Bell 206B-1 Directional Control in Low Airspeed Flight.
AIRCRAFT RESEARCH AND DEVELOPMENT UNIT EDINBURGH (AUSTRALIA)
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Following a number of incidents associated with directional control of the Bell 206B-1 helicopter, Aircraft Research and Development Unit was tasked to conduct a limited investigation to determine critical wind azimuths and the conditions conducive to loss of directional control. The flight tests defined areas of critical wind azimuths and speeds were insufficient directional control margins and deficient directional handling qualities existed which were likely to substantially limit helicopter operations at high gross weight and density altitude. Lateral and directional control inputs in left sideward flight were hindered due to interference of the pilots left thigh and restricted clearance between the collective lever and cyclic stick. Insufficient aft longitudinal control margins were likely during flight in rearward azimuths with extreme forward CGs. Inaccurate and unreliable airspeed indications in the low airspeed flight regime were found to be unsatisfactory. Although dynamic flight tests for loss of directional control proved inconclusive, a study of relevant reports, articles and flight test data revealed several factors which, in some combination, may result in loss of directional control of the helicopter. Of these factors, operation of the tail rotor in the vortex ring state was considered most likely to be the trigger for loss of directional control. Several recommendations are made pertaining to helicopter operations, changes to the Flight Manual and further flight tests. Author