Starting Problems

[Mark]

To me, it kind of looks like a Schmidt Tube philosophy or Houston Maupin design as seen in the plans section of this site. The Houston Maupin has very little volume in the "combustion chamber" if you take into account the valve unit protrusion.
http://www.deutsches-museum.de/en/colle ... -jet-1942/
http://www.preservedaxisaircraft.com/Lu ... %20500.jpg

Here's kind of a Houston Maupin valve philosophy made by Al Belli with 7 petals and another less spectacular one I bought on eBay with 6 petals on the bottom right.

[PyroJoe]

Yeah,
Maupin was the first thing that came to mind. Unfortunately there are few fellas that have run or built those types.

If that CC was any shorter the back of the chamber could be used as a valve retainer.

Would it be possible to construct a section of CC to bolt inbetween the flanges?

[PyroJoe]

About the closest engine in proportion looks to be a Russian design. About 200mm would need to be added to the CC and the tail would need to be trimmed 3mm at a time until it is tuned. Without the smooth cone transition it may need the extra 120mm at the tail to help with the added resistance of the sharp cone angle.

[PyroJoe]

Using Erics calculator, with the matching tail diameters shows about the same:
CC diameter was taken from the valve grid parameter of the calculator.

[WebPilot]

Joe, you're all over the place. I see you're not the only one who thought what I thought.

I don't see a combustion chamber ... er, I guess I should say "your cc is too small."

Nice work otherwise.

I might add that pulse-jet literature suggests a jet nozzle length,

L jet noz = (0.8 to 1.5) × D

where D is the outside diameter of the combustion chamber.

I suspect at a 3" diameter exhaust, petal valves are going to be too restrictive in order to develop MAX thrust.

We're going to find out if his present valve forward resistance is too great in order to establish a limit cycle.

[motthomas]

I didnt get any testing done yesterday after all because I got a bit held up machining the new fuel jet extension and ran out of lab time. But everything is set up now so I can run a test 1st thing monday morning and then keep swapping things and re-testing all day long! Its going to be fun!

I realise my combustion chamber is a bit on the small side but I was under the impression that a pulsejet should be able to function as simply a straight pipe and that the reason for incorporating the wider "combustion chamber" was to allow the required valve area to fit? Hence the smaller combustion chamber diameters on engines that use a v-type valve grid. As for the length of the chamber, I wasnt able to find any correlation between chamber length and any operating parameter when I was doing my research so I just picked a length which represented approximately 20% of the engines volume since Paul Schmidt observed that that was the approximate volume of fresh gases which entered the engine during the intake phase. The steeper reducing cone is merely to ease the machining process since the CC and reducing cone were machined as one part and welded to the tailpipe.

Out of curiosity, has anybody ever made a jet with a small CC similar to mine? I would love to play around with different chambers of different shapes and sizes but time restrictions apply. Plus, because I am writing an engineering report on all of this, any decision I make really needs to be backed up with something solid be it either maths, printed research or manufacturing constraints. If I wasnt building the engine for university I would have more freedom to test out things like this.

Oh yeah... I almost forgot. I eventually got around to uploading a video of the jet on youtube:
http://www.youtube.com/watch?v=QqxhxJyuN3s
As ye can see the engine runs abysmally!

[motthomas]

SUCCESS!!! Well in my book its success anyway. I tested the engine using 0.010" valves and it fired first time! Its pulsing properly but it will not sustain. The longest run I have got from it was 15 seconds before flames shoot out the intake and it stops. Also it will not sustain without an air supply.

I suspect the problem is still with the valves. According to my calculations, 0.010" valves still do not have a high enough natural frequency but I used them anyway because it was all I had today. I actually ran a test earlier this morning with the new retainer plate and new modified fuel jet but still using the 0.006" valve because they were set up in it since friday evening so it was just as easy to just run a test with them and see what happened. What happened was a continuous flame coming out the intake which I think proves your theory, Forrest. When the valve is not stiff enough it will be wide open during the combustion stage and you get blowback.

Im really happy with this result but I have a few problems to iron out before the end of the week.

And a video of todays test: http://www.youtube.com/watch?v=Ts9gCQko9hI
This video is of a second test I did with the 0.010" valves after it started the first time. This is where I attempt to cut the air supply and try throttling it.

[WebPilot]

What happened was a continuous flame coming out the intake which I think proves your theory, Forrest. When the valve is not stiff enough it will be wide open during the combustion stage and you get blowback.

  

[Rocket Man]

You can change the frequency of the reed valves just by changing the reed valve retainer. If you enlarge the flat area of the reed valve retainer the parts that touched the reed valves to a larger diameter it effectively makes the reed valves shorter so the frequency goes up.

[WebPilot]

...
You're going to have to make some valve retainers, or just washers since you're pressed for time, of varying OD's , in order to shorten the free length of each petal valve thus increasing its frequency, or vice-versa.

You must have read my post on previous page, here.