Couple of questions (basic)

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Ratsniff
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Couple of questions (basic)

Post by Ratsniff » Fri Jul 30, 2004 11:24 am

Well, Ive always wanted to build a pulse jet and only recently have I really done research into them. What I intend on doing is building a smallish pulse jet that produces around 15 lbs of static thrust. I even ordered Bruce Simpson's CD (4 weeks ago) but by the sounds of things, I'm not going to be seeing that.

I do have some questions that I cant seem to find the answers for.

Say the valve thingy has 18 holes all the way around it, does that mean the petal has 18 petals on it, and each petal covers a hole?

How far should the reed be able to open? I couldnt imagine it being any more than 10mm.

Going by Simpson's calculator, the exhaust pipe would be around 640mm long. Is this long enough for the pulse jet to run without the aid of compressed air?

Im not actually too sure about that last question but I think I read somewhere that if it is too short or something it wont work unassisted...But then again, I might not have.

Stephen H
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Post by Stephen H » Fri Jul 30, 2004 11:45 am

Say the valve thingy has 18 holes all the way around it, does that mean the petal has 18 petals on it, and each petal covers a hole?
yes!
How far should the reed be able to open? I couldnt imagine it being any more than 10mm.
shouldnt be far, make the valve stopper to the plans
Going by Simpson's calculator, the exhaust pipe would be around 640mm long. Is this long enough for the pulse jet to run without the aid of compressed air?
last time i checked his calculator it was only the early version and not to go by it. try use a plan already availible. there are lots to download on this site http://www.pulse-jets.com
Im not actually too sure about that last question but I think I read somewhere that if it is too short or something it wont work unassisted...But then again, I might not have.
your right. if its to long or to short it will not run!

hope this has helped and welcome to the forum
Stephen

Ratsniff
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Post by Ratsniff » Fri Jul 30, 2004 12:20 pm

Ok, I think I the "Didgeridoo" pulse jet plan is looking like the one I will construct. I have no idea how much thrust it will produce though.

edited the dumb grammar

Stephen H
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Post by Stephen H » Fri Jul 30, 2004 1:03 pm

from memory it is about 4-5 pounds. a easy pjet to build though!

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Post by skyfrog » Fri Jul 30, 2004 1:37 pm

Sorry I intercept, since my question is quite basic too.

After jankyard shopping, I've got a combustion chamber which diameter (D = 6.0cm ) is 3 times bigger than that of a exhaust pipe (d = 2.0 cm), will this combination end up a working pulsejet ?

Eric's program and many other designs indicate that the diameter for my combustion chamber should be around 4.0cm.
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Post by steve » Fri Jul 30, 2004 8:47 pm

skyfrog wrote:Sorry I intercept, since my question is quite basic too.

After jankyard shopping, I've got a combustion chamber which diameter (D = 6.0cm ) is 3 times bigger than that of a exhaust pipe (d = 2.0 cm), will this combination end up a working pulsejet ?

Eric's program and many other designs indicate that the diameter for my combustion chamber should be around 4.0cm.
keep in mind that CC diameter is nothing more then a product of the inefficient use of valve area in most engines. This is why it is possible to have an engine with no defined CC as long as the valve setup has near perfect efficiency. (there is a picture of such an engine somewhere around here) for this reason it is also possible to have an oversize CC as long as the valve area/efficiency is made the same as with the 4.0cm dia as shown on the calculator. However, you cannot make the CC too large or it will start to affect the operation of the engine and possibly prevent it from running altogether. A CC that is only 2cm oversize is probably still small enough that the engine will run (with a lower amount of thrust, though) but I can't promise anything.
Last edited by steve on Sat Jul 31, 2004 3:11 am, edited 1 time in total.
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Ratsniff
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Post by Ratsniff » Sat Jul 31, 2004 1:10 am

Stephen H wrote:from memory it is about 4-5 pounds. a easy pjet to build though!
Yeah, I made some (crude) calculations to get somewhere around that. The (English) plans on this site seem to be mostly for models, with not much thrust. does anyone know of any plans that are around 15-20 lbs?

With the fuel system, where do I get a "fuel jet" from? Im talking about the nozel bit that has a knob on the side to let in the amounts of fuel.

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Post by skyfrog » Sat Jul 31, 2004 6:41 am

steve wrote:[A CC that is only 2cm oversize is probably still small enough that the engine will run (with a lower amount of thrust, though) but I can't promise anything.
Thanks for your info. I think for my first pulsejet, I had better follow existing design. So I went junkyard shopping again, and this time I've got a bigger exhaust pipe, perfectly suits my combustion chamber.
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Post by steve » Sat Jul 31, 2004 6:38 pm

great to hear it! BTW, how will you be making the cone to connect the two pipes?
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Post by Mark » Sat Jul 31, 2004 11:40 pm

A mere formality. A small detail. Or maybe not.
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Post by skyfrog » Sun Aug 01, 2004 2:14 am

steve wrote:great to hear it! BTW, how will you be making the cone to connect the two pipes?
I can't find any suitable reducers for it. So I'll make this cone by sheet metal (0.8mm thick) and by TIG, a little bit costly this time. When taking its shape, I'll get some picures and have it posted, if people here doesn't mind my blowing the website's hard drive a little bit fatter.

Sunday welding day !
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Post by skyfrog » Sun Aug 01, 2004 9:17 am

Ok, this is the completed welded tailpipe. I think this engine, if works, can deliver a thrust ranging from 2.5kg to 3kg. I regret that the materials from the junkyard wasn't 100% utilized ( I mean the larger pipe originally planned for making C.C.), causing wastes.

Tailpipe Length = 56 cm
Tailpipe Weight = 0.5 kg

Next step : Valving
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Post by Mark » Sun Aug 01, 2004 11:25 am

Looks like it's coming along fine.
Mark

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