Collective Limit Characterization of the CH-53D Helicopter

United States Marine Corps CH-53D Sea Stallion operators in Afghanistan identified a limitation to aircraft power that was undocumented in the aircraft's flight manual. The limitation was caused by the mechanical collective high-pitch stop, which prevented pilots from being able to command the engine power available which had been calculated during pre-mission planning. While fleet pilots had occasionally hit the collective high-pitch stop in years past, they generally attributed this to pilot technique. However, with the integration of more powerful engines in 2009, operators reported that the collective high-pitch stop was encountered more frequently. The particular conditions under which the collective high-pitch stop might be encountered were unknown. To characterize this limit to engine power available due to the collective high-pitch stop, Air Test and Evaluation Squadron TWO ONE (HX-21) evaluated the CH-53D for hover and forward flight performance at varying density altitudes (Hd), rotor speeds (NR), and blade angle riggings. This technique yielded obtainable engine power plots that could be overlaid on the power available charts. The test aircraft was not limited by the collective high-pitch stop at 105% NR in a hover. However, at 100% NR, the collective high-pitch stop limited aircraft performance by as much as 10% torque (Q) in a hover and 24% Q in forward flight. Aircraft blade angle rigging had a significant effect on obtainable power, as much as 9% Q within the allowable rigging variation. Main rotor NR had a similarly large effect: Increasing NR to 105% yielded approximately an 11% increase in Q. This paper presents an overview of the flight testing that occurred during the collective limit characterization program at Marine Corps Base Hawaii (MCBH) Kaneohe Bay, the methodologies used to characterize the collective limit, challenges that were encountered during testing, and possible solutions to the collective limit dilemma.