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National Advisory Committee for Aeronautics, Technical Notes - Effective Moment of Inertia of Fluid in Offset, Inclined and Swept Wing Tanks Undergoing Pitching Oscillations

Fluid-dynamics studies were made of simplified model fuel tanks
mounted on a mechanism that simulated a wing undergoing torsional oscil—
lations of a few degrees. The tanks were mounted as follows: vertically
offset (pylon mounted) below the axis of oscillation; inclined to the
horizontal as in a climbing or diving attitude; and swept with respect
to the axis of oscillation as in a centrally mounted.tank on a swept
wing undergoing torsional oscillations. The effective moment of inertia
of the fluid was determined experimentally for the various tank con-
figurations over a tank-fullness range from empty to full and was es-
sentially unaffected by the oscillation frequency of the spring—inertia
dynamic system except when this frequency was near the lowest natural
fluid frequency because of its own wave motionr Comparisons of the

experimental and theoretical inertia solutions for full pylonamounted
tanks and centrally mounted swept—wing tanks showed good agreement.
Moreover, the theoretical solutions for full centrally mounted swept—wing
tanks offer good engineering approximations for partially full conditions
because of the small effect of tank fullness on the ratio of the measured
effective moment of inertia of the fluid to the moment of inertia of the
fluid considered as a solid. For partially full pylon—mounted tanks,
the ratio of the effective moment of inertia of the fluid to the moment
of inertia of the solid was small, and this inertia ratio increased
rapidly with tank fullness greater than 75 percent and approached the
theoretical values for the lOO-percent—full tank.

Studies of the effect of vertical, horizontal, and diffused baffles
in a pylon—mounted tank revealed that the effective moment of inertia of
the fluid was increased above that found in the unbaffled partially full
tank. It was also found that the diffused baffle distributed throughout
the interior of the pylon-mounted tanks produced high damping charac-
teristics for partially full conditions, whereas the use of the diffused
baffle in a partially full, centrally mounted tank did not produce high
damping characteristics.