Proposed Design: Simple Two-Tube Valveless

Moderator: Mike Everman

Re: re: Proposed Design: Simple Two-Tube Valveless

Postby larry cottrill » Mon Nov 28, 2005 5:30 pm

Mike Everman wrote:We found at Burning Grape that the joint where fuel tube met air tube to be excessively leaky, but maybe we were only concerned because compressed air became precious. Some solder there would go a long way.

That's probably true, but when I got the Fo Mi Chin II engine back in my hands, it started up right away (within 3-5 seconds of turning on the propane). Remember, that was the first time I had ever tried to start that engine! (This was with 30 PSIG and small diameter air hose, my "standard" setup for Dynajet starting.)

On the two tube motor: I have great doubts that it will work, there is no restriction aft of the CC. I suspect it will be a torch only, unless you can make a restriction.

I suggested this as a reason why I couldn't get my Black Princess going. In response, Graham stated that he and Nick had gotten perfectly straight pipes to run well. Of course, I think it's harder when the L/D ratio is low, as my recent designs tend to be. I think that makes both the length dimensions and the starting technique a lot more critical.

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re: Proposed Design: Simple Two-Tube Valveless

Postby hagent » Mon Dec 05, 2005 1:28 am

Ok.

I was not able to make it self sustain. It runs pretty good with air but it just does not seem to be able to draw enough air in to keep the cycle going. Also when the mixture is perfect and it detonates it will blow it's self out, even if the cc is red hot. Has a nice tail flame though, and I see the flames coming out from the intake too. The flare at the end of the exhaust at the cc side is probably almost gone because of the heat.

Any ideas?

Thanks,

Hagen
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Re: re: Proposed Design: Simple Two-Tube Valveless

Postby hinote » Mon Dec 05, 2005 1:46 am

hagent wrote:I was not able to make it self sustain. It runs pretty good with air but it just does not seem to be able to draw enough air in to keep the cycle going. Also when the mixture is perfect and it detonates it will blow it's self out,

Any ideas?


Hi Hagen:

Please don't be discouraged.

This is a more exact science than many believe. The fact that some of us have been able to "easily" build operating valveless pulsejets only means that we have operated within known, "safe" parameters.

To me, it sounds like your tailpipe return signal is quenching the combustion process; in simple terms, it's out of time!! The timing of the return (recompression) signal is probably the most important part of making a valveless that runs, vs one that won't. After you can make one that runs, the next step is one that runs with reasonable efficiency (I'm still working on that!!).

Absent the actual facts in terms of numerical values, this is only an opinion.

Bill H.
Acoustic Propulsion Concepts


".......some day soon we'll be flying airplanes powered by pulsejets."
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Re: re: Proposed Design: Simple Two-Tube Valveless

Postby hinote » Mon Dec 05, 2005 2:05 am

hinote wrote: After you can make one that runs, the next step is one that runs with reasonable efficiency (I'm still working on that!!)


Just so you'll appreciate the above statement, I've built an (original design) engine that would start with a mouse-fart--but wouldn't produce 1/2 of its expected thrust. Minor dimensional changes resulted in an engine that would still start easily and produce a range of about 5%-80% of expected thrust--still not acceptable!!

Now I'm quite sure I can further modify the concept to produce 100% of its expected maximum thrust, and still maintain a minimum thrust value below 10% of max. The dimensional changes to achieve this are remarkably small.

Again--don't give up!! Consider your current acitivities as an educational experience, and imagine where you'll be, 2 years from today.

Bill H.
Acoustic Propulsion Concepts

".......some day soon we'll be flying airplanes powered by pulsejets."
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Re: re: Proposed Design: Simple Two-Tube Valveless

Postby hinote » Mon Dec 05, 2005 2:19 am

hinote wrote:Just so you'll appreciate the above statement, I've built an (original design) engine that would start with a mouse-fart--but wouldn't produce 1/2 of its expected thrust. Minor dimensional changes resulted in an engine that would still start easily and produce a range of about 5%-80% of expected thrust--still not acceptable!!


Here's another:

My latest successful engine (M8E) is well-built from stainless steel--and ran, right out of the design software.

BUT!: She's hardly perfect. First, the max thrust created was 16 lbf; I had hoped for as much as 25 lbf (but probably a little less). Additionally, I demonstrated a best TSFC of 1.9--a truly amazing number for a first effort, but not up to my expected 1.6 (duplicating SNECMA's effort with the same general configuration).

