Further Examination of the Vibratory Loads Reduction Results from the NASA/ARMY/MIT Active Twist Rotor Test

15 Pages·2002·1.1 MB·English

by  Wilbur, Matthew L.

FURTHER EXAMINATION OF THE VIBRATORY LOADS REDUCTION RESULTS FROM
THE NASA/ARMY/MIT ACTIVE TWIST ROTOR TEST
Matthew L. Wilbur William T. Yeager, Jr. Martin K. Sekula
[email protected] [email protected] [email protected]
U. S.Army Vehicle Technology Directorate
NASA Langley Research Center
Hampton, VA 23681
The vibration reduction capabilities of a model rotor system utilizing controlled, strain-induced
blade twisting are examined. The model rotor blades, which utilize piezoelectric active fiber
composite actuators, were tested in the NASA Langley Transonic Dynamics Tunnel using open-loop
control to determine the effect of active-twist on rotor vibratory loads. The results of this testing have
been encouraging, and have demonstrated that active-twist rotor designs offer the potential for
significant load reductions in future helicopter rotor systems. Active twist control was found to use
less than 1% of the power necessary to operate the rotor system and had a pronounced effect on both
rotating- and fixed-system loads, offering reductions in individual harmonic loads of up to 100%. A
review of the vibration reduction results obtained is presented, which includes a limited set of
comparisons with results generated using the second-generation version of the Comprehensive
Analytical Model of Rotorcraft Aerodynamics and Dynamics (CAMRAD II) rotorcraft
comprehensive analysis.
Nomenclature mechanisms have been investigated for this purpose. 1 The
most widely explored active material actuation methods
AFC Active Fiber Composite have employed either piezoelectrically-actuated flaps placed
ARES Aeroelastic Rotor Experimental System at discrete locations along the blade, 2-8 or piezoelectric
ATR Active Twist Rotor material distributed along the blade and used to directly
Fx fixed-system longitudinal shear, + aft, lb control deformations (usually twist) in the host blade
Fy fixed-system lateral shear, +right, lb structureY 6 The study of one such concept, strain-induced
Fz fixed-system vertical shear, +up, lb blade twisting, isthe subject of the current study.
Mx fixed-system rolling moment, +left wing up, in-lb The primary design constraint in both the active blade
My fixed-system pitching moment, +nose up, in-lb flap and the active blade twist approaches is the need to
Mz fixed-system yawing moment, + clockwise, in-lb obtain high piezoelectric actuation forces and displacements
IBC Individual Blade Control with a minimum of actuator weight. An additional concern
IDE interdigitated electrode with flap actuation mechanisms is that they must be
PFC piezoelectric fiber composite designed to fit within the geometric confines of the blade
R rotor radius, ft structure. Direct control of blade twisting using embedded
TDT Transonic Dynamics Tunnel piezoelectric materials, although simple conceptually, has
CL rotor lift coefficient proven to be difficult to implement with conventional
rotor shaft angle-of-attack, + nose up, deg piezoelectric materials. Over the past several years,
advance ratio however, piezoelectric fiber composite (PFC) actuators have
/z
been shown to have the proper combination of
conformability and performance characteristics necessary to
Introduction
develop a useful individual blade control system. 8'11-16 In
addition, recent improvements in PFC strain-actuation
Rotorcraft vibration and noise reduction, as well as
capacity 17indicate the potential for far greater blade twist
increasing rotor performance and maneuverability, continue
actuation than has been achieved to date. Thus, the viability
to be a primary concern of the rotorcraft research
of this class of actuators for rotorcraft vibration control is
community. One promising means of attaining such goals is
just now emerging.
to define an efficient way to achieve helicopter
In 1997, a cooperative effort between the NASA
rotor individual blade control without the need for hydraulic
Langley Research Center, the Army Research Laboratory,
power in the rotating system. Numerous electromechanical
and the MIT Active Materials and Structures Laboratory was
approaches exploiting active (smart) material actuation
developed to perform initial feasibility and proof-of-concept
studies of active twist rotor (ATR) technologies. The
Presented at the American Helicopter Society 5g_'Annual Forum, ultimate goals of the ATR program are to provide a wind-
MontrOal,Canada,June11-13,2002. Copyright©2002bytheAmerican tunnel demonstration of an active twist rotor concept that
HelicopterSocietyInternational,Inc.Allrights'reserved.
