Lt Col Paul Fiorenza, USAF
Maj Jeffery Mabry, USAF (M)

Over the last decade USAF Combat Search and Rescue helicopter pilots have increased operations in high density altitude, heavy weight conditions during ongoing conflict in South West Asia.  While executing combat maneuvers in this environment operational pilots have noted under certain conditions in a right-hand turn that the helicopter does not appear to roll out of the turn at the expected roll rate.  Pilots have occasionally reported hitting the left control stop while trying to roll out of steeply banked right-hand turns.  This phenomenon has been reported to be repeatable, primarily during high gross weight, high angle of bank (AOB) turns.  Furthermore, between 2000 and 2008 two “Class A” mishaps occurred during low altitude maneuvering flight in good weather conditions.  In both cases the pilot failed to recover from a steep banked right turn.

In 2007, Det 1, 413 Flight Test Squadron, Nellis AFB, NV began an evaluation of HH-60G lateral flying qualities to support a simulator data collection effort, in partnership with the Army Aviation Test Center, Ft Rucker, Al.   The purpose of the test was to investigate the roll rate characteristics of the HH-60G in forward flight.  The flight test was designed to quantify roll rates of the helicopter and determine procedural and/or material solutions based on the data collected.  Additionally, testing was designed to determine and isolate the variables that contribute to a suspected reduced left roll rate.  The test sequence explored AOB, collective power position, airspeed, control input combinations, magnitude of lateral control input, and recovery techniques.

Follow on data analysis in 2008 and 2009 identified significant asymmetrical roll rates in forward flight.  When attempting left rolls to recover from high load factor right turns, it was demonstrated that steady state roll rates were only 50% (20 deg/sec vs 40 deg/sec) of those recorded when executing right roll recovering from left hand turns.   A clear correlation existed between increased load factor and the reduced left roll rate due to a load factor induced right rolling moment.  This right rolling tendency resulted in a left lateral cyclic stick migration to achieve stabilized right bank turns.   The rolling moment was assessed to be the result of changes in lift across the rotor disk under loaded flight, creating a “coning roll” to the right.  The combination of lateral stick migration to achieve balanced flight and a reduced left cyclic envelope at high collective settings resulted in a reduction in control envelope available to recover from steep bank right hand turns. Collective reduction of approximately 20 to 40%, or 2 to 3 inches, was demonstrated to reduce the load on the aircraft sufficient to regain left roll response; however, this technique resulted in significant increase in altitude lost.

The proposed presentation will discuss Power Point slides comparing lateral flying quality time histories under varying airspeed and load factor flight conditions.  Test pilot narration of composite test video and data recording will also be presented.  Following data presentation, an analysis of the multiple factors leading to the observed roll rate asymmetry will be discussed, including flight control design, tail rotor influence, sideslip, loadfactor effect on control power, and aerodynamic influences on the helicopter’s blades including blade stall, non uniform inflow and coning roll.  The presentation concludes by illustrating how undesirable handling qualities of multiple design and aerodynamic factors conspire to result in a situation where a transient negative stability condition can be achieved while executing maneuvering within flight manual limitations.