suitable final configurations. 
An economic assessment subroutine is then employed to 
determine the best of these alternate configurations, 
and to select the final active control aircraft and 
system designs. To provide the necessary system and 
aircraft inputs for this approach, wind tunnel tests 
and validation flights, using DHC-6 and subsonic 
transport "te^t beds", are planned in 1976 and 1977. 

The next phase of the program involves the 
extension of this initial work into specific short-haul 
and long-haul transport applications. For the short- 
haul application, depicted in figure 11, a ride quality 
and precise trajectory tracking system will be designed 
and installed on a DHC-6 Twin Otter aircraft. Representa- 
tive operational flights of the modified DHC-6 will be 
conducted in 1978 to demonstrate the active control 
system performance and benefits. The system is 
expected to significantly improve ride quality for 
low-wing- loading STOL aircraft, as indicated in the 
figure. Following completion of these tests, a more 
extensive short-haul active control design for powered- 
lift aircraft incorporating envelope limiting, ride 

quality control, gust load alleviation, maneuver load 
control, and flight path control will be designed and 

14 



evaluated for a Tilt Rotor vehicle. Completioft of these 
evaluations is scheduled for the mid 1979 time frame. 

For the long-haul application, represented in 
figure 12, a series of contracted active control aircraft 
designs considering reduced static stability, gust and 
maneuver load alleviation, ride quality and fatigue-life 
control, envelope limiting, and flutter and structural 
mode suppression will be conducted for representative 
subsonic, freighter, transonic, and supersonic missions; 
and the results will be compared with conventional aircraft 
designs. The most promising of these designs will then 
be evaluated in the 1980-1981 time period through design, 
fabrication and flight tests of a scaled research 
vehicle which will concentrate on the demonstration of 
the high-risk technologies essential to validation of 
the ACA design techniques. 

Completion of the ACA program should provide a 
comprehensive design base for the application of 
active control. 

Active Control Configured Transport 
In addition to NASA ' s work on active control 
design procedures and systems, a companion program which 
would carry this technology into practice through actual 
redesign of a jet transport is under consideration for 
initiation in mid 1975. This Active Control Configured 



15 



Transport (ACCT) program, to be managed by the Flight 
Research Center, would make direct use of the digital-fly- 
by wire and active control technology program outputs to 
redesign a small jet transport, such as a Jetstar or 
B-737, to evaluate the resultant benefits and penalties 
in a realistic operational environment. Such a recon- 
figured aircraft could offer major performance improve- 
ments (reference 14) through synergism of active controls 
and advanced aerodynamic technologies. Figure 13 
illustrates some of these potential benefits in terms 
of relative fuel consumption. While individual 
contributions of either control or aerodynamic technologies 
are relatively small, the combination of a fly-by-wire 
active control system with a high-aspect-ratio super- 
critical wing design made possible through maneuver- 
load and gust alleviation can yield appreciable fuel 
savings. Noise footprints for such an ACCT design 
could also be reduced by as much as 90%, based on early 
engineering estimates . 

The ACCT program, represented in figure 14, could 
include an actively-controlled supercritical wing 
located for optimum static margin and a corresponding 
new horizontal and vertical tail. To minimize 
demonstration costs, the associated active control 
system could be mechanized using the all DFBW system 
proven during the F-8 program. Extensive ACCT flight 

16 



tests would verify the fuel savings and performance and 
ride quality improvements obtainable with an integrated 
active control transport design, and would provide 
invaluable experience on the active control system and 
aircraft operations in a representative flight environ- 
ment. 

If warranted by the initial conceptual designs and 
cost/benefit studies, an ACCT test aircraft would be 
selected in 1976. Design of an active control configur- 
ation for this transport could be completed in 1977, and 
the test aircraft could be modified by 1979 for opera- 
tional flights in the 1980-1981 time span. By involving 
potential users throughout the design, implementation, 
and flight test phases of such a demonstrator, airline, 
industry, and FAA acceptance of active controls could 
be significantly accelerated and the associated 
technology could be made widely available for future 
transport applications . 

