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Issue No 54, 22 January 2024
By: Anthony O. Ives
At the time writing the environmental impact of transportation technology such as cars, aircraft, etc was getting a lot of attention in the media. Aircraft noise is also grouped in with environmental impact even though it does not environmentally pollute it is considered a form of pollution somethings referred to as noise pollution. Aircraft noise is one of the reasons why airports face resistance when there are plans to expand them usually from residents who live beside them. Helicopters are perceived as being very noisy however, compared with jet aircraft, a jet aircraft would be a lot noisier than helicopter. Helicopters other the hand operate at lower altitudes so they are more noticeable as being noisy.
Most of the helicopter noise is produced by the rotors although noise will be produced by the engines the noise from the rotors will usually dominate. They may also be some noise produced by aerodynamic flow around the fuselage but in most cases the noise produced by the rotor aerodynamics will make this practically unnoticeable. The rotor noise is produced as the result of four or five different effects:
Vortex Noise - this can also be referred to as turbulence noise or broadband noise, this noise is caused by turbulence and vortices in the rotor wake hence the name that is used for it, it is a very random type of high frequency noise but usually over a wide range of frequencies hence why it is sometimes refered to as broadband noise, high frequency noise dissipates at quite short distances so vortex noise will mainly dominate close to the helicopter, vortex noise is harder to predict usually CFD (Computional Fluid Dynamics) or experimental data is required
Rotational Noise - is produced by the pressure distribution of the rotor blades that is the result of the rotor producing lift/thrust, it is lower frequency noise which is associated with the blade passage frequency of the rotor, lower frequency noise can travel greater distances so it will be one of the first noises that is heard as a helicopter approaches, rotational noise can be predicted using a complex mathematical model [1] but also using CFD or experiment data.
Thickness Noise - is produced by the pressure distribution of the rotor blades that is the result of the rotor slicing through the air typically associated with non-lift dependent drag [2], in terms of frequency, penetration, prediction, etc it will be the same as rotational noise with a complex mathematical being available for prediction [1].
Blade Slap - is produced by a number of sources however, it is more commonly associated with Blade Vortex Interaction (BVI) with it the result of rotor blades tip vortices [3] interacting, contacting the rotor blade, blade slap typically happens when the helicopter is descending it can be a tell tale noise for the pilot, it is louder than rotational or thickness noise, in terms of frequency, penetration, it will similar but can be more challenging to predict usually requiring CFD or experiment data.
Shock Waves - blade slap can sometimes also be considered the result of shock waves although shock waves usually form at the blade tips where airflow velocities approach the speed of sound, in terms of frequency, penetration, prediction it will be similar to BVI
Within the helicopter cabin there may be other noise sources due vibrations and the engine noise can also dominate. The tailrotor can also produce a lot of noise in some cases more than the main rotor, the noises sources will be similar to those for the main rotor as described above.
The simplest way to reduce helicopter noise is to reduce the rotor thrust loading that is use as large a rotor diameter as possible. However, rotor diameter will be generally be limited by structural and weight requirements, so other methods can be looked such rotor blade design. Vibrations and possibly noise can also be reduced by using more rotor blades but again this will be limited by weight requirements. More rotor blades mean less load variation as the rotor spins, which results in less vibration hence why the larger the helicopter the more rotor blades they will have, compare a Robinson R22 with 2 blades to a Mil Mi-26 with 8 blades. Tailrotor noise can be reduced by using a fenestron [4] or in theory eliminated by using a NOTAR (NO TAil Rotor) system instead of a conventional tail rotor.
The above methods would be considered passive however active systems could also be used. An active system is one that is able to measure the noise or vibration and then use an actuator or speaker to create an opposite effect to cancel it out, the one example of this is noise cancelling headphones often used by pilots and passengers of helicopters. Flaps operated at the trailing edge of the rotor blade could also be used to reduce noise and vibration by modifying the pressure distribution of the rotor blade airfoils, a number of technical papers have been published researching this concept [5].
Smart materials [6] are considered more suitable than conventional actuators for controlling something such as trailing edge flap on rotor blade as they can integrated with the structure reducing the space and weight required for an active noise and vibration control system. Smart materials that are considered the most useful for active rotor blade noise and vibration control are piezoelectrics and shape memory alloys. Piezoelectrics are used in some cigarette lighters, they can be made from a various materials, they produce an electric charge if stretched or compressed and either stretch or compress if an electric charge is applied to them. Piezoelectrics can be bonded to a structure and used as sensor to sense a structure move or vibrate, but at the same deflect a structure if an electric charge is applied to it. Hence a piezoelectric can both act as a sensor and an actuator.
Shape memory alloys are a metal alloy that generally return to a certain shape at a particular temperature. The temperature can be applied by conducting an electric current through the shape memory alloy material resulting it returning to it to the 'memory shape'. Shape memory alloys can only really be used as actuators making them not as useful as a piezoelectrics. Bonded on an optium position on the correct structure both piezoelectrics and shape memory alloys can be used as a speaker to cancel unwanted noise within a aircraft cabin, etc. A number of technical papers have been published looking at using smart material as rotor blade flap actuation as well as noise and vibration control [7,8].
Smart materials used for active noise and vibration control can be used to carry out other functions such as rotor blade ice protection and aerodynamic control for boundary layer transition and flow separation.