uses piezoelectric fiber composite actuators, to investigate
the potential benefits of such a system to reduce rotorcraft ::
vibrations and noise, and, to a lesser extent, investigate
potential improvements in rotor performance. This is being
accomplished using a four-bladed, l l0-inch diameter
aeroelastically-scaled wind-turmel model rotor designed for
testing in the heavy gas, variable density test medium of the
NASA Langley Transonic Dynamics Tunnel (TDT). 18 The
TDT is a unique facility that permits full-scale rotor tip
Mach numbers, Froude numbers, and Lock numbers to be
matched simultaneously at model scale. In particular, the
reduced speed of sound in the heavy gas test medium allows
full-scale tip Mach numbers to be matched at lower
rotational speeds and drive motor power.
References 12 through 15 document the design,
fabrication, and bench and hover testing of a single Active
Figure 1. The Langley Transonic Dynamics Tunnel (TDT).
Twist Rotor prototype blade. Subsequently, a full set of
ATR blades was fabricated and forward-flight testing
conducted in the Langley Transonic Dynamics Tunnel to throughout the test envelope, which allows more accurate
assess the impact of active blade twist on rotating- and modeling of the full-scale aerodynamic environment of the
fixed-system vibratory loads and acoustic noise generation. rotor system.
Initial vibration reduction results were presented in reference
16, with an overview of the noise reduction performance
Model Helicopter Rotor Testbed
presented in reference 19. The impact of active-twist on
rotor performance has not yet been evaluated The ATR blades were tested on the Aeroelastic Rotor
experimentally. Experimental System (ARES) model helicopter rotor testbed
The current paper will build upon the promising shown in figures 2 and 3. The ARES is powered by a 47-hp
forward-flight results presented in reference 16, which electric motor through a two-stage, belt-driven transmission
concluded that reductions in 4P fixed-system loads of 60% system. Rotor control is achieved by a conventional
to 95% were achieved with 1.1 °to 1.4° of active blade twist.
hydraulically-actuated rise-and-fall swashplate using three
The capacity of blade twist actuation to reduce or eliminate independent actuators. Similarly, a single hydraulic actuator
rotating- and fixed-system vibratory loads is more fully controls rotor-shaft angle of attack.
explored with additional results and flight condition Instrumentation on the ARES testbed permits
sensitivity studies. Additionally, an initial set of analytical
continuous display of model control settings, rotor speed,
comparisons and power consumption results is presented.
rotor forces and moments, fixed-system accelerations, blade
loads and position, and pitch-link loads. All rotating-system
ATR Model Test System data are transferred through a 30-charmel slip ring assembly
to the testbed fixed system. An additional slip ring permits
Wind Tunnel
the transfer of high-voltage power from the fixed system to
the rotating system for actuation of the AFC actuators
Forward-flight testing was conducted in the Langley
embedded in the ATR blades. A six-component strain-gage
Transonic Dynamics Tunnel (TDT), shown in figure 1, in a
balance placed in the fixed system 21.0 inches below the
heavy gas test medium at a constant density of 0.0047 rotor hub measures rotor forces and moments. The strain-
slugs/ft 3. The TDT is a continuous-flow pressure tunnel gage balance supports the rotor pylon and drive system,
capable of speeds up to Mach 1.2 at stagnation pressures up
to 1 atm. The TDT has a 16-ft square slotted test section
with cropped comers and a cross-sectional area of 248 ft2.
I
f- Model pivot
Either air or R-134a, a heavy gas, may be used as the test point /
medium. The TDT is particularly suited for rotorcraft
aeroelastic testing primarily because of three advantages I OI
associated with the heavy gas. First, the high density of the
test medium allows model rotor components to be heavier;
thereby more easily meeting structural design requirements
while maintaining dynamic scaling. Second, the low speed
of sound in R-134a (approximately 550 ft/sec) permits much
lower rotor rotational speeds to match full-scale hover tip
Mach numbers and reduces the time-scales associated with
active control concepts and dynamic response. Finally, the Figure 2. Schematic of the Aeroelastic Rotor Experimental
high-density environment increases the Reynolds number System (ARES) helicopter testbed. All dimensions are in feet.
usedtogenerahteighvoltagelo,wcurrenptowerforthe
independaecnttuatioonfeachblade.
