Focused wave engine runs!

[larry cottrill]

The short answer is that no kind of rig does that- it is almost entirely done by hand with a hammer and anvil. To start with I use a slip roll to begin to curl the metal but after that there is no machine to help me. I will squash part of it with the vice grip, then then hammer it smooth on the anvil and repeat this many times. I gradually work the two edges together untill they are close enough for me to tack weld.

Steve--you're getting top quality on your cones, IMHO.

Steve -

Yes, they are beautifully done - the check is now in the mail for mine!

However, I'm having second thoughts on my offer to have you do the rolling and tack welding of these in job lots. I had no idea that so much of the process was manual craft of that kind. I'm afraid that a fellow of your obviously creative bent might get pretty sick of hand-smithing cones over and over again, even if there's some money to be made.

Of course, we could agree to let you do them in small batches until you're just sick and tired of messing with it, after which I'd need to go elsewhere. It isn't like I'd sue you or anything ;-) As an apprentice jeweler, I once had to make up 30 or so Masonic rings from gold slab to finished pieces, a job that took about a week, and I'll tell you, the amusement value of such labor wears off fairly quickly - and that involved a large number of new skills to learn, too.

Please comment when you can. I eagerly await my parts shipment!

L Cottrill

[Graham C. Williams]

What about the possibility of choking?

Dear Larry.
Exactly why I suggested pinching the inlet.
The amount of exhaust pipe backflow is directly related to the combustion peak pressure. The Thrust is related to how well you can add momentum to this backflow. I think the main pipe is fine but the inlet pipe is dropping the combustion pressure too much and too early. The classic solutions are:
1) Increase the induction pipe length
2) Chocking the exhaust backflow through the induction pipe.

Pinching the induction pipe so that the pressure losses are controlled by sonic exhaust backflow but still having (just) sonic induction flow could be the answer and maximize the potential thrust.

Graham.

[Bruno Ogorelec]

My, the field is rich with theory and it all sounds very reasonable.

I think the prudent thing is to test the mixture settings first. One should see the thing run on the gamut of settings from very lean to very rich, without changing the geometry.

After that, the trimming and the extending game.

But, it's so much fun already...

[Mike Everman]

Fun to be sure! I'd like to see Steve video an extended run at night, with a c-clamp on the intake to progressively close it down, and see where the hotspot goes. Maybe with a flexible swash plate near the tail for a crude relative thrust indication.
Faaascinating.

[Hank]

This engines pushing the reversion wave out the intake and the fact that all of the exaust duct appears to be cool to the combustion chamber suggests something amiss with tune. I wish to retract my original suggestion of increasing the length of the intake tract and suggest that some one-third of the length of the exaust be cut off. Perusal of the photos suggests that this engine is, in fact, running backwards. Too great a flow inwards from the exaust is creating a back-pressure that is not allowing thrust to flow the course it should.

Hank -

How would you square this hypothesis with the observation that there is, in fact, a small but visible tail flame? If the tailpipe isn't handling the primary wave, wouldn't the large cold mass keep any of the blast mass from ever getting out? If there is in fact "something amiss with tune", how can the engine be as easy to start as Steve asserts? [Of course, I realize that 'tuning' can itself mean a couple of different things. You could, for example, have good acoustic tuning but poor division of mass flows, theoretically.]

Hello- The last part of your last sentence sums up what I believe is occuring in the operation of this engine. The exaust duct, as stated previously, is of incorrect volume, too long. That some of the combustion product is making its way down the duct is no indication that things are as they could optimally be. A simple test of relative mass gas movements would be to fire up the engine and hold a strip of metal foil by first the end of the exaust duct and then the intake. Where is there greater movement and in which direction? If the greater weight of the flow is through the exaust, as I am sure it is, shortening the length in small increments until heat is visibly observed at about half the length of the duct should be about optimal mass flow tuning. Hank

[larry cottrill]

This engines pushing the reversion wave out the intake and the fact that all of the exaust duct appears to be cool to the combustion chamber suggests something amiss with tune. I wish to retract my original suggestion of increasing the length of the intake tract and suggest that some one-third of the length of the exaust be cut off. Perusal of the photos suggests that this engine is, in fact, running backwards. Too great a flow inwards from the exaust is creating a back-pressure that is not allowing thrust to flow the course it should.

