Prettiest FWE (?) - the 'Lady Anne Boleyn'

[Eric]

Its really quite a dramatic demonstration, Larry are you starting to think the FWE doesnt contain all the heat in the combustion chamber? :)

The sound the engine makes is an easy way to tell how the engine is running, if it sounds really really horrible then its probably performing pretty well :D



The power loss is definately from a loss in intake flow, not a higher temperature, I have experienced the same thing. With the flashing on the bottom of the intake there will be less flow, since it cant pull air through the flashing, not just right at the bottom either, the flashing will make it harder for the intake to pull air in over a large percentage of the intake.

This is easily observed by moving the flashing back so that it lets more air in while the duct is still at high temperature and watch the power level increase.


A perfect flare should increase the maximum power, if only slightly, likely the larger gain will be better TSFC, letting the engine produce maximum power with less fuel.



Graham,
I put emphasis on the intake flare since it ultimately the first step for the engine to take in fuel and air, where everything begins, possibly the most direct impact on performance.

With small tailpipe engines like the normal FWE or the dynajet for example, the exhaust velocity is very high and they do benifit greatly from a proper flare for taking air into the tailpipe.

With the expanded tailpipes the exhaust is going quite a bit slower on average, and also the larger size pipes reduces the choking effect, so that they really do not need a flare.

Basically unless you make the flare ridiculously large on an expanded tailpipe engine, there wont be any noticeable change, and would only be considered a good thing durring static / low speed running, if at all.


James could you go one step further next time, like Larry suggested, to keep the flow from cooling the entire tailpipe. A 1" tube would be way to small, something like 2" DIA placed behind the intake that diverts the flow away from the engine should work nicely.

Being able to see the heat distribution right to the end of the tailpipe will be incredibly helpful in studying the engine, and learning how to better improve the design.


Eric

[milisavljevic]

Hello Larry --

The figures that I listed were determined at maximum thrust. Did you notice the ... ultimate thrust?
Did you notice that this thrust level exceeds the Dynajet by some 6%?

Well, no. Part of it is that I'm just not used to seeing forces in grams.

You dimension a motor in mm, but then are caught unprepared when the thrust is measured in grams? ;-)
I cannot help you out with this, Larry; I no longer cite (or reference) non-metric quantities in my posts.

However, there does seem to be something of an "apples and oranges" problem to this.
The [Dynajet] advertised thrust of 4.25 lb is almost certainly not the best it could do -
it is almost certainly a compromise between max thrust and reasonable SFC.

[1] The Dynajet is a piss-poor, albeit popular, example of a valved pulsejet. The Team Helmond's P-90
is superior in every respect (I am not referring to thrust); even the tiny TigerJet is a better valved motor.

The Dynajet's maximum thrust and its advertised thrust are one and the same; its SFC is incidental,
not a result of a careful design process. If thrust were increased, SFC would decrease, but to improve
the thrust would require changing the design - and the resulting pulsejet would no longer be a Dynajet.

[2] The only "apples-and-oranges" aspect to this discussion is that your Dynajet is a well-established,
production-proven design, while James' "Lady Anne" is a one-off built from your first try at a new layout.

Do you feel that these effects [e.g., overexpanded tailpipe] are too subtle for UFLOW to show clearly,
or am I just not seeing the details of behavior that show this?

The effect of overexpansion on a real pulse duct is not subtle: it effectively squelches maximum thrust
at the point where the forward pressure bias reaches its maximum sustainable level, e.g., 2.5:1. What
happens in UFlow1D is a different matter, as this package cannot properly simulate pulse combustion.

That being said, I may suggest (hint) that you compare the simultaneous peak amplitudes of pressure
pulses occuring in the inlet and exit planes; certain geometries will require that you phase shift one of
these two sets of pulses (offset) by 90°/180°. You may also consider comparing absolute magnitudes.

Would this show as a moment of sub-atmopspheric pressure at the tail end, for example?