TheATRbladesg,eneraplarametefrosrwhichare
provideidnTable1,havearectangulpalranformwitha
chordof4.24inchesra,diusof55.0inchesa,NACA-0012
airfoil,andalinearpretwisotf-10° fromthecenteorf
rotationto thebladetip. Testingwasconducteadta
constanrottorspeeodf688rpm(Mtip 0.60)onafour-
bladedarticulatehdubwithcoincidefnlatpandlaghinges
andtrailingpitchlinks.InstrumentatoiontnheATRblades
includetsen4-armstrain-gabgreidgesw,ithresultfsromthe
threemositnboarsdtraingagebridge(sflapwisech,ordwise,
andtorsionp)resenteindtheresultssectioonfthispaper.
ATRbladefrequenciedse,terminebdyacombinatioonf
analysiasndexperimenotablservatioanr,epresenteind
Table2.
Table 1.Active Twist Rotor General Parameters
Propert_ Description Value
Figure 3. The ARES testbed in the TDT with the ATR R Blade radius, ft 4.583
hardware installed.
c Blade chord, ft 0.353
rc Root cutout, ft 1.04
pitches with the model shaft, and measures all of the fixed- Opt Blade linear pretwist, deg -10.0
system forces and moments generated by the rotor model. A N Number of blades 4
streamlined fuselage shape encloses the rotor controls and e Flap-lag hinge location, ft 0.25
drive system; however, the fuselage shape is isolated from -(2o Nominal rotor rotational 688
the rotor system such that its forces and moments do not speed, rpm
contribute to the loads measured by the balance. ,Oo Nominal test medium 0.0047
density, slugs/ft 3
ATR Blades Mtip Blade hover tip Mach 0.60
number
Each Active Twist Rotor blade utilizes 24
Zig Rotor thrust for simulated 225.0
commercially-available Active Fiber Composite (AFC) lg flight, lb
actuators to achieve active twist control. The AFC actuators,
shown conceptually in figure 4, are embedded directly in the
structure of each blade D-spar, spanning a section of
uniform blade structure from 0.30R (30% blade radius) to
Table 2. ATR Rotating Blade Frequencies
0.98R. The AFCs are placed in four layers through the blade
thickness and are oriented such that the active strain is
applied at +45 ° relative to the blade spanwise axis to Mode Frequency
generate maximum torsional control of the blades. Four Rigid Lag 0.29P
dedicated high-voltage amplifiers, one for each blade, are Rigid flap 1.04P
lstelastic flap 2.79P
2ndelastic flap 5.36P
lstelastic lag 6.07P
Elastic torsion 7.59P
directed primarily
intiber axis ('[=
direction)
Computer Control System
Active-twist control of the ATR blades is achieved with
\
a computer control system incorporating a digital signal
IDEused topole
processor, 32 analog-to-digital channels, 6 digital-to-analog
alternately along
channels, and 32 digital input-output channels. The control
Figure 4. Active Fiber Composite (AFC) piezoelectric system may be used in an open-loop fashion with the user
actuator concept. prescribing either: 1) the amplitude and frequency of
collectivtewistmodeactuatioonr,2)theamplitudceo,ntrol
where VZBCbis the actuation voltage for the bth blade
phasea,ndharmonifcrequencoyfactuationF.orthefirst
typeof actuatione,achbladeis senttwistcommandsdetermined by the harmonic amplitude, An; the control
accordintogtheequation: harmonic, n; the control phase, Pn; and the azimuthal
position of the reference blade, gtref(which is assumed to be
blade 1with blades 2, 3, and 4 trailing at 90° intervals).
Vac t = Aac t COS(O/
Results
where V,ct is the actuation voltage determined by the
actuation voltage amplitude, Aact; the actuation frequency, All testing was conducted in the heavy gas test medium
(O,_t;and time, t. This type of actuation is useful for sine of the TDT at a nominal density of 0.0047 slugs/ft 3. The
dwell excitation for the development of system frequency rotor rotational speed throughout the test was held at a
response functions. With the second type of actuation, the constant 688 rpm resulting in a nominal hover tip Mach
active-twist commands are synchronized to the rotation of number, M,_p, of 0.60. The test matrix, presented in Table 3
the rotor system such that proper actuation frequency and in terms of advance ratio,/_, and rotor-shaft angle-of-attack,
control phase are achieved, regardless of rotor speed. For a,, incorporated various steady-state trim conditions
this type of control, either "collective" twist mode or representative of sustained lg level flight and descending
"individual blade control (IBC)" twist mode actuation may flight at advance ratios ranging from 0.140 to 0.367. At
be selected. With the collective twist mode of actuation, each flight condition the rotor was trimmed to a nominal
each blade is sent twist commands simultaneously according rotor lift coefficient, CL, of 0.0066 and the first-harmonic
to a schedule prescribed by the azimuthal position of a blade flapping with respect to the shaft was trimmed to
reference blade: within 0.1 °.
