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National Advisory Committee for Aeronautics, Technical Notes - Tail Design Requirements for Satisfactory Spin Recovery

The design requirements for airplane_tail surfaces
that would.make recoveries from spins satisfactory have
been investigated in an analysis of the characteristics'
of models of approximately 100 military airplanes:'_In"'
the analysis, the relative distribution of mass along the
wings and fuselage and the relative density of the air-
plane were considered as well as the design of the '
vertical surfaces.

A chart is presented that shows an empirical relation-
ship between a tail-damping power factor and the—relative
density and mass distribution for satisfactory spin '
recovery. A formula for computing the tail—damping power
factor is also given.

It was concluded that,if a dgsigner provided a tail-
damping power factor of 600 x 10- for airplanes for which
the relative density at the spin altitude waanot greater
than 20, the probability that the airplane would have
satisfactory recovery characteristics over a wide range
of mass distribution by reversal-of rudder and elevator
would be very great. For larger values of relative _
density, larger values of tail-damping pcwer'ractor'would"“
be required.

The need for a reliable design requirement for air-
plane tail surfaces that will enable a designer to incor—
porate satisfactory recovery characteristics into a new
airplane design has long been recognized.‘ Recent British
and previous American research on this sub act are pre—
sented in references 1 and 2, respectively. The previous
American criterion indicated that a tail- damping power
factor of 150 x 10- -6 .was a minimum requirement for
o'otaining satisiactury recovery characteristics by rudder
reversal.

Spin-tunnel data accumulated since the publica—
tion of reference 2 have shown that this requirement is
inadequate.inasmuch as the effectiveness of_reversal of
the rudder in producing recovery is dependent not—only
upon thetailrdamping power factor but also_upon the rela-
tive mass distribution and relative density—or the air—
plane. Results obtained in reference 5 have indicated
that, as the relative mass-distribution along the wings
of—the airplane is increased, the effectiveness of the
rudder as a spin-recovery device decreases, whereas that
of the elevator increases.