jam jar jet from pvc cement can

thespeedycicada · Wed Sep 05, 2007 10:05 pm

the primer can would work but it may only last a short amount of time like the glass one but with no breaking of the glass.The reason for this is that metal conducts heat very well which disrupts the cycle (hot gas and air blown out,some of the flame and air sucked in alchohol fumes ignite and so on) If you like pulse jets try making a real one you can make a simple lockwood hiller type like i did with steel pipe from the hardware store its very loud and produces quite a bit of thrust using propane as the fuel then make a reed valved one to get serious thrust.By the way the proper name for the jam jar jet is a ryenst pot (after who created it) Well at least thats what they call it here. http://www.pulse-jets.com/

jimmy101 · Thu Sep 06, 2007 10:22 am

Insomniac wrote:...If sucking air generated any appreciable thrust, harrier jump jets wouldn't work because the intakes still face foward while the outlets pivot, which would cause the jet to go flying foward while trying to hover.

A harrier does indeed drift forward because of the thrust generated at the air intakes. To offset that thrust the engine outlets are pointed forward slightly when the pilot wants to hover in place.

For every action there is an equal and opposite reaction. Sucking air into the engine has to create thrust. How much thrusts depends largely on the diameter of the intake, but since the intake is not gigantic, the thrust from the intakes is significant.

Insomniac · Fri Sep 07, 2007 1:18 am

Yes, but think of this. If the sucking air was equal to the thrust, then it would be impossible to hover, because the exhaust pipes would need to face the same way as the intakes. Heres how I'm guessing it works.

With the outlets, most of the gas goes straight out the back of the engine, and very little blows sideways, because of the momentum of the air. On the intakes however, there is a vacuum pulling air from all directions, inculding the air to the side of the intake, which means very little thrust is generated.

Also, try this. Get a bendy straw, and bend it at a 90 degree angle. Put the long end in your mouth and blow. The straw will try and move sideways. However, if you suck on the straw no noticeable thrust is generated.

Here's a pic of what I think happens with an intake compared to an outlet.

meatballs · Fri Sep 07, 2007 5:35 am

i like that insomniac, thats a cool way to look at it. if i did use a primer can does anyone have any idea what size hole i should use? and whats a good way to get the brushy thing off the cap? i can break most off with pliers, but the part thats coiled flat against the cap is annoying, i can't get it with pliers. and it doesn't respond to solvents (surprise surprise) so what should i do?

Insomniac · Fri Sep 07, 2007 5:42 am

I dunno. I tried making one of these a while ago by getting a tin can, drilling a hole in the top and emptying all the stuff inside out, rinsing it and then putting a little methylated spirits in the bottom and lighting it. I tried to get it to work for ages, but somthing just wasn't right. I even got it to go psssht! when it ignited, but it just wouln't start to cycle. Dunno what was wrong, but I'm guessing it was the fuel not evaporating quick enough, or the combustion not generating a strong enough vacuum.

jimmy101 · Fri Sep 07, 2007 11:27 am

Insomniac: You've got the basic idea but the difference in thrust is based on the difference in velocity of the two air flows.

The air intake is usually larger than the exhaust (but it doesn't have to be).
The air intake speed is slower than the exhaust speed, as a result the density of the intake and exhaust gases is different (exhaust has lower density).
The force created by the two thrusts are proportional to the square of the flow velocities. (Mass would normally be in the force equation as well but all the mass that enters the engine also exits so mass cancels. Well almost, there is a small amount of additional mass exiting because of the injected fuel but that is a very small percentage of the total mass.)

Result, it is possible to get net thrust even if the intake and exhaust are pointing in the same direction. (Of course, you get more thurst if the two vectors are working together instead of against each other.)

ALIHISGREAT · Fri Sep 07, 2007 3:15 pm

Also, try this. Get a bendy straw, and bend it at a 90 degree angle. Put the long end in your mouth and blow. The straw will try and move sideways. However, if you suck on the straw no noticeable thrust is generated.

yes but a straw would not have much of an effect, but a huge rolls-royce pegasus engine that sucks in ridiculous amounts of air would have an effect, but it still wouldn't be much "thrust". i think it would work kinda like a propeller does on an airplane.

paaiyan · Fri Sep 07, 2007 3:23 pm

ALIHISGREAT wrote:
Also, try this. Get a bendy straw, and bend it at a 90 degree angle. Put the long end in your mouth and blow. The straw will try and move sideways. However, if you suck on the straw no noticeable thrust is generated.
yes but a straw would not have much of an effect, but a huge rolls-royce pegasus engine that sucks in ridiculous amounts of air would have an effect, but it still wouldn't be much "thrust". i think it would work kinda like a propeller does on an airplane.

I may be mistaken, but I believe that humans can exhale air with more force than they can take it in.

ALIHISGREAT · Fri Sep 07, 2007 3:28 pm

paaiyan wrote:
ALIHISGREAT wrote:
Also, try this. Get a bendy straw, and bend it at a 90 degree angle. Put the long end in your mouth and blow. The straw will try and move sideways. However, if you suck on the straw no noticeable thrust is generated.
yes but a straw would not have much of an effect, but a huge rolls-royce pegasus engine that sucks in ridiculous amounts of air would have an effect, but it still wouldn't be much "thrust". i think it would work kinda like a propeller does on an airplane.
I may be mistaken, but I believe that humans can exhale air with more force than they can take it in.

oh yer didn't think of that :thumbup:

Insomniac · Fri Sep 07, 2007 8:16 pm

Yeah, but even blowing very gently generates more thrust than inhaling as hard as possible. And with aircraft props, intake and exaust are so close together its hard to tell the difference between which is making more thrust, because when you think about it, where on the prop blade is it counted as intake, and where is it counted as exhaust?

meatballs · Fri Sep 07, 2007 11:20 pm

ok how about this. imagine a shop vac that can blow as well as suck. if you put the shop vac on the vacuum setting then you dont feel the hose being pulled foreward. but if you have the thing blow then you can feel the hose pushing back against your hands.

blowing creates more thrust than sucking.