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Slap it or pull it?

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Unread postAuthor: jrrdw » Wed May 12, 2010 4:37 pm

Gun Freak wrote:There is a design in boat motors that uses a propellor under the bow and pulls the boat along, supposedly being more efficient.


I think that would be a 'impeller' and are all ready widely used on jet ski's/wave runner's...
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When life gives you lemons...throw them back they suck!
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Unread postAuthor: Ragnarok » Wed May 12, 2010 5:30 pm

Technician1002 wrote:Again, this is used to eliminate slow startup of my valve.

Um, not to be pedantic*, but that's not really true, is it?

Sure, it might be a beneficial side effect, but I know you gave included that distance so that the piston had distance to accelerate into where it wouldn't hit (/break off) the "reset pin" on the trigger rod.

*Okay, I am being pedantic.

Gun Freak wrote:There is a design in boat motors that uses a propellor under the bow and pulls the boat along, supposedly being more efficient.

From some view points, such a thing is an improvement. Take a boat in a relatively shallow channel.

If the propeller is at the back, it's pulling water from under the boat, forcing it to sit lower in the channel, increasing fluid drag.
Putting the propeller at the front will push water under the boat, lifting it up and reducing fluid drag.

However, in a sufficiently deep channel, such effects become unimportant.
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Does that thing kinda look like a big cat to you?
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Unread postAuthor: iknowmy3tables » Wed May 12, 2010 7:14 pm

Image
Spring vs slingshot rubber? spring definitely, with a catch mechanism the time for the force handle to go form not-applies to being applied is instantaneous, it'll be just as fast as the sling shot rubber, the only difference is that the sling shot rubber has some momentum that gets transferred to the ball valve handle, however there's hardly any momentum because the sling have very little mass.

you might be thinking that nearly all the potential energy in the stretched rubber is going to be applied onto the handle almost instantaneously, but this isn't true. You are dealing with a inelastic collision where energy is not conserved. People make that mistake all the time, even the mythbusters. In fact this collision involves a very light very fast object colliding with a much larger more resistant to move object so I would say only a small fraction of the energy is going to be conserved.

so that collision doesn't help very much and only the elastic force remains to open the ball valve, so in my imagination of a comparable strand of slingshot rubber and spring I would say the spring would be better

EDIT: I missed the part about having a mass on the sling, but still energy is lost, and the collision can really wear out parts of the assembly so my answer remains the same.
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Unread postAuthor: Technician1002 » Wed May 12, 2010 8:42 pm

Ragnarok wrote:
Technician1002 wrote:Again, this is used to eliminate slow startup of my valve.

Um, not to be pedantic*, but that's not really true, is it?

Sure, it might be a beneficial side effect, but I know you gave included that distance so that the piston had distance to accelerate into where it wouldn't hit (/break off) the "reset pin" on the trigger rod.


If you model a very close ratio piston valve in GGDT, you will see that when the pilot is vented, the force to open the valve at the beginning is very low and slows the valve as it creeps open before it cracks enough to pop open. This beneficial side effect was indeed considered in the design of the valve. Without a strike, the very narrow ratio valves can have >5ms from crack to pop. Getting past this slow leak position by having struck the valve changes the numbers drastically bypassing the long duration of the slow leak positions by having the piston up to a moderate speed when the valve cracks open. This changed the valve from ~5 ms from valve seal crack to full open and reduced it to <1ms.

This change permitted opening the valve fully before lightweight items could move very far in the barrel. A slow valve would have given the 4 inch poof foam ball a slow initial acceleration. Before the 4 inch foam ball launch was dropped from the official competition, the speed the valve cracked open was very important. We were getting almost 500 FPS on a 4 inch foam ball at only 60 PSI. A foam ball was busting out the bottom of poly buckets. This is directly due to the minimum crack to full open speed the valve provides.

Note the 3-4 inches of rope length between my foot and the breech. This is acceleration distance before the valve gets yanked open. This used to be much shorter.
Image
This improvement was built into the later designs with the rod. It killed two design items at once.

Due to the complexities of the valve dynamics, the primary design was to avoid hitting the reset pin. The speed the piston is moving when the air seal is broken is secondary and in the design. The engineers judging the papers noticed this design touch and loved it.

