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Author Topic: Silicone exhaust deflectors  (Read 1915 times)

Offline Jim Svitko

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Silicone exhaust deflectors
« on: April 18, 2022, 07:31:30 AM »
I have a RO Jett 67 with the tube muffler.  Due to the rear exhaust, it looks like I have to add one of those silicone exhaust deflectors that will protrude out the bottom of the fuselage.

I have heard that those pre-formed exhaust deflectors can cause quite a power loss.  I have never used one before so I have no experience with them.

Another possibility is to get a length of silicone tubing, of the proper ID, clamp it to the end of the muffler, and then make a gradual curve in it to come out the bottom, without having a rather abrupt change in direction.  If I go this route, how much pressure must the tubing withstand?  The properties of this tubing can vary so I want to make sure I am getting something that will hold up to the operating conditions.

If anyone has experience with this, what do you recommend?  Thanks.

Offline Dan McEntee

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Re: Silicone exhaust deflectors
« Reply #1 on: April 18, 2022, 08:05:52 AM »
   I have never run these on a RO Jett, but can just suggest that whatever you try for an exhaust deflector, make the inside diameter as large or larger than the exit or stinger of the muffler.. Those silicone elbows come is several sizes, so trying the largest would be the place to start, and just keep track of engine performance with a tach. On something as large as a .67, your power loss may be negligible. Excessive length of a deflector will affect RPM the same as smaller diameter also. I think some simple flight tests are what you need to tell you what you want to know. On a rear exhaust you definitely need something to keep things from getting soaked. I think most of the mufflers have the stinger exiting at an angle towards the bottom, with the end cap closed, so that starts things on the direction it needs to go, so just adding a straight extension gets it outside the fuselage.

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Offline Jim Svitko

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Re: Silicone exhaust deflectors
« Reply #2 on: April 18, 2022, 08:47:39 AM »
Dan, this muffler is a straight pipe, other than the bend required at the front to clear the tank compartment.  Everything else is straight, no bend or any direction change to the stinger at the outlet end.

The outlet stinger measures about 0.56 inch OD, and the ID is about 0.46.   So, a section of tubing, or a deflector with 0.50 ID, should work.

If some have experienced power loss with the deflectors, my guess it is due to too small an ID.  And, the 45 degree bend might have something to do with it as well.  I have looked on line for these deflectors and saw one with an ID of 11 mm, which should work OK.  At least, I see no reason to believe it would cause big problems.

With hobby shops now closing up, I have to find this stuff on line.  I would prefer to see the item in the flesh, maybe check the fit, before buying.

Offline big ron

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Re: Silicone exhaust deflectors
« Reply #3 on: April 18, 2022, 08:48:35 AM »
Jim we heated and bent the header muffler on Joe’s Blue Max it kept breaking the deflector for some reason. Was very simple just support the bend where you heat it.
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Offline Brett Buck

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Re: Silicone exhaust deflectors
« Reply #4 on: April 18, 2022, 09:59:26 AM »
I have a RO Jett 67 with the tube muffler.  Due to the rear exhaust, it looks like I have to add one of those silicone exhaust deflectors that will protrude out the bottom of the fuselage.

I have heard that those pre-formed exhaust deflectors can cause quite a power loss.  I have never used one before so I have no experience with them.

Another possibility is to get a length of silicone tubing, of the proper ID, clamp it to the end of the muffler, and then make a gradual curve in it to come out the bottom, without having a rather abrupt change in direction.  If I go this route, how much pressure must the tubing withstand?  The properties of this tubing can vary so I want to make sure I am getting something that will hold up to the operating conditions.


    I don't think the angle vs curved  makes any difference. The diameter and length certainly does. I run the largest thin-wall DuBro or the largest Hattori stinger, and cut it off just long enough to keep the oil out of the fuselage. The DuBro has a pretty sharp angle, the Hattori a bit of a radius, but they work about the same.

