ACTIVE CONTROLS CHANGING THE RULES

OF STRUCTURAL DESIGN

50

By RAYV. HOOD NASA Advanced Technology Transport Program Office

Active controls can substitute for much

weight aircraft carry to provide passive pro­

tection against natural and man-made hazards

as wel l as improve ride and increase maneuverabi I ity

In most current commercial aircraft, mechanical and electronic devices , in combination with the aerodynamic control surfaces, augment inherent stability and control characteristics. Yaw dampers increase d utch-rolI damping for swept-wing air­craft, for example. Considerable current work aims at going significantly beyond stability augmen­tation by using aerodynamic surfaces in com­bination with advanced flight computers and electrohydraulic systems to improve economics, smooth the passenger's ride, reduce terminal-area noise, and ease terminal-area congestion. This emerging technology is becoming known as active controls technology (ACT).

Active controls might improve economics by reducing structural weight, lengthening fatigue life, and improving aerodynamic performance. The Advanced Transport Technology (A TT) systems studies have looked at relaxing static stability, controlling maneuver load , actively suppressing flutter, and aIleviating gust loads. Active controls can al so smooth the ride by improving the aircraft's response to turbulence, through a control function directly related to those used to alleviate gust loads and lengthen fatigue life. The several aims may sometimes conflict, however, forcing trades be­tween them.

The future will see the introduction of advanced operational procedures to aIleviate noise and congestion in the terminal area. And the Air Traffic Control System may demand control of an aircraft in time as weIl as in three spatial dimensions. An aircraft will need a highly automated and precise flight-path control system. In the terminal area, propulsion wilI act as part of the control system, along with aerodynamic controls to exert lift, side force, and drag. The pilot will have a display of his desired and actual four­dimensional position, which , combined with the more precise controls, will improve safety in the terminal area.

Active controls can be added at the end of design, as is normally done with ordinary controls today; but to obtain the maximum benefits, active controls must be integrated into the design early. Consideration of active-control concepts early in the design cycle (F-l) will change the alIowable ranges for some variables in the parametric trade studies . The aircraft becomes a control-configured vehicle (CCV).

The USAF has shown much interest in CCV and has contributed significantly to the development of CCV technology. However, a fully integrated CCV design has not yet been demonstrated in flight. The A TT systems studies investigated partially in­tegrating active controls technology into long­range, high-subsonic-cruise commercial trans­ports. Relaxed static stability appears to offer the

A stronautics & A eronautics

greatest potential benefit. Conventional design practice requires the vehicle

to possess some degree of inherent stability. Thus, if stability augmentation fails , the pilot can safely complete the flight. Static longitudinal stability is the tendency of the aircraft to be restored to its original speed following a disturbance. The speed of a statically unstable aircraft continues to diverge from its original value following a disturbance. The distance between the aircraft's center of gravity (CG) and a position called the neutral point (NP) measures static stability. (Nonaerodynamicists will find it helpful in visualizing the moments affecting stability to consider NP somewhat like the center of lift , CL. NP has a different and more complicated definition than CL and only in specific cir­cumstances coincides with it.-Ed.) As the CG moves aft toward the NP, the aircraft becomes less stable. If the CG continues to move aft past the NP,

'"the aircraft becomes unstable. As speed increases, the NP may move fore or aft

due to Mach number and aeroelastic effects. Conventional design requires transport aircraft to maintain the CG ahead of or at the NP throughout the flight envelope, which sets the maximum aft CG limit. The graph for sizing the horizontal tail by conventional stapdards (F-2) has a plot for zero static margin, which means that the CG and NP are coincident all along the line. As the CG shifts aft (relative to the reference point at the leading edge of the mean aerodynamic chord, MAC), the tail must grow larger and provide increased lift to move the NP aft with it. The parallel dotted line represents an allowable shift in CG of 10% of MAC as the aircraft bums fuel in flight.

Conventional design standards, furthermore , call for insuring that the pilot can trim the aircraft for maximum lift coefficient during takeoff. This requirement sets the farthest forward position of the CG. A given size of tail can counter the moment resulting from a separation of CG and wing-body CL for a CG set only so far forward . Where the curves for takeoff trim and static stability intersect, allowing a 10% CG shift, determines the size of the horizontal tail.

The cruise CG selected in this manner will generally fall somewhat ahead of the CL and require a downward tail load to maintain balance. Increased lift must counter the increased down­ward force on the tail. The resulting increase in wing drag, combined with the tail drag, is called the trim drag.