This is not what I meant, nor is it how the term is normally interpreted in literature; overexpanded may
be taken to mean that a pressure metric in the exit plane passes below the value for its counterpart in
the inlet plane, after dividing the inlet pressure metric by its maximum sustainable bias: Px < Pn / Rfpb.

Overexpansion in the tailpipe is a relative expression; you could state that the inlet is underexpanded.

But please do not! ;-)

Hope this helps!
M.

[milisavljevic]

One note: James' interpretation of the intake design is best.

Well, I am not in love with the curved pipe. Could you clarify, though, why you feel strongly about this?
Is it just that he gets a little more length in the pipe, a better angle going in, or is there some benefit
to having the 'sharp' elbow, or what?

The intake 'pipe', as you call it, actually serves as two (2) functional units, not one -- and this is why
James' Chinese-style intake is the way to go with your "Lady Anne". The first unit is the intake; this
is comprised of a (typically) straight tube with an appropriate flare (usually 25%-37%). The second
unit is the intake transition, and it is functionally equivalent to the 45° cone that connects the intake
to the combustor cylinder in the classic Lockwood-Hiller series of pulse reactors; the presence of an
acute (sharp) join between the two segments only serves to enhance the functionality of this pairing.

That's it! ;-)

Hope this helps!
M.

[larry cottrill]

You dimension a motor in mm, but then are caught unprepared when the thrust is measured in grams? ;-)
I cannot help you out with this, Larry; I no longer cite (or reference) non-metric quantities in my posts.

Well, of course by unprepared, I didn't mean I couldn't deal with it, just that the meaning of the value doesn't jump out at me "at a glance", as it would in pounds ;-)

The Dynajet's maximum thrust and its advertised thrust are one and the same; its SFC is incidental,
not a result of a careful design process. If thrust were increased, SFC would decrease, but to improve
the thrust would require changing the design - and the resulting pulsejet would no longer be a Dynajet.

I thought I remembered Jerry commenting on this, to wit that increasing the fuel flow would, in fact, gain a little thrust, without other modifications. But I may have dreamed it, or misinterpreted what was said.

The only "apples-and-oranges" aspect to this discussion is that your Dynajet is a well-established, production-proven design, while James' "Lady Anne" is a one-off built from your first try at a new layout.

Fair enough.

The effect of overexpansion on a real pulse duct is not subtle: it effectively squelches maximum thrust
at the point where the forward pressure bias reaches its maximum sustainable level, e.g., 2.5:1. What
happens in UFlow1D is a different matter, as this package cannot properly simulate pulse combustion.

That being said, I may suggest (hint) that you compare the simultaneous peak amplitudes of pressure
pulses occuring in the inlet and exit planes; certain geometries will require that you phase shift one of
these two sets of pulses (offset) by 90°/180°. You may also consider comparing absolute magnitudes.

OK, that I can understand.

Would this show as a moment of sub-atmopspheric pressure at the tail end, for example?

This is not what I meant, nor is it how the term is normally interpreted in literature; overexpanded may
be taken to mean that a pressure metric in the exit plane passes below the value for its counterpart in
the inlet plane, after dividing the inlet pressure metric by its maximum sustainable bias: Px < Pn / Rfpb.

I guess the 'bias' part is the concept I'm missing. This is because I am an uneducated man. I have always assumed that the best exit condition would be where the exhaust gas has been reduced to exactly the static pressure of the atmosphere around it. If we have a visible exhaust flame, it should neither be "blooming" (wasted post-ejection expansion due to higher than atmospheric pressure) nor "pinched" (separation and inward vortexing at the exit plane due to lower than atmospheric pressure - like you would get from a badly-designed, over-long deLaval nozzle, for example). The 'bias' might be something I can understand, or it may be a case where I just have to 'take your word for it' ;-)

At any rate, yes, all of your discussion is helpful, and appreciated!

L Cottrill

[milisavljevic]

Hello, Larry!

We cannot go on meeting like this... ;-) I can hear Mike *choking* on his donut as he reads this...just kidding! ;-)

I thought I remembered Jerry commenting on this, to wit that increasing the fuel flow would,
in fact, gain a little thrust, without other modifications.