This deviates from noise but is looking at secondary or even primary functions for a smart material operated noise and vibration control system. Rotor ice protection is probably the most practical secondary function an active noise and vibration system could also perform. Ice protection is not required on a lot helicopters due helicopters operating at low altitude hence below the freezing level and the fact that the vibration and centrifugal forces of the helicopter rotor system would generally shred most ice. However, helicopters that operate in colder climates particularly for offshore or search and rescue missions will require ice protection. Airliners common used thermal anti-ice systems which bleed hot air from the engine compressors, most helicopters equipped with ice protection systems would use a similar system. An anti-ice system does not allow ice to form whereas a de-ice system allows some ice to form then removes it once a certain amount accumulates. Electric thermal ice protection tend to be de-ice systems as they would usually require to much electric power to anti-ice like a bleed air thermal ice protection system. Thermal ice protection systems are not ideal for helicopters as they require a lot space, add weight and make the rotor manufacture more complex, they also give other problems such as corrosion, fatigue, etc.
The ideal ice protection system would be an ice phobic coating as it requires very little space, adds very little weight and requires no power. However, ice phobic coating wear away over time due corrosion so there still being researched as ice protection method [9]. Next up would be an ultrasonic ice protection system which could use piezoelectrics to create a vibration which breaks the bond between the rotor blade surface and the ice accumulation [10]. The ultrasonic ice protection requires very little power and is one such system that could be combined with an active noise and vibration control using the same system to do both tasks. In reality the ice phobic coating could be combined with the ultrasonic ice protection system where the ultrasonic is used as failsafe ice protection system as its primary job will be reducing noise and vibration.
Another option is using a pulse jet which is actuated by a smart structure design [11], this could use a pulsed bias liner [12] technique to deflect water droplets away from the rotor blade surface. Pulse jet system could also be used for aerodynamic flow control [13] as could the ultrasonic system. You could look at combining all three systems to carry noise, vibration, ice protection and aerodynamic flow control as they may be more economically viable if the combined systems can perform multiple functions. Advanced noise reduction systems in particularly are generally more likely to be accepted by a manufacturer if have another secondary or possibly primary function [12].
Please leave a comment on my facebook page or via email and let me know if you found this blog article useful and if you would like to see more on this topic. Most of my blog articles are on:
Mathematics
Helicopters
Woodworking and Boatbuilding
If there is one or more of these topics that you are specifically interested in please also let me know in your comments this will help me to write blog articles that are more helpful.
References:
[1] Helicopter Theory, Wayne Johnson, 1980, Dover Publications
[2] http://www.eiteog.com/EiteogBLOG/No35EiteogBlogDrag.html
[3] http://www.eiteog.com/EiteogBLOG/No29EiteogBlogDrag.html
[4] http://www.eiteog.com/EiteogBLOG/No1EiteogBlogDrag.html
[5] 'Development of a resonant trailing-edge flap actuation system for helicopter rotor vibration control', J-S Kim, K W Wang, E C Smith, Journal of Smart Materials and Structures 16 (2007), pages 2275-2285, October 2007, Institute of Physics Publishing
[6] Smart Structures: Analysis and Design, A. V. Srinivasan, D. Michael McFarland, 2000, Cambridge University Press
[7] 'Design, construction and characterization of a flightworthy piezoelectric solid state adaptive rotor' , Ron Barrett, Phillip Frye, Michael Schliesman, Journal of Smart Materials and Structures 7 (1998), pages 422-431, October 1997, Institute of Physics Publishing
[8] 'Development of a piezoelectric actuator for trailing edge flap control of full scale rotor blades', F K Straub, H T Ngo, V Anand, D B Domzalski, Journal of Smart Materials and Structures 10 (2001), pages 25-34, November 2000, Institute of Physics Publishing
[9] 'A silicone based ice-phobic coating for aircrafts', Summer L. Sivas, Bill Reigler, Rob Thomaier, Kelly Hoover, 39th International SAMPE Technical Conference - From Art to Science: Advancing Materials and Processing. Engineering, 29 October - 1 November 2007, Society for the Advancement of Materials and Process Engineering
[10] 'Ultrasonic shear wave anti-icing system for helicopter rotor blades', Jose L.Palacios, Huidong Gao, Edward C. Smith, Joseph L. Rose, AHS International 62nd Annual Forum - Vertical Flight: Leading through Innovation - Proceedings, 9- 11 May 2006, American Helicopter Society (Vertical Flight Society)
[11] Lumped Element Modeling of Piezoelectric-Driven Synthetic Jet Actuators, Quentin Gallas, Jose Mathew, Anurag Kasyap, Ryan Holman, Toshikazu Nishida, Bruce Carroll, Mark Sheplak and Louis Cattafesta, AIAA-2002-0125, 2002, American Institute of Aeronautics and Astronautics
[12] ‘Heat Transfer Through a Single Hole Bias Flow Acoustic Liner’, Anthony O. Ives, Jian Wang, Srinivasan Raghunathan and Patrick Sloan, Journal of Thermophysics and Heat Transfer, Vol. 25, No. 3, July – September 2011 https://doi.org/10.2514/1.T3637
[13] Rotorcraft Retreating Blade Stall Control, P. Lorder, D. McCormick, T. Anderson, B. Wake, D. MacMartin, M. Pollack, T. Corke, K. Breuer, Fluids 2000 Conference and Exhibit, 19-22 June 2000, American Institute of Aeronautics and Astronautics
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