Robinson R44 Flight Manual Supplement

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Effective Helicopter Noise Abatement Operations Eric Greenwood Department of Aerospace Engineering The Pennsylvania State University What help do pilots get to fly quietly? Bell 212 HAI Fly Neighborly Guide Robinson R44 Flight Manual Supplement “Blade slap” is usually Blade-Vortex Interaction noise Shallow Steep Steeper • Occurs when rotor tip vortex wake passes near rotor blades • Rapid change in airloads results in impulsive noise • Tends to dominate annoyance metrics when it occurs • Magnitude and directivity strong function of rotor operating condition Δα Vz 2006 NRTC Bell 206 Acoustic Flight Test PPGD 95 100 105 110 115 -1800 -1500 -1200 -900 -600 -300 0 300 Advancing Side Microphone AIRSPEED ~ 60KT ( m = 0.155) VERTICAL SPEED, FPM BVISPL, dB -500 FPM -1000 FPM -1300 FPM -1700 FPM LEVEL FLIGHT ONE REVOLUTION You can’t trust your ears to tell you what’s happening outside the aircraft! BVISPL, dB Acceleration has a powerful influence on BVI noise 2007 NRTC Bell 206 Acoustic Flight Test Rapid maneuvers are often v e ry noisy 2011 NASA/Bell/Army Maneuver Noise Test Command & Control Trailer Microphone on Ground Board WAMS Eglin Maneuver Array When you can look at it the right way, maneuvering noise doesn’t seem different than steady flight Can we use the steady flight noise data we normally collect to model maneuvers? Yes! By mapping the rotor aerodynamics to acoustics! Pitch Up Advancing Side Roll Sickenberger, R.D., Modeling Helicopter Near- Horizon Harmonic Noise due to Maneuvers, PhD Dissertation, University of Maryland, May 2013. Can we use our models to develop useful guidance for pilots? Descending turns can be very loud! Sound Exposure Level, dBA Level turn 6° descending turn 6° climbing turn Deceleration also tends to increase noise. Sound Exposure Level, dBA Steady 0.05 g decel 0.05 g accel Noise Abatement Guidance • Avoid tendency to decelerate during maneuvers, such as turns or pull-ups • Sustained acceleration leads to high noise levels; however, a little acceleration can add margin to BVI onset during noise sensitive maneuvers • Keep noise sensitive areas inside of turns • Avoid descending during turns toward the advancing side of the rotor • Steeper climbs reduce noise on the ground, so long as the pull-up into climb is gradual But, is this good advice for all helicopters? 2017 NASA / FAA Noise Abatement Test R44 Noise Abatement Approaches Least Effective Approach Flown Shallow Decelerating Approach -3.5°Flight Path Angle 80 KIAS – 20 KIAS, 0.05g decel 100.3 EPNdB 97.2 dBA SEL Prior Noise Abatement Guidance Normal Decelerating Approach -4.5°Flight Path Angle 80 KIAS– 20 KIAS, 0.05g decel 96.3 EPNdB 93.1 dBA SEL Best Approach Flown Steep Approach to Flare -6°Flight Path Angle 80 KIAS until 0.15g flare to 20 KIAS 90.2 EPNdB 85.4 dBA SEL 10.3 EPNdB / 12.0 dBA SEL benefit re. Least Effective 6.3 EPNdB / 8.0 dBA SEL benefit re. Historical Guidance R44 Maneuvers X, feet X, feet dBA Level Turn Descending Turn Y, feet 2019 NASA / FAA Medium Helicopter Noise Abatement Test Noise Abatement Guidelines • General-purpose rules of thumb • Validated for all ten aircraft • Now included as a part of HAI’s Fly Neighborly Curriculum • Feedback from early adopters indicates that it’s working! The Best Laid Plans… … may be obsolete tomorrow due to changes in: • Mission objectives • Vehicle configuration • Weather • Air traffic Real Time Noise Awareness On Demand Mission Planning Area: • Spans 20 x 40 km, similar area to Las Vegas, Rome, or Beijing Mission: • Takeoff at Y = 0 km • Land at Y = 40 km • Closed airspace from 0 - ∞ m Trajectory: • About 20 minutes of flight time • About 10 seconds computer time • About one minute between nodes Dynamic Replanning eVTOL Noise • Electric motors don’t help, the engines weren’t the problem! • Lower harmonic noise is less important with lower tip speeds • Fluctuations in airloads cause loading noise at higher frequency harmonics: • Blade-Vortex Interaction (BVI) • Rotor-Rotor Interactions • Rotor-Wing Interactions • Rotor-Stator Interactions • Rotor broadband will also be more significant: • Airfoil Self Noise • Blade-Wake Interaction Rotor-Rotor Interaction (e.g. Boeing PAV) Rotor-Wing Interaction (e.g. Cora) Rotor-Stator Interaction (e.g. Bell Air Taxi) Vehicle Configuration Control • Most eVTOL have nonunique trim • These nonunique trim states can be exploited for noise reduction • “Acoustically-aware” flight controls should optimize vehicle configuration for best balance between noise exposure and performance XV-15, 70 KIAS, -6° FPA Nacelle @ 60 ° Nacelle @ 80 ° Concluding Remarks • A decade of sustained research in low noise helicopter operations is yielding real reductions in community noise exposure • Real-time noise modeling will soon provide operators with effective tailored guidance • “Acoustically-aware” autonomy will eventually enable ultra-low-noise VTOL operations in and around communities Fin 2016 NASA / Army Altitude Variation Test AS350 EH-60L 4000 feet Amedee Army Auxiliary Airfield 7000 feet USMC Mountain Warfare Training Center Sea Level US Navy Salton Sea Facility ARC Ambient conditions can change noise for the “same” condition by a factor of two! -30 -20 -10 0 10 p'n, Pa Sweetwater, 7000 ft, Mid GW -23.75 (15 Runs) -30 -20 -10 0 10 Sweetwater, 7000 ft, Mid CW -16.93 (14 Runs) -30 -20 -10 0 10 p'n, Pa Amedee, 4000 ft, Mid GW -15.74 (19 Runs) -30 -20 -10 0 10 Amedee, 4000 ft, Mid CW -14.23 (20 Runs) 0 0.01 0.02 0.03 0.04 0.05 Time, sec -30 -20 -10 0 10 p'n, Pa Salton Sea, 0 ft, Mid GW -10.30 (15 Runs) 0 0.01 0.02 0.03 0.04 0.05 Time, sec -30 -20 -10 0 10 Salton Sea, 0 ft, Mid CW -13.27 (15 Runs) 130 KIAS Nondimensional Nondimensionalization to the rescue! Motion moves the wake up into the rotor… and then right back down through it But, with a good wake model, we can predict the noise 1700 - 1000 ft 0 16 - 26 16 - 26 Acoust ic Pressure Tim e FRAME Measured Advancing Side Retreating Side