For each condition tested, data were acquired with
Vco=uA.cos(n<e¢-0.)
active twist control disabled. These sets of data are referred
to throughout the paper as the "baseline" conditions.
where Vcwl is the actuation voltage determined by the Typically, several sets of baseline data were acquired to
establish a set of well-determined baseline loads and
harmonic amplitude, An; the control harmonic, n; the control
conditions. The effect of active twist control was then tested
phase, On; and the azimuthal position of the reference blade,
by selecting the type of actuation: collective or IBC twist;
gel For the IBC mode of actuation, each blade is sent twist
the actuation voltage amplitude, typically 500 V, 750 V, or
commands according to a prescribed schedule associated
1000 V; and the harmonic frequency of actuation, typically
with its own position in the rotor azimuth:
3P, 4P, or 5P. The ATR computer control system was used
to automatically actuate the rotor system with the prescribed
amplitude, control type, and frequency parameters at a
sequence of control phases beginning with 0° and
progressing to 360 ° in 20° increments. At each control
Table 3. ATR Forward Flight Test Matrix
_t= 0.140 _t= 0.170 _t= 0.200 _t= 0.233 _t= 0.267 _t= 0.300 _t= 0.333 _t= 0.367
0_s= +8° X
0_s= +5° X
CZs= +4° X X X X X
CZs= +2° X X X
CZs= +1° X
0_s= 0° X X X X
C_s= -1° X X X
0_s= -2° X X X
0_s= -4° X X
0_s= -6° X X
0_s= -8° X
....:. the data set is presented in figure 6, in which only the 3P
.....'." : "..
response is presented for clarity. The upper plot in figure 6
so1.....i.....i i.....i. i".
presents the magnitude of the response as a function of
20.1..i....... ....i.. i i active twist control phase. The solid horizontal line
spanning the plot represents the baseline response. The
lower figure presents the response phase as a function of the
E t :_ ": : control phase, with the horizontal line indicating the baseline
phase. The results as presented in figures 5 and 6 are useful
for providing an overview of the effect that active blade
LT_0_'" " " '" twist has on system loads. However, important details are
unavailable with the plot formats of figures 5 and 6 that are
4
critical to a thorough understanding of the vibration
Control Phase, deg 360 Harmonic
reduction potential of the active-twist concept. For example,
Figure 5. Sample ribbon plot. Blade flapwise bending
figure 7 presents the 3P response results from figures 5 and 6
moment (0.29R) for 3P IBC control. ]_ = 0.140, CL = in another, generally more informative, format.
0.0066, _zn=-1.0 °, and 1000 V actuation amplitude. Figure 7 presents the rotating-system 3P response in a
response map or "circle plot" format for the baseline (no
phase increment the control conditions were held, permitting control) condition, and 3P active twist actuation voltages of
the rotor and fixed-system transient responses to dampen, 500 V, 750 V, and 1000 V. The plots present the 3P sine
then data were acquired automatically on the data component of the response as a function of the 3P cosine
acquisition computer systems. Measurements were made for component of the response, so that both response magnitude
19 control phases with the first measurement made at a and response phase are evident in the results. The solid
control phase of 0° and the final measurement in the circle represents the baseline condition, while the three open
sequence (360 °) being arepeat of the first. symbols represent the response measured during twist
Figure 5 presents the typical method used to convey actuation. A radial line is presented on the plot to reference
ATR rotating-system response in reference 16. The figure the location of the response due to 0° control phase. Control
presents a "ribbon plot" of the blade flapwise bending phase angles advance around the plots counterclockwise.
moment measured at 0.29R for 3P IBC active twist control The advantage of this plot type is that it directly shows the
at F = 0.140, CL = 0.0066, an= -1.0 °, and 1000 V actuation relationship between the baseline response, the response for
voltage amplitude. Each of the "ribbons" represents the varying actuation voltage amplitudes, and the zero harmonic
changing 3P, 4P, and 5P harmonic blade responses as a load condition represented by the origin of each plot. A plot
function of active twist control phase. The vertical bars on in which the "circle" of response points encompasses the
the rear surface of the plots (at 0° control phase) are origin represents a condition for which sufficient control
representative of the harmonic loads generated by the authority exists to eliminate that particular harmonic load.
baseline (control oft) condition. Another means for viewing As shown in figure 7, 3P harmonic loads for the blade
flapwise bending moment may be "zeroed" for the condition
,°3221o p5o5onse presented (_ = 0.140, CL = 0.0066, an = -1.0 °, 3P IBC
actuation). Thus, significantly more information is
conveyed by the "circle plot" format when compared to the
"ribbon plot" format, thereby permitting a more thorough
evaluation of the test results.
