Document
IMPLICATIONS OF CONTROLTEXHNOLOGY ON AIRCRAFT DESIGN Steven M. Sliwa and P. Douglas Arbuckle NASA Langley Research Center Hampton, Virginia First Annual NASA Aircraft Controls Workshop NASA Langley Research Center Hampton, Virginia October 25-27, 1983 ABSTRACT New controls technologies are now available for implementation with aircraft systems. Many aircraft with state-of-the-art technology in the fields of aerodyna- structures,and propulsion'require extensive augmentation merely for safety of mics, flight considerations in addition to potential performance improvements. The actual performance benefits of integrating the new controls concepts with other new technol- ogies can be optimized by including such considerations early in the design process.
several advanced aircraft designs have run into considerable problems Recently, related to control systems and flying qualities during flight test, requiring costly redesign and fine-tuning efforts. It is no longer possible for the aircraft design to be completed prior to getting the controls specialists involved. The challenge to the control system designer has become so great that his concerns must be considered at the conceptual design level. A computer program developed at NASA for evaluating the economic payoffs of integrating controls into the design of transport aircraft at the beginning will be described.
o NEW CONTROLS TECHNOLOGIES ARE AVAILABLE o MANY NEW AIRCRAFT REQUIRE ADVANCED CONTROLS o EXPENSE OF FINE TUNING CONTROL SYSTEMS FOR CURRENT STATE- OF-THE-ART AIRCRAFT HAS RISEN DRAMATICALLY 8 INTEGRATING CONTROLS INTO DESIGN PROCESS IS BENEFICIAL o A TOOL HAS BEEN DEVELOPED TO EVALUATE THE PAYOFFS OF CONTROLS INTEGRATION INCREASE IN CONTROLSOOMPLEXITY During the past 20 years, the control systems being used on state-of-the-art aircraft have improved significantly. In the 1950's and 1960's, simple control laws were being applied to improve the flying qualities. In contrast, current configura- tions may require extensive augmentation for safety of flight as well as for good flying qualities. Because of this, and because of the increased complexity of all aircraft systems, it has become extremely difficult to fine-tune or adjust control laws during flight test. Redesign efforts currently require significant amounts of engineering, which result in costly delays. Previously, very simple control schemes were used merely for improving flying qualities, and mechanical back-up systems were always utilized in the event of electronic component failure. Now, highly complex laws which rely on the improved reliability of digital and analog circuits use redun- dant systems for back-up modes. These examples illustrate some of the fundamental issues facing a control system design engineer today.
SIMPLE
c\
AUGMENTATION III a USED FOR GOODFLYI NG QUALITIES EXTENQVE AUGMENTATION . EASY TO ADJUST CONTROLLAWS REQUIRED AT FLIGHT TEST FOR SAFETY OF FLIGHT 0 S IMPLE CONTROL 0 VERY EXPENSIVE CONTROLS LAWS WITH TO ADJUST COMPLEX IT\r / MECHANICAL CONTROLLAWS AT BACKUP FLIGHT TEST COMPLEX CONTROLLAWS WITH MULTI PLY L REDUNDANT SYSTEMS -I I ~.. ~~ I I w 1960 1970 1980 COMPARISONOF CONTROLSTECHNOLOGY A comparison of some of the characteristics of early automatic control systems for aircraft and those being applied to current configurations is shown below. Ini- tially, control systems were designed using simple single-loop analyses for aircraft with limited envelopes where rigid airframe assumptions were adequate.
Now flexible aircraft with expanded envelopes have significant aeroservoelastic interactions that cannot be ignored during control system design. The current tendency is to develop digital fly-by-wire control systems utilizing complex multi-input, multi-output design techniques with sophisticated redundancy management.
Clearly, to achieve the full potential of applying these technologies, the controls integration must occur early in the design process.
