Even the Cheapest Jet Should Breathe Like a Reynst!

Moderator: Mike Everman

Postby larry cottrill » Fri May 14, 2004 3:47 am

Mark wrote:I was thinking you might like to try a coil of 1/8 inch or 1/16 inch is it, that really small copper tubing you can buy at some hardware store, it even comes with fittings for it too,anyway if you could coil it around some region that gets hot, preheating the propane can go a long way to getting a peachy keen flame. The added heat not only gives the fuel more velocity which aids in mixing, but also the propane will react with the oxygen much faster too.
Best of luck whatever you come up with. The one picture of your jet looked kind of strangely stylish sitting on its little stand.
Mark

Yes, that might make some sense -- one more thing I could try, if I live long enough.

Glad you like the test stand -- when the jet makes the cinder block creep along the driveway toward the garage, I'll know it's time to stop testing and start manufacturing. I keep hoping someone will comment positively on that nice braided fuel hose. And, you didn't say what you thought of the NoDump Propane Stand -- don't tell me you didn't notice it!

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Old tubing and hodgepodge

Postby Mark » Fri May 14, 2004 1:37 pm

I did feel envy when I saw your braided hose. I don't like the one I have, it's old and black and all my fittings are ways of getting around not having the best equipment, I just had to use what necked down and fit on those weird fittings, it's a jumble of poverty. Then of course I think my fittings leak a tiny bit but that doesn't matter does it? LOL
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Postby mk » Sat May 15, 2004 7:37 pm

Hello!

Larry wrote:...it runs with forced air, but dies immediately after taking the air source away.

(I think it is written in one of the picture discriptions.)

Have you already tried to pre-heat the forced air?

My small engines (Lockwood types) will only start by the "blowing-the-flame-into-CC-methode" if the forced air is pre-heated. Even when the CC and tailpipe sections are hot.
I pre-heat the forced air flow by heating the intake noozle and air source noozle with a torch. The engine begins to run just after a few seconds of pre-heated forced running mode.
A "hot-air gun" would be the most simple starting air source, delivering a relatively huge and hot (~400°C) volume of air (preheating with a torch unnecessary). I was able to start my engines (~60...~110mm CC ID) in a few seconds, without an igniton source (of course a lighter is needed, but no spark plug etc.)...till it broke down! But torch porwered pre-heating works nearly as good.

I hope it'll work at the Elektra I as well!
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Postby Mike Everman » Sun May 16, 2004 4:25 pm

Thanks, Marten and wie gehts?
I'm trying to run the 50mm Kazooenstein today. I'll have to remember what you've said, I think!
Mike
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Elektra I Site Revisions

Postby larry cottrill » Mon May 17, 2004 1:15 pm

Just did a revision this weekend -- better top of page photos on Page 1 and several testing-related photos on Page 2.

Let me know what you think of it!

http://www.cottrillcyclodyne.com/ElektraI/ElektraI.html

I hope to be able to test with varying tailpipe lengths this evening. I will have extensions of 8-, 4-, 2- and 1-inch length available, so in combination I will be able to set up any length between 15 and 30 inches with a granularity of 1 inch.

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Postby Mark » Mon May 17, 2004 2:58 pm

Now that's a perfect blend of copy with the photograph. A nice picture with bit of truth lingering in the humor. I like the what every man wants line... This is some really good advertising for our field.
I was wondering Larry if you aren't secretly working another job in the media as well?
http://www.radiohof.org/news/charlesosgood.html
Good job.
Mark
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Postby larry cottrill » Mon May 17, 2004 3:38 pm

Mark wrote:Now that's a perfect blend of copy with the photograph. A nice picture with bit of truth lingering in the humor. I like the what every man wants line... This is some really good advertising for our field.
I was wondering Larry if you aren't secretly working another job in the media as well?
http://www.radiohof.org/news/charlesosgood.html
Good job.
Mark

Mark -

Whenever I do one of these, I always have in mind the idea of selling them as photo grade posters. I think the one at the front of the Maggie site is better than this one, though. To make photo posters I would have to go back to the negative and have it re-scanned, since 1000 x 6000 pixels doesn't blow up very large without looking pretty crude. The shot of Elektra getting warm might be a good full-color poster, in landscape format.

My cost for a poster size photo would be dollars per square foot, not pennies, so these would not appeal to everyone. [Of course, in reality, a picture of an old guy holding a piece of conduit might not appeal to anyone! A girl in a string bikini would do better, I'm sure: "What every girl really wants ..." I always like those ads of a beautiful lady smiling at her favorite carburetor, reversible pump or exhaust header.]

Anyway, thanks. If it doesn't rain tonight, I should know a lot more about resonance by tomorrow than I do now.

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Hypothesis on getting a length that works

Postby larry cottrill » Mon May 17, 2004 5:19 pm

Larry Cottrill wrote:Anyway, thanks. If it doesn't rain tonight, I should know a lot more about resonance by tomorrow than I do now.

Since I now have a crude way of finely dividing the tailpipe length possibilities, it seems to me that you need to know when you're getting it right [rather than just accepting the first thing that seems to resonate].

It seems logical to me that once you get a resonant length for cold starts, you should work your way out gradually until you get the longest variation that will still easily catch hold and run. The idea is that as the engine heats up, you'll be running at higher gas temps and faster wave speed, thus shortening the pipe, acoustically speaking. This could result in dropping out of resonance after warmup if you start short. So, getting the longest pipe that will sustain on a cold start should come into even better resonance as it heats up, and be able to keep going.

What do you think, guys?

