The critical problem of stall-spin characteristics of high performance aircraft and the need for experimental data in this area are reviewed. A rotary mechanism for obtaining this aerodynamic data in a conventional wind tunnel is presented. The intricacies of the drive systems and the articulation available through such a mechanism are described

Mancini, R. E.

Matsuhiro, D. S.

Vallotton, W. C.

English

NASA Technical Reports Server

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ROTARY btE(711ANISblFOR WIND TUNNEl,
STAI,I,/SP IN S'I UDIES
By Ronald I!. Mancini, Dennis S. Matsuhiro,
and Wilbur C. Vallotton
NASA Ames Research Center
ABSTRAC'F
The critical problem of stall/spin characteristics of high performance
aircraft and the need for experimental data ip this area are reviewed. A ro-
tary mechanism for obtaining this aerodynamic data in a conventional wind
tunnel is presented. The intricacies of the drive systems and the articula-
tion available through such a mechanism are described.
INTRODUCTION
With the advent of higher-performance aircraft, a great deal of research
effort has been expended to improve the aircraft's performance capabilities
and safety for the crew. One area of increased concern is in the stall/spin
characteristics associated with highly swept, low aspect ratio, high angle-of-
attack capabilities of current and future aircraft designs. A yearly toll of
aircraft and human lives can be directly attributed to loss of attitude con-
trol resulting from a stall/spin entered during a high performance maneuver.
With new military aircraft being designed for expanded flight envelopes that
require even higher angles-of-attack for routine flights, a method of pre-
dicting the full-scale spin modes and recovery techniques is needed.
For years researchers have desired a method of obtaining experimental
wind tunnel data from which a rigorous analytical approach could be devised
for computing the spin characteristics of a given aircraft design, Early at-
tempts at this approach included the building of a free-spin vertical tunnel
facility at NASA Langley Research Center. ttowever, these facilities are
greatly limited in their Reynolds and Mach number capability. Furthermore, ,,
' conventional wind tunnel test techniques, where realistic Reynolds and Mach
numbers can be achieved, are not sufficient to fully describe the aerodynamic
characteristics of an aircraft during a spin.
_, 6Z $PL Tec_ieal Memorandum 33-777
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https://ntrs.nasa.gov/search.jsp?R=19760021190 2020-03-20T02:34:30+00:00Z
SYMBOI,S "x
model angle-of-attack
model angle of side-slip or yaw
X offset angle of spin axis from air stream axis
axis for model positioning
axis for model positioning
spin axis
CW counterweight axis
HP horsepower
GPM gallons per minute
psi pounds per square inch
rpm revolutions per minute
ROTARY MECHANISM FOR DATA ACQUISITION
Because of the high Reynoids number capabilities of the NASA Ames wind ,
tunnel facilities, a concerted effort was directed into the design of a rotary -:
balance apparatus for use in a conventional wind tunnel. Such a device would ienable aerodynamic data to be obtained under the same local flow conditions
as in the full scale spin. The mechanism was to be capable of providing the
motions required for generating the force and moment derivatives due to con-
ing as well as damping in both pitch and yaw. Those configurations which are
important in understanding the stall/spin problem art illustrated in Figure 1.
BASIC OPERATION
The "Dynamic Stability RigJ' seen in Figure 2, supports and rotates anL"
aircraft model inside a transonic wind tunnel test section. The angle o_ in-
cidence of the model with respect to the air stream can be remotely varied to
a maximum of 30". This is accomplished through rotation about a set of axes
(_,_) which pass through the center of the model and intersect the spin axis
(fl). These axes are defi ed in Figure 2. By proper rotation about the _ and
axes, a particular combination of angle-of-attack (a) and angle of side-
slip or yaw (B) may be obtained. The envelope of nossible a-8 orientations
consists of discrete positions within a cone whose apex angle is 60". This
limitation of discrete rather than continuous positions will be discussed
later. Using a rear-mounted straight sting model, the maximum a or 8 is £30 °.
JPL Technical Memorandum 33-777 63 _'
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, 1
ttowever, by use of bent stings and top mounted models, this envelope can be
expanded to meet the desired range in a of -5 ° to +100 °
Changes ill model orientations will be made remotely and prior to spinning
the balance in the airstream. The model position is set and the counterweight
assembly {see Figure 3) is driven to a position which compensates for the un-
balanced static moment about the spin axis (s2). The entire apparatus is then
rotated in the air stream at a rate from 0 to 400 rpm. This high rotation
rate, much larger than the full scale spin, is a requirement resulting from
one of the modeling parameters in which spin rate is inversely related to
model size. For pure coning motion, the spin axis (fl) of the mechanism is
aligned with the tunnel stream axis. To obtain the moments resulting from
• damping in pitch and yaw, the spin axis (fl) is slightly offset from the air
stream axis by pitching the rotary balance assembly with the tunnel model-
support system. This offset is shown as angle 7 in Figure 3.
DRIVE ASSEMBLIES
The main drive for the spin rotation is powered by a servo valve control-
