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7-JZ_~~/~~ ~~ ~"--- ~~-=-__e~ ~/2-6/G s /¥ .,flSC- ~/-#'S#/275
Navy Department Bureau of Ordnance Contract NOrd 9612
PITCHING MOMENT BALANCE FOR THE
HIGH SPEED WATER TUNNEL
By
R. W. Kermeen
Hydrodynamics Laboratory California Institute of Technology
Pasadena, California
Memorandum Report No. EM-12. 4 April 15, 1952
Copy No. IZ
J. T. McGraw Project Supervisor
ABSTRACT
The measurement of model drag in the High Speed Water Tunnel
has been difficult because of the interaction of pitching moment and the
measured drag forces. Due to the mechanical arrangement of the force
table used. pitching moment will produce reaction at the drag gage caus
ing an erroneous reading. An internal balance has been fabricated which
measures pitching moment. allowing a correction to be applied to the
drag gage reading.
Introduction
Discrepancies in drag measurement have been observed in the High
Speed Water Tunnel which have been traced to the interaction of pitching
moment with the drag measuring apparatus. Tunnel models are rigidly
mounted to a vertical spindle which is pivoted in the center and restrained
by a load cell attached to the lower end of the spindle, as shown in Fig. l•.
Because of the mechanical arrangement, it is possible for pitching moment
to produce a force at the drag cell. Pitching moment can be produced by
shield interference, mechanical deflection, etc. A marked change in the
character of the drag curve has been produced by changing the lengthwise
position of the support point of the model.
Some pitching moment seemed to be unavoidable so an internal
strain gage balance was fabricated to measure the pitching moment pres
ent. A correction could then be applied to the drag reading.
De scrietion of Balance
The pitching moment balance which is used in conju:tlction with the
tunnel three-component balance consists of a l-in. diameter cylindrical
section l-3/8 in, long. It is attached to the top end of the spindle and forms
a part of the model cylindrical body section. As shown in the sketch, Fig. 1,
the pitching moTnent balance, hence the model. is attached to the spindle
thr~ugh ball bearing pivots, A triangular, unif-o~~' strength, cantilever
beam extends outward from the spindle. Any pitching moment on the model
is transmitted through a knife edge to the end of this beam. Drag forces
are transmitted through the ball bearing pivots to the spindle. A pair of
DRAG CYLINDER
TO DRAG GAGE
KNIFE EDGE
UNIFORM STRENGTH BEAM
PRELOAD SPRING
STRAIN GAGES
FLEXIBLE DIAPHRAGM
-2-
PITCHING MOMENT BALANCE
DRAG PRELOAD SPRING
STRAIN INDICATOR PRESSURE COMPENSATING BELLOWS
BALL BEARING PIVOT
Fig. l - Schematic of tunnel balance and pitching moment balance
-3-
Baldwin SR-4 resistance strain gages is cemented to the beam and the
wire leads are taken out through the spindle. The entire assembly is
made water tight and filled with oil. A thin neoprene diaphragm seal is
used around the spindle and a pressure compensating bellows is installed
in one end of the section. The body section and beam are shown in Fig. 2.
l .
11'1'1
Type SR 4 strain gages on uniform strength beam
Ball bearing pivot
Fig. 2 - Pitching moment balance section before assembly
The Baldwin SR-4 Type L strain indicator is used to measure the
bending of the beam. This instrument was calibrated against the tunnel
balance drag gage and the strain indicator readings are applied as a cor
rection to the drag gage data. The limits of operation of the pitching
moment balance are ± 17 in-lbs of moment which correspond to 1. 75
lbs of drag on the three-component balance. To insure proper alignment
of the model in the working section and to have maximum range with the
pitching moment balance, the models were statically balanced about the
support point before installing them in the tunnel. The range of operation
of the pitching moment balance can be increased by preloading the model.
- - - - ·~
-4- ~ - -
Results
· The balance was first used with a long model which had produced
erratic results. Figure 3 is a plot of drag coefficient versus Reynolds
number. The uncorrected drag data is represented by the lower curve,
while the upper curve represents data corrected for pitching moment
interaction. In general, the pitching moment correction was in the same
direction as shown in this figure, that is, a negative downward pitching
moment which causes a smaller drag gage reading. The negative pitching
moment is apparently caused by the balance spindle deflecting downstream
causing a small upward pitch of the model. This results in a negative pitch
ing moment on the balance as the stable model used in these runs attempts
to align itself with the stream. It has been found that the opposite is true
and a positive pitching moment results when an unstable shape such as a
pointed large calibre ogive nose is used on the pitching moment balance.
As a test of the effectiveness of the pitching moment balance a
series of runs was conducted in which the 5-in. A. S. Projectile was sup
ported at various points along the cylindrical body section. Figure 4
shows the results obtained on the three -component balance without the use
of the pitching moment balance. In these runs the model was supported at
the 0. 30 L, 0. 35 L, and 0. 48 L points. The curves (Fig. 4) were obtained
by making one drag run with a single spindle shield and then repeating the
run with the addition of a second image shield supported above the model.
This method is based on the assumption that the interference caused by
the image shield is the same as that caused by the spindle shield, and the
diffellence between the curves for these two runs is applied to the no
image run as a shield correction.
Figure 5 shows the results obtained on the pitching moment balance
with the 5-in. A. S. Projectile supported at the 0. 30 L, 0. 35 L, and 0. 41 L
points. Image shield runs were made using the pitching moment balance
but in every case the results were identical with the no-image runs.
The procedure in the High Speed Water Tunnel has been to support
all models as near the 0. 50 L point as possible. For the runs without
the pitching moment balance, the image shield correction for the o. 48 L
support point was negligible and it is interesting to note that this curve is
almost identical with those obtained on the pitching moment balance.
.3 5
0 (.) .30
...... z w - .25 (.)
lL lL w 0 (.)
.20
"' <(
"' 0
. 15
. 50
0 (.) .40
...... z w
~ lL lL
.30 w 0 (.)
"' <( a:: 0
.20
. 50
0 (.) .40 !-"" z w 0 lL lL w . 30 0 (.)
"' <(
"' Cl
.20
0 C; 0
0 L\
C;
2
-5-
) ~RRECTED FOR PIT CH ING MOMENT
~ u L C; C;
UNCORRECTED
l 4
REYNOLDS NUMBER X I 0- 6
l
v 0 0 C; C; C;
6 8
Fig. 3 - Pitching moment correction for model of 6-in. projector, 0. 22L support point .
I I I
10
I SUPP ORT PO I NTS
=
I 2 4
REYNOLDS NUMBER X 10-6
/ / ,
I I ~.48L Vr . 35L
'/ 30L
-
6 8 10
Fig. 4 - Effect of mode 1 support point -on drag coefficient of 5-in. A. S. Projectile without pitching moment balance. Curves corrected for shield interference .
61
SUPPORT POINTS
0 . 41 L
C; . 35 L
0 .30 L
i 2
.@ &SID eP
4
REYNOLDS NUMBER X 10- 6
~ ~ E
6
~ ~ ~ 4iD ~
8 10
Fig. 5 - Effect of model support point on drag coefficient of 5-in. A. S. Projectile. Model mounted on pitching moment balance. Image shield runs are included.
12
12
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