View Full Version : Main Gear Sound Waves

07-19-2012, 12:17 PM
OK i hope this is in the right place. Currently building a 450 jet ranger using helical main gear and belt drive and would like to try to cancel out some more of the drivetrain noise. Can anyone elaborate on the orientation of the sound waves generated by the main gear? Suggestions for sound deadening materials and design are welcomed.

08-16-2012, 05:34 AM
welll... you could put a mini protos in it
that will about kill all the sound thing is dead quiet
put some Rail Blades on and it might as well be a stealth heli

08-17-2012, 11:05 PM
:rotf231 views... and not a single suggestion(that didnt require replacing the heli:face)

Mods please feel free to remove this thread.

Funny, I posted the same question on another forum and got quite a few responses (and an actual answer) in a couple of days. I apologize to those who wasted their time reading this.

08-17-2012, 11:22 PM
Wow aditude. Since its a scale question. One would think it might be better asked in the scale forum

Sent from my iPhone using Tapatalk

08-18-2012, 05:07 PM
No attitude intended. Sorry if it came across that way. While my project is a scale project, I was actually hoping someone a bit more educated than myself could shed light on the physics side of the question. Since this is the "physics" forum, I thought it fit. :cheers

08-18-2012, 10:23 PM
It's a tough question. In a gear mesh, the sound energy originates from the repeated impact as each tooth engages, but the vibrations travel through the gears, bearings and frame of the helicopter and emit from a variety of surfaces.

If I were attacking this problem, I would start by trying to isolate the fuse from the heli frame so it doesn't transmit and emit the gear noise.

08-20-2012, 04:12 PM
thanks for the reply. Actually hadnt considered isolating fuse from frame,etc. I suppose the noise IS going to transmit through(and possibly even be amplified?) the fuse if mounted solidly. Excellent suggestion. :thumbup:

I am also looking into some way to dampen the actual main gear itself, possibly through a very small amount of weight, much like a harmonic balancer on an auto engine.
It was suggested I might try coating the center section of the main gear with a VERY light coat of rubber, as in auto undercoating, spray on bed liner material, etc. Can anyone see pros and/ or cons to this?
One idea I had was to drill small holes in the outer end of the "spokes" of the main gear and insert steel bb's or lead air gun pellets using epoxy or similar. Thoughts?

Because this seems to be viewed as a "scale" only discussion, if the mods would like to move this to the scale section, please feel free to do so.

08-20-2012, 06:51 PM
I am also looking into some way to dampen the actual main gear itself, possibly through a very small amount of weight, much like a harmonic balancer on an auto engine.

I doubt such a thing would be very effective without adding a lot of weight. the noise comes from the contact between the teeth so is difficult to eliminate.

Can you get helical (slant) cut hears for your heli? Those are considerably quieter.

08-20-2012, 08:41 PM
I don't understand why you want to add things to the main gear so badly. Harmonic balancer in piston engine is to balance an unbalanced system. The main gear is perfectly symmetrical however? So whats the ball bearing for.

08-20-2012, 08:52 PM
Can you get helical (slant) cut hears for your heli? Those are considerably quieter.

The OP has stated that he is using helical gears.

08-20-2012, 09:39 PM
Yes, I am currently using a helical gear.

Let me start by saying I find the majority of your posts to be very informative/helpful. Im not looking to add things arbitrarily, quite the opposite as Im trying to keep weight down as much as possible. I realize some of you guys are engineers and so forth, but I have over 20 yrs building race motors(drag and road) and have seen firsthand the different effects design of harmonic balancers can have. Anyone familiar with the older 'Rattler' balancers( common on smal block engines running high rpm) will know why this idea makes sense. They used a series of "freefloating" weights (small mallory metal slugs installed in bores just slightly larger than the OD of the slug/hence the 'rattler' name) that had an enormous impact on high rpm vibes, far better than the elastomer/ ring type found in passenger cars. Im only looking for NEW ideas, suggestions.

