Proposed Design - The Short Lady Revealed

Moderator: Mike Everman

Re: Proposed Design - The Short Lady Revealed

Postby larry cottrill » Thu Sep 16, 2004 3:01 am

steve wrote:
Incidentally, the length shown for that tube is supposed to be the acoustic length, which means the real piece should be .6 x ID shorter in total physical length overall


...wait, what????

so what you are saying is 133 - (.6 x 19) = 121.6mm ?

the intake should be 121.6mm and not 133mm long?

Seems like a lot, doesn't it. Yet, that's what our friend Bill Hinote says to use. An open end behaves as if it is .6R farther out than where it really stops. On a two-ended tube, that's shortening the tube total length by .6 x the ID! The reason it's such a large value, of course, is because the intake tube is such a large ID pipe on my design.

It's still a fairly small fraction of the total length. Nobody has ever been able to explain to me what the tolerance of all these measurements is. A few mm is a tiny part of the primary frequency's wavelength -- but remember, it can be a huge fraction for the higher harmonics, which seem to be very important to have on your side. So, take this into account on the intake tube and just hit it as close as your methods will allow. But, try to get a good measurement of the final result, so we'll know how close to theory we were able to get. Scientific honesty.

Don't worry about the weight or smooth appearance on this one -- the important thing is tight welds and smooth interior surfaces as much as possible. We'll fly the next one, after we learn some things!

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Re: Proposed Design - The Short Lady Revealed

Postby Mike Everman » Thu Sep 16, 2004 3:34 am

I think that's too much. I believe the acoustic properties of the intake will have a node .6r out from the inlet, true, but it will also have an antinode on the closed end of the cc. I believe the intake tube doesn't act as a true open end on the inside, but that the inlet to the closed end of the cc acts as a closed cylinder, where only odd modes are possible. Strangely, I think that all modes are supported by the whole tract because, while the exhaust and intake share an antinode at the end of the cc, the nodes needn't be perfectly coincedent, allowing the tail to have odd and even number of nodes.

On another note, if UFLOW isn't already correcting for the open ends and showing me real geometry, I may just open a vein!
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Re: Proposed Design - The Short Lady Revealed

Postby larry cottrill » Thu Sep 16, 2004 12:33 pm

Mike Everman wrote:I think that's too much. I believe the acoustic properties of the intake will have a node .6r out from the inlet, true, but it will also have an antinode on the closed end of the cc. I believe the intake tube doesn't act as a true open end on the inside, but that the inlet to the closed end of the cc acts as a closed cylinder, where only odd modes are possible. Strangely, I think that all modes are supported by the whole tract because, while the exhaust and intake share an antinode at the end of the cc, the nodes needn't be perfectly coincedent, allowing the tail to have odd and even number of nodes.

On another note, if UFLOW isn't already correcting for the open ends and showing me real geometry, I may just open a vein!

Mike -

All right, now we have Steve [the only one of us who is ready to butcher steel on this thing] waiting in suspense while we fight out where to make the cut. What a test of nerve and resolve! Hang in there, Steve ... we'll get it right yet!

This time, unless I'm misunderstanding what you've said above [and that is certainly possible], I think I'm closer to the truth. Here's why: The intake pipe in this type of engine has to work both ways! Remember that Bill laid out two conditions for the intake: a length of L/5 and an outer node locus of L/3, both relating to the acoustic length L of the pipe as a whole. I think the premise of these two criteria is that the pipe has to behave itself in both modes -- its own characteristic open pipe [two node] oscillation has to blend with its operation as a closed pipe [single node] horn against the reflection of the front wall.

What that has to mean [I think!] is precisely this: The acoustic length of the pipe has to satisfy the L/5 criterion, while the acoustic position of its outer lip has to satisfy the L/3 criterion, within some fairly tight tolerance. We can surely do that.

NOTE TO BILL H: Please correct me publically if this is a misinterpretation of what you meant to state! We need to properly understand this.

I perhaps did not make it clear that my L/8 'derivative' is just a convenient approximation -- it does not serve us the same critical way that the Hinote Criteria do, since these are actually based on the way the harmonics fall into line in the pipe, while my L/8 is not. It is definitely these two that have to be hit within some [as yet undiscovered] close tolerance. But in saying that, I think we have to admit that the intake pipe has to function both as an oscillator in its own right and as an element of the larger picture. And we can only do that by making it suffer the indignity of being chopped off at its open-pipe acoustic length.

Unless you can refute this reasoning on sound theoretical grounds, I have to let my claim stand for the radically shortened length.

NOTE TO STEVE: Looks like I owe you a drawing with corrected dimensions. Hang on ...

As to UFLOW concerns, I think that the correction is almost surely accounted for by the way it works in simulating the action of the real pipe. I can't imagine that we need to overlay a correction of our own on top of what it does. If that weren't true, it would be nearly useless in situations where the L/D ratio is low [short pipes]!