You may be right; I would put the emphasis on "a little thrust", and then ask, "but at what price?", e.g., valve life.

I guess the 'bias' part is the concept I'm missing.

I understand that the concept may not be intuitative, but so little about pulsejets ever is! I will keep trying, okay?

I have always assumed that the best exit condition would be where the exhaust gas
has been reduced to exactly the static pressure of the atmosphere around it.

This never happens in real-world propulsion systems...nor is it ever a design goal. How do I explain this concept?

Hmmm... Sound of gears grinding... ;-)

Let us rephrase your statement -- replacing "exit condition" with "operating point", "exhaust gas" with "radiator temperature", and "static pressure" with "ambient temperature". Did this example change your perspective any?

At any rate, yes, all of your discussion is helpful, and appreciated!

That makes it worthwhile for both of us! =)

Best regards,
M.

[larry cottrill]

The intake 'pipe', as you call it, actually serves as two (2) functional units, not one -- and this is why
James' Chinese-style intake is the way to go with your "Lady Anne". The first unit is the intake; this
is comprised of a (typically) straight tube with an appropriate flare (usually 25%-37%). The second
unit is the intake transition, and it is functionally equivalent to the 45° cone that connects the intake
to the combustor cylinder in the classic Lockwood-Hiller series of pulse reactors; the presence of an
acute (sharp) join between the two segments only serves to enhance the functionality of this pairing.

M, I feel like you are running rough-shod over me with this one!

How is a bend in a pipe, with no change in cross-sectional shape or area, a transition in any sense? Yes, we have a transition at the nose of a Lockwood. But that is a VERY blunt cone. Yes, we have a transition in the case of a Chinese engine's intake. But that is a VERY fancy piece of sheet metal, meticulously crafted to form a "just so" pseudocone that flares out into the chamber wall (even though I have seriously disputed that the "original" Chinese drawing even shows such a thing, that's the way EVERYONE builds them).

In each of these other cases, the nature of the transition is clear. It does at least three things: (a) Acts as a diffuser for incoming airflow; (b) Acts as a nozzle for outflowing gas mass; and (c) Forms a pressure node at the inside end of the intake pipe. But the FWE intake has never been like this at all; it has never comprised a straight pipe followed by a cone. It is NOT a "Chinese style" intake at all - it is a rotation of a straight-pipe intake like the ones on the front end of the Kentfield engine. The only really important difference is that its juncture with the chamber is at an angle. This, of course, reduces the "sharpness" (i.e. the 'Q') of the tuning of this pipe, but that's the only basic difference. The bend should be acoustically inconsequential - if I welded two half-metre pipes together like that and blew across the end, would it sound the pitch of a half-metre pipe? I wouldn't bet on it.

The importance of this difference is that the resonance of the pipe has to be viewed as determined by the full bent centerline length. There is surely no formation of a pressure node at the bend - the inner node is where the pipe breaks into the chamber.

This is the basic assumption I have always used when 'unfolding' the FWE design. If this is just wrong, I will need a prettty good explanation to be convinced why it should be so. How can that little bent piece, formed from the VERY SAME tubing as the rear-facing pipe, act as any sort of acoustic 'transition' element?

Please state your case for this.

L Cottrill

[milisavljevic]

M, I feel like you are running rough-shod over me with this one!

Not at all. It is simply a collision between fact and supposition, and I doubt that I will ever be able to alter your opinion.

How is a bend in a pipe, with no change in cross-sectional shape or area, a transition in any sense?

Now you are arguing my point. Your premise does not correspond to the physical intake as used on James' pulsejet.*
You are heading off into repeating the same mistaken interpretation that you have wrt. the Chinese intake transition.

If I welded two half-metre pipes together like that and blew across the end,
would it sound the pitch of a half-metre pipe? I wouldn't bet on it.

Now you are misinterpreting my statements; I have never implied anything that suggests this should be the case.

The importance of this difference is that the resonance of the pipe has to be viewed as determined
by the full bent centerline length. There is surely no formation of a pressure node at the bend.