2O
5
O° __750 v lOOOV
0; , ' 9'o , , 18o, , 2_o, , 3;0 15 \
ControlPhase,deg
180 Baseline -
p
n_
"o
0 ,m
I1=.=-9900 0
-180! - J
0 90 180 270 360 i , , , | | , , , , , , , , , ,
ControlPhase,deg -_-25 -20 -15 -10 -5 0
Cosine Response
Figure 6. Sample response plot. Blade flapwise bending Figure 7. Sample response map or "circle plot." Blade
moment (0.29R) for 3P IBC control. ]_= 0.140, CL = 0.0066, flapwise bending moment (0.29R) for 3P IBC control. /_ =
_zn= -1.0 °, and 1000 V actuation. 0.140, CL = 0.0066, _ = -1.0 °, and 1000 V actuation.
Low-Speed Flight actuation voltage amplitudes (less than 300 V) may offer
greater fixed-system load reductions. Although these
Generally, the highest baseline fixed-system loads observations specifically address the results presented in
were observed for the transition flight speed,/_ = 0.140. It figures 8 through 10, similar trends were observed at other
was also observed that the highest fixed-system loads were advance ratios. In general, 3P IBC actuation was shown to
predominately at the more negative (shaft forward) angles of provide the greatest potential for vibration reduction across
attack, and that the IBC blade control was the most effective the flight speed range tested, although control phase and
in minimizing the majority of the system loads throughout actuation voltage for component load minimization was
the flight conditions tested. This paper, therefore, shall shown to vary more than is evident in figures 8 through 10.
concentrate on the test results acquired at/_ = 0.140, CL =
0.0066, and am= -1.0 °, as this is considered to be the "worst Effect of the Rotating-System on Fixed-System Loads.
case" condition. Likewise, the results presented will be Of specific concern in evaluating the results presented in
those acquired with the IBC active twist control to provide figures 8 through 10 is the effect that loads changing in the
an indication of the maximum vibration reduction control rotating-system have on those in the fixed-system. As has
authority achieved. been well-documented throughout the literature, a rotor
Figures 8, 9, and 10 present results in the circle plot system with a matched set of N equally-spaced blades will
format for 3P, 4P, and 5P active twist actuation, act as a filter such that only the harmonic loads that are
respectively, at the maximum, 1000 V, IBC actuation multiples of the number of blades are transferred to the
amplitude. Results presented in each figure include the fixed-system. Thus, for a 4-bladed rotor system such as the
blade flapwise (a) and chordwise (b) response at 0.29R, the ATR, one may expect to observe in the higher-harmonic
blade torsional (c) response at 0.34R, the pitch link (d) fixed-system: 4P loads, 8P loads, 12P loads, etc. These
response, and the six fixed-system balance (e through j) loads are generated in the rotating-system by harmonic loads
responses. Rotating-system (flap, chord, torsion, and pitch at nN and nN+l, where n = 1, 2, 3, ... Academically
link) plots include 3P, 4P, and 5P response results speaking, one would expect that 4P rotating-system loads
represented by circles, squares, and diamonds, respectively. will most greatly impact the 4P fixed-system vertical shear
Fixed-system results present only 4P responses. For each (Fz), and the 4P fixed-system yawing moment (Mz).
plot, solid symbols indicate the baseline, no control, Likewise, the remaining fixed-system loads -- the 4P
response. Where possible, radial lines are shown to indicate longitudinal (Fx) and lateral (Fy) shears, and the 4P pitching
the 0° control phase response. (My) and rolling (Mx) moments -- should be most affected by
Examination of figures 8 through 10 shows that the 3P and 5P rotating-system loads. It is useful, therefore,
significant blade torsional loads (subplot c) are generated at to identify evidence of such phenomena within the results.
