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National Advisory Committee for Aeronautics, Technical Notes - Notes on Aerodynamic Forces - III - The Aerodynamic Forces on Airships

The results of the two preceding notes are applied to air—
ships and checked with wind tunnel tests.

The Air Forces Observed on an Airship Model.

In the first two notes of this series I discussed the dynam—
ical forces of bodies moving along a straight or curved path in
a perfect fluid. In particular I considered the case of a straight
and very elongated body and as special case again if bounded b9 a
surface of revolution.

The hulls of modern rigid airships are-mostly surfaces of rev-
olution and rather elongated ones too. The ratio of the length to
the greatest diameter varies from 6 to 10. With this elongation,
particularly if greater than 8, the relations valid for infinite
elongation reguire only a small correction, only a few percent,
which can be estimated from the case-of ellipsoids for which the
forces are known for any elongation. It is true that the trans-
verse forces are not only increased or decreased uniformly, but
also the character of their distribution is slightly changed.

But this can be neglected for most practical applications, and
especially so since there are other differences betneen theoreti-
cal and the actual phenomena.

Serious differences are implied by the assumption that the
air is a perfect fluid. it is not, and as a conseQuence the air
forces do not agree with those in a perfect fluid. The resulting
air force is by no means a resulting moment only; it is well known
I that the airship hull experiences both a drag and a lift, if in-
clined. The discussion of the drag is beyond the scope of this
note. The lift is very small, less than one percent of the lift of
a wing with the same surface area. But the resulting moment is
comparatively small too, and thus it happens as it appears from
model tests with hulls, that the resulting moment about the center
of volume is only about 70% of that expected in a perfect fluid.
It appears however that the actual resulting moment is at least of
the same range of magnitude and the contemplation of the perfect
fluid gives therefore an explanation of the phenomenon.