Handling Qualities of Large Rotorcraft in Hover and Low Speed

Summary and Discussion Handling Qualities of Large Rotorcraft With ACAH The analysis above showed that implications of the large vehicle size are notable and have two key aspects. First, the large size of the vehicle confers high mass and moments of inertia that introduce increased delay to the response of the vehicle to both pilot inputs and external disturbances alike—an aspect that factors into the pilot workload required to stabilize the aircraft and reject disturbances in hover and low-speed maneuvering. The trade-offs between disturbance rejection bandwidth (DRB) and closed-loop stability (characterized by the phase margin) were examined. The balance shifted to a preference for increased DRB and reduced phase margin as the vehicle size increased—this was attributed to the aforementioned workload issues, where for the larger vehicles, pilots preferred the increased automatic rejection disturbances—allowing them to “keep out of loop” and not have to work as hard or make as many corrective inputs to stabilize the aircraft attitude or position. Underlying all these outcomes was the second key factor—that part of the reason pilots wanted to minimize their control inputs was to keep attitude disturbances, particularly in pitch, to a minimum. The reason was that the conventionally located cockpit at the front of the aircraft, some 40 ft from the center of rotation at the center of gravity, led to a number of motion-related ride and handling qualities issues. Pitching the aircraft created heave motion at the cockpit, and yaw created sideward accelerations. Such responses would not be conducive to passenger acceptance. These factors were investigated further through a comprehensive analysis of the short-term attitude response requirements in hover and low-speed maneuvering for a selection of rotorcraft of different sizes. The analysis also included a specific study on the effect of the pilot offset in isolation, and showed that by simply changing the offset location, and thus modifying the motion and visual cues presented to the pilot, a vehicle with the same dynamics would get progressively better HQRs with decreasing longitudinal offset location. The levels of attitude bandwidth in particular, were “too hot” at the 30 to 40 ft offset location and above. Following the ADS-33 recommendations for Level 1 HQs for cargo/utility class helicopters caused pilots difficulties when trying to perform hovering tasks using attitude control. Reducing the bandwidth of the attitude response ameliorated the problem somewhat, but ultimately the handling qualities for the modified Hover MTE remained Level 2 at best. There is an open question about the direct applicability of the ADS-33 Hover MTE to the civilian role envisaged for an LCTR2 type rotorcraft. The experimental assessments in this report made some allowance for this by increasing the size of the