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National Advisory Committee for Aeronautics, Technical Notes - Analysis and Modification of Theory for Impact of Seaplanes on Water

An analysis of available theory on seaplane impact
and a proposed modification thereto are presented. in
previous methods the over— —all momentum of the float and
virtual mass has been assumed to remain constant during
the impact but the present analysis shows that this
assumption is rigorously correct only when the resultant
velocity of the float is normal to the keel. (The proposed
modification chiefly involves'consideration of the fact
that forward velocity of the seaplane float causes
momentum to be passed into the hydrodynamic downWash)(an
action that is the entire consideration in the case of
the plan-ins float) and consid.eration of the. fact that,
for an impact with trim, the rate of penetration is -
determined not only by the velocity component normal to
the keel but also by the velocity component parallel to
the keel, which tends to reduce the penetration.

The analysis of previous treatments includes a
discussion-of each of the important contributions to _'
the solution of the impact problem. (The development of "
the concept_of flow in transterse planes, the momentum35
equations, the aspect-ratio corrections, the-effect of
the generated.wave on the virtual mass, the distribution
of surface pressure, and-the conditions for maximum _ '
impact force are discussed in detail} Impact treatments?"
based on flow in longitudinal planes, as for bodies of
very high aspect ratio, have -been omitted since they
seemed to be of no interest for the problem of the typical
float.

The momentum passed to the downwash is evaluated as
the product of the momentum of the flow in the transVerSe’
plane at the step by the rate at which such planes slide'
off the step. Simple equations are given that permit the
use of planing data to evaluate empirically- the momentum'
of the flow in the transverse plane at the step. On
the basis 01 such study, modification 0- (the general
equat:.ons of the previous theory is supplemented by
substantial improvement in the formula for the momentum
of the flow in the plane element) This improvement can
be made because the flow in the plane is independent—of
the fl.ight- path angle.