THEN NOW o MECHANICAL LINKAGES o DIGITAL 6 DOF FLY BY WIRE o SIMPLE YAW DAMPER WAS ONLY m COMPLEX CONTROL LAWS WITH AUGMENTATION HIGH-ORDER COHPENSATORS o SIMPLE SISO DESIGN TECH- . COMPLEX MIMO DESIGN TECH- NIQUES USED DURING CONTROL NIQUES USED DURING CONTROL SYSTEM DESIGN SYSTEM DESIGN a RIGID AIRFRAME ASSUMPTION o AEROSERVOELASTIC INTERAC- TIONS IMPORTANT GOOD o LIMITED ENVELOPE o EXPANDED ENVELOPE o SIMPLE CONTROL MODES o NEW. COMPLEX CONTROL MODES o REDUNDANCY THROUGH MECHANI- o REDUNDANCY THROUGH MULTIPLE CAL STRENGTH SYSTEMS o NEED FOR CONTROLS INTEGRA- TION EARLY IN DESIGN EFFORT EVOLUTION OF CONTROL SYSTEMS The control system components have undergone considerable change and refine- ment. Originally, simple mechanical linkages using cables, pulleys, and push rods were used. As hydraulic boost became popular, it became possible to improve the fly- ing qualities in certain flight regimes by feeding back a sensed variable, such as yaw rate. The control system with simple augmentation still maintained full author- ity through mechanical connections between the pilot and the control surface. In the event of a failure of a control system component, the pilot still maintained control, but with reduced flying qualities. The current trend of fly-by-wire control systems requires redundancy of critical elements since there will no longer be mechanical connections between the pilot and control system as a backup. The concepts of fault tolerance, detection, and isolation are new areas of important research.
CONTROLSTICK CONTROLSTICK n SENSOR t COMPENSATOR SIMPLE AUGMENTATIONUSED FOR MECHANICAL GOOD FLYING QUALITIES CONTROLLER FORCESTICK EXTENSIVE AUGMENTATIONREQUIRED FOR SAFEI-Y OF FLIGHT APPLICATION OF RSSAS To A CURRENT TRANSPORT CONFIGURATION Relaxed Static Stability Augmentation Systems (RSSAS) for transport configura- tions is one application of advanced control systems that may result in significant benefits. Immediate performance gains can usually be realized through a reduction in trim drag. Further gains can be achieved by resizing the horizontal tail due to a reduction in the stability constraint for the inherent aerodynamic stability of the aircraft. Good flying qualities will be achieved by the active control system. The reduction in tail area results in a decrease in aircraft operating weight and drag.
All of these benefits yield fuel savings of 2 to 4 percent for most transport config- urations.
30% DECREASED RESIZED TRANSPORTTO TARE ADVANTAGEOF RSSAS can be achieved by introduc- The greatest benefits of utilizing a RSSAS system ing the concept at the conceptual design stages. A reduction in tail area results in Hence, the wing and engine can be resized, resulting in weight and drag savings.
more weight savings. Additionally, the fuselage and landing gear structure can be redesigned for the lighter weight. In fact, after the airframe modifications, a fur- ther reduction in tail area may be possible, resulting in another round of changes.
These benefits continue to cascade through the design but generally converge rapidly, resulting in a design which takes maximum, synergistic advantage of applying this new technology. If the concept is not introduced soon enough, the full benefits of RSSAS cannot be achieved. In the case of transport aircraft, fuel savings of 6 to 9 per- cent are possible.
ACTIVE CONTROL DESIGN4,
‘\\
PROGRESSIN DESIGN The actual integration of multiple decentralized control systems into a single centralized control system has also been a recent development which will result in augmented operational safety, performance, and capability as well as improved In present practice, each component of the vehicle is designed indepen- economy.
dently. Certain advanced designs require control systems for various aspects, such as flying qualities, engine performance, structural damping, and weapon control. Each subsystem typically has an independent controller which is directed by the crew or flight management computer. It is conceivable that independent controllers could work in harmony; but, it is just as likely that they will conflict with each other.
to integrate all the controls and design each subsystem A preferred approach is controller simultaneously. Such a system will tend to work in harmony in response to crew or computer commands.
INTEGRATED DESIGN OF COMPONENT DESIGN OF PROPULSION, AERODYNAMICS, PROPULSION, AERODYNAMICS, STRUCTURES AND CONTROLS STRUCTURESAND CONTROLS DIRECT CONTROL OF FLIGHT COMPUTER ENGI NE . CREW
I
CREW CONTROL FULL POTENTIAL OF INTEGRATED USE OF CONTROLS Once the use of advanced integrated controls has been hypothesized, there are Modern control theory allows the use of multiple many avenues that can be explored.
effecters allowing such things as wing warping, rolling tails, spoilers, leading-edge devices, Unconven- and thrust vectoring for control and performance enhancements.
tional flight modes, such as target alignment independent of flight path or side can then be contemplated. All of these functions cannot be force excursions, Instead, a total inte- properly used if a separate controller is designed for each.
grated control system design approach should be used to minimize the conflicts and optimize the overall performance.