the frequency of twist actuation. Pitch link loads (subplot One example of the effect of rotating-system loads on
d), while remaining small for 3P twist actuation, increase the fixed-system is presented in figure 11. In this figure, the
greatly at the actuation frequency for 4P and 5P actuation, 3P blade flapwise bending moment, the 3P pitch link load,
and are always higher in response magnitude than the and the associated 4P fixed-system rolling and pitching
corresponding baseline case. Blade flapwise bending loads moment responses due to 3P blade actuation at 1000 V are
(subplot a) are shown to be most sensitive to actuation at 3P presented for a flight condition of//= 0.140, CL = 0.0066,
and 5P due to the proximity of the first and second blade and am = -1.0 °. For this condition, the 3P blade chordwise
flapwise elastic frequencies. A similar phenomenon is also moment and the 5P rotating-system loads were found to
identifiable in the blade chordwise bending loads (subplot b) contribute little to the overall 4P fixed-system response. As
that are shown to be increasingly sensitive to the higher presented, the 3P blade flapwise moment (figure lla)
actuation harmonics. The 3P blade twist actuation (figure 8) reaches a minimum load along the 180 ° control phase radial
provides generally the greatest control authority over the line. At this condition, the 3P pitch link load (figure 1lb),
fixed-system vibratory loads, where reductions of 70% to which is already relatively small, is also nearly minimized.
95% are evident for all fixed-system loads except for yawing The associated effect in the fixed-system is presented for the
moment, which is generally unresponsive to 3P actuation. 4P rolling and pitching moments (figures l lc and l ld,
Further, fixed-system vibratory loads are very nearly respectively), which shows that each of these loads are very
minimized simultaneously when the blades are actuated at nearly minimized along the 180 °, as well. Similar trends
3P for control phases between 180° and 200 °. Figure 9 may be observed in the experimental results for other flight
shows that the 4P blade actuation provides the least control conditions, actuation frequencies, and rotating- and fixed-
authority over the fixed-system loads, even for the vertical system load combinations.
load where the 4P actuation was expected to provide the
greatest load reductions. Figure 10 presents the results for Vibration Sensitivity Studies
5P blade actuation, which is effective in providing control of
fixed-system lateral forces and yawing and rolling moments Clearly, a vibration reduction method is impractical if
although does not offer the depth of load reductions evident its application is limited to a small region of the flight
in the 3P actuation results (presented in figure 8). It is noted envelope; therefore, the variation in vibratory loads with
that the 5P actuation appears to over-excite the 5P blade changing flight conditions is of considerable interest.
flapwise and chordwise bending responses so that lower 5P Testing conducted to date on the ATR has not included
2O
2O
o
= 5P
o 0O
_10
Q. 0
o_
n,
,ci 0 '--=10 4P
if)
0o 3P
4P
-1030 .... -20.... -10.... 0 .... 10 .... 20
-2020 ...........-.1..0.. 0 10 20
Cosine Response
Cosine Response
(a) Blade flapwise moment (r/R = 0.29), in-lb (b) Blade chordwise moment (r/R = 0.29), in-lb
2O
10
10 _...-o-<>-o..
5
° ¢O-
J
o0 3P
._==° I ,w /
= 0
,i
-5
-2f_20-._..-. -10............0 10 20 -1010 .... -5 .... 0 .... 5 .... 10
Cosine Response
Cosine Response
(c) Blade torsion moment (r/R = 0.34), in-lb (d) Pitch link load, lb
10 50 10
• 40 0
=" 5 _ __
o o 30 _ ._ _ -10
0 20 --_
==•-2o
__.10 r --
50
-30
-1010'' '-5.... 0.... 5....10 -1010''0"i 0'26"30"4(3"50 -4010" 0"i0";20"30"4-0
Cosine Response Cosine Response Cosine Response
(e) 4P longitudinal shear (Ex), lb (f) 4P lateral shear (Fy), lb (g) 4P vertical shear (Ez), lb
2OO
600
0
o _ ,- 400
_ 0
400 = <
n, 0 .i -400 --
_ _ • 200
•-= 0o
-200 ........ -600 ............. 200
-200 0 200 400 600 -200 0 200 400 -200 0 200 400 600
Cosine Response Cosine Response Cosine Response
(h) 4P rolling moment (M), in-lb (i) 4P pitching moment (My), in-lb (j) 4P yawing moment (M), in-lb
Figure 8. Rotating- and fixed-system response maps tbr/J = 0.140, CL = 0.0066, _ = -1.0 °, and 3P, 1000 V IBC actuation.