My very first QDV did not have this fast start and some kids would crack the valve and due to the tight o rings, the valve would hiss instead of popping open. This was fixed simply by adding a longer pull rope to the golf ball. Now when the cord is stepped on and the cannon is lifted, the lift provides considerable momentum before the rope comes taunt and yanks the valve open. I learned a lot from the first cannon. The sliding rod does in fact have 2 important design properties. One is the speed the rod is moving as it hits the piston.

Sometimes kids are gun shy and still slowly pick it up to pull the rope taunt gently. The difference in the launches is noticeable as a slow pull will often allow the projectile to have considerable movement in the barrel before the valve pops open. I get the kids to do a ready aim fire sequence to ensure they get the sudden lift and resulting cord yank on that cannon.

The floating o ring cannons tend to just pop, but with light projectiles and a light touch, it is possible to get a slow opening that is noticeable. A moderate pull speed is all that is required to get high performance.

Edit, the steel nut on the rod in the larger cannon is an elastic collision. :D The steel on HDPE is a bouncy collision. I don't mention it much as not too many people will notice the benifit. The rope pulled design is inelastic.

Found another photo showing the long cord between the piston and the golf ball. This length is to provide a good yank start. Photo is at the bottom.

I just visited the official contest website to verify this impact opening was mentioned. It is there.
The initial opening speed was considered to get the opening avalanche started faster. It was decided to decouple the trigger rod from the valve core to reduce moving mass further and permit an impact to start the core motion. This operation is much like a cue stick hitting a billiard ball for a short acceleration time.

https://inteltrailblazerschallenge.wikispaces.com/The+brag+zone
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Attachments
PICT238.JPG
Rope from golf ball, through the bumper, and through the piston with length left over.
PICT238a.JPG
Another photo showing the rope length on the ABS apple cannon.
PICT238a.JPG (23.48 KiB) Viewed 258 times
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Unread postAuthor: Brian the brain » Thu May 13, 2010 6:16 am

A nice discussion wich could go on for ever and ever....untill you simply try it.

Try both mechanisms and tell us which is best and why.
This is going nowhere.
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Gun Freak wrote:
Oh my friggin god stop being so awesome, that thing is pure kick ass. Most innovative and creative pneumatic that the files have ever come by!

Can't ask for a better compliment!!
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Unread postAuthor: iknowmy3tables » Thu May 13, 2010 6:38 am

but what spring would be comparable to what length of slingshot tubing? technically since sling-shot tubing can be stretched further it would have more energy potential than a similarly long spring, you can't do a controlled experiments if you don't know what variables to control
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Unread postAuthor: Technician1002 » Thu May 13, 2010 2:01 pm

Just a quick note on the boat thing.. This has been modeled a lot. A prop in the back puts the high speed prop wash behind the boat. As far as the hole goes, the boat is going downhill into it. When reversed, the flow along the hull from prop wash is much higher for more drag, creates a "mountain" of water you are trying to push the boat up onto, and creates turbulant water so steering and handeling becomes difficult. Take a decent displacement type hull such as a sailboat, install the prop backwards (to keep effeciency instead of running it backwards) and see how much slower this is. FYI, marine transmissions generally are 1:1 in both forward and reverse. Reverse is not a "Low Gear".

My model RC boats are much slower in reverse and difficult to maintain heading.
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Unread postAuthor: boyntonstu » Thu May 13, 2010 6:06 pm

Technician1002 wrote:Just a quick note on the boat thing.. This has been modeled a lot. A prop in the back puts the high speed prop wash behind the boat. As far as the hole goes, the boat is going downhill into it. When reversed, the flow along the hull from prop wash is much higher for more drag, creates a "mountain" of water you are trying to push the boat up onto, and creates turbulant water so steering and handeling becomes difficult. Take a decent displacement type hull such as a sailboat, install the prop backwards (to keep effeciency instead of running it backwards) and see how much slower this is. FYI, marine transmissions generally are 1:1 in both forward and reverse. Reverse is not a "Low Gear".

My model RC boats are much slower in reverse and difficult to maintain heading.


The longer the non-planing boat/ship is, the higher the maximum theoretical speed.
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