Relaxing the static stability requirements allows moving the CG aft near CL, where the horizontal tail can be smallest and the trim drag approaches zero. Instead of using static stability to set the aft CG limit, the CD/ Convair study used the ability to trim the clean configuration to the maximum August 1972

PROPULSION DESIGN

AERODYNAMI C DESIGN

STRUCTURAL DESIGN

ADVANCED FLI GHT CONTROL DES I GN

CONFI GURATI ON

'-'-----'-'I

AND PERFORMANCEi-----------' OPTIMIZATI ON

F-1 Maximum benefits from active oontrols can come only by considering controls early in the design cycle, at bottom, rather than last as at present , top.

PROPULSION DESIGN

AERODYNAMIC DESIGN

STRUCTURAL DESIGN

COOFI GURATI ON AND PERFORMANC E 1----------' OPTI MIZATION

F-2 SIZING THE HORIZONTAL TAIL For Convair Mach 0.98 configuration.

PRODUCT . DEFI N III ON

PRODUCT DEFINITION

• RELAXED Sl ATIC STABILITY • CONVENTI ONAL BALANCE

401----u ... ::;;

'" ' 20 ~ > ... cr <..:> ~ DI-----f'!". cr ~ t5 u

-201-----1

Req. HTV

-40 0'" --..... 4---'-...... 8'----'-:---'--' .4 .8

HORIZONTAL TA IL VOLUME, (SHISREFI ( t HlcREFI

F-3 HORIZONTAL-TAIL DRAG BY CONVENTIONAL DESIGN For Convair Mach 0.98 configuration , CL= 0.40.

TOTAL HORIZONTAL TAIL DRAG - '-... t

IFrom Ref. J)

HORIZONTAL TAIL SKIN FRICTION DRAG

~ o~-~~~-----~~~----~~~

0.6 0.8 1.0 1.2 HOR I ZONTAL TAl L VOLUME

51

52

F-4 TAIL-SIZE SAVINGS

u-n

Advanced Technology Flight Critical Stability Augmentation Maneuver Load Alleviation Gust Load Alleviation Optimum Cruise C. G. and Reduced C. G. Ra nge

usable wing-body lift coefficient (CL:::::- 1.0), as might be necessary during maneuvers in cruise. The plot (F-2) slopes up to the right because as the CG moves aft of the CL a larger tail is required for trim . The study allowed either of two other requirements to set the maximum forward CG position:)that the pilot be able to trim to maximum lift coefficient during takeoff and landing and that he be able to unstick the nose gear at 1.05 Vstall. Unstick turned out to be the limiting case.

Relaxing the stability margin saved 25% in tail area with corresponding savings in drag and weight. Following conventional practice, a designer could also reduce trim drag to zero (F-3), but only at the cost of increasing tail volume, which would increase skin-friction drag and keep total tail drag essentially constant.

Boeing investigated the possibility of also reducing the vertical tail's size. They based this size on the more critical of two criteria-directional

40

"" 30

10

100

Slable OStll lalory

motion

200 300 Equivalent alrspee<l, Vet kn

UnSlable

500

o o

Conventiona l Technology Statically Stable Fail-Safe Yaw Da mpe r I nherent Accepta ble Sta ll

stability or engine-out control. For wing-mounted­engine configurations, engine-out control set minimum size. For the configuration having three engines mounted aft, they selec:ted a fin to provide a minimum directional static stability (CnP = 0.002/deg). The minimum fin size for this con-' figuration combines with unfavorable inertial characteristics to produce a significantly unstable dutch roll over most of the flight envelope.

For transports with three aft-mounted engines, drawings here (FA) compare size of tail for planes designed with conventional and active controls technology. The relaxed stability configuration has about 11% less takeoff gross weight and a total drag coefficient reduced by 0.0007. The stability­augmentation system required for an airplane with relaxed static stability must have a high enough reliability to provide a level of confidence in flight safety acceptable to manufacturer, airlines, and the FAA. Plots of the unaugmented longitudinal and

40

=30

10

100 200

Unslable unaU<jmenled Dulch roll region

3IXI EqUivalent airspeed, Ve. kn

500

F-5 Unaugmented logltudlnal , left , and lateral-directional , right , dynamic stability for trijets with relaxed stabili ty shows the craft unstable over a large porti on of the fli ght envelope.

Astronautics & Aeronautics

lateral-directional dynamic-stability characteristics for the relaxed-stability trijets (F-S) show the craft unstable over a large portion of the flight envelope.

Active controls can also save weight by red ucing the structural strength necessary. According to conventional design practice, civil-transport aircraft must withstand 2.S-g maneuver loads, withstand peak gust loads, have a sufficiently low fatigue damage rate, and not flutter up to a speed 20% greater than the dive speed . In practice, the designers layout a wing strong enough to withstand maneuver and peak gust loads and then investigate fatigue and flutter characteristics. They may have to add weight to stiffen the structure or provide mass balancing for flutter .

