Helicopters, due in large part to their rotor design, produce an inordinate amount of noise. Though a percentage of this noise comes from the engine of the aircraft, a significant amount of the noise is produced by the aircraft's rotors (Marte 1970). There is a variety of different noises produced by a helicopter's blades such as thickness noise (the time between each peak in noise), blade vortex interaction (the sound of each blade hitting turbulence from the last blade), and high speed impulsive noise ( the noise created from the rotational speed of the blades). These are all forms of aerodynamic noise, which is noise created as air passes over the body or airfoils of an aircraft.
Currently, reducing the amount of aerodynamic noise created by a helicopter increases its cost by a great deal and requires an extensive amount of resources making these stealthier aircraft uneconomical and often inaccessible. Stealth helicopters with a reduced acoustic signature are currently only in use by the military because of their immense cost. There is very little information, such as the exact cost, available to the public due to the military keeping their technology very secretive. Even with this technology available there is still a very limited amount of stealthy helicopters in service. These stealth aircraft are used exclusively for missions that are of high importance and must be carried out without detection . They use a variety of things to reduce their acoustic signature such as using blades shaped to reduce turbulence that can reduce the noise created by the blade by up to 4 decibels (dB) (Nusca 2010), quieter engines, and special materials that absorb radar signals (Status Report 2015). All operations that use helicopters could benefit from this stealthy technology making them a safer mode of transportation and more effective in combat; however because it is uneconomical, most military operations are not able to put it to use.
In a civilian setting the acoustic signature of helicopters is a problem for a few different reasons. In large cities noise pollution is a serious problem, only worsened by the extensive use of tour, news, and private helicopters. Noise pollution is not only a nuisance but can lead to serious health issues such as cardiovascular disease, stress, and hearing loss (Jaffe 2015). An economical modification to any helicopter already in use could be implemented to reduce the noise pollution in large cities creating a safer and more peaceful environment.
To reduce the noise pollution created by helicopters the aerodynamic noise produced by them must be mitigated. Aerogels may be the solution. An aerogel is a solid material with an extremely low density. They are created by synthesizing a gel and removing the liquid components of the gel leaving behind a solid structure with microscopic pockets of air within the structure where the liquid was once contained. It’s because of these microscopic air pockets that the aerogel has an extremely low density. Along with their low density, aerogels have many other properties such as low thermal conductivity, low speed of sound through the material, and a high specific surface area (Norris 1996). These properties make aerogels effective insulators of both sound and heat.
There is a variety of aerogels that have been synthesized using different materials all having slightly different properties. For example, organic aerogels tend to be more rigid and compressible than silica aerogels which are often very brittle. There are also polyurethane aerogels which are extremely flexible. These different aerogel not only differ in properties but also in appearance (Norris 1996).
The purpose of this study is to create an effective and easily implemented way to reduce the acoustic signature of an aircraft. It was Hypothesized that by applying a polyurethane aerogel coating over the airfoils of a helicopter the sound signature of the aircraft will be reduced because the aerogel will deafen the aerodynamic noise created by the airfoil.
1. Presence of an aerogel coating on an airfoil.
1. The amount of aerodynamic noise created by the airfoil.
1. Speed at which the airfoil is rotated.
2. Size of the airfoil.
3. Thickness of the aerogel coating.
4. Temperature of the environment.
5. Type of motor used to rotate the airfoil.
MATERIALS AND METHODS
Synthesizing a Polyurethane Aerogel
Originally when searching for the most suitable type of aerogel for my project I considered the use of a silica aerogel. Silica aerogels are the most common type of aerogel and are relatively easy to synthesize. After seeking help from researchers at Missouri Science and Technology in Rolla, Missouri It was suggested to me to not use a silica aerogel because of their brittality. I was introduced to a polyurethane aerogel while visiting their lab and decided they would be a far superior option. Polyurethane aerogels are a relatively new type of aerogel developed in 2013 (Leventis 2013). Polyurethane aerogels are both flexible and compressible making them very suitable for use on helicopter blades as the blades often bend and flex, especially while stationary.
After deciding upon using polyurethane aerogels, a graduate student at Missouri Science and Technology graciously offered to assist me in the synthesis of the aerogels. The following chemicals were used to synthesize the polyurethane aerogels: triethylene glycol (TEG)-(Sigma Aldrich), anhydrous acetonitrile (CH3CN)-(Sigma Aldrich), acetone (Flinn Scientific), DBTDL, (Sigma Aldrich) and N3300A (Desmodur N3300A)-(Sigma Aldrich).
