Any Input?

[Fred]

OK, here is an update design. seems very long, but based on Pyro Joes info. the only tinkering i think i should have to deal with is the inlet.

anyone out there have any thoughts on it, i would appreciate any ideas or concerns.

200# pulse jet Model (1).pdf

Foa Pulse jet

(71.71 KiB) Downloaded 646 times

thanks.

[tufty]

Yeah, you'll probably have to do some "tinkering" with the inlet.

Foa's design looks really nice on paper, but concentric / annular inlets are *really* hard to grok. You have a massive amount of boundary layer relative to the amount of inlet you want, and it's hard to adjust (basically, "tinkering" pretty much equates to "making a whole new annulus every time you want to change something"). It's probably worth searching for "annular" on the forum, there was at least one such design built and run in living memory.

Fuelling is going to be an issue as well, you probably want an annular fuel feed.

If you're gonna be tinkering, you might want to consider finishing up the tailpipe with a straight (rather than tapered) section, it makes life much easier with regards to adjusting tailpipe length - with a slip fit sleeve you have a fair range of adjustment.

As for the design overall, I can't really say. If you've pulled the dimensions out of your hat, or from the valved PJ calculator, there's a better than 50% chance it won't fire up even with a conventional inlet setup. You probably want to start with CC and tail dimensions that are known to work, and go on from there. By which I mean, "build a conventional engine, run it, be sure it works, cut the front off and graft your front end on". And even that's no guarantee.

[Fred]

Tufty,

I redesigned based on Pyro Joes info posted earlier in this section.

as for annular inlet, i will look up and read what i can find. it could mean more changes before i build it.

as for fuel, i was considering three injectors mounted right behind the stand offs that will support the cone section. but perhaps it will require more? not sure yet.

now for the tail pipe; do ou think a i am a bit confused about tapering to begin with. seems most design have a tapered section, but what you are saying is make it work with a straight section the tinker with the conical tail section?

thanks.

[metiz]

I think he meant that, if you pulled the design out off your ass (wich you havn't) there's a good chance it won't run. If you'd want to adjust the tailpipe in that situation, a straight pipe would be a lot easier.

But since you designed it of off Pyro's recipe you should be good and you won't need to change the tail. The intake might be a problem though, like Tufty said. However, when people say "that won't work" you should see it as a challenge and not a limiting factor

[Fred]

the intake is the challenge, how to get resonance? i can't find any info on Foa's design anywhere. Googling had nothing except what has been posted here on Pulse-Jets.

very odd that you can find any info.

[PyroJoe]

Enjoy seeing stuff like this tested.
The size of the engine is rather big so there is considerable investment involved.

I would say it would be good to review the pressure that is created from ram pressure, versus the peak pressure from a typical valveless pulse jet. It will definitely help on intake, but during the pressure cycle.....I'll leave to your own powers of reason.

I guess the tail diameter could be increased, and an attempt to pressure dump out the tail, but that isn't necessarily a good performance option.

It would be good to do testing on something considerably smaller and work your way up in scale so theres not so much money, fuel and perspiration involved. There is absolutely no shame in working with scaled down engines.

Blended bodies are showing good possibilities for shorter more compact and powerful engines, but there is more testing to be done before I can say for certain. There are so many projects in the works its hard to find time to post.

[Fred]

i agree, and have considered doing a third scale of this? and make it from sheet steel for testing.

i am concerned about ram pressure vs the dyamic pressure of the pulse cycle. at some point ( i am assuming over 200MPH) the ram would take over, but slower than that, will the pulse resonance be enough to overcome the incoming air stream until you reach ram effect?

seems to me that it would work under static conditions, by i have my doubts when it starts to move. maybe i am wrong??
any thoughts?

[PyroJoe]

hmmm
I would think it would depend on the flame speed of your fuel air mix and the ability to maintain ignition. What is your approx. max airspeed?

[Fred]

200 to 250. and for a glider that is huge. i considered faster speed, like up to 450, like a U2, but that is out of my realm.

do you think ram effect will even work that slow a speed?