CONCLUDING REMARKS 
The successful completion of the NASA programs 
touched on in this brief overview should permit a major 
step fojTward in the application of advanced control 
concepts by providing a better understanding of the 
associated system and aircraft design problems and 
benefits. Maximiom participation by industry in the 



17 



definition and implementation of these programs, and wide 
dissemination of the resultant design, development, and 
flight test data will - we hope - be instr\amental in 
bringing about the early realization of the potential of 
active controls. 

We stand on the threshold of a revolution in air- 
craft design, if we can learn to practically harness 
the capability of digital avionics and advanced 
controls. With the increased emphasis on cost- 
effectiveness and fuel-economy, we must take full 
advantage of this capability for the development of 
more efficient and competitive future transports to 
maintain our leadership role in the marketplace - 
and in the air. 



18 



REFERENCES 

1. Meyer, G. : Methodology for Design of Active Controls 
for V/STOL Aircraft. NASA Symposi-um on Advanced Control 
Technology and Its Potential for Future Transport Aircraft, 
1974. 

2. Deets, D. A.; and Szalai, K. J.: Design and Flight 
Experience with a Digital Fly-By-Wire Control System Using 
Apollo Guidance Systems Hardware on an F-8 Aircraft. AIAA 
Paper No. 72-881, 1972. 

3. Deets, D. A.: Design and Development Experience with 
a Digital Fly-By-Wire System Control System in an F-8 
Airplane. NASA Symposium on Advanced Control Technology 
and Its Potential for Future Transport Aircraft, 1974. 

4. Szalai, K. J.: Flight Test Experience with the F-8 
Digital Fly- By-Wire System, a Forecast for ACT. NASA 
Symposium on Advanced Control Technology and Its Potential 
for Future Transport Aircraft, 1974. 

5. Lock, W. ; Peterson, W. ; and Whitman, G. : Mechanization 
and Experience with a Triplex Fly-By-Wire Backup Control 
System. NASA Symposium on Advanced Control Technology and 
Its Potential for Future Transport Aircraft, 1974. 



19 



6. Van Dierendonck, A. J.: Practical Optiinal Flight 
Control for Aircraft with Large Flight Envelopes. 
AIAA Paper 73-159, 1973. 

7. Athens, M. ; and Willner, W. : A Practical Scheme 
for Adaptive Aircraft Flight Control Systems. NASA 
Symposium on Parameter Estimation, 1973. 

8. Kaufman, H. ; and Berry, P.: Digital Adaptive 
Flight Controller Development. Twelfth Symposium on 
Adaptive Processes, 1973. 

9. Montgomery, R. C; and Caglayan, A. K. : A Self- 
Reorganizing Digital Flight Control System for 
Aircraft. AIAA 12th Aerospace Sciences Meeting, 1974. 

10. Montgomery, R. C; and Price, D. B. : Management 
of Analytical Redundancy in Digital Flight Control 
Systems for Aircraft. AIAA Mechanics and Control 

of Flight Conference, 1974. 

11. Roskam, j. ; Barber, M. R. ; and Loschke , P. C: 
Separate Surfaces for Automatic Flight Controls. SAE 
Paper 730 304, 1973. 



20 



12. Benz, C. E.; and Zeller, John R. : Integrated 
Propulsion Control System Program. SAE Paper 730359, 
1973. 

13. Berry, C. T.; and Gilyard, Glenn B. : Some 
Stability and Control Aspects of Airplane/Propulsion 
System Interactions on the YF-12. ASME Winter Annual 
Meeting, 1973. 

14. Lange, R. H.; and Deets, D. A.: A Study of an 
ACT Demonstrator With Substantial Performance 
Improvements Using a Redesigned Jet Star. NASA 
Symposium on Advanced Control and Its Potential for 
Future Transport Aircraft, 1974. 



21 



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FIGURE 14