Noise Cancellation Corner

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larry cottrill
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re: movie of new type of pulsejet? how to name it?.

Post by larry cottrill » Thu May 26, 2005 1:13 pm

Actually, Leo, I just thought of something that's pretty good, if I do say so myself! It would, however, create potential interference problems, and you might have to shield the exhaust outlets [and possibly the intake] from each other. BUT, this is so good , it is almost a "must do", whether for you or someone else:

Bend one of the pipes in an "S" curve, with two 90-degree bends, so that the exit is still pointed rearward, but the exit face of the bent pipe is up around the halfway point along the length of the straight pipe. Something like a dance hall girl with one knee up. [Incidentally, the engine design you have there is wonderfully anthropomorphic, and would lend itself nicely to various sculptural interpretations!] You will now have an engine that, theoretically, accomplishes some noise cancellation [to an observer behind the engine]. This is because the two emerging wave fronts will not join together along the engine axis, but will be two smaller fronts at some time interval dependent on the temperature of the outside air. Effectively, the sound energy will be "spread out" over time.

Unfortunately, as Eric found out, the blend of wave fronts from an engine having multiple wave exits will not be the same in all directions. It will always form an interference pattern of some kind, like the illustration I posted for Eric a while back, and there will always be zones where the waves reinforce to produce high noise levels. You can see this by scrolling down to my first post on this page:
http://www.pulse-jets.com/phpbb2/viewto ... 7&start=15

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Re: re: movie of new type of pulsejet? how to name it?.

Post by larry cottrill » Thu May 26, 2005 1:24 pm

Mike Everman wrote:so very cool, Leo. I like your style!
I too would love to see the effect of spreading the tails apart a bit. it could boost thrust, even.
or you could make a turbine!
Image
Mike -

This exact idea was illustrated by Dr Robert Goggard back in the 1930s [with a valved pulsejet in mind] as "tip jet" motive power for an airplane propeller!

Yes, Goddard did actually theorize a valved pulsejet design, although his valve mechanism was far too massive and slow-acting to be practical - it would have limited a Dynajet size engine to a few cycles per second. Part of the problem was his mistaken belief that the tailpipe should be valved off during the intake cycle. He was just better at rockets, I guess ...

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re: movie of new type of pulsejet? how to name it?.

Post by Mike Everman » Thu May 26, 2005 1:55 pm

Cheezits Crust, Larry, you've just exposed my pet project for the last several months! I knew, considering the volume of ideas you spew forth, (and I mean that in a good way) that you'd force me at some point to disclose what I've been working toward. It's been hard keeping a lid on it, even through the meet, because I wanted to demonstrate it at the same time as disclose it! Nice job, but Aaaaarg! I've been bouncing this stuff off of Graham for a while now. I'm betting you are furiously writing and drawing for a huge post, running wild on this as I have been.

Yes, you have something there I'm sure, but I do not know yet if it kills the engine to do cancellation this way. It must, but it must be tried, too. The main advantage over pairing of engines that are out of phase is that a twin tail engine cannot get out of phase with itself! And it has the same wave character coming out of two sources. My concept puts the motor inside another duct so the cancelled or semi-cancelled waves are forced to come out the ends, and can maintain somewhat equal intensity when they do encounter each other.

For waveform alignment, I just posted the simple equation for the distance between the noise sources, that is, D=1/2f * Vs which is oddly perfect to experiment with a straight 36" long pipewood (200 Hz, and Vs assumed to be 350m/sec) inside another pipe, which I have also not gotten around to doing. This would use the intake noise to cancel the tail noise. Not perfect, but they are theoretically in phase with each other, and the right distance from each other to hear the effect. The wave alignment will go out of whack as the air around the motor warms up, so you want some flow around.

I need to split this topic, if no one minds...
Mike
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re: movie of new type of pulsejet? how to name it?.

Post by Mike Everman » Thu May 26, 2005 2:22 pm

I sent this group of sketches to Graham last month in an attempt to document some phone conversations we'd had on the subject. I'd like to now disclose this to you fellas and open the public discussion on the concept. I'm almost glad you brought it up, Larry, and I, of course, think you are brilliant! I was almost not going to post my twin tail turbo because of this "secret" work of mine! Though anyone that knows me knows that my prime focus has been noise reduction, before moving on to bigger motors.

The first sketch is what I was just talking about, the next was some history, the third is similar to what you suggested, Larry. The fourth is the whole reason I've been creating reliable straight-pipe engines, so I can slide this closed end tube along the tail, and the one at bottom I've played with without thrust feedback, and can get more or less noise depending on position of the tube.

Yikes, so much to do!
Attachments
cancellation techniques.JPG
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re: movie of new type of pulsejet? how to name it?.

Post by Graham C. Williams » Thu May 26, 2005 2:45 pm

Dear Leo.

Another good European Motor.
Why am I seeing flames out of the exhaust pipe? Much of the tailpipe looks cool so I can assume cold air is being drawn down the pipes. Perhaps it's a trick of the camera or could you be burning unused fuel? Another possible answer could be a very thick boundary layer in the twin pipes.

Graham.

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Re: re: movie of new type of pulsejet? how to name it?.