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Re: Proposed Design - The Short Lady Revealed

Postby Mike Everman » Thu Sep 16, 2004 1:46 pm

I am lacking sound theorectical grounds, press on! (Bill is road tripping in Oregon, I believe.)
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Re: Proposed Design - The Short Lady Revealed

Postby steve » Thu Sep 16, 2004 7:32 pm

I rolled the cone today but havent gotten a chance to weld it up yet. Ill weld it up then connect it to the tailpipe tomarrow. Then over the weekend I will smooth out the transition between the two.

I would like to make an autoCAD model of the intake for my own reference and to determine the angles to cut etc. so the sooner you post the updated dimentions the better!

any ideas on the best way to make the dome? I was thinking I would just cut a slit in a disk and curl it slightly to make a very shallow cone.

If you want I can probbably post some construction photos over the weekend.
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Re: Proposed Design - The Short Lady Revealed

Postby larry cottrill » Thu Sep 16, 2004 8:50 pm

steve wrote:I rolled the cone today but havent gotten a chance to weld it up yet. Ill weld it up then connect it to the tailpipe tomarrow. Then over the weekend I will smooth out the transition between the two.

I would like to make an autoCAD model of the intake for my own reference and to determine the angles to cut etc. so the sooner you post the updated dimentions the better!

any ideas on the best way to make the dome? I was thinking I would just cut a slit in a disk and curl it slightly to make a very shallow cone.

If you want I can probbably post some construction photos over the weekend.

Steve -

Just finished up the detail dimension drawing. As you'll see, in this one the bend is 45 degrees rather than 60 as in the Elektras. All the dims you need should be shown, except the size and shape of the hole in the cone. The intake is illustrated as not flattened on the inside end, to facilitate measuring for cutting and welding. After fabrication, I'd still flatten the end just a bit, as I did for the Elektra II: squeeze it to a bit less than .75 inch across the outside, making it somewhere close to 1 inch major axis. All dimensions shown are to the nearest .01 inch and nearest whole mm.

I see no reason that the front dome can't be a blunt cone of the same height. Shouldn't make a bit of difference in terms of it working. If you want it fancier, make the cone first and then work it out into a domed shape by heating and hammering. An old trailer hitch ball might be just the thing for whacking it to round it out; I'm not sure what the best way would be to support it, though -- maybe one of those big holes that's cut in the wood base of the Bukowski Easy-Mount.

Anyway, sounds like you're keeping up an unrelenting pace; good luck this weekend, and here are the dimensions. This is kind of big, so you'll need to click on it and maybe print it for readability. Post any questions you might have, any corrections or whatever. And, are you kidding? Of course everyone will want to see the photos!

L Cottrill
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Intake detail dimensions for the Focused Wave Engine, conical chamber version. Drawing Copyright 2004 Larry Cottrill
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Re: Proposed Design - The Short Lady Revealed

Postby steve » Thu Sep 16, 2004 9:35 pm

WOW! that was more then I was expecting!

guess I won't have to make that CAD model after all...

maybe one of those big holes that's cut in the wood base of the Bukowski Easy-Mount.


hehe I knew someone would make a comment about those eventually. I just grabbed a random piece of scrap wood to make the mount because I was in a hurry and it was convienient.
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Re: Proposed Design - The Short Lady Revealed

Postby steve » Fri Sep 17, 2004 12:38 pm

I am in school right now and I just finished welding the cone and the transition to the tailpipe. the transition was welded with an Oxy torch and the inside is very smooth. I dont think I will even need to file it down at all.
All the dimentions seem to be very close to the ones on the plan except for the cone diameter at the front which is 63mm instead of 64. Oh well, close enough!

I'll try to get some pictures of the inside before I weld the top on and post them later.
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Re: Proposed Design - The Short Lady Revealed

Postby larry cottrill » Fri Sep 17, 2004 1:17 pm

steve wrote:I am in school right now and I just finished welding the cone and the transition to the tailpipe. the transition was welded with an Oxy torch and the inside is very smooth. I dont think I will even need to file it down at all.
All the dimentions seem to be wery close to the ones on the plan except for the cone diameter at the front which is 63mm instead of 64. Oh well, close enough!

I'll try to get some pictures of the inside before I weld the top on and post them later.

Steve -

Incidentally, I forgot to mention a couple of things before:

First of all, I didn't indicate the degree of bend on the drawing. It is 45 degrees, of course. Because the dimensions are rounded to the nearest hundredth inch / nearest mm, you should use a 22.5-degree angle in making the cut, rather than the dimensions shown for the cut edges.