While I am aware the noise is not going to be eliminated entirely, or simply, I'm also aware of there being 'more than one way to skin a cat' so to speak. I like thinking outside the box and trying new things. There is a good thread somewhere here on HF(dont recall where exactly) about a fella making alloy main gears for his 8003D. Post after post about too heavy, too noisy,etc. When he posted the vid of the initial run it was astoundingly quiet(spur cut gears no less) which got me thinking about my own project. I dont care too much for naysayers or the idea of what the world would be if we didnt go outside the box occasionally. After all," Freak" IS right in the forum name, no?:peace

08-20-2012, 10:37 PM
A fairly simple approach is just to use a decent mic and an audio processing app like Audacity; you can run a Spectrum/FFT plot, which will help you isolate both the frequency and magnitude of the annoyance(s). Knowing the motor RPM (Im assuming you have a tach), with head and tail removed in test, you can back that spectrum analysis into motor unbalance, pinion/main tooth interface, tail drive, or belt drive, etc.

I suspect you will find the most upside in damping the chassis/canopy. You might also get away with a damping compound like polynorbornene on the lower main gear face, if that is a primary acoustic source. May be a bit of a trick to find an appropriate Shore value that will not expand like pizza dough; perhaps a simple backplate could be tied in.

If you look at just a standard equal loudness contour for human hearing, you will find we are very insensitive to 100hz and below. So while the vast majority of acoustic energy is coming from our airfoils, our perception is generally different for larger machines.

08-20-2012, 11:01 PM
Bladebender, thinking differently is a true gift i'm in full support of your experiment to reduce gear noise. I feel like a douche for dismissing your idea, hopefully you can understand how often half baked ideas pop in and put of forums, including some of mine.

I think attempting your advanced gear mods probably come after the other suggestions here such as isolatin the canopy, in terms of overall impact. The way i think of your problem, is that gear vibrations are low impedance low amplitude, and air vibrations are high amplitude high impedance. Dampening low amplitude low impedance oscillation is quite difficult, so i would work on the interface - ie how gear vibes eventually transmit into air vibes.

There is a tight coupling between gear and main shaft

There is tight coupling between shaft and frame

There is tight coupling between motor and frame

Then frame/canopy is the converter between the source and output

So if you can uncouple any of these you should see a significant reduction in noise.

The other side of the coin is reducing the source of vibes to begin with, which is what you asked about and what i think is hardest. The only simple thing is helical gears with heavy grease, which given your much greater experience than mine, i trust you've figured that all out. As extrapilot mentioned, analysing the audio will accurately locate the source, and quantitate the vibration. You must apply the human frequency response curve to the measurement to decide what particular vibrations are the ones of concern.

08-21-2012, 12:33 AM
Okay, if you're headed out of the box, let's take a walk. Some ideas I've been tossing around:

New, precision gear train with specially-cut metal gears. Maybe wider to increase the number of contact points, reducing the pressure per point.

Eliminate the tail output shaft by routing a longer tail belt around a large gear on the main shaft with idler guides to route it into the tail.

Mount the motor and main shaft on a subassembly that is isolated from the frame. Getting the power to the tail might be tricky because the belt tension will compress the isolators.

Replace the main gear train with a belt.

Find a large-diameter, high-torque, low-kv motor and run it inline with the shaft with no gear reduction. You're probably looking at around 325KV, and quite heavy, but also very quiet.

Enclosed, lubricated main gear box.

Lower head speed with a multi-blade rotor.

08-21-2012, 01:04 PM
Thanks for the heads up on Audacity. I'll look into that, though to be honest most of this 'science' is well beyond my education level, hence asking in the smart guys forum. I'm liking the backing plate idea as well(simple). I'm just a glorified grease monkey with a love of power and a penchant for doin things differnt from the next guy. Thank you Mr. Erik Buell for that...

No harm, no foul and certainly no need to feel like feminine products:hug:. I understand that alot of :quotesideas come across internet forums. To be honest, most probably have some validity IF people would follow thier own path and not cave to the 'can't be done' mentallity of laziness. My initial approach was actually to try isolating sub assemblies, but I feared making the entire platform unstable. So I decided to use the KISS approach and focus on what I felt like was the primary contributor... the main gear. However, after looking at the list of numerous interfaces you posted I will revisit that arena as well.

I like how youre thinking, sadly the current economy is limiting my R+D funds. Custom parts will have to wait a bit, but you've sparked a few ideas I hadnt thought of, thank you.

