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National Advisory Committee for Aeronautics, Technical Notes - Water Impact Theory for Aircraft Equipped with Nontrimming Hydro-Skis Mounted on Shock Struts

This paper deals with theoretical methods for treating oblique water
impacts of aircraft equipped with nontrimming hydro-skis mounted on shock
struts. The shock-mounted hydro;ski has become of interest in recent
years primarily as a landing device for high—performance aircraft capable
of operation from water, snow, ice, or sod bases. In addition to softening
the impacts encountered in operations from the solid-material runways,
the shock strut allows a wider ski to be used on the water runways with-
out increasing the loads over those encountered with the narrower rigidly
mounted ski. Since the wider ski permits easier take-off because of its
increased lift-drag ratio, the shock strut indirectly improves take-off
performance without increasing the landing load.

Although several ways have been conceived to mount hydro-skis on
shock struts, such as, for example, the translating ski mounting, the
trimming ski mounting, and the verying-dead—rise ski mounting described
in reference 1, this paper is concerned only with the simple translating
ski mounting. This design (see fig. 1) incorporates a ski which is fixed
in trim relative to the aircraft and which translates upward under load,
telescoping the shock strut. It is the purpose of this paper to derive
and solve theoretical equations for this case.
The theoretical equations derived in this paper employ the
hydrodynamic-force terms of references 2 and 5 in combination with the
shock-strut spring and damping terms. The equations employing the force
term of reference 2 are simple enough so that with suitable spring and
damping restrictions they can be solved and plotted in nondimensional
form for use in design-trend studies. Such a study has been made for a
broad, practical range of aircraft landing conditions and is included
herein. The more accurate equations employing the force term of ref—
erence 3 were too complex for expression in dimensionless form and so
are presented in the form suitable for dynamic calculations involving
a wide range of bottom shapes, spring types, and damping exponents. These
more accurate equations might be employed for final design calculations.