Wind Tunnel Balances Page: 23 of 51
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ANNUAL REPORT NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS.
In the case of the Langley Field balance, substituting 0.57Xh for Wxo, and the values previously
specified for I and e, the limiting value of the sensitivity is found to be
0.57. x -e
w 0.57 0.57 x 0.02
The same method maybe applied to the lift and lads tthe conclusion that, with a model
having a lift of 20 kg., the initial sensitivity must not b9. greater than 1.5 gms. This would be an
extreme value of the lift, and it is seldom necessary to reduce the sensitivity below 0.5 gm. on
account of the variation in lift, but-on the other hand, it is seldom that actual use could be made
of the sensitivity of 0.1 gm., previously'taken as the standard for which it was nesessary to provide.
Only on stream-line bodies, struts, and similar objects of small resistance would the possible
accuracy of measurement be.as great as this. It is in some respects a disadvantage of the N. P. L.
type of balance that its "statical sensitivity" must be t1e same in respect of lift and drag.
Y, the third of the threeforces acting on the model,is perpendicular to the plane of symmetry,
and does not exist so long as the wind direction is parallel to that plane. As soon as the balance
moves from its position of equilibrium, however, the model assumes an angle of yaw, and this
gives rise to a force. Y which is almost always negative for a positive angle of yaw and vice
versa. The magnitude of Y for a given angle of yaw varies widely with the type of model and
with conditions of test, generally being largest, relatively to the lift, at small angles of attack.
The absolute values-of Y are virtualUy _ndependent. of the angle of attack. For an angle of
yaw of 1, Y may be as high as 2 per cent of the lift for complete models at an angle of attack
of 00, or about 1 per cent of the lift at 40. This force is largest when the wings have a considerable
amount of dihedral or sweep back. In the case of fair-shaped objects, such as airplane bodies
and airship envelopes, Y at an angle of yaw of 10 is usually from 10 per cent to 35 per cent-of X.
With models of the size used in the Langley Field wind tunnel, and with a wind speed of 50 m.
per second, Y has a maximum value of about 50 gms. for bodies and 100 gms. for complete
models..
If the originof thereference axesis drectly-over tepivQt when in equilibrium Yhasno effect,
as its line of action always passes through the pivot. If, however, as is usually the case, the
model is set up with the origin forward of the vertical through the pivot Y will tend to produce
instability in respect of the drag measurements, while not affecting the movements of the lift
arm. If the origin is above (in the model, not in the tunnel; i. e., nearer to the upper wing
than) the vertical through the pivot Y will tend todecrease the sensitivity in lift, assuming
that the two arms oscillate synchronously, without affecting the measurements of drag. The
opposite positions will, of course, have opposite effects. The magnitudes of these effects are
very small. They would seldom modify theensitivity by more than 0.02 gm., and they need
not be taken into account, provided that the model is so supported that the origin is reasonably
close to- (within 8 cm., in the case of, a. tunnel 1.5 meters in diameter) the vertical through the
main balance pivot.
There remain only the yawing and rolling moments to be considered. Both of these, denoted
by N and L, respectively, make their appearance, like Y, as a result of the assumption of an
angle of yaw, and do not exist while the wind direction is parallel to the plane of symmetry.
The analysis of the action of these moments need not be followed through in detail. The first is
unimportant, while the rolling moment, whichmay assume a considerable value in the case of a
model or a wing with marked sweep back or dihedral acts to increase the sensitivity in lift, and
is therefore opposed to the effect of the change of lift itself for motions in one direction, while
acting with it for motions in the opposite direction from the central position. The maximum
value of the effect of the rolling moment is about i5 per cent of the maximum unstabilizing
effect which may arise due to changes of the lift with angle of yaw.16
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Warner, Edward P. & Norton, F. H. Wind Tunnel Balances, report, 1920; (https://digital.library.unt.edu/ark:/67531/metadc65722/m1/23/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.