2O
2O
10
= 5P
10
0
m 0o_3P 0o 3P
.E-10 4P
p
-1030 ............-..2..0.... -10 0 10 20 -2£ ................
-10 0 10 20
Cosine Response
Cosine Response
(a) Blade flapwisemoment(r/R= 0.29), in-lb (b) Blade chordwise moment (r/R = 0.29), in-lb
2O
10
4P
10 / 0_
c
O
°°-0 \ °- /
.E 4F 1-
-10 / -5
0o
-2_20 .... -10.... 0 .... 10 .... 20 -1010 .... -5 .... 0 .... 5 .... 10
Cosine Response
Cosine Response
(c) Bladetorsionmoment(r/R= 0.34),in-lb (d) Pitch link load, lb
10 5O 10
40 e 0
5
OQ. o. 30 oo. -10
0
n_ n,, _ n,, -2o
_10
.E -5 .E t-
0 _ -30
-1°16"'-5....o....5....lo -1010o'"iO"'_0'3' 0'',;0''50 -4°16'''0'''io'' "_63"0"40
Cosine Response Cosine Response Cosine Response
(e) 4P longitudinal shear (Fx), lb (f) 4P lateral shear (Fy), lb (g) 4P vertical shear (Fz), lb
2OO
600
0 _ 0°
400 =
o 0o _400--_----
_ _ O
& 200 _-200 _ _'
0 ._ -400
_ _ 200 0
-200 ....... -600 ............. 200 ................
-200 0 200 400 600 -200 0 200 400 -200 0 200 400 600
Cosine Response Cosine Response Cosine Response
_) 4Prolling mome_(M_,in-lb (i) 4Ppitching moment(My),in-lb _) 4Pyawing moment(M),in-lb
Figure 9. Rotating- and fixed-system response maps for y = 0.140, Cc = 0.0066, _ = -1.0 °, and 4P, 1000 V IBC actuation.
2O
2O
0o 5P
10 \ x_-- _"_
,-. _ 0°\
,10n o
• 0
_0
.E
-10 _ p
0°
i i i i
Ill I
-1y30 -20 -10 0 10 20
-2020 .... -10.... 0 .... 10.... 20
Cosine Response
Cosine Response
(a) Blade flapwise moment (r/R = 0.29), in-lb (b) Blade chordwise moment (r/R = 0.29), in-lb
2O
10
/,...O-O --_ /0 o
10
o P
Q,. &
0
0
.E
if)
-5
-10 \
-2020 .... -10' ' ' '0 .... 10 .... 20 -1010 ...........-.5.... 0 5 10
Cosine Response
Cosine Response
(c) Blade torsion moment (r/R = 0.34), in-lb (d) Pitch link load, lb
10 50 10
"" 5 ,40 L [_'_. "e 0
o 30 -- q- -0' o_10
I1_ _ I1_-20
=_-5 _ =_10-- _,
_ 0 m _-30
0 _ 20 D- _
-101()"'-5 .... 0 5 "'10 -1010 O"iO'":)d"30 40 50 -4010"0 "i0'")0"30"4-0
Cosine Response Cosine Response Cosine Response
(e) 4P longitudinal shear (Fx), lb (f) 4P lateral shear (Fy), lb (g) 4P vertical shear (Fz), lb
2OO
600
0 ,_
400 = _
o 0o = 400
_" O _ _0 o
o 200 _ _ -200 r"e_
_ L0c I_Z IZ_ 200 _
n, 0 I_ _ .----400
_ _ ._ 0
-200 ........ _ '" -600 ............. 200 ..............
-200 0 200 400 600 -200 0 200 400 -200 0 200 400 600
Cosine Response Cosine Response Cosine Response
(h) 4P rolling moment (Mx), in-lb (i) 4P pitching moment (My), in-lb (j) 4P yawing moment (M), in-lb
Figure 10. Rotating- and fixed-system response maps for/a = 0.140, Cc = 0.0066, g, = -1.0 °, and 5P, 1000 V IBC actuation.
20 2 = 0.200 and F = 0.267), with a zero condition for rolling
8o,
moment extending up to F = 0.333. By inspecting figure
16c, it may be concluded that a voltage amplitude of
approximately 500 V at an active twist control phase of 150 °
is sufficient to eliminate the rolling moment at F = 0.267.