    Brett

Offline Lauri Malila

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Re: Silicone exhaust deflectors
« Reply #5 on: April 18, 2022, 10:21:07 AM »
 
    I don't think the angle vs curved  makes any difference. The diameter and length certainly does.
  Brett

It’s a common joke among the acoustic engineers, how several books have been written about how brass horns like saxophone should be bent for the best sound. In reality it’s all about volume and the critical factor is the lenght of artists hands😂.
If you look at F2A for example, you’ll get an idea how critical the stinger dimensions can be. So I think we would also have a place for optimizing there but we should first figure out what we need. Honestly, I have no idea.
But the average noise spectrum & pressure is such, that you could for sure make a choice between a positive, negative or neutral pressure addition.
Our silencers have in general so large volume that I doubt it would really matter the engine run, but a well optimized stinger can have a noticeable effect on the engine noise.
Sorry for a slight off-topic. L

Offline Brett Buck

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Re: Silicone exhaust deflectors
« Reply #6 on: April 18, 2022, 11:53:37 AM »

It’s a common joke among the acoustic engineers, how several books have been written about how brass horns like saxophone should be bent for the best sound. In reality it’s all about volume and the critical factor is the lenght of artists hands😂.
If you look at F2A for example, you’ll get an idea how critical the stinger dimensions can be. So I think we would also have a place for optimizing there but we should first figure out what we need. Honestly, I have no idea.
But the average noise spectrum & pressure is such, that you could for sure make a choice between a positive, negative or neutral pressure addition.
Our silencers have in general so large volume that I doubt it would really matter the engine run, but a well optimized stinger can have a noticeable effect on the engine noise.
Sorry for a slight off-topic. L

   All it has to be in this case is "big enough". I think the ID on the Hattori stinger is nearly 1/2" = that's not consequentially restricting the flow, and it is far to short for any tuning effects (not to mention that the any motion is highly damped), so I think big enough is good enough

       Brett

  p.s. probably does have some effect on the sound, shooting it into a angled wall is slightly different than shooting it into a curve, but, the wavelengths we are talking about for the primary mode are almost 3 feet long. Maybe some much higher harmonic.
« Last Edit: April 18, 2022, 11:47:44 PM by Brett Buck »

Offline Walter Hicks

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Re: Silicone exhaust deflectors
« Reply #7 on: April 18, 2022, 03:18:42 PM »
Jim, #1 Listen to Brett carefully.

       #2 I ran the RO Jett 67 RE for several years with the header muffler. I used the Big Silicone deflector cut off as short as I could and still divert the
            exhaust from the plane. Worked fine.

Offline FLOYD CARTER

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Re: Silicone exhaust deflectors
« Reply #8 on: April 21, 2022, 11:07:31 AM »
I use the DuBro silicone exhaust deflector on my RO Jett 61.  The RO Jett muffler has a very short exhaust tube at the end, which points in the wrong direction.  Keeping the DuBro from falling off is the problem.  I finally got mad and drilled through for a 2-56 bolt with washers to hold the darn thing on.  Power loss?  Probably.  But the engine has excess power, so a slight loss is no big deal.
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Offline Brett Buck

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Re: Silicone exhaust deflectors
« Reply #9 on: April 21, 2022, 11:32:50 AM »
I use the DuBro silicone exhaust deflector on my RO Jett 61.  The RO Jett muffler has a very short exhaust tube at the end, which points in the wrong direction.  Keeping the DuBro from falling off is the problem.

   Clean both of them off with acetone, until it is absolutely clean bare metal and absolutely clean bare silicone, use the small tie-wraps over the grooves, and after a few flights, it will be nearly welded in place, the problem being that you may not be able to get it back off without cutting it.

    Same with couplers on tuned pipe- rough it up with 80 grit, clean it antiseptic clean on the inside of the coupler and roughed-up header, one run, and it will never come off again without some effort. 

       Brett

Offline FLOYD CARTER

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Re: Silicone exhaust deflectors
« Reply #10 on: April 24, 2022, 11:43:22 AM »
You can clean it all you want.  How long will it stay clean?  Isn't exhaust sort of oily?
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Offline Lauri Malila

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Re: Silicone exhaust deflectors
« Reply #11 on: April 24, 2022, 11:51:10 AM »
If the surfaces are dry and clean from oil during assembly, oil won’t go between them. It’s exactly as Brett said; after some time you won’t be able to remove the silicone tube without cutting it away. But if surfaces are even sightly oily, it won’t stay in place. Same logic as with glue, btw😂 L

Offline Brett Buck

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Re: Silicone exhaust deflectors
« Reply #12 on: April 24, 2022, 03:08:04 PM »
You can clean it all you want.  How long will it stay clean?  Isn't exhaust sort of oily?