Active controls could shift the maneuver loads inward on the wings so the wings would need less strength (F-6). The spanwise lift distribution for a 2.S-g maneuver , shown in the upper drawing, produces the same total lift as conventional loading but less bending moment at the root. The controls create this distribution by deflecting inboard and / or outboard surfaces, such as flaps or spoilers, in response to maneuvering accelerations.

Gust-load alleviation reduces fatigue damage due to gusts and is related to improving ride qualities discussed later. In this case, the controls deflect wing surfaces in response to acceleration and / or angle of attack. A typical system, shown in schematic (F-7), may simultaneously deflect the elevator to maintain airplane attitude.

Active flutter suppression works by deflecting wing trailing and / or leading-edge · surfaces in response. to wing deflection and deflection rate. Determining suitable control-surface locations and sensors awaits further development. The system­study contractors did not consider leading-edge surfaces, as no data for them on a supercritical wing were available.

The ATT system studies found , in general, that the critical wing design loading occurred during the 2.S-g maneuver. Little or no material had to be added to meet fatigue or flutter requirements. After active maneuver-load control allowed lightening the structure for the 2.S-g maneuver load, some weight (F-8) had to be restored to meet flutter and fatigue requirements. Active gust-load alleviation added to the maneuver-load control greatly red uces the weight that has to be restored for fatigue (F-9). Adding active-flutter suppression would yield a similar improvement. Since they are so closely related , these three active-control func­tions must be integrated in application.

In a slightly di fferent form, the measures taken for gust-load alleviation improve ride quality. Deflections of control surfaces in response to aircraft motion due to gusts and turbulence would

August 1972

F-6 EFFECTS Wing OF MANEUVER Loading LOAD

, , \,...- Maneuver Load Control

\

CONTROL

~\," Shear

" .... _--=== Root Tip

Bending Moment '"

' ... ,

Root Tip

t.ngust

F-7 Gust-load alleviation system would reduce fatigue damage. Elevator movements would maintain att itude despite flap deflection .

c '" E E u c:

o OL...--~:='-'---'"2--..J......----L4 ---'-----'6 x 103

Strength Design Weight Reduction (MLA). lb.

F-B After maneuver-load control lightened structure some weight had to be added to withstand fatigue and flutter, at left . F-9 Adding gust-load alleviation reduced weight that had to be added for fatigue.

3500

3000

2500 Fatigue

inc rement 2000 -Ib

1500

1000

500

0 1000 2000 3000 4000 S<XXl 6000 Strength design weight reduction -Ib

53

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"" .... ..... '" ~ 0.04 ~ on :E "" e 0. 02

~ 8 N

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F-1Q ACTIVE RIDE CONTROL IMPROVEMENT

_ NASA BASEliNE CONFIGURATION _ REDUCED LOADS CONFIGURATION

_ An TO INBOARD FlAPERON

)

42,1'-RED UCTI ON

D WIOACS

. W/ACS M • 0. 99 ALT ' 40,000 Ft 112,192MI GW • 209,000 Lbs 194, !l2 kg l

FUSELA GE STATION IN.

10 20 30 50 Meters

(From Ref. 3)

F-11 ECONOMIC BENEFITS OF ACTIVE CONTROLS o PAYLOAD ' 40.000 LB ° 3·ENGINES AIRPLANES OMCR • . 98

l.5~-

oDESIGN RANGE ' 3000 N.MI . oALL ALUMINUM CONS TRUCTI ON oFAR 36 NOISE LEVEL

60 All Profi ts are Corrected to the Ret.rence Airpl ane

50 Total I nvestment Base

~40 ~ ~ 3O

~ e 20 s~ .

q 10

• REFERENCE A I R PLANE -

54

oStatically Stable With Trim Drag o FAR 36 NOise Level

oMCR •. 98 oBaslc ROI • 13. 79

minimize the accelerations acting on passengers and crew. In turbulence, those aboard feel both rigid-body accelerations and accelerations from flexing of the structure. For the subsonic long-haul transport, rigid-body accelerations predominate. In aircraft having long, slender fuselages, fuselage bending accelerations become more important. For a typical subsonic long-haul configuration, an active ride-control system could reduce rigid-body acceleration about 20-40% over the length of the aircraft (F-IQ). The predicted unagumented ride qualities resemble those of current wide-body transports.

Among the several aspects of a concerted attack on noise and congestion in the terminal area , the curved decelerating approach offers an opportunity for active controls to contribute. Directing aircraft through a curved approach will keep them away from areas sensitive to noise, sequence traffic better and allow reducing the spacing between parallel runways. Direct drag, direct side force, and direct lift would enable the aircraft to follow the desired flight path independent of fuselage angle , making steep approaches more comfortable for passengers. Direct-lift control has already been

investigated and found useful. As a byproduct, it may produce a pitching moment which would have to be controlled by automatic deflection of the pitch-control surface. Automatic flap controls in conjunction with automatic throttle controls might also become necessary on takeoff and approach.

