gschuld
Well-Known Member
I apologize ahead of time for a very long post. I am currently in rehab for excessive wordiness and I am making steady progress:grin:. I have read all the articles and threads that I have found(always looking for more) on the subject of reducing the acoustic signature of a small, single engine aircraft. I'll try to summarize the basics as to what I have absorbed so far. I realize that my statements are broad generalities and each one applies differently depending on the airplane/setup. As my personal focus is a composite two place side by side (KR-2s), I will make these generalities in relation with this type of plane in mind.
Assuming an aircraft that has not attempted to lower noise levels.
1. When at or near full throttle, the propeller generally produces the most prevelant noise from both the ground and from the cockpit. The consensus is that the bulk of the propeller noise is transmitted into the cockpit through the canopy.
2. When at cruise settings, the propeller's volume tends to decrease noticeably and the engine exhaust noise becomes roughly equal to or greater than the propeller volume noticed from cockpit, and likely the ground as well.
3. Mechanical engine noise(lifters, intake suction noise, PSRUs, cylinder fin "ringing", etc.) contribute to noise but are generally drowned out by either the exhaust or propeller noise unless significant measures are taken to reduce noise from both these sources.
4. Wind noise may be from very minor to very noticable depending on several factors including the planes aerodynamic shape, air speed, how air tight the canopy, etc. seals. Metal planes appear to suffer somewhat more than composite.
5. Exhaust driven airframe noise(from very little to deafening). Buffeting may be caused to the underside of the fuselage behind the engine by the exhaust pulses unless if the exhaust pipe(s) exit parallel or near parallel to the underbody. Metal plane appear to suffer more than composite.
6. Vibration induced noises (minumal to frustrating) from everything from engine sourced airframe vibration to linkages rattling.
I'm sure that there are others and please feel free to add...
As I will be building from scratch, I am interested in developing the best and most realistic plan before I start. Four big areas of interest for me are aesthetics, good performance(160+mph cruise speeds), noise reduction, and a good overall speed to fuel efficiency ratio. I realize that like everything else in the world, noise reduction efforts will need to be a compromise of sorts. I feel no need to compete with the Vietnam "quiet plane" with it's highly geared engine turning a big 6 blade slow turning prop with a full fuselage length exhaust system utilizing big auto mufflers etc etc. Am I willing to give up 40mph to go silent, absolutely not. Am I willing to give up perhaps 5mph to achieve a cruise speed noise level drop of 50% or more, probably.
So far, I have read about great suggestions to improve the acoustic levels.
-Propeller. There have been many reports of "drastically" different noise levels between prop manufacturers and designs. For example, I understand that the Prince P-tip and some of the Whirlwind propellers have a good reputation for being noticably quieter. A quieter performance propeller wouldn't likely be a noticable weight penalty, but I am curious as to whether a performance penalty would result.
-Engine exhaust. The european regulations have forced the issue abroad. From the long homemade "swiss muffler" to high tech "ceramic foam" experimantal mufflers (used by Dave Anders' modified RV-4 along with a Whirlwind propeller to take the quietest plane section win in last years NASA challenge contest)
The challenge, of course, would be to have an exhaust system that would be as free flowing as possible, yet be able to reduce the sound signiture significantly while trying to keep weight and aero drag to a minumum. I'd like to find out more about Dave Anders' custom muffers:lick:. Turning the exhaust outlet(s) downward atleast 10-15 degrees from the fuselage bottom sure seems to help the buffeting and overall exhaust tone noise heard from the cabin.
-Ensuring good airtight seals would be helpful everywhere around the cockpit.
-I am not sure whether motor mount technology can be worked with for aircraft engines. I know that in the marine industry, quality vibration reducing motor mounts have been fantastic at reducing both vibration and noise.
