Hi all, looking at suggestions to reduce prop noise on a C185. I'm guessing there is more than one way to 'skin this cat' so looking at options and real world experience by others. I fly out of a 1200' private strip with VERY close neighbours, noise is high on my list of flying 'friendly'. Currently looking at a C185 with standard 2 blade and IO 520 - full noise out of home is not an option!
How would this prop compare noise and performance wise getting out at say 2500 rpm with something like a 3 blade MT at same rpm. I was pretty much set on getting a 3 blade MT and just running it 2600 or so until clear of houses. Anyone had any feedback how an MT sounds like at these setting compared with the Mac or Hartzell 3 blade equivalents?
I have had an IO 550 C185 with hartzell scimitar prop out of the strip and asked the neighbours later, they didn't seem overly concerned, more of a different tone than the Maule they are use to. So perhaps it is just a matter of dropping back the rpm's with the 2 blade, unless the 3 blade is going to offer significant advantage at these lower rpms? I'm told the 3 blade MT pulls pretty damn good at 2500, but would like to hear from those using them as it's a significant investment just to keep the locals happy - what would you do in this situation (other than just buy a Mall )??
BTW I always operate light out of this strip - power wires at BOTH ends.

I believe there are tip speed slide-rule type calculators, or quick formulas you can use on a hand calculator. Some of the engineers here can probably quote it from memory. But the real formulas also have to include density altitude and "true airspeed" of the prop tip.

Once you have this all figured out, and have a quick way to calculate your tip speed Mach number vs. RPM, you can do this on runup using the actual temps and pressures and altitudes at that moment.

Then back out the propeller until you have 50 RPM less than whatever the critical Mach number is, and you should be out of the horror-noise range that the 185 is famous for.

Sound costs energy, by the way, and I'm not sure that the extra drag at the prop tip doesn't take away whatever extra engine horsepower you get from running the engine at max RPM. That's a powerplant engineer question, somewhat above my level of education.

Hacksaw solves most problems! However, short of that, try slotted washers of whatrever thickness works on the prop until the noise is reduced. Then make one thickness washer w/ a suitable leash, and use it routinely until you need the extra noise(horsepower).

The 3 blade MT is astonishingly quiter than a 2 blade Mac. Not just a diffent pitch ( although that too ) but quiter .
I can't say on the IO, but on my O-520, the T/O thrust actually Improves when I give it a couple twists back... This is weird to me, as well as counter- intuitive. But it is repeatable, and very obvious on my home strip.
Performance will be better than the two blade, and weight will be almost a wash. Cost will be significantly more.
Take care, Rob

Thanks Rob, that's the kind of info I'm after. Noise is a difficult thing to quantify, what exactly does annoy people, how much would i need to back it off to appease the non-aviator neighbours I guess I'll have to figure out over time.
So the washer idea is like the clothes peg idea for the prop, I think you'd get a feel for it over time then have those spare rpms in case you need em.
I guess I'm just trying to find the best way to the friendliest 'noise signature' while still having enough performance to get out and clear of my power lines.

Just a reduction of 200 or 300 RPM makes a HUGE difference in the amount of noise of the 2 blade Mac. Takes the snarl right out. The tips are actually working and not cavitating.

Rob wrote:The 3 blade MT is astonishingly quiter than a 2 blade Mac. Not just a diffent pitch ( although that too ) but quiter .
I can't say on the IO, but on my O-520, the T/O thrust actually Improves when I give it a couple twists back... This is weird to me, as well as counter- intuitive. But it is repeatable, and very obvious on my home strip.
Performance will be better than the two blade, and weight will be almost a wash. Cost will be significantly more.
Take care, Rob

NT, second Rob's comments. I am based on the same airport as several other 180s. On in particular has the 470 and I have the 520. He has the 2 blade sea prop and I have the 3 blade. My bird is definitely quieter than the 470 (notice I said quieter, not quiet). 180 is never going to be quiet.

However... suggest you see prior posts on comparisons of 2 blade vs. 3 blade. If it were me, I would try to make nice with the neighbors and chose the prop that suited my mission best. Hopefully the two coincide, but if they don't I wouldn't go spending money or changing just because one was quieter.

Good Luck!!

Ok I might just try reduced rpm on the 2 blade to start with and see how it performs. I'm not familiar with the io 520, any mechanical operating problems at full MAP and say 2500 rpm for takeoff to be aware of?