I never said that there was (a pressure node). Moreover, whenever you use 'bend', you are no longer addressing
my argument(s), and you are in essence making my point (again). There are no 'bends' for us to talk about here.

This is the basic assumption I have always used when 'unfolding' the FWE design. If this is just wrong,
I will need a pretty good explanation to be convinced why it should be so.

Well you are wrong, but you will need someone more patient(!) than me to pursue this with. I suggest Mike Everman.

Note: by 'wrong', I am referring to the assumptions behind 'unfolding' the FWE-type duct into a linear configuration.

Please state your case for this.

As I can see this going on forever, based upon my following previous threads, e.g., wrt. the Chinese intake transition,
I will leave my case with what I have written thus far. Of course, I should point out that when I modeled James' motor,
the length of his intake (that, btw., totally nailed this duct's acoustics and performance) was approximately 70 mm; this,
if you care to measure the length of the actual part (and not your drawing), takes you from the flare up to the transition.

Hint: measure the intake pipe along the edge closest to the main tube, going from the inlet plane up to the join (weld).

I know that this will not satisfy you, but I can live with that. Sorry, Larry! :-(

Best regards,
M.

*For the record, my discussion is limited to James' build of the "Lady Anne" release of Larry's FWE-series motors.

[larry cottrill]

But I am an ignorant man. What is an ignorant person for, if not to try the patience of the wise and learned?

Perhaps you, in your blunt and direct manner, are answering one of my oldest questions about such engines: How EXACTLY does the wave front work its way from the chamber into the intake pipe (and vice versa for the reflected front)? I admit that I have NEVER understood the exact details of this, in the case of a 'short transition' like this (in my original 'Short Lady', where the pipe penetrates deep into the chamber, the case is simpler). I think, however, you have settled the issue, in a way that perhaps even I can understand.

My mental picture has always been that the intake pipe assembly, comprising the two sections, acts approximately like a 'waveguide', i.e. the wave front can be visualized as in effect tracking along the centerline of the pipe regardless of any twists and turns. I have, of course, always understood that this is an oversimplification at best: You simply cannot have that neat a resolution where the duct width is a tiny fraction of the wavelength. Period.

What you are saying, I think, is that the pressure does not travel in this neatly duct-directed way within the short segment that you have called the 'transition'. Instead, what really happens is that pressure rises and falls in the short transition zone in more of a 'directionless' sense. In effect, this would mean that the straight pipe begins to 'feel' the pressure as if it were simply 'plugged into' the chamber (the picture being like what I did with Fo Mi Chin II and the Smooth Lady, or some of the Thermojet engines). Putting it another way, my pictured wave front does not move 'up' the transition (like a boat being winched up a ramp), but rather, simply 'along' it (longitudinally) until it enters the intake stack. This is why you have said "There are no 'bends' for us to talk about here." It is as though the lateral displacement doesn't exist!

The transition itself (assuming it is sized so that it doesn't somehow get in the way) matters only in its function as properly locating and positioning the straight pipe. (This of course would not be true if it were a long piece, like the angled part of the intake on the original Short Lady.)

If I have finally arrived at the correct picture, you can rest assured that I am humbly and truly grateful! (You surely can't really believe that I am too stubborn to even pay attention to try to learn something ;-)

If not, then I'll probably never figure it out, and will continue to proceed intuitively ...

L Cottrill

[milisavljevic]

Hello Larry --

This post is a place holder, to let you know that I have read your reply and that I will post again on Thursday.

But I am an ignorant man. What is an ignorant person for, if not to try the patience of the wise and learned?

I am wise enough to realise that I am not. =)

You surely can't really believe that I am too stubborn to even pay attention to try to learn something ;-)

Where did you ever get an idea like that? ;-)

M.

[larry cottrill]

OK, after the EXTREMELY helpful discussions with M (see several previous posts), I think I finally know what I'm doing with the intake on an FWE !!! This first revision of Lady Anne ought to come WAY closer to proper tuning (unless my guesses about temps are way off, or some such). Following the way I now think the intake works (and, yes, the transition, too) I have revised the basic layout of the intake as shown here in the Rev 01 drawing. I probably won't revise the full-scale plans until we arrive at what we might consider a "final" solution.

