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A BRIEF REVIEW OF AIRCRAFT CONTROLSRESEARCH OPPORTUNITIES IN THE GENERAL AVIATION FIELD Eric R. Kendall Gates Learjet Corporation Wichita, Kansas First Annual NASA Aircraft Controls Workshop NASA LaFgley Research Center Hampton, Virginia October 25-27, 1983 CONTROLSTECHNOLOGY REVIEW The review process itself is part of a feedback control system (Figure 1).
The work already accomplished by NASA on flight test programs (Block A) and on trade studies (Block B) must be reviewed to determine the potential controls technol- ogy benefits available to the general aviation industry (Block C). General aviation industry constraints (Block D) must be defined and applied to determine the currently useable controls technology (Block E). Any shortfall between the required technology (Block F) and the useable technology shows up as a technology deficiency (Block G) and identifies the future research opportunities (Block H). Additional future research by NASA (Block J) can increase the controls technology benefits and ulti- mately nullify the technology deficiency.
GENERAL AVIATION NASA INDUSTRY (A) MILITARY 8 LARGE COMMERCIAL TRANSPORT AIRPLANE TEST PROGRAMS
h
NASA-SPONSORED
I
(J) (H) $ (G) v I FUTURE FUTURE TECHNOLOGY RESEARCH* RESEARCH DEFICIENCY OPPORTUNITIES Figure 1 MILITARY AND COMMERCIALTEST PROGRAMS A significant number of flight test programs related to ACT have been conducted during the last 25 years. Some of these are shown in Figure 2. Most are The technology is related to very large airplanes with flexible structures.
'acronym saturated'. Richard Holloway defines the more commonly used acronyms and explains system functions in Ref. (1). Those used here are:
ccv . . . . . ..Control Configured Vehicle
FMS. . . . . ..Flutter Mode Suppression GASDSAS . ..Gust Alleviation & Structural Dynamic Stability Augmentation System ALDCS. . . ..Active Load Distribution Control System AS . . . . . . ..Augmented Stability WLA. . . . . ..Wing Load Alleviation Figure 2 NASA-SPONSORED TRADE STUDIES A few of the many trade studies sponsored by NASA are shown in Figure 3. These all relate to commercial transports or commuter airplanes. The STAT program, which started in 1978, was reported by Louis Williams of NASA Langley at the 1982 SAE Commuter Aircraft and Airline Operations Meeting in Savannah, GA (Ref. 4). The report contains much material relevant to the application of advanced technologies in the general aviation industry. ACT benefits were explored on two candidate airplane designs. Controls technology benefits need to be separately identified.
1970 - LOWWING LOADING STOL STUDYt2) ,,,,.,,,,,,,.,,,,.,,,NASA/BOEING WICHITA 1972 - APPLICATION OF ADVANCED TECHNOLOGIES TO LONG-RANGE TRANSPORT AIRCRAFTt3),,,,,,.,,..,,,.,NASA/BOEING SEATTLE 1978 - SMALL TRANSPORT AIRCRAFT TECHNOLOGY NASA/INDUSTRY (STAT) PROGRAM(') IIIIIIIIIIIIIIIIIIIIIIIIII~IIIIIII 1982 - INTEGRATEDAPPLICATION OF ACTIVE CONTROLS TECHNOLOGY TO AN ADVANCED SUBSONIC TRANSPORTt5) Illll1llll111I111111111111111, NASA/BOEING SEATTLE Figure 3 CONTROLSTECHNOLOGY BENEFITS (TEST RESULTS) Some of the benefits which have been obtained as a result of the flight test programs are listed in Figure 4. Active control systems on the B-52 and on the C-5A are incorporated as retrofits to production airplanes. The L-1011 systems permit increased wing span which leads to improved cruise performance.
l ECP 1195 ON B-52 REDUCES FATIGUE 8, ALLEVIATES GUST LOADS
. ALDCS ON C-5A IMPROVES FATIGUE LIFE BY GUST AND MANEUVER
LOAD REDUCTIONS
. L-1011 SYSTEMSREDUCE LATERAL GUSTDESIGN LOADSAND PERMIT
WING TIP EXTENSIONWITH NO BEEF-UP
Figure 4 Cotd~RoLs TECHNOLOGY BENEFITS (TRADE STUDIES) NASA-sponsored trade studies have shown significant synergistic design benefits for a wide range of commercial airplane types. Some results from these studies are shown in Figure 5 for STOL, commuter, and subsonic commercial transport airplanes.