To synthesize polyurethane aerogels nitrogen N3300A (Desmodur N3300A, 37.8 g, 504 mmol) and the alcohol triethylene glycol (TEG, 16.875 g, 225 mmol) were dissolved in a mixture of two solvents namely anhydrous acetonitrile (209.55mL) and acetone ( 16.425 mL). The solution was stirred using a stir plate in an Erlenmeyer ﬂask at 23 °C under N2 for 10 min, and DBTDL ( 375 mL) was added. The resulting solvent was stirred using a stir plate for another 5 min and was poured into syringes used as molds which were sealed using parafilm and kept at room temperature. The gelation time was 20 minutes.
Figure 1: Aerogel going through gelation process
Once the solutions gelled, they were left in the molds for 24 hours to further solidify and set. After the 24 hour setting period the gels were removed from their molds and placed in an acetone wash containing 4 times the volume of the gel. The gels were left to soak in the acetone wash for 8 hours and they taken out and re washed 4 times. This is done to replace any liquid within the gel with acetone. The washed gels were then placed in a supercritical drier which completely removes all liquid from the gel creating the finished aerogel.
Figure 2: Completed Aerogel
The completed aerogel was then cut into slices approximately 2 mm thick and the shape of the airfoil. These slices were then adhered to the airfoils using a sprayable adhesive.
For my airfoils I used the blades of a remote control helicopter. The blades measure 117.475mm long and it 22.225mm at it’s widest point. There are two separate blades that connect to a shaft. They worked very well as they are scaled down but functional helicopter blades. Though these blades are from an r/c helicopter they still exhibit the same aerodynamic properties and characteristics an actual helicopter blade would have.
Figure 3: airfoils
Creating a Stable Testing Chamber
To accurately measure the amount of aerodynamic noise produced by the airfoils of the helicopter the noise was measured in an acoustically insulated testing chamber to ensure that no outside noise interferes with the data. To create an acoustically sound chamber I researched anechoic chambers. An anechoic chamber is an enclosed space that absorbs all sound given off within the chamber making it sound proof and reducing the amount of reverb throughout the chamber. To absorb the sound within the chamber, the inside of the chamber was lined with a sound absorbing studio foam with many wedge patterns.
Figure 4: Acoustically Insulating Foam
These wedges cause the sound waves within the chamber to bounce into the foam and become absorbed preventing them from ricocheting back throughout the box. By using this technique some anechoic chambers are able to create an environment with negative decibels.
To construct my own acoustically sound chamber I implemented the same wedge technique these anechoic chambers use. These wedges are sold in 30.4cm x 30.4cm sections online. I used plywood to construct a 70cm x 70cm x 70cm cube. My local hardware store sells these plywood sections precut so I then screwed them all together to create my chamber. I chose these dimensions for my acoustic chamber because my airfoils themselves have a 10” radius. By making my chamber 24” this gives the airfoils 9” on each side of the box.to ensure that there is ample room for air to circulate throughout the box without excess turbulation since it is a closed system.
Figure 5: Insulating Chamber Under Construction
After constructing the box I then cut two small circular holes on opposite sides of the box. One hole was for the motor to be able to mount to the box and allow the spinning motor shaft to enter the box. In the other hole, opposite of the motor hole, a 4.76mm radial ball bearing was placed. The shaft of the airfoils being 4.76mm fits snuggly through the radial ball bearing allowing the shaft to rotate very stably with minimal friction. After mounting the motor on the bottom of the chamber the shaft of the airfoils was then attached to the motor shaft using a coupler device I created using a metal lathe. Adhesive was then placed in the coupler to ensure the two shafts stayed attached to it. The other end of the shaft was then inserted into the ball bearing.
Figure 6: Completed Acoustically Insulated Chamber (side panel removed)
Driving the Airfoils
To rotate the airfoils within my acoustic insulation chamber I used a RS395-12v electric motor. I chose this motor as it provided enough torque to rotate the airfoil and shaft without providing an excess amount of RPM (revolutions per minute). The RPM still needed to be reduced so I did this by reducing the amount of voltage supplied to the motor by using a 3 volts to power the motor rather than a 12 volts. This reduced the RPM of the motor from 13,110 to 350 rpm, a more realistic rpm for an actual helicopter blade to rotate at. Though most remote control helicopters have a much faster RPM, I chose this lower RPM to better replicate the pitch and rotor speed of an actual helicopter’s blades. When the blades spin at a faster RPM, such as in a remote control helicopter, they create a noise with a higher frequency. I wanted to produce a pitch as close to an actual helicopter as possible because different pitched noises travel through materials differently. With a lower RPM the effect of the aerogel coating on the acoustic signature of the airfoil will more closely replicate that of a full size helicopter.