-Insulating the firewall, fuselage sides/bottom, etc with reasonably lightweight materials would certainly be worth looking at and seems to give good results(depending to who you ask). A lightweight, thin sound absorbing material placed on the inside of the cowling should reduce the rattling tin can effect by not allowing the noises to keep bouncing around all the hard smooth surfaces surrounding the engine looking for a way out. I understand that about a 40% coverage of the "interior walls" of an enclosed noise generator would be enough to make a big difference.
Assuming all these things are somehow improved successfully without incurring too much of a weight/performance penalty, I have a feeling that prop noise through the windshield would be one of the biggest noise hurdles left. I have read a number of studies about residential home window heat insulation and sound reduction technology. It appears that the trapped airspace between a sealed double pane not only has a huge effect on it's thermal efficiency, but noise reduction properties are enhanced dramatically as well. According to one study, a double pane window with 2 layers or 1/8" glass set(and sealed airtight) 1/2" apart is said to permit less than half the sound level(depending on the pitch,etc) through than a single 1/4" glass panel of the same overall weight.
So for example, a KR-2s factory canopy is 3/16" thick. Many builders have instead opted for the Dragonfly canopy as it has a sleeker profile and is 1/8" thick. So in theory, if I used two dragonfly canopies, glassed in the main (mildly tinted) one normally and mounted the other (clear) one inside the first with high end double sided 1/2" thick by 1" wide acoustical tape(made for this very purpose) I would not only(in theory) be able to reduce prop noise dramatically but have roughly triple the thermal efficiency. The weight penalty of using two layers of 1/8" compared to one layer of 3/16" called for in the plans can't be more than a pound or two.
My most obvious concern would be the possibility of a fogging problem between the panes that occasionally was a problem before window manufacturers started using an inert gas between the panes rather that air. Trapped moisture with "regular air" was the problem, and it can't be wiped away from the outside if it fogs on the inside! I have a window manufacturer not too far from me and I may ask them if it would be feasible to seal a double pane canopy with the same sealed gas as the modern windows. After the window guy stops laughing at my rediculous idea, I might even get an answer:gig:. Can't hurt to ask! Regardless, being that the outer canopy section would be mounted in exactly the same way as a stand alone canopy, the inner canopy section could be easily removed if deemed unsuccessful for some reason leaving the normal canopy intact and saving a small amount of weight. Perhaps I have too much time on my hands...
If you're still awake after reading this, thanks for your interest!
George
Assuming an aircraft that has not attempted to lower noise levels.
1. When at or near full throttle, the propeller generally produces the most prevelant noise from both the ground and from the cockpit. The consensus is that the bulk of the propeller noise is transmitted into the cockpit through the canopy.
2. When at cruise settings, the propeller's volume tends to decrease noticeably and the engine exhaust noise becomes roughly equal to or greater than the propeller volume noticed from cockpit, and likely the ground as well.
3. Mechanical engine noise(lifters, intake suction noise, PSRUs, cylinder fin "ringing", etc.) contribute to noise but are generally drowned out by either the exhaust or propeller noise unless significant measures are taken to reduce noise from both these sources.
4. Wind noise may be from very minor to very noticable depending on several factors including the planes aerodynamic shape, air speed, how air tight the canopy, etc. seals. Metal planes appear to suffer somewhat more than composite.
5. Exhaust driven airframe noise(from very little to deafening). Buffeting may be caused to the underside of the fuselage behind the engine by the exhaust pulses unless if the exhaust pipe(s) exit parallel or near parallel to the underbody. Metal plane appear to suffer more than composite.
6. Vibration induced noises (minumal to frustrating) from everything from engine sourced airframe vibration to linkages rattling.
I'm sure that there are others and please feel free to add...
As I will be building from scratch, I am interested in developing the best and most realistic plan before I start. Four big areas of interest for me are aesthetics, good performance(160+mph cruise speeds), noise reduction, and a good overall speed to fuel efficiency ratio. I realize that like everything else in the world, noise reduction efforts will need to be a compromise of sorts. I feel no need to compete with the Vietnam "quiet plane" with it's highly geared engine turning a big 6 blade slow turning prop with a full fuselage length exhaust system utilizing big auto mufflers etc etc. Am I willing to give up 40mph to go silent, absolutely not. Am I willing to give up perhaps 5mph to achieve a cruise speed noise level drop of 50% or more, probably.
