FWE with increased Throttle Range?

[PyroJoe]

Not sure how this 50/50 length one will develop. The fussy nature of the engine must be resolved before it fabs into the super sized. More testing ahead. This baffle plate adds about 3 inches to the length.
Joe

[PyroJoe]

Tested a augmentor this weekend. In a word, WOW.

Before the addition I could start this engine and run in the lower throttle range without earplugs. Now ear plugs are a must.

It continues to start easily and have very much the same throttle range, but the sound has increased considerably. It feels as if the intake has something to push against now.

This one will slide into a 5" tube. Have spotted a 5" stove pipe section from the hardware store. May give it a go.
Joe

[larry cottrill]

Not sure how this 50/50 length one will develop. The fussy nature of the engine must be resolved before it fabs into the super sized. More testing ahead. This baffle plate adds about 3 inches to the length.

Ha -- that looks like it would fly by itself, sort of like a mini Space Shuttle launch, if you could get a streamlined fuel supply anchored out in front.

Tested a augmentor this weekend. In a word, WOW.

You got a WOW because the design is pretty good, I think. It's arguably too small to maximize static thrust, but would be an excellent compromise of performance vs drag for low-speed operation (model plane or boat).

Before the addition I could start this engine and run in the lower throttle range without earplugs. Now ear plugs are a must.
It continues to start easily and have very much the same throttle range, but the sound has increased considerably. It feels as if the intake has something to push against now.

Really interesting. The power available at the intake is often underestimated (at least by beginners) and is almost always underutilized. Back when I was first trying to start my old Electra I with my leaf blower, and I was using it up too close, the most impressive thing I noticed was the loud bang and strong "kick" back into the blower -- like a .22 calibre pistol, at least!

This one will slide into a 5" tube. Have spotted a 5" stove pipe section from the hardware store. May give it a go.

YES! Have a go at that, definitely. It should only cost you three bucks or something to try it. If you can isolate the shell with some strategically located little streamlined pads, it could even be aluminum, if you can find aluminum duct that size. Of course, mounting could be a bit tricky with the shell on.

L Cottrill

[larry cottrill]

Joe -

Thought of something this morning that I should have said:

The design of the front end of your shell (outer shroud) is important. The reason is that you do NOT basically want high speed air flow around your engine. Guys that fly piston-powered speed planes have known for years that SLOW air cools much more effectively! This is counterintuitive, but known/proven by a lot of speed flight experience (by others, not me ;-). So what they do for good cooling is enshroud the engine tightly around the cylinder fins, then provide a really small inlet in front of, and somewhat larger exit port behind, the cylinder. The area of the inlet is far less that the cross sectional clear area inside the cylinder fins. Thus, in essence, they develop a DIFFUSER to slow down the air to a higher pressure and density as it gathers the heat.

With our engines, we have the problem that heat radiation isn't fully concentrated in one small zone, but is spread out (albeit non-uniformly) over the whole pipe. That means there's a lot longer path (and time!) for heat pickup, and it also means that heat gathering will be less effective at the rear because of the heat charge already assimilated into the flow. So, you don't want to go TOO far in slowing things down. But, you could concentrate on having the slowest flow around the chamber where you need maximum heat transfer.

So what I'm saying is, a simple cylindrical shroud, while probably useful, cannot be expected to be anywhere near optimal. You will need a carefully contoured shroud (eventually) to optimize cooling (if that's even what you're after). And, as if that weren't enough: A well-contoured design could yield some "passive ramjet" augmentation -- wouldn't THAT be something to get working for you? What a coup de grace for a pulsejeteer; a consummation devoutly to be wished. Ha ;-) Be the first on your block, as Mark likes to say.

Seriously, wasting all that heat (energy) is ridiculous, but none of us ever do anything about it. Sheesh.

L Cottrill

[PyroJoe]

Probably the first test will be in a relatively long straight tube. To observe how the engine performs in a "free wheeling" condition. Will install 1/4" tubes at various locations around the shroud and attempt what we did with Annes test wand to measure how far the liquid will lift at various locations. Hopefully it will be measurable. This will also give a feel of the engine characteristics when mounted inboard.

The second test will be to slowly alter the intake/exit dimensions of the shroud a few degrees between runs. Progressing towards a ramjet configuration. The PJ expanded tail/augmentor will probably result in a less than typical geometry at the shroud exit.

There should be a broad range of flow speeds to test with this engine. It may be interesting to see the thrust readings at the lowest possible throttle range compared to full on. Very much would like to see a reasonable amount of thrust at moderate throttle. ha, wishful thinking.

All this is several weeks in the future, way ahead of myself. Need to test the thrust of the bare engine and optimize the fueling. Maybe even test liquid fueling of this one. Still trying to grasp the changes the augmentor has created.

Joe

[PyroJoe]

It would be funny to strip the augmentor off and construct/apply a ramjet to the intake, then slip that in a ramjet shroud. doublerammy. ha

[larry cottrill]

Great stuff! Be sure to hammer a smallish flare on the front end of that diffuser, to help smooth things out on the way in. The right ramjet back there will do wonders, as Eric has already shown.

Because of wave action, "some experimentation required" on the end points of that outer shroud (but, you knew that ;-). The rear edge will be the real challenge. Maybe what you want to do is start with a really LONG outer duct, so you can experiment with just the relationship between the rear ends of engine and shell. Then once it seems right, pare down the front end somewhat "gradually" until it gets good again. A textbook case of "stepwise refinement" - ha.

L Cottrill

[PyroJoe]

The engine with the augmenter makes a little over 3.5 lbs.
Thrust can easily be raised or lowered about a 1/2 lb at a time.