Hank -

How would you square this hypothesis with the observation that there is, in fact, a small but visible tail flame? If the tailpipe isn't handling the primary wave, wouldn't the large cold mass keep any of the blast mass from ever getting out? If there is in fact "something amiss with tune", how can the engine be as easy to start as Steve asserts? [Of course, I realize that 'tuning' can itself mean a couple of different things. You could, for example, have good acoustic tuning but poor division of mass flows, theoretically.]

L Cottrill

[jmhdx]

Hi all, thought it was time I chimed in with my unqualified and often worthless opinion.
Are we forgeting what the "exhaust" is for? A reynst pot or jam jar relies solely on rapid cooling and therefore contraction of the remaining gases in the chamber for refilling by the then higher pressure of the 1 atmosphere outside. Adding an exhaust, I thought, was primarily to augment the low pressure by having a gas mass travelling away from the chamber stretching the vacumn for a more complete filling. If tuned it has the secondary function of compressing the new mixture on its return and more evenly igniting the mixture resulting in a more rapid combustion of available fuel/air. The only evidence I can provide is that in my experiments a two port combuster runs at a higher frequeny than the same chamber with a single hole. An audible blat-blat rather than a put-put is also evident.
Why would you want any gas to escape from the "exhaust"?
I hope that blocking the "exhaust" off will result in a "gas spring", stretching the vacumn then recompressing the chamber while allowing the mean total mass of hot gas to escape through the single opening.
Can we test this with uflow?
I have no idea wether a long tube or conical chamber would be best and do not have the facilities to try it. A running metal jam jar would be the place to start.
Fustrated,
Mike.

[Hank]

This engines pushing the reversion wave out the intake and the fact that all of the exaust duct appears to be cool to the combustion chamber suggests something amiss with tune. I wish to retract my original suggestion of increasing the length of the intake tract and suggest that some one-third of the length of the exaust be cut off. Perusal of the photos suggests that this engine is, in fact, running backwards. Too great a flow inwards from the exaust is creating a back-pressure that is not allowing thrust to flow the course it should.

Hank -

How would you square this hypothesis with the observation that there is, in fact, a small but visible tail flame? If the tailpipe isn't handling the primary wave, wouldn't the large cold mass keep any of the blast mass from ever getting out? If there is in fact "something amiss with tune", how can the engine be as easy to start as Steve asserts? [Of course, I realize that 'tuning' can itself mean a couple of different things. You could, for example, have good acoustic tuning but poor division of mass flows, theoretically.]

L Cottrill

Larry, I doen't understand why you reposted this.
Regarding the ease with which Steve asserts his engine starts, obviously the criteria for combustion are being met. That these criteria are not in the proper proportions for a tuned propulsive duct is evidenced by the lack of heat along the exaust duct. See above post. Hank

[Bruno Ogorelec]

Mike, I don't think a gas 'spring' of that kind is viable. What you propose is essentially a tube closed at both ends, with the proper intake/exhaust branching off, like the Chinese intake. The combustion zone would be at one end of the closed tube, while the 'spring' would be formed at the other end.

It won't work. If you plot the sum of the standing waves in a closed chamber, you will see why. Take the primary half-wave then its first even harmonic then the second etc. etc. and then add the pressure values of all those waves at every point along the tube, to get the plot of the sum of pressures. You will see how quickly the sum decays. There will be no punch at all stored in the 'spring' I'm afraid.

I was amazed to see that. I never liked the idea of the reflection chamber or pressure reservoir or whatever you may call it, but several people in the forum did, so I tried to keep my mind open. But, the pressure graph of the closed-end tube removed the last of my reservation. I am now completely convinced that it doesn't work.

Bruce Simpson argued -- years ago -- that it would just 'water down' the combustion event and he was completely right.

[larry cottrill]

Larry, I don't understand why you reposted this.
Regarding the ease with which Steve asserts his engine starts, obviously the criteria for combustion are being met. That these criteria are not in the proper proportions for a tuned propulsive duct is evidenced by the lack of heat along the exhaust duct. See above post. Hank

Hank -

I didn't re-post it - it showed up as a wild post that was a mess made up of my quote and your response, and I saw that you had already posted your response above. I couldn't figure out a way to delete it so I just cleaned it up by editing.