TRAJECTORYOPTIMIZATION LIFE/ CYCLE EXTENS IONS UNCONVENTIONALFLIGHT MODES NGINE CON TROL ENHANCEDFLYING QUALITIES NOZZLE CONT ROL FLIGHT-PATH CONTROL WEAPON GUIDANCE AND CONTROL CONFIGURATION BENEFITS OPTIMUM PRELIMINARY DESIGN OF TRANSPORTS OPDOT (Optimum Preliminary Design of Transports) is a computer program developed at NASA Langley Research Center for evaluating the impact of new controls technolo- It provides the capability to look gies upon transport aircraft (see reference 1).
at configurations which have been resized to take advantage of active controls and provide an indication of econoniic sensitivity to its use or the requisite assump- tions. Although this tool returns a conceptual design configuration as its output, it does not have the accuracy, in absolute terms, to yield satisfactory point designs for immediate use by aircraft manufacturers. However, the relative accuracy of comparing generated configurations while varying technology assumptions has been demonstrated to be highly reliable making OPDOT a useful tool for ascertaining the synergistic benefits of active controls, composite structures, improved engine efficiencies, and other advanced technology developments.
ACTIVE -=EEL WITH FOR RElAXED STABlUTY ACTIVE CONTROLS OPTIMAL DESIGN METHODOLOGY The approach that is used by OPDOT is direct numerical optimization of an econo- mic performance index. A set of independent design variables is iterated given a set of design constants and data. The design variables include wing geometry, tail geometry, fuselage size, engine size, etc. This iteration continues until the opti- mum performance index is found which satisfies all the constraint functions. The analyst interacts with OPDOT by varying the input parameters to the contraint func- tions or to the design constants. The optimization of aircraft geometry features is equivalent to finding the ideal aircraft size, but with more degrees of freedom than classical design procedures will allow.
WING AREA ECONOMICS WING ASPECT RATIO FIXED GEOMETRIES TECHNOLOGYLEVEL ‘$;:;;EA;N’$;.
ETC.
L/D WT.
GEOMETRYCONSTRAINTS CONTROL POWER \ FUNCTION / ETC.
I- 1
NO + YES CONSTRAINED
I SOLUTION I
PERFORMANCE FUNCTION FLOW DIAGRAM The performance index in OPDOT is computed by having a candidate configuration "fly" an entire mission while satisfying reserve fuel requirements. Industry statis- tics are used for estimating weights and costs. The stability and control analysis is similar to Datcom-type capabilities, and the program computes the interference drag in a general way, making OPDOT sensitive to tail sizing considerations. The flight profile is a multiple-step model of a suboptimal cruise/climb for optimum fuel effi- ciency. The program.is fairly flexible to use and has graphics output to illustrate each configuration.
rPERFORMANCE c ROI * DOC 1 INDEX .
/ COMPUTATION COST I I nwt FEATURES 1 CRUIS; STEP 1 l INDUSTRY STATISTICS FOR COSTS AND WEIGHTS ENGINE
I I
l DATCOM-TYPESTABILITY AND CONTROLDERIVATIVES l MULTIPLE-STEP,SUBOPTIMAL CRUISE/CLIMB @GENERALIZED INTERFERENCE DRAG METHODOLOGY FOR CONDUCTINGSENSITIVITY STUDIES A study is performed by inputting a set of problem parameters and selecting an initial set of independent design variables. OPDOT finds a solution, and that configuration is saved for later comparison.
The analyst then systematically varies a design constant or constraint function, andeach optimum design is stored.
Then a locus of optimum designs can be plotted as a function of the parameter in question.
This plot can be used to determine the sensitivity of a design to applying a new technology, for example, and each point includes the maximum synergistic benefits available for the set of inputs specified.
OPDOT OPTIMAL (Optimize design based on DESIGNS DESIGN CONSTANTS J an economic index) I I- - L- I DESIGN CONSTRAINTS ’ t I CHANGE A DESIGN CONSTANT I OR DESIGN CONSTRAINT - x 103 r INCOME CONFIGURATION 1 REQUIRED FOR A
J
FIXED ROI, $/FLIGHT -- I I I I l I 5000 6000 7ooO 8000 9000 10000 FIELD LENGTH, ft FL,YING QUALITIES STUDY One study that was made with OPDOT (references 2,3) was the evaluation of the impact of minimum acceptable flying qualities upon aircraft design. This is the prime factor which influences aircraft design when RSSAS systems are considered. It is assumed that an RSSAS system will augment the flying qualities up to more than acceptable levels, but provisions must be made in the event the autopilot/augmenta- tion system fails. Transport aircraft will generally have mechanical backups, so they should have sufficient unaugmented stability to assure the flight can be completed after a set of failures. Clearly these requirements, in effect, specify the inherent aerodynamic stability characteristics of the configuration. OPDOT will give the designer and regulators economic sensitivities to these criteria, enabling a proper compromise between safety and economy to be made.