Post by Graham C. Williams » Thu May 26, 2005 3:01 pm

Mike Everman wrote:I sent this group of sketches to Graham last month in an attempt to document some phone conversations we'd had on the subject. I'd like to now disclose this to you fellas and open the public discussion on the concept. I'm almost glad you brought it up, Larry, and I, of course, think you are brilliant! I was almost not going to post my twin tail turbo because of this "secret" work of mine! Though anyone that knows me knows that my prime focus has been noise reduction, before moving on to bigger motors.!
Some months ago Mike Everman asked me to hold backup copies of his work pertaining to pulsejet noise reduction. This note is to confirm that he has been working in this area for some time now. Personally I'd have just bit my lip and got on with it.
I'll not disclose any further information. Mike, you can do that as you see fit.

Regards
Graham.

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Re: re: movie of new type of pulsejet? how to name it?.

Post by larry cottrill » Thu May 26, 2005 3:03 pm

Mike Everman wrote:The first sketch is what I was just talking about, the next was some history, the third is similar to what you suggested, Larry. The fourth is the whole reason I've been creating reliable straight-pipe engines, so I can slide this closed end tube along the tail, and the one at bottom I've played with without thrust feedback, and can get more or less noise depending on position of the tube.
Holy pipe flange, Batman! I didn't mean to knock all the cats out of the bag with a suggestion for a single experiment ...

But like I said, I knew it was too good an idea for somebody not to be working on it!

Incidentally, Mike, your turbine design would be an excellent plan for somebody to work up on an old spindle and brake drum. Just change it so the chamber is more of a broad pot-shaped thing [sort of like Eric's 'quiet engine'] that just bolts on with the lug nuts and have the intake looking up from the center. At first, I thought of using the brake drum itself for the chamber, but they're cast iron and thus almost impossible to weld anything onto - you'd also have a bearing that runs kind of hot!

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re: Noise Cancellation Corner

Post by Eric » Thu May 26, 2005 4:27 pm

Hmmm I am also working on noise reduction. Grrrr I hate competition ;)

Eric
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Talking like a pirate does not qualify as experience, this should be common sense, as pirates have little real life experience in anything other than smelling bad, and contracting venereal diseases

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Re: re: Noise Cancellation Corner

Post by larry cottrill » Thu May 26, 2005 5:03 pm

Eric wrote:Hmmm I am also working on noise reduction. Grrrr I hate competition ;)

Eric
No, competition is good, at least up to a point. The problem with competing inside a community is knowing where to draw the line between cooperating and trying to cut each other's throats ;-) It can be tricky later on, figuring out who said what when, first real disclosure of an idea, and so on ...

Tread carefully, gentlemen, and remember: ten years from now, whatever you come up with will probably be owned by P&W or General Electric, while we all still drive in every morning to our day jobs.

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re: Noise Cancellation Corner

Post by Mike Everman » Thu May 26, 2005 5:15 pm

I'm happy to work it out in a public forum, but was holding back 'cause I dreamt it would work, which is most likely, usually, wrong. It has been my pet project that I wanted to spring on you guys full blown with a working, or non-working example when the time was right.

I wanted here to disclose the approaches I feel most worthy of exploration, and have always had a policy of "consideration where it is deserved", should I like to persue it later in an IP or manufacturing sense. I'm showing you where I'm at conceptually, which isn't far, but there are things to try that make sense to me.

The downside to a disclosure like this is that it becomes a leaping off point for some of you brilliant people, but alas, I do just want this community to make it's mark, ultimately.
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re: Noise Cancellation Corner

Post by Bruno Ogorelec » Thu May 26, 2005 7:11 pm

We need someone who speaks good technical French badly. I have a strong feeling that somewhere in the Ecrevisse documents there's a sentence saying, in effect "oh, by the way, this layout cuts noise in half".

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re: Noise Cancellation Corner

Post by Mike Everman » Thu May 26, 2005 8:15 pm

Yeah, there's neat stuff they did with clusters of many engines, and I can't tell if it's fantastique or not-so.
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Re: re: Noise Cancellation Corner

Post by hinote » Thu May 26, 2005 9:09 pm

Mike Everman wrote:Yeah, there's neat stuff they did with clusters of many engines, and I can't tell if it's fantastique or not-so.
Interesting to note--the original engine installation on the Emouchette had the engines clustered in pairs. They subsequently changed the cluster to 3, on each wing.

One could argue they wanted more thrust (probably true), but I'll bet they found the tripling was a much smoother configuration. The exhaust character (multiple thrust nodes) would appear to conform better to triple-engine phasing.

SOMEBODY needs to try a cluster of engines, to see what happens.

Bill H.
Acoustic Propulsion Concepts

".......some day soon we'll be flying airplanes powered by pulsejets."

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re: Noise Cancellation Corner

Post by Mike Everman » Thu May 26, 2005 9:22 pm

Snecma did a cluster of 30! Look how whacky the power distribution got.
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re: Noise Cancellation Corner

Post by Bruno Ogorelec » Thu May 26, 2005 10:05 pm

No, such clusters will not work that easily. There's just too much 'noise' for the engines to work properly. I am a firm believer in clusters but just as firm on crafting them very carefully so that the engines do not hinder each other. The SNECMA cluster and the even worse cluster proposed by Tharratt in his 'Propulsive Duct' are just stabs in the dark.

I am working with Al Belli on a 'proper' cluster. We are still talking the concept over but may cut metal soon, depending on the production evaluation that's under way.

We may also have come up with a neat way to couple two engines, too, which is actually rather more difficult than having three or five or seven together.

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