Help me design a super port

[AndonD454 10+ year member]

i just got a new assignment for physics.. were supposed to design soem type of mechanical air cannon.. and you can imagine what i thought of when he suggested using a box with a hole in it. the object is to get the biggest,. most concentrated puff of air across the room to knock over paper turkies.

i know that a lot of you have good knowledge of port velocities and moving air, so i figured i would get any help i could from you.

he said most people use a box with a hole in the front. but the back is a flexible plastic-like material with a handle in the middle. you pull the handle and the elactic straps inside the box, and create tension which is released when you let go, thrusting this plastic membrane back towards the box quickly. the air that was sucked in the box while pulling back on the handle is then forced quickly out of that hole, creating a gust of air which travels in the direction the hole was pointing.

NOW, how could i incorporate a port to increase distance, accurate, and velocity?

im thinking abotu using PVC .. maybe 3".. on the outside of the box from the hole out? and maybe building the inside of the box to force as much of the gust into the port hole as possible? would this give me an advantage over somebody who just uses a hole?

what would be the best size for the highest velocity? and what abotu length? does a longer port give the air coming out more distance or accuracy?

thanks for help guys

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[swimfreak26 10+ year member]

The smaller you make the "pipe" the higher your velocity will be since you're trying to fit the same amount of air through a smaller volume in the same amount of time. Draw back is that you lose some of the, we'll call it "shotgun effect," as you'll also be shooting a more direct column of air.

 

[skmfkr 5,000+ posts]

just buy one lol they got them at spencers

 

[AndonD454 10+ year member]

swimfreak.. if i use a smaller pipe will the air start to get choked off at any point (like if i use 2" pvc)? will all the same air come out as if i were to use a larger diameter? or will the same amount of air come out, just in a longer, narrower form?

i liek the shotgun effect idea, so i would liek to make the diameter as big as i can, but if i cant get a good amount of force at 5 meters then it will be useless. what would eb a good size to use as a compromise?

also, would it be better to have the box get narrower into the tube, or will it matter? for exapmle.. if i used a large cone, with a 4" hole at the end, would it help direct more air out, or would it just be better to use a long straight 4" pipe?

and skmfkr, i wish i could just use one of those cannon things, my friend has one. thats abotu the amount of air power im looking for, but we have to make it ourselves. maybe i could create a larger-scale version of that?

 

[Heresy 10+ year member]

The smaller/longer the port, the more you have to worry about viscosity and the friction of the port walls. Eddies start to develop which will reduce the force of the air that is being outputted. There was a paper at the AES convention in San Francisco last year that suggested having a port flared at a 45 degree angle to reduce the eddies; however, that the paper was concerning SPL vs audible distortion, I think. Still, it might be useful to know.

 

[AznRevlAzn 10+ year member]

make a milled barrel.have he back part (about 30-40% of your pip) with a slightly smaller bore size. Throw in a 4500-5000psi carbon fiber wound tank and your ready >//content.invisioncic.com/y282845/emoticons/biggrin.gif.d71a5d36fcbab170f2364c9f2e3946cb.gif

 

[AznRevlAzn 10+ year member]

oh yeah, have all your bores polished or coated with teflon, or even better Nylatron) //content.invisioncic.com/y282845/emoticons/biggrin.gif.d71a5d36fcbab170f2364c9f2e3946cb.gif

 

[swimfreak26 10+ year member]

I don't think he's going to have to worry too heavily about eddies in this application, it won't slow down the air enough unless he chooses a really small pipe diameter, with substantial length.

Umm, as far as what to use I would assume that's part of the assignment, figuring out the physics behind it and determining which is best. If you're not ready to do those calculations (or attempt to), just do trial and error with a homeade setup.

 

[AllStar1500bd 5,000+ posts]

You should see the mystbuster's episode that they did this on. They launched turkeys into airplane windshields.

 

[AllStar1500bd 5,000+ posts]

oh no 666 posts... had to double post to get off that EVIL number, sorry I did it in your thread.

 

[XtrmAudioCncpts 10+ year member]

It was actually raw and frozen chickens

 

[AndonD454 10+ year member]

thanks swimfreak ill mess around with a couple designs and see how they do