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Postby jmhdx » Mon May 17, 2004 7:30 pm

Thankyou Larry your recent contributions have been most excellent.
I think your absultely right as usual.
I have considered using a cardboard ring, possibly in the inlet, to restict the gas flow making the pipe longer in relation to gas mass volume.
This would of course burn away allowing more fuel in and shortening the relative length to accomodate higher gas speeds.
Regards,
Mike.
Oh nearly forgot, none of this is fact.
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Postby Rossco » Tue May 18, 2004 4:10 am

? My tiny engine was always the other way! Long to start and then i took the extention off the tail when hot and the frequency and power went up?
Only measured by ear and feel.
Unless im misunderstanding.
Ill have to do some further experiments with this, when i do some with the variable intake for start up.

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Postby larry cottrill » Tue May 18, 2004 4:32 am

Rossco wrote:? My tiny engine was always the other way! Long to start and then i took the extention off the tail when hot and the frequency and power went up?
Only measured by ear and feel.
Unless im misunderstanding.
Ill have to do some further experiments with this, when i do some with the variable intake for start up.

Rossco

You're right -- that seems to counter my hypothesis, at first glance. If what I said is basically correct, it would mean that for some reason, your design is capable of starting easily in a highly detuned configuration, and once you physically shorten it, it falls into better resonance at the higher temperature, even though it managed to run at a significantly longer pipe length with poorer resonance. Seems a bit far-fetched as an explanation. My hypothesis could just be wrong, or at least overly simplistic.

Incidentally, I never got to test tonight -- rain all afternoon and thunderstorms all evening. Looks like more of the same off and on for the next few days, probably.

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Re: Hypothesis on getting a length that works

Postby milisavljevic » Tue May 18, 2004 6:07 am

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Re: Hypothesis on getting a length that works

Postby larry cottrill » Tue May 18, 2004 2:39 pm

It is good that you have spared me the maths and yourself the cramped finger joints. I start to 'glaze over' pretty quickly when the sigmas and backwards sixes start to appear. However, in this case, even you verbalization of the situation leaves me wondering what's wrong -- you say I'm '180 degrees out of phase' but then you seem to re-state my premises almost exactly ;-) To wit:

I have said that you should use an overlength pipe for starting:
It seems logical to me that once you get a resonant length for cold starts, you should work your way out gradually until you get the longest variation that will still easily catch hold and run.
You stipulate that you may find it advantageous to use an overlength pipe for starting:
It is easier to start an over-long duct from "cold iron".
Isn't that the same thing?

And again, my saying
So, getting the longest pipe that will sustain on a cold start should come into even better resonance as it heats up, and be able to keep going.
seems to me remarkably like your assertion that
... as the temperature within a duct rises, resonances are only enhanced ...
It does not seem to me that we basically disagree on this point at all.

The assertion of mine which is truly suspect is
This could result in dropping out of resonance after warmup if you start short.
What I mean by this is that, if your engine is some design which relies on a phase relationship between the combustion tube [i.e. chamber and tailpipe] and another resonant element [say, a long tubular intake duct], this phase relationship could be right for a cold engine, but not right when the combustion tube heats up, if you start with the shortest possible overall length. If such a 'phase balanced' design is correctly proportioned cold, there could be a significant phase shift, because the average gas temperature of the air-cooled intake will not rise in keeping with the combustion tube, i.e. its natural frequency might hardly rise at all. [The specific engine design will determine how true this really is in practice.]

Yes, the phrase "dropping out of resonance" was silly and misleading. It is the phase relationship between resonant elements of different temps and dimensions that I was trying to get at. It seems to me that you want to arrive at the perfect phase relationship as you gain temperature, not have the perfect relationship when you start cold and have it degrade as you get running. An extension for starting is a useful "cheat" to get the relationship right for the cold start and then restore that relationship again by removing it after warmup starts to throw it out of whack. This, I believe, is exactly what Rossco has observed.

Another positive effect of an extension for starting might be that it gives you a more massive 'piston' to work against for better compression during the early 'constant volume expansion' phase of combustion.

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Postby milisavljevic » Tue May 18, 2004 5:16 pm

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Postby larry cottrill » Tue May 18, 2004 5:51 pm

milisavljevic wrote:Hello Larry,

Yes, I should have read through all of your posts in this thread, and thereby wisely avoided responding only to the content
of the post from which I quoted you. I offer my apologies for this obvious oversight on my part. Having said that...

Your response contradicts itself. The points which you have introduced (i.e., taken from your prior posts) that parallel my statements (i.e., the ones you have quoted) are entirely contradicted by the statements that I took exception to
(i.e., the second half of your response). What I can say, when you are arguing from both sides of the fence?

I can only say that I must withdraw. The field is yours. Best of luck to you in your future endeavors.

Regards,
M.

M -

Actually, a little while after I posted that, I started to see what you mean. If I believe that
... you want to arrive at the perfect phase relationship as you gain temperature, not have the perfect relationship when you start cold and have it degrade as you get running
... then it doesn't make sense that you would be able to do that by physically shortening the pipe as you gain temperature!!! As the speed of sound increases, the frequency of the pipe would be trying to increase relative to the intake, so you'd have to lengthen it to maintain the starting frequency and preserve any "ideal" phase relationship, NOT physically shorten it. Duh.

So, I have to concede that on this one, I'm really not sure what I'm talking about any more. The only explanation I have left for using an extension for starting is the "larger mass" thing -- that might be valid ... maybe.

Truce! Truce! ...

Seriously, thanks very much for your thoughts and discussion on this! It's much appreciated, even when I seem to misunderstand half of it ...

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