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National Advisory Committee for Aeronautics, Report - Methods for Obtaining Desired Helicopter Stability Characteristics and Procedures for Stability Predictions

In the first part of this report a brief review is presented of
methods available to the helicopter designer for obtaining desired
stability characteristics by modifications to the airframe design.
The discussion is based on modifications made during the
establishment of flying-qualities criteria and includes sample
results of theoretical studies of additional methods.

The conclusion is reached that it is now feasible to utilize
combinations of methods whereby stability-parameter values are
realized which in turn provide the desired stability chamo—
teristics.

Part 11 reviews some of the methods of predicting rotor sta-
bility derivatives. The procedures by which these rotor deriva—
ti ves are employed to estimate helicopter stability characteristics
have been summarized. Although these methods are not always
adequate for predicting absolute mines of the stability of the
helicopter, the’eflects on stability of changes in individua
derivatives can‘generally be estimated satisfactorily.

The problems relating to stability of helicopters have been
the subject of numerous published works. Requirements
established by the military services for satisfactory helicopter
stability are specified in reference 1. Some of the pertinent
work on this subject by the National Advisory Committee
for Aeronautics is described in references 2 to 21. The
purpose of part I of this report is to summarize some of the
physical methods available to the designer for obtaining
desired stability values by changing the airframe design.
Although the direct application considered is that of meeting
flying-qualities criteria, it may be worth pointing out that
other reasons often arise for designing a configuration so
that specific amounts of stability are provided; for example,
the most efficient combination of autopilot and airframe
design may be desired.

In order to predict helicopter stability, as for example to
estimate theoretically whether a proposed helicopter will
meet the flying-qualities requirements, both the applicable
equations of motion and the necessary stability derivatives
must be determined.