To anyone else, if you have any ideas not already mentioned, no matter how simple or silly please feel free. I'm VERY open minded.

Creativity is the fuel for progress.

08-25-2012, 10:17 PM
Changed helical main for straight cut gear(for testing) and loaned to a friend a bit more in the know with the audio apps(thanks again ExtraPilot) for finding"loudest" source of noise. using 3500kv/13t brass/stk150 main(governed to 2500HS no main or tail blades). the pinion/main mesh was in fact the 'noisiest' area he could find, however after changing to a steel pinion(bushed traxxas pinion?not sure, found it in tool box and lucked up on pitch) he noted the the main gear/tail driver gear area was now loudest. assuming the helical gear is at least as quiet as the steel mock up, that would mean the majority of the sound is coming from the tail driver gear. He says he believes the sheer mass of the motor probably dampens most of the noise there. Since the tail drive gear is very light and "floats in small bearings it most likely the main culprit in transmitting vibes to frame. going to install original helical setup and let him loose on it again. update to follow
Thanks for all the info/ideas guys, :peace

12-30-2012, 11:48 AM

Any updates? Noise is often a problem for RC, and any progress is welcome.

01-06-2013, 01:32 PM
Rubber or other 'bouncy' materials will indeed reduce or deaden the sounds from your gears, although I doubt spraying it actually on the gears will help your helicopter fly since the gaps between gears are normally accurately machined and matched I suspect that this will only serve to increase the friction between the sprockets and reduce the mechanical efficiency of the drive system.

If you had boundless resources then you could analyse the frequency / frequencies that you wanted to deaden and make a compound that critically dampened the frequency. Since you have access to rubber foam material you could try making a case to shroud your gear mechanism out of plastic, carbon fiber or another light material. Spray your rubber material to this if you do this and isolate the canopy you should start to get rid of much of the noise from your gears

If possible you should try some different grades of rubber materials for the two systems as you may not be isolating the correct frequencies at your first attempt. If you can alter the compound this will help, as a rule of thumb plyable soft rubber absorbs higher frequency sounds, solid rubber absorbs low frequency sounds.

If your 450 helicopter sounds anything like mine you will need rubber that is soft to the touch to get rid of the sound. Hope this helps

02-16-2013, 07:34 AM
BB, a simple solution to actual gear noise is putting a bit of lithium grease on them, a very small amount with no excess at all. This creates a hydraulic film at the mesh.

I've often wondered what could be the absolute simplest way to deaden the generally high pitch scream of a motor and gears. There are two things but the first is as easy as buying a roll of rubber foam door jamb tape from your local hdwr store and applying it to the inside of the fuselage. Make large panels from the strip without leaving spaces.

First, your fuse is acting like an echo chamber and amplifying what is emitted by the motor/gears. The rubber foam is extremely light, will lower the resonant frequency of the fuselage, as stated earlier rubber foam absorbs high pitch sounds. How that is?

You know the sound that comes from a car tire? That's the resonant frequency of rubber. Foam has little air cells that capture high frequency and slow the rate of deflection. Different thicknesses will produce varying results, thicker beside the gears, thin everywhere else.

Tap on the heli chassis with your fingernail, what do you hear? Sound lighter in different places? You could add a bit of foam tape to those areas too.

Heard of SONO-MAT foam? I built lots of PA boxes and use that and or automotive sound insulating mat all the time. Densely packed fiberglass insulation too, but that would be too thick for the interior of your heli and you don't want to deal with glass fibers, but you could always glue down some fine mesh or cotton over it in panels if you found favorable results with it.

Whatever you use to accomplish your goal weigh everything before and after you complete the job.

I could tell you where to get a beautiful foam rubber sheet with adhesive backing in a variety of thicknesses, google/bing and you'll find it. It's used quite extensively in the sporting goods market and is known as NEOPRENE.


02-19-2013, 09:14 AM
Belt drives instead of gears will remove a ton of noise. There are lots of belt systems around for machines of all sorts of sizes, including lots in cars/trucks that could be adapted.

The other end of the 'out of the box' type thinking is a sound system with a loudspeaker and an active noise cancelling processor :p That would get rid of system noise, and allow you to pump out a true scale sound instead :banana