I'm holding off a while to see how many different approaches to the problem we can accumulate. The common thread (at the moment) seems to be that we are not getting (and/or maintaining) adequate pressure to accelerate the tail slug reasonably in the time available. This is quite interesting to me, since in the case of the Elektra and Fo Mi Chin engines, I was fearful of having inadequate tailpipe "piston mass". When I got the Elektra I to run by lengthening it, I assumed that along with reaching the point of resonance, I had gotten the mass up to where it needed to be. I now think this was probably wrong, and the engine is probably barely able to shove it out the door! For all I know, the tailpipe could also be in half-wave resonance - it is almost exactly twice as long as the Dynajet tailpipe [including the chamber] for the same frequency of operation, approximately. So, I'm now pretty sure that it's way off!

L Cottrill

[Mike Everman]

It's interesting to watch this animation, the sum of the travelling waves (of a single harmonic), in a closed-closed pipe.
http://faraday.physics.utoronto.ca/IYea ... dwave.html

It's eye opening to consider that in these engines, the standing wave is made up of several travelling waves.

[hinote]

I'm holding off a while to see how many different approaches to the problem we can accumulate. The common thread (at the moment) seems to be that we are not getting (and/or maintaining) adequate pressure to accelerate the tail slug reasonably in the time available.

L Cottrill

Here's my 2 cents worth:

I think the darned thing should be run some more, and 2 things verified before cutting-up a good-running, easy to start engine.

1. Some sort of thrust reading should be taken, to get a baseline value against which the proposed modifications can be compared.

2. A reduction in fueling should be attempted, while observing thrust values. This would verify the overfueling previously discussed.

Without some sort of instrumentation and documentation and a plan to do some constructive research, the whole effort will be a waste of time.

Bill H.
Acoustic Propulsion Concepts

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

[steve]

Here's my 2 cents worth:

I think the darned thing should be run some more, and 2 things verified before cutting-up a good-running, easy to start engine.

1. Some sort of thrust reading should be taken, to get a baseline value against which the proposed modifications can be compared.

2. A reduction in fueling should be attempted, while observing thrust values. This would verify the overfueling previously discussed.

Without some sort of instrumentation and documentation and a plan to do some constructive research, the whole effort will be a waste of time.

Bill H.
Acoustic Propulsion Concepts

How ironic that you would post that as I have just completed preliminary thrust measurements! I converted the engine to the fueling setup that Bruno suggested and rigged up a primitive thrust stand using a spring scale borrowed from school.

With the new setup throttability was vastly improved. It will throttle much lower then before as well as a bit higher. On the lowest throttle setup the flames completely dissappear from the tailpipe and almost completely dissapear from the intake with the exception of the occasional little pop. Thrust at this level is verry little, around 100g while thrust at the highest setting is around 600g (1.3lbf) At about 3/4 throttle the thrust is about 500g (1.1lbf).

I believe that the reason for the low thrust is partly due to my very crude thrust rig and partly because the engine is not augmented, although I intend to fix the augmentation problem soon.

Larry: your cones/domes are finally done and I will ship them out as soon as I can!

[larry cottrill]

How ironic that you would post that as I have just completed preliminary thrust measurements! I converted the engine to the fueling setup that Bruno suggested and rigged up a primitive thrust stand using a spring scale borrowed from school.

With the new setup throttability was vastly improved. It will throttle much lower then before as well as a bit higher. On the lowest throttle setup the flames completely dissappear from the tailpipe and almost completely dissapear from the intake with the exception of the occasional little pop. Thrust at this level is verry little, around 100g while thrust at the highest setting is around 600g (1.3lbf) At about 3/4 throttle the thrust is about 500g (1.1lbf).

I believe that the reason for the low thrust is partly due to my very crude thrust rig and partly because the engine is not augmented, although I intend to fix the augmentation problem soon.

Larry: your cones/domes are finally done and I will ship them out as soon as I can!

Steve -

Nice, direct attempt at thrust measurement. "Crude but effective" is one of my favorite phrases, as you must know by now. However, crude as it seems, the only things that would cast doubt on the numbers would be instrument error and friction. You're supposedly using good scales, so if your wheels and axles are low loss, those values should be pretty good. That's a nice range of thrust values at this point, I think! Changing the fuel delivery setup was also a good thing to try.

Sometime, just for fun, see if you can cold start it without auxiliary air, just by starting your spark and quickly hitting it with a sharp blast of fuel gas immediately followed by throttling back - who knows, you may be surprised!

Thanks for your work on the parts - I will be eagerly awaiting them, and I sent your check this morning as promised.

L Cottrill

[Hank]

Hello- This thread has my interest.
Steve, what is the weight of your engine, less the stand?
Hank