Another interesting tidbit on noise....yesterday I flew the system over trees and grass and the suppressed sound came back. Could still hear the flapper valve but the noise was back to where it had been.

Comparing that to what it sounded like at the Jackson contest, there was no comparison. The Jackson field is located in the middle of a sand basin. Ed thought that sound reflection and maybe even resonance off the sand was a significant contributor to the loudness at the contest. He probably has something there

Matt,

Good to hear of your success with the custom 17-3/8x12.

Maybe sometime before the Pocono contest I can head down your way on a nice Friday to compare results and swap props. I plan to fly at Black Dirt this coming Friday, before the Conyngham event.

Bob,

This prop is more load than the 17x12 apc but now that the engine is showing signs of full break in, it works pretty well. In your Focus set-up, you probably don't need any more prop than what you are using so I wouldn't change anything.

Throttle curve finding was interesting. I can't imagine why I never looked before yesterday....the curve had throttle accelerated at low settings rather than retarded. Once I got that curve better, the engine improved its character immediately. The original curve looked like a hook, curving to the top of the screen on a JR tx. The present curve is curving slightly down...nearly a straight line actually.

We need to tame the noise signature

Matt,
RC Showcase used to sell carb intake filters that bolted directly onto the Walbro carb. Something like that would work here to eliminate both the intake noise and the flapper noise.

I just found it at Troybuilt. here is the link: http://www.troybuiltmodels.com/items/ZDZCPA057.html
And at Aircraft International (scroll down): http://www.aircraftinternational.com...s/MIntake.aspx

The foam should muffle the sound a bit.

Another option if you don't want to go with these, as they are a bit expensive, would be to get a cheap intake trumpet, wrap a piece of filter foam over the end, and hold it on with an O-ring. This intake will work if you want to try that, and it is very inexpensive. I don't remember which carb is on the SAP-180, but if it is a WA or WT weries carb this intake will fit:

http://www.horizonhobby.com/Products...ickAccessories

The choke baffle is the problem with any add on device. It is not a conventional baffle. This requires some engineering to get right

Forgive me in advance if this is a stupid question, but what is the concern about noise?

Pattern has a noise limit that is checked at National events. The idea is to come up with a viable gas power setup that provides ample power and also is quiet enough to pass the noise tests.

Archie

The gasoline set up I am using will pass the current noise test I think. Have not checked it with a calibrated meter in a plane, however when we did several months ago, we were getting values in the 89-91 dB, as is on the bench (not in a plane)

The point of all the noise discussion is that the gasoline set-up is less quiet than glow right now at this early stage of development. Many, maybe all others will decide not to do anything else to their gasoline set-ups and accept the slightly noisier flying.

But I think we can get the gasoline set-up as quiet as the glow stuff. Waits to be seen

Just thinking with my fingertips here, but is there any chance that the intake noise could be muffled by drawing the air from inside the fuselage?
On Rick's airplane there is a hole in the center of the soft mount already. Maybe a big hole in the firewall and something to shroud the carb a little better? Maybe box off the front of the fuse to serve as an intake muffler?

[link=http://www.modelaircraft.org/files/events/rulebooks/rcaerobatics.pdf]AMA Pattern Rules[/link], see rule #4.2

FWIW, I will test mine soon with a Radio Shack sound meter, but I am pretty sure it will pass the test. It is pretty quiet in the air. A bit more noise than an OS 1.40RX on a pipe, but not at all objectionable and it doesn't sound like a gasser, and also no smoke trail.

Exactly. I am doing exactly that except the choke baffle gets in the way of sealing the carb to sponge interface. I have a hole in the firewall in my plane so some air is being sucked from the fuse. A bicylce inner tube that surrounds the stand offs is the next thing I will play with. It will cover the engine backplate to the soft mount front face

If someone gets to fly this idea before I do next weekend, please report the findings here

I didn't realize the engine has a choke baffle. Is it a flapper, or just the typical in-carb butterfly?

Do you have a picture?

I have on order a velocity stack with a built in choke.  I have been running with out the choke because I am using rubber blind rivets to attach the stand off to the firewall.  Unfortunately the choke flapper hits the firewall.  I could shorten my stand offs to get it past the firewall but then it will hit the blind nuts.  Right know I am using a thumb choke if I need it.   I am not entirely sure that this going to work.  You guys that are working on sound signature could then just mount the air filter that is design to mount on the out side of the velocity stack.  Just some thoughts.

Sounds too much like SCCA to me...

Yep.

Ever had a neighbor with a Harley?

To be honest, it has been the most rewarding experience of my RC hobby thus far to assemble a 2-meter gasoline powered pattern plane that easily meets weight and noise restrictions, and also has the coconuts to fly the sequences, and didn't cost an arm and a leg, and runs on cheap fuel.

Doesn't get much better than that!

I agree, and I think excessive noise is unnecessary these days. I am just trying to get a feel for the limits, do any electrics ever fail? Would an OS glow engine with stock muffler pass?

Todd says that using a starter will do no harm. So maybe the choke issue is a moot point. I pride myself on hand starts but if it takes a starter for the first flight of the day, I would be willing to swallow my pride in the name of simplicity and lightness.

another stray thought... I have those often...As Scott asked...How much quieter could the set up be if the intake were ducted from the outside of the airframe from the top so as not to reflect noise off the ground? Also makes a thumb choke possible.

RC

Yes, electrics do fail - as evey propulsion system does from time to time - and while the batteries are getting better and lighter all the time, it is still difficult to meet weight requirements, and there is the issue of field-charging the batteries.

The main reason I was on the verge of going electric (before the SAP-180 came along!) was because of the ever-escalating cost of glow fuel. Had I gone electric, I would have gotten a Honda generator as well. If you want to practice a lot, gas is the way to go. And the SAP-180 has made this possible.

I don't know whether or not an OS 1.40 or 1.60 would meet sound restrictions. On pipes, they can be extremely quiet. Also don't know, but perhaps the power curve would also not be ideal with a stock muffler.

All my humble and occasionally ill-informed opinion, of course.

I suggest using a sound meter to measure the dBs per AMA instructions and go from there. If it is within the limit, why bother?

That's my plan![sm=teeth_smile.gif]

I don't think my current starter has the torque for it.  Its the Torque Master 180.  Yeah, it will spin a 180 glow engine... I attempted it on another similar size gasser and it barely loped it over.  What are you using?

It's a butterfly type that slides over the top of the carb. It is not an inside the carb type. Yes we have a bunch of pictures much earlier in the thread

Guys look...I didn't want to make this thing a big deal but I see how it does get to be one. As I stated earlier, the engine should meet the sound requirement. It did meet it at Todd's shop when we were only bench running pattern props. The only prop that failed was the 18x6 Vass that the sport guys would typically run...then again that prop turned 9500 on the pipe which is of little use to the pattern competitor.

I am looking for a certain quality to the sound is all. I don't want to get too bogged down on this issue because it's not really that big a deal. The setup works great as we've discussed

Sullivan Dynatron with a couple of Hobbico 12-volt gel cells in series (for 24 volts) will start it. Will start a 50cc no sweat, probably even 85cc. Just got to be careful not to try to start a flooded engine because it could break a con rod.

Mangum geared on a 4C 2350 is a super nice little starter. Put the big cone on it from Tower and you are good to go.[ If you go to the Which Starter thread on the Gas engine forum, you can see pictures of my set up. Even petec likes it for firing up his new 170DZ.

RC

I would move the pics if I knew how.