Hey mate,

My 2c
Growing up beside the Makarora airstrip (maybe 80m away), Southern Alps Air operated FMA (185F 3-blade) and CGH (185C 2-blade). Occasionally in the mornings one plane would be away about first light.

Our house had thick wooden walls, but there was a very noticable different between the two - you always knew which 185 it was without opening your eyes. CGH would wake you up if you were still sleeping, FMA could sometimes sneak away. As the plane was abeam the house the 2 blade peaked, screamed and reverberated for a moment then the noise dyed back, whereas the 3 blade just continued the drone of the engine. That was a Mac or Hartzell depending on the year. There was a very meaningful difference in terms of how late you slept. But I always liked planes, so the noise was just an excuse to look out the window.

Given most of the neighbours are abeam the hanger end of the strip, or short final on western side, you could always dial the RPM back when you are around those areas, while using full power on the western side of the runway. The noise is definitely worst when the plane is abeam.

NZ,

My 185 is IO 520 D powered with a 86" 2-blade seaplane prop. I fly out of a 1,500 foot turf strip.

In the interest of early morning takeoffs I decided to try and quiet the bird down. I first tried the clothes pin trick to bring the takeoff rpm back to 2700 to quiet the tips, but that proved to be annoying.

I then realized my index finger is just the right thickness so now use it as the "prop stop". She's still a loud plane cause she's sporting a 330hp motor, but the prop tips are no longer going supersonic so she's not as annoying to persons outside the cockpit.

However. Two things occur when I use this technique instead of pushing everything to the wall.

1. She takes longer to get into the air.
2. She gets hotter on climb out and quicker - even though the fuel flow is identical. The only thing I can figure is there is more load on the engine at 2,700 then there is at 2,850.

So I reserve this technique for times when noise is really something I should care about, and I end the takeoff climb and go lean of peak as quickly as possible to cool the engine down, then I resume my climb.

Otherwise it's everything to the wall and a big grin on my face.

Get rid of that two bladed. Obscenity, and install a MODERN three blade Mac, MT or Hartzell.

ANY of those will pull as hard or harder than that slab of aluminum, and be much quiter in the process. Modern three blade props really do perform well.

As to pulling that thing back for takeoff, consider how you'd answer this question after you failed to clear the obstacle at the end of the strip:

"Tell us again why you chose to use partial power on takeoff, and why you failed to transition to full power after it became apparent that the takeoff wasn't going well"

That is, of course, assuming you and your pax survived the "incident"

Full rated power on takeoff is a good thing. Sometimes, MORE than full rated power is even better. A good three bladed prop will give you full rated power AND your neighbors peace.

MTV

Barnstormer wrote:2. She gets hotter on climb out and quicker - even though the fuel flow is identical. The only thing I can figure is there is more load on the engine at 2,700 then there is at 2,850.

It's because the engine is effectively having it's timing retarded. As you pull back the RPM, because the mag timing is fixed but the piston is moving slower, it's further behind TDC when the combustion pressure starts to increase, and that means the engine is developing higher cylinder head pressures at TDC as a result (pressure and heat are interconnected, increasing the pressure increases the temperature of the gas) and the engine develops hotter CHTs as a result
.
These higher CHTs are a warning sign. High cylinder head pressure (and thus temp) can kill engines.

Because it's at full throttle & developing full power at low DA, these higher cylinder pressure 'spikes' are much harder on the engine (e.g. big end bearings) and cause increased wear and tear. It can lead to detonation or preignition if you reduce RPM too far. So while its fine at lower power setttings, at full power it's very risky. You want to avoid those kind of operations for increased engine longevity.

(edit for more details ... and my bad grammar)

Battson,

Ah ha! Thanks for the info. Yea I tried all kinds of various ideas including climbing at a much higher airspeed and nothing really helped except going to the firewall. Appreciate your insight much.

Cheers!

Is there a difference in noise level and quality between the Hartzell 3-blade and the Mac 3-balde?

Yeah, good question Denalipilot.

I see that a 3-blade MT prop is being considered as a possible option. There's a guy on my field who bought a new 200hp Husky a few years ago, not long after that he put it on a set of new Wipline amphibs. He's been through several props, 2 and 3 blade Hartzells as well as a 2 and now a 3 blade MT. I don't know why but that 3 blade MT makes a helluva racket and it is unlike most any other airplane I have ever heard, it sounds like it is beating the air into submission. The noise preceeds the airplane on takeoff, almost like a pusher-configured flying boat. It sure ain't neighbor friendly, but He says it performs great.