On the positive side, I was able to start this engine and go into self-sustaining operation with ZERO deflection on the thrust stand!! (same setup, that measured the max value of 16 lbf). That may not be an ultimate accomplishment--but it is certainly a first, as far as I'm concerned. BTW it's also another indication of off-optimum configuration.

I know where I need to make some minor adjustments/improvements.

Your first goal should be to create operating pulsejets, without regard to how well they run. You'll be in the 80th -percentile, at that point.

The operating complexity of these engines is far greater than imagined, and includes issues of fuel "injection"--at least as important as configuration design issues.

Bill H.
Acoustic Propulsion Concepts

".......some day soon we'll be flying airplanes powered by pulsejets."
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re: Proposed Design: Simple Two-Tube Valveless

Postby larry cottrill » Mon Dec 05, 2005 2:09 pm

hagent -

Don't overlook the effects of the relatively small chamber volume in this design. Specifically, make sure you try to minimize the starting air to as low a volume as you can get that will make it buzz. The biggest problem I had in my first successful designs was getting the starting air just right - I kept making the mistake of thinking I needed a large air flow for starting, when all it took was just a touch of air in the intake. I'm sure the basic reason my starting air tubes work well for me is that they give fairly high velocity but very limited flow, perfectly aimed right down the center.

Of course, if the lengths / proportions are just wrong, it won't work no matter how good your technique is! So far, I have never been able to get a really small essentially 'straight pipe' engine to sustain. It is possible that the Hinote Criteria, while a good starting point, just aren't close enough to the real values needed for these critical engines. That's why I made up a graduated set of extension pipes that socket tightly onto a (non-flared) 1.25-inch tailpipe (and onto each other).

Sounds like you're making progress to me!

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re: Proposed Design: Simple Two-Tube Valveless

Postby hagent » Tue Dec 06, 2005 6:49 am

Thanks for all your guys help. I just have to think about what could be the main problem. I think the exhaust length is good because adding a longer section didn't seem to improve things. It's possible that the intake size/volme is wrong. I may have to try and crimp the whole CC along with the exaust and intake down a bit. This will restric everything but effectively giving the CC a greater volume ratio as compared to the intake and exhaust. That's a one way change though, but at least it didn't take long to build.

Hagen
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re: Proposed Design: Simple Two-Tube Valveless

Postby Mike Everman » Tue Dec 06, 2005 2:52 pm

Larry makes the point of low CC volume, but more importantly it's CC cross-sectional area vs. total exit area. I just don't see how this thing can be anything but a burner. The most "open" valveless I've studied, the AS-1 has exits totalling only 62% of the CC area. The SNECMA engineers were very proud of this low level of physical confinement, and everything has to be right in it's LONG duct to a level that we are unlikely to duplicate by cut and try.

I don't want to discourage, and love to be proven wrong! I just feel that there is zero confinement going on here, and no opportunity to create any. You're better off making something that is known to work first, rather than beat your head on this, Hagen, IMHO.
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re: Proposed Design: Simple Two-Tube Valveless

Postby hagent » Sun Dec 18, 2005 7:30 am

Here is the sound file for the pulse jet that I made. As before it will only run on air. I would guess that this is about 50% max throttle.
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re: Proposed Design: Simple Two-Tube Valveless

Postby larry cottrill » Mon Dec 19, 2005 2:01 am

Hagen -

Try this for us: Leaving the intake as-is, gradually lengthen the tailpipe with a loose-fitting extension. You might be surprised!

Or, I might be surprised. :-(

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re: Proposed Design: Simple Two-Tube Valveless

Postby hagent » Mon Dec 19, 2005 4:10 am

Hi Larry,

Actually I did try that. I used about a 1 foot extension and it did not seem to help at all. I would say that it even ran worse. I think I'm going to try and squeeze the intake a bit and make the dia smaller.

You wouldn't happen to know what the frequency is on the file I posted?

I'm guessing that it's around 300 hz which seems a bit high.

Thanks,

Hagen
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re: Proposed Design: Simple Two-Tube Valveless

Postby Mike Everman » Mon Dec 19, 2005 5:28 am

It's about 171, though the sound file only shows the 342Hz second mode. An artifact of the file being so clipped.
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re: Proposed Design: Simple Two-Tube Valveless

Postby hagent » Mon Dec 19, 2005 6:52 am

Thanks Mike!
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