Basically, if I now understand how it works, the intake pipe I originally showed (the curved pipe) would have been oscillating as an approximately 88mm pipe, with an open pipe frequency somewhere around 11.5f (11.5 x the main pipe fundamental frequency), and set pretty far back in the chamber. Also, my poor understanding of how the pipe worked led to a very inaccurate "unfolding" of the engine for my original UFLOW analysis, I believe. The new intake pipe has been lengthened to 111mm for (I hope) a frequency of 9.0f, and if my unfolding technique is now correct, its new forward position should operate from a pressure antinode that is right at the rear of the chamber dome. The engine 'runs out' fantastically well in UFLOW - but again, this assumes that I'm guessing somewhere close to accurate internal temps.

Using a transition piece and locating it so far forward from the previous location forced me to increase the distance from the mounting plane to the engine centerline slightly. I also eliminated the little 32mm diameter cylinder by simply extending the middle cone forward to the end of the chamber. That little piece was doing nothing for us, and just created an extra butt weld around the pipe. Dimensions for this revised cone sheet are:
- Outer radius: 542 mm
- Inner radius: 413 mm
- Subtended angle: 13.9 degrees
Note also that the longer straight section of intake allows for a second weld to secure the starting air tube, for better assurance that it will stay aligned with the intake centerline.

Anyone desiring to build this version might want to hold off until some ongoing comments have been aired. My own feeling is that, at the intake, it's easily 100 percent better than what I first presented, and should be a winner for wringing performance out of this pipe.

L Cottrill

[Eric]

When I have some free time this winter I will probably give one a try, for now we are just going to have to bug James every time we want any experimental data.

I think the tailpipe if it is allowed to build up heat over the entire length with an intake diversion tube, the resulting colors would greatly help any future simulations since you wouldnt have to guess about heat so much.

How would you feel about designing an FWE with an advanced chinese style tailpipe? It would increase the length a bit but it might be worth it to get 4-5 pounds thrust out of it.

Eric

[Mike Everman]

I like the new intake transition location.
Larry, what someone should be doing is making the linear version of this to get the full benefit of all you UFLOW work!

[Eric]

Werent Grahams 07 fwe's linear types?

Eric

[larry cottrill]

Werent Grahams 07 fwe's linear types?

Eric

That is correct - Graham designed them that way (using just UFLOW1D in those days, of course) and Nick built them as 'linear' motors, just exactly as modelled by UFLOW (IF I remember the discussion correctly - I haven't gone back to the thread, and I don't remember if any physical tweaking was done).

This working model proves that there's no problem folding it, though, once you get the velocity node in the right spot (which I define as right on the front chamber cone edge - but maybe that's just me ;-).

L Cottrill

[larry cottrill]

I like the new intake transition location.
Larry, what someone should be doing is making the linear version of this to get the full benefit of all you UFLOW work!

Mike and Graham -

Note that there is a large discrepancy between Graham's intake layouts and mine. The only simple explanation I can think of offhand is that either I am putting too much faith in the cool air breathing capability of that rear cone, or Graham is putting too little faith in it. By my way of thinking, the gas inside the cone is far cooler than anywhere in front of it - i.e. there is a big temperature transition just aft of the rear 'choke' throat. This makes the engine behave (acoustically) as if it is SEVERAL INCHES LONGER than its physical length. Thus, for me, the intake and transition came out pretty close to what I would design today for a full-length 'Short Lady' (26 inches long with straight tailpipe, for those too new to be familiar with it). However, I can assure you that my UFLOW model does use the actual lengths shown! I just use very cool temps in the tail cone.

Of course, as I've said before, all the temps I set up in a UFLOW model are just guesses. I generally produce models that cycle far slower than the real engines built from them - which suggests that I consistently guess too low on all internal initial temperatures.

Only if someone will build it will we know for sure. But if I'm just wrong, how could my original design have run so well? And I think this layout is FAR closer to proper tuning!

L Cottrill