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National Advisory Committee for Aeronautics, Technical Notes - Landing Gear Impact

This report deals with the impact forces-in landing gears. Both
the landing impact and the taxying impact have been considered, but drag
forces have been so far excluded. The differential equations are devel-
oped and their numerical integration is shown, considering the nonlinear
properties of the oleo shock strut. A way is shown how the dimensions
of the metering pin may be determined from a given load-time diagram.
A review of German literature on landing-gear impact is also presented.

The objective of this report is to study the impact forces acting
on the wheels and shock struts of an airplane. For practical reasons
the investigation has been limited to the vertical forces and does‘not
consider the effect of the drag load which_aets on the wheels during ~
the spin-up time. Within the limits_drawn by this restriction, an attempt
has been made to develop a method for numerical computations which, it
is hoped, will be useful in practical design work.

The oleo-pneumatic shock strut which is now in general use and
which has attained a high degree of perfection exhibits_a rather com—
plicated relation between the force, the stroke, and the rate of stroke.
For practical work, it is imperative to express this relation in mathe-
matical form and to develop a method for the numerical solution of the
ensuing differential equations. A detailed discussion of this subject
will be found in the section "Intentional Nonlinearities.

Nevertheless it is sometimes useful to consider a highly idealized
type of landing gear_which has linear differential equationsa Although
such a model will never correctly reproduce the details of the real
landing impact, it admits of easy mathematical treatment and permits
study of questions of a more general character. This has been done in
the section ”Linear Spring-Damper Systems" and the usefulness of the
results obtained there lies in the fact that they do not depend on the
more or less incidental details of real landing gears which unavoidably
enter the computations of the nonlinear theory.

The later sections of the report are deVoted to refinements of the
method. Encept for the simple mass correcti_ons, they will not be used
in daily routine work, but they may be of particular importance when
airplanes o_f unusual design are built.