Title: Periodic Helicopter Response in Turbulent Ship Air Wake
Author(s): Matthew Bornemeier, Murray Snyder, Gabriel Karpouzian
Abstract: An experiment consists of a remote-controlled T-REX 600E Pro helicopter with a rotor diameter of 1.37 m (4.45 ft) piloted above an underway 32.9 m (108ft) Patrol Craft's flight deck. The aircraft is hovered in positions which closely match actual takeoff/landing positions of MH-60 Seahawk helicopters above flight decks of US Navy air-capable ships to analyze the effect of turbulent Re = 3.4×10^5 ship air wake on helicopter angular motion. Modern naval helicopter flight training focuses on minimizing excess relative motion between the ship and the helicopter by holding constant positions during shipboard takeoff and landing profiles. Ship air wake turbulence and its interaction with helicopter rotor airflow create chaotic fluctuations of velocity within the flow field which in turn cause apparently random pitch, roll and yaw aircraft motions. These aerodynamic disturbances are effectively additional to the pilot’s control inputs. Due to the characteristic length scales of the ship’s flight deck height, H, and the relative wind freestream (streamwise) velocity, U_8, the majority of energy in this air wake turbulence is contained in a frequency bandwidth that is very nearly identical to the bandwidth of pilot control inputs to the helicopter. The result is an increased level of pilot workload which makes the evolution more difficult. In the experiment helicopter motion is assessed via VectorNav© VN-200 inertial measurement unit mounted on the helicopter transmitting variably-spaced data with an average sampling rate of 150 Hz in a 3-D Cartesian reference frame. Data is collected with the helicopter hovering relative to the ship’s flight deck in initial hover, perch, and 45 degree positions with heights scaled to full-size H-60 helicopters in the same positions above air-capable ships (CG/DDG/FFG). Pilot inputs to the T-REX helicopter are in the form of five PWM signals to servo actuators on the helicopter. Three signals correspond to three swashplate actuators, one throttle control actuator to maintain a constant main rotor shaft RPM and a tail rotor servo actuator to change the pitch on the tail rotor blades. PWM signals were converted from individual channel servo data into values that are a percentage of total available control input of collective, lateral cyclic, longitudinal cyclic and yaw to illustrate in all control axes both pilot workload in the time domain and the predominant input spectra of pilot inputs in the frequency domain. Fourier analysis is performed with the Welch power spectral density estimate on helicopter angular velocities to determine predominant frequencies of motion. Frequencies of this motion are compared with both pilot input frequencies and vortex shedding frequencies of incompressible, subsonic flow around 2-D and 3-D backward-facing step geometries. The spectra of pilot inputs in roll, pitch and yaw were found to have cutoff frequencies of 0.5-0.7 Hz, which agrees well with full-size helicopter pilot autospectra. A 3 Hz, non-pilot initiated disturbance is noted; the source of this disturbance may be attributed to the characteristic scale of the turbulent air wake resulting from the set motion of the ship and its geometric configuration. Of particular note, in the yaw direction the magnitude of this disturbance is on the same order as the pilot’s inputs. Because the frequency range is in the physiological range, the motion and magnitude of helicopter motion is even readily confirmed in video footage of the flight tests. Variability in the ship’s motion and its geometry may produce a turbulent air wake containing distinct structures characterized by scales with frequency content different than the 3 Hz scale. If present at full scale in an MH-60 Seahawk hovering above air capable ships during launch and recovery, this phenomenon may have significant implications regarding human piloting performance and spatial orientation, especially for night flights and flights in extreme operating regimes both of environment and pilot condition.