Figure 7: RS395-12v Electric Motor
Measuring Aerodynamic Noise
To measure the aerodynamic noise created by the airfoils within the chamber I used the Pop Voice Lavalier Lapel Omnidirectional Condenser Microphone™(Amazon). This device is a microphone compatible with smartphones. The microphone can record any noise above 30 decibels (dB) and between the frequencies of 20- 20kHz. It can measure these noises with an accuracy of ± 2dB. I used an app on my smart phone named Decibel 10th Pro to record the data collected by the microphone. This app allowed me to accurately read the data collected by my microphone on my smartphone. The app also allows the user to change the frequency weight. The frequency weight affects the range of frequency that the microphone records. This was set to frequency weight A (20Hz-20kHz) as that is that records the frequencies that can be heard by the human ear.
Figure 8: Pop Voice Lavalier Lapel Omnidirectional Condenser Microphone
All data was imputed into Microsoft Excel. A t-Test: Two Sample Assuming Equal Variances with an alpha value of .05 was used to determine significance.
I hypothesized that when applied to the airfoils of a helicopter, aerogels would reduce the acoustic signature given off by the airfoils. After collecting data and analyzing stats, I found that my hypothesis was supported. The average decibels given off from a non-aerogel coated airfoil was recorded at 92.775 dB. The average decibels given off from an aerogel coated airfoil was recorded at 85.845 dB (Graph 1). The aerogel coating reduced the acoustic signature of the airfoils by 7.5%. These results are supported by the acoustically insulating properties of the aerogels.
TABLE 1: t-Test Two-Sample Assuming Equal Variances
|t-Test: Two-Sample Assuming Equal Variances
|Aerogel Coting dB
||Non Aerogel Coating dB
|Hypothesized Mean Difference
|t Critical one-tail
|t Critical two-tail
It should be noted that due to the small diameter of the shaft connected to the airfoils there was instability in the shaft when rotated at high speeds. The instability may have affected the acoustic signature of the airfoils; however, this instability was perpetuated throughout all testing groups. It was assumed that because all groups experienced this instability, the acoustic signature was affected the same throughout.
GRAPH 1: Comparison of Coated & Non-coated Airfoil
P-value = less than 0.001; N-value = 40
DISCUSSIONS AND CONCLUSIONS
It can be concluded from my data that an aerogel coating over an airfoil does reduced the acoustic signature of said airfoil. The 7.5% reduction of noise is extremely promising for future studies using aerogels to reduce the acoustic signature of helicopters. The average helicopter at a 100 ft. hover gives off 100 dB. With an aerogel coating in place on the helicopters airfoils, that 100 dB could potentially be reduced to 92.5 dB. To put 92.5 dB into perspective the average motorcycle gives off 90 dB from 25 ft. away(“Noise Sources and Their Effects” 1992). Though this data appears promising, more testing on variables such as durability and creating aerogels in large quantities would need to be done before they could be implemented on a helicopter.
A few improvements could have been made to get better and more reliable data. A more stable testing apparatus could have been created. The instability in the airfoil shaft gave off excess noise, which could have interfered with data collection. Additionally, a more sensitive microphone could have been used to record data. This would have ensured more accurate and consistent data.
The data collected during this experiment suggests that the use of an aerogel coating on an actual helicopter would reduce its overall acoustic output. This could be implemented in both a civilian and military setting. Military helicopter could be made safer and more effective by reducing their acoustic output keeping soldiers onboard safer. Civilian helicopter could also be made quieter, reducing the noise pollution they give off and making an urban setting a safer and healthier place.
In a future study the following aspects will be explored:
Suraj Donthula, Graduate student at Missouri Science and Technology
-Donated much of his time and resources on helping me synthesize aerogels. He allowed me to work in his lab with him and shared his vast knowledge on aerogels.
Dr. Nicholas Leventis, Professor at Missouri Science and Technology
-Got me in contact with Suraj Donthula and gave his thoughts and suggestions on my project.
Mr. Chris Reeves, Advisor and Teacher
-Helped to make sure my project was a success and donated his own personal time and resources. He provided lots of motivation and was always there when I needed guidance.
Mrs. Alison Bowman, Advisor
-Provided feedback on my paper and ideas. She also drove me to Missouri S&T to synthesize my aerogels.
Mr. Randall Driver, Advisor
-Helped gather material and gave feedback. He also drove me to Missouri S&T when needed.
Mark and Andrea Ellefsen, Parents
- They supported me throughout my project and helped with anything they could.
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