GLA. . . ..Gust Load Alleviation PAS. . . ..Pitch Augmentation System AAL . . . ..Angle-of-Attack Limiting
l REFJ2) STOL STUDY SHOWS 10% TO 30% GROSS WEIGHT
REDUCTION WITH MECHANICALFLAP 'GLA'
(F.L, x2500 FTJ
l REF,(3> MACH0,98 AIRPLANE GROSS WEIGHT REDUCED
BY 11% THROUGH USE OF 'AS'
. REF, (4) 'ACT' BENEFITS CONVAIR& LOCKHEED COMMUTER
AIRPLANE DESIGNS
. REFJ5) 10% IMPROVED FUEL EFFICIENCY ON SUBSONIC
TRANSPORT THRU USE OF PAS, AAL & WLA
Figure 5 GENERAL AVIATION ‘ACT’ CONSTRAINTS The constraints listed in Figure 6 are typical of some which might be specified by the general aviation industry. Coordination within the industry is required before these can be considered as an official industry input. However, judging from the complexity levels of systems in use today, constraints such as these will produce significant future research opportunities. The need to be compatible with manual (unpowered) primary flight control systems was addressed by Dr. Jan Roskam and others in Ref. (6).
This considered the use of a separate surface stability augmentation system for general aviation aircraft. Design philosophies and hardware implementation schemes were defined and evaluated in Ref. (7).
l MUST BE SIMPLE
l MUST BE EASY TO MAINTAIN
. MUST NOT BE SAFETY CRITICAL
. MUST BE COMPATIBLE WiTH MANUAL
(UNPOWERED) PRIMARYFLIGHT CONTROL
SYSTEMS
. MUST IMPROVEAIRPLANE SALES
POTENTIAL
Figure 6 GENERAL AVIATION FLIGHT TESTS G.eneral aviation constraints such as those just mentioned have been recognized for some time. As a result, several NASA 'ACT' programs have been directed specifi- A recent review has been pre- cally towards the general aviation type of airplane.
sented in Ref. (8) by Dr. David Downing and others of KU. A few of the programs are listed in Figure 7.
VRS........ ..Vertica 1 Ride Smoothing GPAS....... ..Genera 1 Purpose Airborne Simulator RSS........ ..Relaxe d Static Stability SSSAS...... ..Separat e Surface Stability Augmentation System c-45 111.....1,1...0.1,1. VRS c-140 . . . . . . . . . . . . . . . . . ..GPAS/RSS BEECH99 . . . . . . . . . . . . . . ..SSSAS Figure 7 1o-3 BUMP SIZE The potential need for ride quality control on some commuter airplanes can be seen by comparing the estimated ride qual'ty of various types of unaugmented air- !I planes. Figure 8 shows the estimated lo- bump size for three types normalized .to that of a commercial transport flying at an altitude of 35,000 feet. It is apparent that the commuter with its relatively high response to vertical gusts flying in the more gust-prone lower altitude bands will present a rougher ride in turbulent condi- tions. Technology trends in many of the emerging new commuters are towards simplicity, and it may be some time before ride quality systems are generally accepted. Research should continue to take advantage of the rapid developments in ' electronics and controls to make these systems more attractive for future high- technology commuter airplanes.
1 COMMERCIALTRANSPORT, 35,000"
2 BUSINESS JET, 45,000' 3 COMMUTER, 10,000' I TECHNOLOGYTREND b .SIMPLE FLAPS .PARTIAL GEAR RETRACTION l CONVENTIONAL CONTROLS -----_ --- -a
LCL -
Figure 8 -. - . -.._ ._.- -. - -.~L_-. _ TECHNOLOGY CHOICES General aviation airplane designers tend to use simpler control technologies than those employed on military and large commercial transport airplanes. As a result, a larger reserve of well-proven controls technologylis available as an alternative to the adoption of advanced state-of-the-art controls technology. This is depicted in Figure 9.
Figure 9 - SOME GENERAL AVIATION PREFERENCES A brief list of some general aviation airplane desjgn preferences is.presented in Figure 10. This is included to emphasize a point that in many instances a simple technology is chosen over a more complex and more effective one. The tendency to use simple.flap systems and low wing loadings is an example qf the 'trade towards Quite often there is a tendency to reject any beneficial simplicity' approach.
external features if they are considered detrimental to styling, and the preference is to eliminate avionic systems rather than to add them. Clearly much research will be required to produce ACT benefits which are marketable in the general aviation sector.
