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National Advisory Committee for Aeronautics, Report - Studies of the Lateral Directional Flying Qualities of a Tandem Helicopter in Forward Flight

An investigation of the lateral-directional flying qualities of a
tandem-rotor helicopter in forward flight was undertaken to
determine desirable goals for helicopter lateral-directional flying
qualities and possible methods of achieving these goals in the
tandem-rotor helicopter. 0n the basis of comparisons between
flight measurements for various configurations and correspond-
ing pilots’ opinions, it is concluded that some important
considerations are: the presence of pedal-fired directional
stability, no reversal in rolling velocity during a turn following
a lateral step displacement of the control stick with pedals
fired, and reasonably well damped lateral—directional oscilla-
tions. These conclusions are also expressed in the form of
desirable flying-qualities goals.

Comparison between directional stability as measured in
flight and rotor—of model tests in a wind tunnel shows qualita-
tive agreement and, hence, indicates such wind-tunnel tests,
despite the absence of the rotors, to be one eflectioe method of
studying means of improving the directional stability of the
tandem heliocopter.

Flight-test measurements of turns and oscillations, in con—
junction with analytical studies, suggest possible practical
methods of achieving the goals of satisfactory turn and oscil-
latory characteristics in the tandem helicopter.

For the past few years the National Advisory Committee
for Aeronautics has been studying the flying qualities of
helicopters in order to set up flying-qualities criteria and to
provide means of improvement. The initial flying—qualities
work was mainly concerned with single-rotor helicopters.
Although the lateral-directional flying qualities of single-
rotor helicopters in contact flight were considered generally
satisfactory, familiarization flights by NACA pilots in
tandem-rotor helicopters indicated the need of studying this
type of helicopter.

In this report lateral-directional flying-qualities stud1es of
a tandem helicopter in forward flight are reported. These
studies, experience with single-rotor helicopters, and studies
made of airplanes such as those reported in reference 1
provide a basis for lateral—directional flying-qualities goals
applicable to the tandem helicopter as well as to all other
types of helicopters. In addition, these flight results, sup-
plemented by analytical studies, provide a basis for improve—
ment of the tandem type of helicopter in order to achieve
these goals.