Annular M20

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metiz
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Annular M20

Post by metiz » Tue Aug 29, 2017 6:25 pm

I build an annular version of the M20 about a year ago. I could not get the injectors out far enough to get it to run. A couple of weeks back I fixed the issue and today I tried again. Although it runs, it's a huge pain in the ass to get going. Thrust is rather disappointing too: 11.4 pounds (original is 26.4)

My intention is to put an intake augmenter on the engine with the hot tailpipe at its center, and build a tailpipe augmenter in such a way that it is "pre-loaded" by the intake ejector gasses. The idea is that all that heat is going to contribute to the thrust. It would be even cooler to see what the engine would do with a stiff breeze at its rear, basically providing the intake ejector with ram-air.

Some pictures:
https://i.imgur.com/PomViAz.jpg
https://i.imgur.com/CQ2vAEV.jpg
https://i.imgur.com/eoLBNda.jpg

Video (ten minutes of fiddling with injectors and starting attempts omitted)
https://www.youtube.com/watch?v=irV-aEUCZik
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Mark
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Re: Annular M20

Post by Mark » Wed Aug 30, 2017 1:27 am

That jet looks very well made. Could it be with annular pulse jets that the airflow is harder to keep flowing in a coherent manner as air skirts in around the neck, it seems like the repeatability of inflow has so much freedom to vary from side to side instead of being more confined and directed say from a front end intake? Also the fuel lines probably create/kick up a fair amount of turbulence as well. It was a good demonstration with all that dust and I can appreciate the hassle of trying to get it to start, pulsating combustion in many forms can be so fragile if all the mysterious conditions aren't met.
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metiz
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Re: Annular M20

Post by metiz » Wed Aug 30, 2017 2:13 pm

Thanks, Mark.
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Mike Everman
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Re: Annular M20

Post by Mike Everman » Wed Aug 30, 2017 4:49 pm

Very nice build, Joost! You can ignore my youtube request for pics, obviously. should have checked here first.

The intake exhaust impinging on the tail cone can't be good, either. I've made a crappy one and so did Bill Hinote. I so far have never seen a good performing annular, but it's so compelling to try. I don't know why.
Mike
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tufty
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Re: Annular M20

Post by tufty » Wed Aug 30, 2017 6:00 pm

The idea is, as you say, compelling. Surely all that symmetry should make for better performance...

I never got anything more than very unconvincing fart noises from my attempts.

Mike Everman
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Re: Annular M20

Post by Mike Everman » Thu Jun 21, 2018 11:05 pm

I think I've boiled the "why annulars suck" to three things.

1. It violates a simple axiom of having the least surface area for a given volume, the king there being a cylindrical tube. Annular skin drag on the flow is always higher in the all important intake (2x+).
2. If you could, you'd have your intake pipe(s) ice cold so incoming gas is as dense a charge as it can be, and does not expand until you heat/expand it where you want to. The annular intake always has a super hot inner wall, being the outside of the exhaust tube.
3. As the annular diameter increases, so does the chance that as Mark says, the exhale is not uniform, but could even go out on one side and in on the other (not really out and in, but out and "less out" 180 degrees across, which will screw with/soften the return rarefaction wave.

I need to get pulsejets back in my life. I'm going to to to Bonneville salt flats in Sept, and bring a noisemaker. Screw this workin all the time!
Mike
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