Remember also that the lines represent the INSIDE surfaces of the tubes. If you think about it, this also affects the cut point for that angle cut at the 'elbow', at least if you're using 3/4-inch EMT [conduit] for it, since the thickness is fairly significant.

On the other hand, an error of a mm or two can't possibly throw the thing out of whack. Don't make a time-consuming fetish out of exactness, just use REASONABLE care to try to achieve the dimensions shown.

The detail drawing is very close to a scale drawing, though I did not indicate it as such, and shows the relative position of edges much better than the small-scale one [where I apparently did not achieve very good scaling of the intake pipe!]. Note that the intake flare is actually up beside the cone surface a bit. Note also that the lip of the flare runs very close to the INSIDE surface of the cone! This was not anticipated, only discovered when I scaled it on the drawing. Here's the thing: What's critical [besides the length of the intake pipe itself] is the proper positioning of the inside acoustic point at the correct location on the engine centerline. So, hit that point as precisely as you can, even if you have to slope the intake outward a degree or two to get the flare to clear the cone. Or, you can ding the bottom edge of the cone inward slightly at that point -- it shouldn't make any difference to the engine's operation.

If you have access to a copier that has an infinite range of enlargement [as many of them do now] you should be able to copy the drawing as a full-size pattern to locate the intake pipe precisely. Just use a percentage of enlargement that makes the longest dimension on the drawing come out right; all smaller dimensions will then be correct as well. Or, you may be able to scale it properly with some program that will print it from the JPEG [for example, I find that I can print the drawing with MS Photo Editor at 97% and get it almost perfectly sized]. The drawing was made with a scale of 100 pixels per inch, using MS Paint.

Again, don't be afraid to double-check everything on that drawing and report back with any errors you find!

Best of luck!

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Re: Proposed Design - The Short Lady Revealed

Postby steve » Fri Sep 17, 2004 6:51 pm

I finished welding the dome to the cone and it looks pretty good so far.

and now for the moment you've all been waiting for: PICTURES!

It kind of reminds me of a panzerfaust rocket launcher.
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Re: Proposed Design - The Short Lady Revealed

Postby larry cottrill » Fri Sep 17, 2004 7:30 pm

Steve -

I am laughing aloud with sheer glee -- that sucker looks like a million bucks! What a wonderful execution of the plan, so far. Great work!

Man, if this thing could only work as good as it looks ...

Congratulations on a fine looking job!

L Cottrill
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Re: Proposed Design - The Short Lady Revealed

Postby Bruno Ogorelec » Fri Sep 17, 2004 8:00 pm

steve wrote:It kind of reminds me of a panzerfaust rocket launcher.


Panzerfaust? Yes, though to me it also looks like the 175 mm recoilless cannon -- the best piece of artillery I have had the displeasure of being acquainted with.

Splendid work, Steve. I am amazed at what you're doing. If this doesn't work out as a great engine, I'll really be disappointed.
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Re: Proposed Design - The Short Lady Revealed

Postby Anthony » Fri Sep 17, 2004 9:27 pm

Bruno Ogorelec wrote:
steve wrote:It kind of reminds me of a panzerfaust rocket launcher.


Panzerfaust? Yes, though to me it also looks like the 175 mm recoilless cannon -- the best piece of artillery I have had the displeasure of being acquainted with.

Splendid work, Steve. I am amazed at what you're doing. If this doesn't work out as a great engine, I'll really be disappointed.


I see what you mean by displeasure...

Well, just make the head rounder and yes, you get a nice Panzerfaust rocket.

Now, we want to see it run! Hehe.
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Re: Proposed Design - The Short Lady Revealed

Postby larry cottrill » Sat Sep 18, 2004 1:18 pm

Steve -

Again let me say that I am mightily impressed with the craftsmanship you have put into this beast.

Would it be possible for you to make two sets of domes and chamber cones for me? No welding, just the cutting and rolling. This is the part I just can't do with the tooling I have. I would pay for materials, shipping and a reasonable labor charge. If possible, the cone sheets should be no thicker than .050 inch and the domes should be formed with a center hole no larger than 7/16 inch diam. The domes could be whatever thickness you're comfortable with, and I can handle piercing the cones for the intake pipes later. What do you think?

What I want to do is go forward with development of a liquid-fueled lightweight version. Of course, at this point it wouldn't be prudent to move very far ahead until you test fire yours, since you're so far along -- but, I can't imagine the need to change the chamber geometry much, no matter what happens.

Please let me know if you'd like to do this.

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Re: Proposed Design - The Short Lady Revealed

Postby steve » Sat Sep 18, 2004 5:29 pm

Of course I would be interested!

the sheet metal I am using is about .037 inches thick mild steel (sorry, no stainless!) I can weld them together if you want me to, I'll probably tack weld them anyway so they hold their shape.

total weight at the moment is 22.7 oz.
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