So far, I have read about great suggestions to improve the acoustic levels.
-Propeller. There have been many reports of "drastically" different noise levels between prop manufacturers and designs. For example, I understand that the Prince P-tip and some of the Whirlwind propellers have a good reputation for being noticably quieter. A quieter performance propeller wouldn't likely be a noticable weight penalty, but I am curious as to whether a performance penalty would result.
-Engine exhaust. The european regulations have forced the issue abroad. From the long homemade "swiss muffler" to high tech "ceramic foam" experimantal mufflers (used by Dave Anders' modified RV-4 along with a Whirlwind propeller to take the quietest plane section win in last years NASA challenge contest)
The challenge, of course, would be to have an exhaust system that would be as free flowing as possible, yet be able to reduce the sound signiture significantly while trying to keep weight and aero drag to a minumum. I'd like to find out more about Dave Anders' custom muffers:lick:. Turning the exhaust outlet(s) downward atleast 10-15 degrees from the fuselage bottom sure seems to help the buffeting and overall exhaust tone noise heard from the cabin.
-Ensuring good airtight seals would be helpful everywhere around the cockpit.
-I am not sure whether motor mount technology can be worked with for aircraft engines. I know that in the marine industry, quality vibration reducing motor mounts have been fantastic at reducing both vibration and noise.
-Insulating the firewall, fuselage sides/bottom, etc with reasonably lightweight materials would certainly be worth looking at and seems to give good results(depending to who you ask). A lightweight, thin sound absorbing material placed on the inside of the cowling should reduce the rattling tin can effect by not allowing the noises to keep bouncing around all the hard smooth surfaces surrounding the engine looking for a way out. I understand that about a 40% coverage of the "interior walls" of an enclosed noise generator would be enough to make a big difference.
Assuming all these things are somehow improved successfully without incurring too much of a weight/performance penalty, I have a feeling that prop noise through the windshield would be one of the biggest noise hurdles left. I have read a number of studies about residential home window heat insulation and sound reduction technology. It appears that the trapped airspace between a sealed double pane not only has a huge effect on it's thermal efficiency, but noise reduction properties are enhanced dramatically as well. According to one study, a double pane window with 2 layers or 1/8" glass set(and sealed airtight) 1/2" apart is said to permit less than half the sound level(depending on the pitch,etc) through than a single 1/4" glass panel of the same overall weight.
So for example, a KR-2s factory canopy is 3/16" thick. Many builders have instead opted for the Dragonfly canopy as it has a sleeker profile and is 1/8" thick. So in theory, if I used two dragonfly canopies, glassed in the main (mildly tinted) one normally and mounted the other (clear) one inside the first with high end double sided 1/2" thick by 1" wide acoustical tape(made for this very purpose) I would not only(in theory) be able to reduce prop noise dramatically but have roughly triple the thermal efficiency. The weight penalty of using two layers of 1/8" compared to one layer of 3/16" called for in the plans can't be more than a pound or two.
My most obvious concern would be the possibility of a fogging problem between the panes that occasionally was a problem before window manufacturers started using an inert gas between the panes rather that air. Trapped moisture with "regular air" was the problem, and it can't be wiped away from the outside if it fogs on the inside! I have a window manufacturer not too far from me and I may ask them if it would be feasible to seal a double pane canopy with the same sealed gas as the modern windows. After the window guy stops laughing at my rediculous idea, I might even get an answer:gig:. Can't hurt to ask! Regardless, being that the outer canopy section would be mounted in exactly the same way as a stand alone canopy, the inner canopy section could be easily removed if deemed unsuccessful for some reason leaving the normal canopy intact and saving a small amount of weight. Perhaps I have too much time on my hands...
If you're still awake after reading this, thanks for your interest!
George