Some of the technical benefits of active controls would increase return on investment (ROn and raise profits. An evaluation of the predicted economic gains assumed a three-engine Mach-0.98 aircraft made of aluminum and just meeting FAR-36 noise regulations. The results presented, though obtained by use of advanced analytical methods, are subject to the uncertainties that still are associated with economic predictions, as indicated in Braslow and Alford's article in this issue. The reference aircraft, without active controls, had an ROI of 13.79%, Profits were calculated for a fleet of 280 airplanes operating between 128 city pairs.

The chart of economic benefits (F-ll) includes no evaluations of economic advantages for im­proved ride qualities or for improved operations in the terminal area. Aircraft having improved ride qualities , however , may have more appeal to the passenger. Improving operations in the terminal area should significantly increase economic benefits; and a follow-on study will quantify these expected benefits.

Routine terminal-area operations under con­ditions approaching zero visibility, a greatly in­creased number of operations per hour, and automation of the Air Traffic Control System will reduce costly delays experienced by the airlines. The future terminal-area environment may have such a large economic impact on airline operations that many aircraft, particularly medium- and short-range aircraft, will be configured to optimize operations in the terminal area rather than in cruise-in substantial part through active controls.

Many questions remain to be answered before active-controls technology can be successfully applied . NASA is defming an extensive research program (F-I2) which , if implemented, should bring active-controls technology to commercial application in the early 1980s. The proposed

RAY V. HOOD acts as technology manager, control of flight, in the ATT program office with responsibility for supervising studies in active controls technology and terminal compatibility. He had previously served as an engineer on space and aeronautical research simulation projects after joining the Space Mechanics Division at NASA Langley Research Center in 1965. He received a B.S. and, in 1964, an M.S. in aerospace engineering from the Univ. of Alabama. He continues graduate work at VPI.

Astronautbs & A eronautics

F-12 PROPOSED ACTIVE CONTROLS RESEARCH PROGRAM

Design Studies Criteria Dev. /Econ . Payoff

Aerodynamic Control Dev .

Advanced Control Laws

Rei iable FIt. Computer

Systems Simulation

Fly-by-Wire

ACT Demonstrator

Aeroelastic Analyses

Term inal Area DLC. DDC. Direct -Side-Force

Assure Commercial Systems Rei iabil ity

Demonstrate Performance and Benefits

Propuls ion I ntegrated En gine Control

Provide L------------~===~ Supporti ng

Technology

program, coordinated with other agencies , will marshall aerodynamics, structures, electronics, propulsion, flight controls, and operational ex­pertise behind a system effort. It will take a broad view, considering applications of active controls to STOL, SST, and military aircraft, as well as to long-haul subsonic transports.

At this time, the primary obstacles to application are inadequate definition of design criteria and design requirements , lack of reliable systems , and lack of confidence in predicted benefits and performance.

The cons~raints of trad itional aircraft-design criteria will prevent fully realizing the potential benefits of active-controls technology. Many existing criteria aim at insuring sufficient strength, stiffness , and stability to provide passive protection against the natural and man-made hazards of flight. In some cases, active controls can provide equivalent protection with reduced strength, stiffness, or stability. The tradition al criteria must be examined in detail to identify criteria which unduly restrict the benefits of active controls. And those criteria must be restated in more fun­damental terms, which will make their real intent clear, and then criteria for control-system ef­fectiveness must be developed to fulfill their basic aims.

The program must also determine design requirements for each of the configurations of interest. Analysis and wind-tunnel tests must establish control-surface type, size, and location. Analysis and simulation must determine sensor type, location, frequency response , and accuracy August 1972

requirements and actuator frequency, accuracy, and power requirements. Control laws for the active-control functions must be derived and computation requirements established.

Many of the things active controls can do could have been done a long time ago, but were not more for a lack of reliable systems than for any other reason. Digital-controls technology will provide the necessary reliability through a combination of reliable hardware and software. The program recommends investigating fault -tolerant computers and reconfigurable computer systems. Software will provide performance monitoring. data logging, and self-testing to improve maintainability as well as reliability. Development of digital fly-by-wire technology will constitute a major element of the proposed program.

Many ground-based tests, including "iron-bird" simulations, are very useful in building confidence in active controls. Nevetiheless, a flight demon­stration will probably prove necessary to eliminate lingering doubts. The same aircraft selected for digital fly-by-wire development may serve for the ACT demonstration . It could have a new reduced­strength wing, incorporating active controls, and possibly a new empennage section reflecting reduced stability considerations. A reliable computing system and digital fly-by-wire control system would complete the conversion.

As we see it , vigorous pursuit of advanced flight­control and avionics technology could lead to future generations of subsonic, long-haul transport aircraft that are lighter, more comfortable, and more economical to operate. •

~5