During the course of this study, it was found that many of the criteria being considered for unaugmented flying quali- ties of transports with RSSAS were inadequate or inappropriate for specifying airplane design parameters.
o LEVEL OF UNAUGMENTED FLYING QUALITIES DETERMINES INHERENT STABILITY CHARACTERISTICS o ECONOMIC SENSITIVITIES FOR THESE CRITERIA WERE FOUND o MANY CRITERIA WERE INADEQUATE FOR PROPERLY SPECIFYING THE UNAUGMENTED FLYING QUALITIES IMPACT OF STATIC MARCIN considering the impact of relaxing the static stability A study was made requirement for transport aircraft. A locus of optimum designs is plotted. For the configuration being considered, a savings of 2.5 percent in direct operating cost is possible when compared to a baseline configuration with 5-percent static margin.
This corresponds to a fuel savings of 6 percent. At a certain point, in this case at -7 percent static margin, reducing the static stability constraint yields no further improvements. This is because the control constraints (typically nose-gear unstick during takeoff) override the tendency to make the tail smaller. A certain minimum size tail is required for control, and the center-of-gravity cannot be moved any further aft without sacrificing nose gear steering traction.
5-
\ \ \
4-
\
3-
2-
l-
SAVINGS
O-
IN
-1 -
DOC, %
-2 -
OVERR I DE
-4
0 BASELINEWITH NOMINAL SM (5%)
-5
I I I I
I I
-6
-;0 -15 -L -25 -10 0 5 10 15 20
MINIMUM ALLOWABLE STATIC MARGIN, %MAC
IMPACT OF LOADABILI'N UPON LXX Implied in the static margin sensitivity study was a range of allowed center-of- The control constraints are usually critical on the forward c.g.
gravity travel.
limit, and the stability constraints are usually critical on the aft c.g. limit.
Reducing this range results in savings for all static margins under consideration.
However, most benefits are achieved during the first 50 percent of reduction, indi- cating that if more careful center-of-gravity control is possible, a fuel savings of 2 percent or more is possible.
ALLOWABLE CGTRAVEL
w 1.22 METERS
----a .61 METERS
---a 0 METERS
~=-=-=p&.~-
Q--Q-
8-o
6-
PERCENT SAVINGS
IN DOC
4-
2-
I I I I I I
I
L I I
-15 -10 -5
O -20 0 5 10
15 20
STATIC MARGIN.% mat
IMPACT OF LANDING GEAR LOCATION UPON DOC Also implied in the static margin study was an aft limit for placing the landing gear. Studies have shown that the maximum aft placement for transport aircraft, where the gear and wing are collocated for structural efficiency, is about 65 percent of the mean aerodynamic chord. This is a critical constraint for RSSAS aircraft since it limits how far aft the center of gravity can travel before traction for nose gear steering is lost. Savings of nearly 1 percent in direct operating cost are pos- sible if the gear could be located further aft without structural weight penalty.
This corresponds to a fuel savings of over 2 percent.
M IL-F-8785B CRITERIA
B LEVEL I
10 - -------II LEVEL71 -----t7 LEVELIII 8- PERCENT SAVINGS 6 _ ---,*-” IN DOC ~-- ---=~===4= --
4-
2-
n 0 I Q I I I I o- (y-
.65 .70 .75 .80 .85 .90 .95 1.0
LOCATION OF LANDING GEAR, XLG/C DOC SAVINGS VERSUS TIME-TO-DOUBLE Another unaugmented flying qualities criterion that may be of interest is time-to- This plot illustrates the possible importance of economic sensi- double amplitude.
tivity to a proposed criteria. If a designer or regulator is considering applying a it is easy to see that the economic benefits of constraint of 30 or 40 seconds, relaxing the constraint from 30 to 40 seconds is of little economic consequence.
However, the opposite is true if considering an arbitrary boundary ranging between 2 and 6 seconds. The economic sensitivity information should be used before establish- ing the flight qualities criteria boundary since it significantly impacts the air- craft design.
