Testing the sound output of a Small Pulsejet

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RC Man
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Testing the sound output of a Small Pulsejet

Post by RC Man » Wed Oct 01, 2014 8:30 pm

I just got this one going, after a 1 year break, Hooked it up to the tank, lit it up with a Lighter, and ran it up to Full Throttle.
Loud little sob. Used my DB Meter to see just how-many Decibels it would put out, My meter is calibrated to a Police DB Meter, so I know it is Accurate.
This PJ is my first I ever built, using PyroJoes formula for Newbees, Built it with his Caliber Method and added a cone of about 20deg. to the CC, on the exhaust side.
Put a bigger intake pipe on it so I could change size and length to fit the power needed, Kind of. As I dont really know what Im doing quite yet with a PJ Motor.
The exhaust is at 39mm, and the rest according to PyroJoes Formula.
I placed a smaller, flared pipe into the Intake, about 8mm smaller, leaving the pipe end flush with the CC end, just as its supposed to be, and the flared end almost flush with the bigger intake pipe.Leaving a small space between the 2 Diameters.
Its hard to explain, but I think it did the trick, as now it starts without any air help, I just open the gas, light it, and off it goes.
Ill make a plan for how I did it later, I used a Flowbench to measure the backflow on the 2 pipes, as I thought I was getting to much blow back through the Intake, not letting it run without starting air. Guess I hit the right nail this time.

Vid of the DB Meter going wild, as it went over 120db, youll see it bouncing around at the end of the meter, LOL, but the sound was still increasing, I think its about 135-150db
(I have to buy a BIGGER DB Meter :x )

https://www.youtube.com/watch?v=nkhv5v5 ... e=youtu.be

I will be posting in here more often now, as I had troubles last year (Not the Noise Police either) LOL, and will have more time to endeavor into the realms of Pulse jets.

Frank
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RC Man
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Re: Testing the sound output of a Small Pulsejet

Post by RC Man » Thu Oct 02, 2014 6:51 am

Here is a makeshift drawing for the Intake I made, Of-course, all radians should be the same, difference between outer pipe ID. and intake OD. about 4-5mm.
It all depends on the ID. of your intake.
The radiused Intake pipe is than welded to the end of the outer pipe, leaving a chamber between the outer and inner pipe.
Backblow is reduced, and starting was easier. (For me anyway)

If you try it, Post your results here. I figure we all could use some help in getting a Pulsejet running, especially us beginners.
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metiz
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Re: Testing the sound output of a Small Pulsejet

Post by metiz » Thu Oct 02, 2014 3:47 pm

Put your dB meter at a distance large enough so that the measured volume <120dB. Then measure the exact distance in metres between the meter and the engine exhaust. That way you can acurately calculate the volume.
http://hyperphysics.phy-astr.gsu.edu/hb ... nvsqs.html
Quantify the world.

RC Man
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Re: Testing the sound output of a Small Pulsejet

Post by RC Man » Thu Oct 02, 2014 4:21 pm

I did that too, It pegs the needle at 20 meters also, Its LOUD!!
I have to do this in my garage, its 20 meters long, and I was at the other end.
My son said not much difference. I didnt check between the 20, cause If I run it that long, the neighbors cry.
This motor starts right up, COLD,surprised me cause I opened the bottle wide open, and it went full loud.
Then the chamber got red. That aint normal, is it? Or is this a real good first motor.

Frank

PyroJoe
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Re: Testing the sound output of a Small Pulsejet

Post by PyroJoe » Thu Oct 02, 2014 7:03 pm

That's a pretty good first motor, the combustion chamber sounds a little long (13mm maybe?).
The CC is supposed to glow, as well as half the tailpipe nearest to the chamber.

RC Man
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Re: Testing the sound output of a Small Pulsejet

Post by RC Man » Thu Oct 02, 2014 7:49 pm

Hey Joe,
How do you hear that the CC is long? The reason I ask is, because Ive only heard PJs on the Internet and never one in person.
Is there a special tone to them, cause, I think some sound different than others.
My cam was maybe a foot or 2 from the motor.

PyroJoe
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Re: Testing the sound output of a Small Pulsejet

Post by PyroJoe » Thu Oct 02, 2014 8:32 pm

There is alot of throttle from the 110dB up to about 120dB then you can start hearing the tail being driven above 120. I find it better to decrease the CC length so that it drives the tail sooner. It typically will lock in faster and can be pushed harder.

RC Man
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Re: Testing the sound output of a Small Pulsejet

Post by RC Man » Thu Oct 02, 2014 8:48 pm

Oh, Now I think I know where your coming from. I was just opening the gas flow real slow. I opened it real fast today, and it took right off to full throttle, valve about 3/4 open (I think)
The CC started to get red first, but the cone is from thinner material than the CC or tail pipe, so that was red first.
How I started it was, fire up the torch, aim it into the intake, open the gas, remove the torch, and open the gas valve. If I do it faster, it just goes right up to full power.
I made the vid right after the first sustained run, you remember your first time, bet you took it real slow too, :o :lol:
Ill do another vid, from a cold motor, to show yall what I mean. But Ill use my sparkplug next time.
I have a real simple setup for a sparkplug driver, just need a power transistor, a relay, a old psu from a pc and a coil with the plug wire. Im using a Accel Super Coil, Overkill, but it was just laying there all lonesome asking to be abused.

Oh, I just looked at the vid again, and think I know what ya mean, Ill need the higher pitched sound(like at 120db)?
But at a lower gas flow? Shorten the CC and that changes the expansion of the gasses to the tail pipe.

As I see it , these things work kinda like a 2stroke pipe, am I right?

Frank

RC Man
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Re: Testing the sound output of a Small Pulsejet

Post by RC Man » Thu Oct 02, 2014 9:23 pm

Heres how I modified the Intake on the Lockwood.
I think a pic does wonders, after this mod, it ran. I never had it so easy to start.
Might be a fluke, but it starts right up.

Maybe a vid tomorrow, will have to buy a coupla beers for my neighbor so they wont get mad at me for running it longer than a few seconds.
It will be worth it though, for me and Yall.
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