To eliminate the vertical shear (figure 16a) would require a
-1-030' ' ' "20 .... -10.... 0 .... 10.... 20 voltage amplitude of approximately 700 V at a control phase
Cosine Response -2__.... -2.... -1' '' 0 '' '1
Cosine Response of 180 °. This illustrates the need to apply a closed-loop
(a) 3P blade flapwise moment (b) 3P pitch link load, lb control scheme that can effectively optimize the active-twist
(0.29R), in-lb actuation commands to yield a minimized vibration index.
This vibration index would be generated by combining the
weighted responses of multiple fixed-system loads to suit the
particular requirements of the encountered flight condition,
and could be tailored to a variety of vibration reduction
m 0
-200 goals.
=_ / =_ -400
180° _ 0°
-200 ' ' '
-200 0 200 400 -600 ............ Overall Vibration Reduction Capacity
Cosine Response -200 0 200 400
Cosine Response
(c) 4P rolling moment (Mx,), in-lb (d) 4Ppitching moment (My), in-lb Figures 8 through 19 present a sampling of the results
Figure 11. Rotating- to fixed-system loads comparison for/_ obtained during the initial forward flight test of the
= 0.140, CL = 0.0066, o_n= -1.0% and 3P, 1000 V IBC NASA/Army/MIT Active Twist Rotor in the TDT. As
actuation. presented, significant vibration reduction or vibration
elimination has been demonstrated for all 4P fixed-system
balance loads, with the exception of yawing moment,
thrust variation, however, a range of shaft angles-of-attack throughout the flight speed range. In general, actuation
and advance ratios has been assessed. Each of these will be
schedules using 3P IBC active blade twist were found to be
examined in the following sections. the most effective in minimizing 4P fixed-system loads. Of
the 26 different forward-flight conditions tested, all but three
Shaft Angle-of-Attack Sensitivity. Figures 12 through 15 offer combinations of actuation harmonic, voltage
present the circle plots for three 4P fixed-system loads (Fx, amplitude, and control phase that will eliminate at least three
Fz, and My) at four different shaft angles-of-attack (an = components of the 4P fixed-system loads, although not
+8.0 °, +5.0 °, +2.0 °, and -1.0 °) for F = 0.140, Cc = 0.0066,
necessarily simultaneously.
and 3P IBC actuation at 1000 V amplitude. As presented in
figures 12 through 15, the shapes of the control authority Active Twist Power
regions remain relatively circular until the rotor-shaFt tilt (an)
becomes negative. It is also evident that the total area of the Of significant concern during the development of an
individual control authority regions remains approximately
active vibration suppression system is the amount of power
the same as the rotor shaft angle-of-attack changes.
necessary for effective vibration reduction. The RMS
Therefore, it is the location of the baseline condition in the
electrical power required to actuate the ATR blades was
response map that determines whether a particular response
determined to be primarily a linear function of actuation
can be eliminated at a given flight speed. It is also noted frequency, as presented in figure 20. Minor variations in
that a zero condition may be achieved for all of the positive power required to actuate the blades were noted with
shaft angle-of-attack (rearward shaft tilt) conditions,
changes in rotor shaft pitch and forward flight speed,
therefore, active twist control appears to be applicable for
however, these variations were clearly secondary to the
the reduction of descent condition vibratory loads.
power required due to actuation frequency. The amount of
power required to operate all four blades simultaneously is
Flight Speed Sensitivity. Figures 16 through 19 present the presented in figure 20 as a function of actuation frequency.
circle plots for the three 4P fixed-system loads (Fx, Fz, and The total power absorbed by the blades for 5P actuation (73
My) at four different flight speeds (_ = 0.140, 0.200, 0.267, Watts RMS) has been found to represent less than 0.9% of
and 0.333) for CL = 0.0066 and 3P IBC actuation at 1000 V the maximum rotor power necessary for the flight conditions
amplitude. The shaft angle-of-attack was varied through the tested.
speed range to simulate a lg sustained flight condition. As
presented in figures 16 through 19, a substantial increase in Analysis Comparison
the size of the control authority regions is evident as the
flight speed increases. Some change is evident in the shape The practical development of active blade twist
of the control authority regions, however, no more than that technology for vibration reduction hinges largely on the
noted for the shaft angle-of-attack sensitivity. A zero capacity of analytical methods to capture the response
condition is within the twist control limits for each of the
characteristics of such rotor systems. Throughout the
fixed-system loads through the middle flight speed ranges (_ development of the NASA/Army/MIT Active Twist Rotor