6-
PERCENTSAVINGS
IN DOC
4-
2-
I I I I I
I
0 10 20 30 40 50 60
TIME-TO-DOUBLE, T2' set
IMPACT OF LOAD ALLEVIATION Gust load alleviation and maneuver load alleviation are active controls concepts Utilization of these technologies impacts the that have potential economic payoff.
design because the structure could be designed to a lower limit load factor resulting Plotted is the savings in empty weight and direct operating in a weight savings.
The dotted line just reflects cost for incremental reduction in limit load factor.
The solid line benefits of the lighter structure for the baseline configuration.
includes resizing the airframe to take advantage of the weight savings from active controls.
OPTIMALLY RESIZED
-- - BASELINE CONFIGlJRATION
E R
”
SAVINGS
7 - OPERATINGEMPTY WEIGHT
WITH
6-
RESPECT
TO
BASELINE,
%
.7 l 8
.5 .6
.l .2 .3 .4
INCREMENTALREDUCTION IN DESIGN LIMIT LOAD, m’s
I I I,,,, I 11.11 I I I I I, mm111.11. _.I. I --I ..,, . ,, ,., . -. . ..--__. --- -.--- --- RESEARCH USING OPDOT Other studies have been performed using OPDOT, including the investigation of the relative benefits of applying general technology improvements to transports and the evaluation of required economic and mission assumptions. Recently, a study was completed which determined the economic viability of canard transports when compared to conventional aft tail configurations. Future studies planned include the comple- tion of vectored thrust integration for transports; multi-body and multi-surface configuration with canard, wing, and aft tail evaluation; and commuter transport technology requirements.
OTHER STUDIES o RANKING OF OTHER GENERIC TECHNOLOGY IMPROVEMENTS
(REFS. ‘-I,51
o EVALUATION OF ECONOMIC AND MISSION ASSUMPTION
(REF. 5)
o DETERMINATION OF ECONOMIC VIABILITY OF CANARD TRANSPORTS (REF. 6) FUTURE STUDIES o COMPLETION OF VECTORED THRUST STUDIES o MULTI-BODY AND MULTI-SURFACE TRANSPORT EVALUATION o STUDY COMMUTER TRANSPORT TECHNOLOGY REQUIREMENTS o EXTEND PROGRAM TO OTHER AIRCRAFT TYPES SUMMARY The integration of controls early in the design process is important because the implication of unaugmented flying qualities during control system failures impacts the aerodynamic design;because it is a requisite for the proposed technology improve- ments to achieve their full, synergistic potential; and, because flight test expense can be saved. Adjustments to the control laws after an advanced technology prototype has been built is no longer -an easy proposition. Hence, it has become increasingly important to include control technologist and design considerations during conceptual design. In this discussion, a computer program developed at NASA Langley was described which utilizes optimization techniques to evaluate economic sensitivities of applying new technologies at the preliminary design level of transport aircraft.
o IT IS BENEFICIAL TO INTEGRATE CONTROLS CONSIDERATIONS INTO BEGINNING OF DESIGN PROCESS CONTROL SYSTEM AND FAILURE MODE ASSUMPTIONS IMPACT INHERENT AERODYNAMIC DESIGN FULL POTENTIAL OF ALL TECHNOLOGIES CAN BE REALIZED fLIGHT TEST EXPENSE WITH RESPECT TO FLYING QUALITIES CAN BE SAVED o A TOOL USING OPTIMIZATION TECHNIQUES HAS BEEN DEVELOPED FOR TRANSPORT AIRCRAFT REFERENCES 1. Sliwa, Steven M.; and Arbuckle, P. Douglas: OPDOT: A Computer Program for the Opti- lnum Preliminary Design of a Transport Airplane. NASA TM-81857, 1980.
2. Sliwa, Steven M.: Impact of Longitudinal Flying Qualities Upon the Designof a Trans- port With Active Controls. AIAA Paper No. 80-1570, Aug. 1980.
3. Siiwa, Steven M.: Economic Evaluation of Flying-Qualities Design Criteria for a Transport Configured With Relaxed Static Stability.
NASA TP-1760, 1980.
4. Sliwa, Steven M.: Sensitivity of the Optimal Design Process to Design Constraints -7nd Performance Index for a Transport Airplane. AIAA Paper No. 80-1895, Aug. 1980.
5. Sliwa, Steven M.: Use of Constrained Optimization in the Conceptual Design of a Medium-Range Subsonic Transport. NASA TP-1762, December, 1980.
6. Arbuckle, P. Douglas; and Sliwa, Steven M.: Parametric Study of Critical Con- stralnts for a Canard Configured Medium Range Transport Using Conceptual Design Opti- mization. AIAA Paper No. 83-2141, Aug. 1983.