naca-report-993
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National Advisory Committee for Aeronautics, Report - An Introduction to the Physical Aspects of Helicopter Stability
In order to provide engineers interested in rotating-wing air-
craft but with no specialized training in stability theory some
understanding of the factors that influence the flying qualities
of the helicopter, an explanation is made of both the static
stability and the sticlcgixed oscillation in horering and forward
flight in terms of fundamental physical quantities. Three
significant stability factors—static stability with angle of attack,
static stability with speed, and damping due to a. pitching or
rolling velocity—are explained in detail.
Most of the published literature on helicopter stability is
written for the specialist in stability theory and is somewhat
difficult for the average engineer to understand. An
explanation of the fundamental ideas underlying helicopter
stability in terms of the basic physical parameters involved
rather than in specialized mathematics therefore appears
desirable.
The subject is introduced by discussions of the means of
helicopter control and the origin of the forces and moments
which act on the helicopter as a result of deviations from
trimmed flight conditions. These fundamental ideas are
then applied to the hovering helicopter and to the helicopter
in forward flight. An understanding of the stability of the
helicopter in both of these conditions is aided by analogy with
the stability of the fixed—wing airplane. This comparison
is made possible in hovering because of the fact. that the zero
lateral velocity of the trimmed airplane in forward flight is
analogous to zero translational velocity of the hovering
helicopter. In forward flight, helicopter longitudinal sta-
bility may be directly compared with the corresponding
motions of the airplane.
The handling qualities of an aircraft are those stability and
control characteristics that affect the ease and safety of flying
the aircraft. This report. is primarily restricted to a study
of helicopter stability which, aside from its direct efi'ect on
handling qualities, must- also be studied in order to under-
stand control characteristics. Thevfirst phase of stability
considered is static stability, which has an obvious influence
on the handling qualities of the helicopter.
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