. SIMPLE FLAPS 8 LOWWING LOADINGS
. CONFIGURATION FEATURES WHICH ARE BOTH
BENEFICIAL & STYLISH (E,G,,WINGLETS)
. MINIMAL DEPENDENCE ON AVIONIC SYSTEMS
(E,G, YAWDAMPERS,STALL PREVENTION)
Figure 10 SIMPLE FLAPS AND LOW WING LOADINGS The 'trade-towards-simplicity' tendency just noted is seen from the data pre- This compares the stall speeds and wing loadings of some general sented in Figure 11.
aviation airplanes with those for commercial transports and advanced high-lift air- In general, the landing CLMAx for planes. the general aviation types is around 1.8, which is achievable by simple single-slotted partial span flaps. The commercial trans ports have a landing CLMAX close to 2.8, which requires more complex flap arrange- ments. The general aviation landing stall speeds are kept to an acceptable level by using lower wing loadings than are commonly used by the commercial transports.
Clearly there is some tradeability towards more complex flaps to obtain the cruise benefits of a higher wing loading. This is an example of an available technology not being fully exploited due to the reference for simplicity. Similar trends were noted by Dr. Jan Roskam, Ref. (9 P, who proposed new airfoils, higher wing loadings, and a new look at general aviation airplane design.
, K-IS , I I I I I I 1 1 I I I 1 I, 40 50 60 70 so 90 Icm IJO MAX LANDING WING LOADING (LB/FT~) Figure 11 THE MAGNITUDE OF AIRPLANE PERFORMANCE BENEFITS .*.
Quite often the magnitude of airplane cruise performance benefits available While such improvements cannot be ignored, from ACT is small (e.g., 2 or 3 percent).
the cost effectiveness of systems needed to obtain them must be carefully considered.
A basic requirement must be that the improvement will be sustained throughout the life of the airplane and that the benefits definitely will be felt by the airplane owner.
To give some indication of performance improvement 'detectability', data on airplane fleet performance variability are presented in Figure 12. This is a histo- gram of incremental percentage fuel flow gathered from forty-one new production airplanes all of the same model designation and all flown on the same route by Data were corrected for observed ambient conditions.
production flight test crews.
About half the measurements are contained within +2% of the nominal value.
'improver' ACT system might make a small Even though a cruise performance but statistically significant improvement to the fleet picture, it may not be of any practical significance to a particular one-airplane operator.
A . - - . . . . - ..---_I--
I
._-_-_-.- __-.-_-.---- ..,__ --.-
t
I
I
-XI
-;5
+6 +4
l b
a %.FUEL FLOW
'BETTER
WORSE
Figure 12 ACT USED IN TOUGH COMPETITIVE SITUATIONS competitive pressures will accelerate the Williams (Ref. 4) wrote that "...
use of technological advances...". The data in Figure 13 confirm this statement and show how ACT was introduced in a tough competitive situation between two general aviation business airplanes. The cruise performance of each airplane is comparable with airplane A's passenger miles pe-r pound of fuel used being better than B's on the short range. The higher wing loading and winglet used on airplane 'A' more than offsets the low wing loading and supercritical section of airplane 'B'. Then, airplane A's balanced field length was made comparable with B's by introducing automatic performance reserve (APR) and automatic spoilers. These change engine thrust and spoiler setting without a direct command from the pilot and therefore can be classified as active control systems.
3,420 4,200 'WINGLET EFFECT IIJCLUDED, (1) BFL IMPROVED WITH APR 8 AUTOSPOILERS.
Figure 13 GENERAL AVIATION ACT SUGGESTIONS Based on the very brief discussion of some general aviation design trends and preferences (still to be coordinated within the industry sector), Figure 14 It’seems that avionic presents a summary of suggestions for ACT activities.
cruise performance improvers will not sell easily unless the advantages are large.
Since many general aviation airplanes have low wing loadings and fly at relatively low altitudes,the emphasis should probably be on ride quality improvement and gust alleviation systems. Retrofittable systems could be attractive since few airplanes are likely to be designed with optimal structures and no.growth capability.