led hydraulic motor operating a pinion and gear assembly (Figure 2). The mo-
tor is capable of producing 64 HP at approximately 1500 psi and 70 GPM.
Th- overall designs of the drive assemblies for the _, _, and counter-
weight (CW) axes are illustrated in Figures 4 and 6. Figures 5 and 7 show
the details of the assemblies. All three drives are similar in design. Ro-
tation is powered by small D.C. electric motors incorporating planetary gear ,_
reducers and brakes. A roller chain couples a sprocket mounted on the motor
output shaft to the sprocket-and-wave generator assembly of a harmonic drive
speed reducer. The output of the harmoni_ drive is, in turn, mounted to the
main shafts of the _, _, and CWaxes. Because of the large gear reductions
involved, rotation rate about all three axes is approximately 30" per minute.
All assemblies incorporate solid bearings for space saving considerations.
The CWand ff drives include a sliding spline-lock mechanism. Centrifugal
accelerations at high spin rates acting on particular orientations of the coun-
terweight (CW) or the _-_ assembly-model combination can result in extremely
high moments about the ¢ and CWaxes. A maximum torque of 70,000 inch-pounds
is possible, this being several times the rated static capacity (10,000 inch=
pounds) of the harmonic drive units. The remotely operated spline-locks re-
lieve this load on the units by transmitting this torque directly to the main
housings (see Figure 5}. An internal threaded shaft, rotated by a chain - DC
motor drive, operates the spline to engage or disengage the lock of the main
shaft to the housing. Indexing for spline engagement is determined by a cam-
switch position detector, incorporated in the main harmonic drive wave genera=
tor-sprocket assembly. Each revolution of the _ave generator operates this
cam-switch. The primary axis positioning device is the potentiometer coupled
to the rear of the main shaft (Figure 5). When driving to a new position, the
rough positioning is performed by use of the potentiometer. The fine adjust-
ment is achieved by activating the cam-switch detector so as to allow align-
ment of the spline teeth for lock engagement. Spllne position overtravel in
either engagement or disengagement is prevented by a limit switch assembly,
operated by a chain sprocket in the spline-lock drive system.
_,:. 64 3PL Technical Memorandum $3-777
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The _ axis drive (see Figure 7 for details) is similar to the _ and CW
drives described above, tlowever, no spline-lock is included in this assembly.
The torque load values from aerodynamic and inertia forces about this axis are
within the static capacity of the harmonic drive unit. Shaft position again
is determined by a potentiometer coupled directly to the main shaft.
Positioning the model in a particular a-6 angle combination involves ro-
tation about both the _ and _ axes. The _ drive, which includes no spline-
lock, is continuous and may assume any angular position, ttowever, due to the
discrete allowable positions of the _ drive, to permit spline-lock engagement,
angular motion about this axis is restricted to 2.25 ° increments or 160 pos-
sible lock positions per main-shaft revolution. This results in a limitation
that only certain angular combinations of a-6 may be obtained. The allowable
incremental steps of a or 6 are dependent on the a-6 combination desired.
Electrical power for the drive systems and balance-data acquisition is
derived by use of a slip-ring assembly. This is a low-level signal slip-
ring unit containing dual brushes and 72 channels. On the rear of the slip-
ring unit is mounted an encoder for fi axis position information and a tacho-
meter for spin rate determination.
CONCLUI)ING REMARKS
The stall/spin characteristics of modern high performance aircraft and
increased capabilities of future aircraft make it highly desirable to obtain
experimental data which will enhance the use of analytical techniques for pre-
dicting these cri.tical modes. This paper has described a rotary mechanism ,._
for wind tunnel studies which should provide the motions necessary for genera-
ring this aerodynamic data, specifically, force and moment derivatives, in
support of stall/spin analytical programs.
/
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JPL Technical Memorandum 33-777 65
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66 JPL Technical Memorandum 33-777
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+
+.. _ ............ ++ + _ . + . ......
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_B 3PL Technical Memorandum 33-777
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__ _ COUNTERWEIGHT
SOLID BEARING
L'"'-d- al
--,.....2
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Figure 4. Overall view of Ck' axis counterweight drive
CHAIN SPROCKET-WAVE
SPLINE LOCK GENERAIOR ASSEMBLY HARMONI:C DRIVE
ASSEMBLY GEAR REDUCER
POTENTIOMETER
IR. IN! LOCK
INOEXING
CAM _MTCH
i I_INI LOCK 0¢ MOTORS-
_TCH AIUIY CHAIN DRIVE
f_'j', / /
Figure S. Counterweight or _ axis drive details
JPI._ Teel_iea! Men-_randum 33-777 69
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- SPIN AXIS
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%,..j ,,_,_
Figure (>. Overall vle_, of _ and $ axi._ drive-_
DC MOTOR CHAIN DRIVE
.ARUoN)COAIV=
Figure 7. $ axi._ :rive dctail._
JPL Technical Memor&ndum 33-7"1"
) 70
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Rotary mechanism for wind tunnel stall/spin studies

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