[PyroJoe]

Ram effect does make a difference, even at lower speeds near 100 mph. It is tempting to pick up an extra 0.5 psi of intake pressure but it comes at an expense.

Most valveless pulse jets like a little quantity of what I call "drift" velocity of their airflow. Some won't even run without it, but if that drift velocity becomes to great, whatever is keeping the fire lit is blown out the back of the engine. Much akin to a ram jet without a flameholder.

Keeping the flame lit with a high drift velocity will probably be a more difficult task than producing the required flame speed.
"some testing required"

[Fred]

in the back of the nose cone, i planned a concave section to act as a flame holder, but it might have to be deep recession to hold the flame? or anotehr option is introduce a annular type of combuster like a turbine, or like the Messerschmitt design posted on this site. something to hold the flame, but to allow free flowing of air.

I guess experimentaion is the word of the day huh?

[PyroJoe]

What is considered cruising speed?

[Fred]

normal operating speed, typically about 65% to 80% of VNE (speed, never to exceed)

why?

[PyroJoe]

There will probably need to be a balance in the tail expansion.
Very little expansion to reach your top speed. (highest exit velocity)
More expansion to help lift off the ground. (highest static thrust)

At cruise speed you would want to match as closely as possible the exhaust velocity to air speed to achieve some measure of propulsive efficiency.

Its probably easier than it sounds, tinkering a bit with the expansion till you get what you like.

[tufty]

I redesigned based on Pyro Joes info posted earlier in this section.

Ah, that's good news. You should at least have a chance of getting it running :)

for the tail pipe; do ou think a i am a bit confused about tapering to begin with. seems most design have a tapered section, but what you are saying is make it work with a straight section the tinker with the conical tail section

If you're looking for thrust, which I assume you are, then the taper is going to help you, and you should certainly keep it. I would, however, suggest going straight-taper-straight rather than finishing with a tapered section. Here's why:

I predict that you will have trouble attaining resonance regardless of the form of the exhaust. It's extremely unlikely that a "first cut" annular inlet will be matched closely enough to the frequency that the motor "wants" to run at, so you're likely to find yourself in a situation where you want to tweak the motor's operating frequency. The easiest way to do that is to start with an exhaust that is "short" relative to what you're expecting, and use a sliding sleeve on the end of the exhaust to adjust the length. This gets you a motor with a range of operating frequencies, and a far better chance of locking the exhaust to the inlet without doing a lot of cutting and welding...

For the inlet, there's a *load* of variables in play. Here's a list of some I pulled from my hat (or arse, if you prefer).

- Inlet area (ram air alongside the "bullet" + fuel-air mix coming through the annular part
- relationship of ram air to fuel-air mix at various speeds (it seems to me that the motor will tend to "lean out" as speed increases)
- length of annular inlet path and ram air inlet path (direct effect on motor frequency)
- relationship of inlet path lengths (a variance between the two *may* increase operating frequency range, but might well also affect the effective area of one path of the other)
- form of the annular inlet path - by changing the form of this, it should, in theory, be possible to speed up or slow down the inlet "slug" of fuel-air mix
- form of the rear of the "bullet", and its effects on mixing of the ram air and the fuel-air mix
- difference in velocity of the 2 inlet flows at different airspeeds, and the effect of this on mixing.
- relationship of "inside" to "outside" lengths of annular inlet path
- relationship of terminating "inside" edge of annular inlet path to position of the rear of the "bullet" (where does the length of the annular inlet part start, does it differ on inlet and exhaust cycles, and so on)

...and that's only for starters.

Personally, I think mixing of the 2 inlet streams is going to be your biggest problem. As it stands, it looks a lot to me as though your fuel-air mix is liable to stay more or less stuck to the combustion chamber walls, and that's not a good thing.

Regarding scaling down - you have a *lot* more latitude with a bigger motor, but as you get bigger you have more problems getting enough fuel to the damned thing. If I'm reading it right, you're looking at a combustion chamber diameter of a foot? That's frickin' huge.

I'm not saying it's not going to work, and I sincerely hope it *does* work, but I think you've got your work cut out for you. It's a very interesting experiment.