. 'CRUISE IMPROVERS'SHOULD:
A) COMPLY WITH CONSTRAINTS
B) COMPETE WITH FLAP/WING-LOADING
TRADE-OFF.
c> PRODUCE SIGNIFICANT REDUCTIONIN
CRUISE FUEL-FLOW,
. RIDE CONTROL 8, GUST ALLEVIATION SYSTEMS MAY BE
MOST LIKELY 'ACT' FUNCTIONSTO FIND APPLICATON
. RETROFITTABLELOAD ALLEVIATORSMIGHT BE ATTRACTIVE
FOR PROVIDING AIRPLANE GROWTH CAPABILITY WITH
MINIMUM STRUCTURAL BEEF-UP, Figu.re 14 GENERAL AVIATION CCV Information and discussion presented so far might give the impression that the general aviation industry is ultra-conservative and not likely to adopt any significantly new controls concept in the forseeable future. However, even though there may be a natural reluctance to adopt a complicated avionics ACT system, the field of 'non-electronic' CCV technology might be regarded differently. The new designs introduced by Beech and by the Gates-Piaggio team at this year's National Business Aircraft Association (NBAA) show in Dallas show once again that . ..competitive pressures will accelerate the use of technological advances..."
{'Ref. 4). Figure 15 shows the competitive 'canard' and 'three-surface' designs relative to a conventional configuration. NASA research has been strongly sup- portive of these unconventional designs. Much is left to be done.
THREE-SURFACE CONVENTIONAL ___---_ Figure 15 GENERAL AVIATION RESEARCH OPPORTUNITIES IN THE FLIGHT CONTROLSTECHNOLOGY The research opportunities in the fields of ACT and CCV for general aviation This review has attempted to forsee some of these opportunities are enormous.
by assessing general aviation needs and trends relative to the currently available Coordination within'the general technology. A few ideas are listed in Figure 16.
aviation industry and between industry and NASA should be intensified in the near term to try to provide NASA with a more complete and representative feedback.
A) OVERALL - CONTINUE STAT FOR SMALLER G.A, AIRPLANES (40 PAX,) - DETERMINE POTENTIAL 'ACT' CONTRIBUTION - CATALOGALL ACT BENEFITS, B) AVIONIC SYSTEMS - EXPLORE FEASIBILITY OF RETROFITTABLE LOAD ALLEVIATORS & RIDE QUALITY IMPROVERS, - RUN SIMULATOR STUDIES & FLIGHT TESTS.
- DEVELOP ANALYTICAL METHODS AFFORDABLETO GENERAL AVIATION USERS C> AIRPLANE CONFIGURATIONS - INCLUDE CANARDS& THREE-SURFACEAIRPLANES IN A) & B) ABOVE, - CONTINUE WIND TUNNEL TESTS TO DETERMINE THE STABILITY AND CONTROLCHARACTERISTICS OF UNCONVENTIONAL AIRFRAME/ PROPULSIONARRANGEMENTS Fiaure 16 REFERENCES 1. Hol.loway, Richard B., "Introduction of CCV Technology Into Airplane Design," AGARD-CP-147, vol. 1, June 1974, pp. 23-'1 to 23-16.
2. Holloway, R. B., Thompson, G. O., and Rohling, W. J., "Prospects for Low-Wing- Loading STOL Transports With Ride Smoothing," Journal of Aircraft, volume 9, no. 8, August 1972, pp. 525-530.
3. Boeing Co., "Study of the Application of Advanced Technologies to Long Range Transport Aircraft," NASA CR-112093, May 1972.
4. Williams, Louis J., "Advanced Technology for Future Regional Transport Aircraft," SAE 1982 Transactions, vol. 91, section 3, 1982, pp. 2481-2497.
5. Boeing Co., "Integrated Application of Active Controls (IAAC) Technology to an Advanced Subsonic Transport Project," NASA CR-165963, December 1982.
"An Investigation of Separate Surface Stability Augmentation 6. Roskam, J., et al., Systems for General Aviation Aircraft," University of Kansas, Center for Research, Inc., August 1972.
7. Roskam, J., "Design Philosophy & Hardware Implementation of Separate Surface Automatic Flight Control Systems," University of Kansas, Center for Research, Inc., May 1974.
8. Downing, D., Hammond, T., and Amin, S., "Ride Quality Systems for Commuter Aircraft," NASA CR-166118, May 1983.
9. Roskam, J., "New Airfoils and Higher Wing Loadings: A New Look at General Aviation Airplane Design," University of Kansas, Center for Research, Inc., May 20, 1974.