I understand it's the tip shape/profile which determines noise.....

Please correct me, because audible vibrations is not my engineering speciality... but i think from loudest to quietest:

Square tip
Rounded tip
'Swept' tip (e.g. Hartzell Scimitar)

That's the tip shape, not leading edge - not much sound generated there.
i am sure there are exceptions, the exact shape will make a big difference.

For comparison, from the Scimitar press-release for 82" 3-blade:

Noise emission is at a globally accepted level of 77.5 dB (a) as measured by the latest FAA and international standards.

MT doesn't seem to say what their noise level is, other than it "meets the standard" - the standard appears to be somewhere 84dB-88dB. There seems to be a wide variety of answers - this sheet has a lot of noise level data:
http://easa.europa.eu/certification/type-certificates/docs/tcdns-databases/MAdB%20Light%20Prop-13-14032013.xlsx

I did find this compared to a 2-blade McCauley

Pilot: Steve Lloyd
Tests Results Recorded by Owen Bell
ADVANTAGES of the 3 Bladed M.T. Propellers vs 2 Bladed McCauley Props:
1. Noise Reduction at the same power settings:
•1 Decibel less noise during Taxi
•3 Decibel less noise during Climb
•4-5 Decibel less noise during cruise at 6500 ft

He doesn't say if that's measured inside or outside, the "at 6500ft" comment may suggest from the ground? Although dB is a logarithimic scale, 3dB is pretty pathetic. For comparison, again remembering the log scale, just covering your ears reduces the noise by about 20-30dB.

Anything under 85dB you can have exposure >8hrs/day. I dont know what the "good neighbour" limit is, but most planes are probably around 85 at a very poor guess. Some of the -550 Skywagons with 2 blades may be quite a lot higher, again guessing...

OK - nevermind that example - found this:


Cessna Aircraft Company A185E 1,520 Teledyne Continental IO-520-D MT-Propeller MTV-15-D/210-58 79.7dB 88.0dB LIMIT
Cessna Aircraft Company A185E 1,520 Teledyne Continental IO-520-D MT-Propeller MTV-9-D/210-58 83.0dB 88.0dB LIMIT
Cessna Aircraft Company A185F 1,520 Teledyne Continental IO-520-D MT-Propeller MTV-15-D/210-58 79.7dB 88.0dB LIMIT
Cessna Aircraft Company A185F 1,520 Teledyne Continental IO-520-D MT-Propeller MTV-9-D/210-58 83.0dB 88.0dB LIMIT

I also found (not that you're looking at a 180) the O-470 is always 78.1dB with the McCauley, no matter what kind of 3-blade was installed of the certified combinations listed - suggesting the engine is acutally generating more noise than the propeller in that case. The above statement applied to props like 2A34C66()/90AT-8 and 2A34C203/90DCA-8, but it's a long list.

That does also suggest that indeed the MT is (at best) only 3dB-4dB better than the stock-standard McCauley prop. That is a very expensive 3dB given the smallest change in tip speed makes a big difference to volume...

Also, according to the same measurements - the McCauley D3A34C403/80VA-0 on an A185F running the IO-520-D generates 84.8dB
(to qualify the previous post, each of those few (3) dB louder the Mac is compared to MT, is doubling the sound 'energy' radiating in 3 dimensions from the source, so that is actually 1.2x louder as perceived by a bystander - not accually that minor. I think the result is it sounds about "twice as loud" for every 10dB increase in that case - to make a horribly general statement, or 1.5x as loud for every 6dB). Still, that's a personal experience thing - for scale, in that definition putting on earmuffs would make things 8x quieter. So to me, a 0.2x reduction in "loudness" isn't the difference between night and day, it's closer to the 'minor change' end of the scale.... how good is your neighbours' hearing???

If you want to see all the numbers, just take that spreadsheet linked in my previous post and filter by the engine / prop / aircraft model you want. There is more data there than you can shake a stick at......

Gee Battson you got too much time on your hands today
Excellent info.
Thanks for the pm Barnstormer. Things are starting to put me off the MT a bit (see recent MT thread). Given we are an 'isolated' country down here some of the service issues with the MT have me worried. I think price wise everything is pretty close with the MT given any product is imported from afar anyway (MT, Mac, Hartzell) but servicing could be a nightmare, given only 1 MT dealer in NZ.
Will have to think it through carefully, can't be dealing with an U/S aircraft for any major periods of time.