3.5 lbs doesn't sound like much, but is the strongest smallish jet that has been on the thrust stand. It was attempting to pushing the metal thrust stand off the concrete blocks. One of those comical had to be there moments. Open up the valve and away she goes... stand and all. ha. Fastened the stand to a large steel plate to keep things stable.


Drafted in a few dimensions of the Dynajet and scaled up to this size(blue). One of the first things I noticed is the Midway cylinder section is about 1/2" longer than the comparable cylinder section of the Dynajet.

The Midway has a volume about halfway between a CC that will start easily and have wide throttle range, and a CC with little to no throttle range and a bit fussy to start airless.

A difference of about 20% in volume. The midway resides in the middle with about 10% below and 10% above.

If I were to cut the 1/2" CC section from the current Midway dimensions it would subtract 9.1% (3.54 cu. inch) from the volume (going from 38.74 to 35.20 cu. inch CC). This would give a DynaJet front cylinder mated to a 20 degree FWE back cone section. Might be a fun engine. ha

If the 2 radius in the back of the Dynajet CC is averaged it becomes a cone of approx. 26 degrees, which is somewhat wide for use as a valveless FWE section. This may be a stumbling block for those looking to convert a Dynajet over to valveless operation.

Joe

[larry cottrill]

If the 2 radius in the back of the Dynajet CC is averaged it becomes a cone of approx. 26 degrees, which is somewhat wide for use as a valveless FWE section. This may be a stumbling block for those looking to convert a Dynajet over to valveless operation.

Joe -

I think this kind of 1-dimensional theoretical comparison speaks too loudly. Recall that there have been pretty good valveless engines with "harder edged" chambers than the Dynajet. The Kentfield would be a perfect example of what I'm talking about.

In a hard-walled "can" like the Kentfield, the internal high frequency reflections right after the explosion are very important to driving the masses, especially the intake air mass. In a smoothed-out chamber like the Dynajet, these reflections are somewhat de-emphasized and "blended". If you stretch the cone of the can chamber enough, they become seriously stretched and blended, with their amplitudes significantly reduced. In the true FWE chamber, they're almost entirely gone, with the entire blast energy stretched broadly over time, gradually (though non-linearly) tapering from maximum to minimum. But, you knew that. My hypothesis has been that the more de-emphasized and fully blended these reflections are, the more "forgiving" the design will be to variations in the "front end" (mostly meaning, the intake duct). But, I think there have been good arguments against this being a net "benefit" to overall performance.

It also seemed to me that good throttleability should be more difficult to get with a more hard-edged can chamber, basically because of the fact that all the frequencies generated will be affected by changing temperature. However, recent experience of many builders seems to show otherwise -- we have now seen fairly good throttleability achieved in a wide variety of designs, I thnk.

L Cottrill

[PyroJoe]

The Kentfield engines that I have seen (thus far) have more cylinder length than cone length. This is not the case in the Dynajet.

The first FWE type engine (non-runner) in this post was a 26 degree engine, strange coincidence.

The FWE 20 degree engines appears to be a good engine at first blush, even if some yea-who starts monkeying around with its volume.


Joe

[PyroJoe]

Brought in the dimensions of the Atom jet this morning. Scaled it up to 3" diameter on the cylinder section. She is about .28" shorter than the Midway (in the cylinder section), it appears to be using a 20 degree angle on the back cone. Hitting pretty close to home. ha
Joe

[PyroJoe]

Tested a CC volume & intake length near to the DJ scale up.

The engine starts and runs fine with only a slight decrease in throttle range.

Opened up the tailpipe at the same point the DJ ends. Used a augmentor matching the tailpipe ID just for kicks. I was hopeing this would give some resistance at the intake, a nice captured column of air for the intake to push against. It will start and run, but as the augmentor is moved close it becomes unstable and extinguish, think it may be re-breathing a portion of the previous pulse.

Time to taper the augmentor and tail and see how she does.

Joe

[PyroJoe]

Added a expanded tail. Extended the intake also, she wouldn't start at first, so the intake was trimmed back in 1/8" increments until she started to lock in for brief periods. Switched over to a fast flow 3 point (crimped, Rossco style)injector to finally make her sustain. After the engine heated up, I quickly changed out the crimped 3 point for the one with the 3 holes and was able to make her sustain with slower flow and higher fuel volume. This one is probably right on the cusp of being able to start airless (with expanded tail).

On another note, the frequency is higher and the noise increased dramatically(compared to the midway), there was some throttle range at the bottom end, with the higher frequency maintained. Although it could maintain the higher frequency, the driving strength felt as though it was lost.
Joe

[PyroJoe]

Since the expanded tail didn't work well with the DJ styled/volume CC, I cut it off and went back to a straight pipe. Also trimmed the intake. She has her pep back and starts easily now, is fast becoming a favorite. The shorter intake allows the air charge to sink deeper back into the CC cone.

Noticed a few months ago, that Bruce S. was using some small cones on his valved engines, thought it would be a good fit for this one. It worked great, and IMO starts just as easy and has more kick! While everyone else was setting fireworks on the 4th, I had her singing a sweet tune under the stars. Still have to test her thrust. There is some other stuff, but it will have to wait for now.

[PyroJoe]

The middle one has a CC cone angle of 19 degrees.
Didn't test it unfolded, just built it folded from scratch.
The CC cone extends all the way forward, with the bottom(front 2.3") of the cone opened up and welded to the pipe below.
She is 46" long with a CC of 4.5" diameter. Main tail pipe is 2.3" I.D.
13" long tail cone with 19 degree angle(cone opens up to about 6.75") ha
No thrust tests yet, but she sounds like a thumper. Hopefully this weekend will wrangle some figures.
Joe