   For goodness sake - it won't get oily if the silicone seals it up!   

     Brett

Offline Steve Helmick

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Re: Silicone exhaust deflectors
« Reply #13 on: July 06, 2022, 05:04:37 PM »
My preference is to leave enough of the exhaust deflector length to allow shutting off the engine by pinching the deflector shut. Found out how handy that was when I fumbled the glow ignitor into the prop. WAY faster shutdown than tipping the nose down...(especially with a clunk tank!)...or finger over the uniflow air inlet, or pinching the muffler pressure hose. Have a plan, just in case.  y1 Steve
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In 1944 18-20 year old's stormed beaches, and parachuted behind enemy lines to almost certain death.  In 2015 18-20 year old's need safe zones so people don't hurt their feelings.

Offline kevin king

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Re: Silicone exhaust deflectors
« Reply #14 on: October 23, 2022, 07:32:40 PM »
I use the largest size Dubro Exhaust deflector on my RJ67, but i also had    Dub shorten the header muffler length down to half its original length.

Offline Jim Svitko

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Re: Silicone exhaust deflectors
« Reply #15 on: October 23, 2022, 08:15:34 PM »
I use the largest size Dubro Exhaust deflector on my RJ67, but i also had    Dub shorten the header muffler length down to half its original length.

How much louder is it with the shorter muffler?

Offline kevin king

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Re: Silicone exhaust deflectors
« Reply #16 on: October 24, 2022, 11:44:11 AM »
How much louder is it with the shorter muffler?
Because Ive only run it with the short header/muffler, i cant say how much louder it is. I also used a sealant on the header - deflector joint with a couple of tie wraps around it. Its important as well to use the correct sealant on the face of the header and bolts where its attached to the engine. A couple of videos is all i have for you to judge the sound levels.
https://youtube.com/shorts/SainM0N_X40?feature=share


« Last Edit: October 24, 2022, 12:02:41 PM by kevin king »

Offline Jim Svitko

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Re: Silicone exhaust deflectors
« Reply #17 on: October 24, 2022, 01:07:45 PM »
It does not sound much different from what mine is like with the full length muffler.  Maybe the only way is to get a db meter and measure both.

I was looking at side mounting the 67 but with the rear exhaust the original muffler is too long and no way to fit it in.

Offline Robert Zambelli

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Re: Silicone exhaust deflectors
« Reply #18 on: October 25, 2022, 10:33:01 AM »
I tried the Du-Bro extension on a four-stroke engine.
It melted.

Offline Steve Helmick

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Re: Silicone exhaust deflectors
« Reply #19 on: October 30, 2022, 08:48:12 PM »
I tried the Du-Bro extension on a four-stroke engine.
It melted.

Was it fastened to the stock muffler, or straight to the header pipe without the muffler?  :X Steve
"The United States has become a place where professional athletes and entertainers are mistaken for people of importance." - Robert Heinlein

In 1944 18-20 year old's stormed beaches, and parachuted behind enemy lines to almost certain death.  In 2015 18-20 year old's need safe zones so people don't hurt their feelings.

Offline Robert Zambelli

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Re: Silicone exhaust deflectors
« Reply #20 on: October 31, 2022, 05:31:44 AM »
Hi, Steve - it was attached directly to the header pipe.
As I recall, it was an ENYA 46 4S.

Bob Z.

Offline Steve Helmick

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Re: Silicone exhaust deflectors
« Reply #21 on: November 01, 2022, 10:29:29 AM »
Yeah, I am not surprised it melted when attached direct to the header pipe. Mufflers work best when they cool the exhaust temperature before it leaves the outlet. Having fins on the muffler makes sense to me. Big Art had the right idea with his finned tube mufflers, IMO. Are those still being made?  D>K Steve
"The United States has become a place where professional athletes and entertainers are mistaken for people of importance." - Robert Heinlein

In 1944 18-20 year old's stormed beaches, and parachuted behind enemy lines to almost certain death.  In 2015 18-20 year old's need safe zones so people don't hurt their feelings.


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