Perfecting the Air Cannon | Project Woodwind

My brother and I have decided to explore new designs for air cannons. We've built plenty, but we've never settled on the best design. Project Woodwind is our first look at optimization and starts with the barrel to tank ratio.
This experiment wasn't perfect but it gives us a starting point for future designs.
Build: (0:00)
Testing: (1:09)
Trials: (1:42)
Results: (2:09)
Destroying Stuff: (2:26)

[@scottmcglashan5001]

So would you say a barrel to tank ratio of around 4:1 is ideal. Just trying to make sense of your results. Great video though

[@ConHathy]

We saw the fastest speeds between 25-30% barrel to tank, meaning 1:4 or 1:3 where the barrel is shorter than the tank. I agree this seems way too small but it's just because of this particular cannon. There was a lot of friction between the projectile and the barrel even after we shaved some size off of it and added lubricant. Most cannons are probably using valves and loose fitting projectiles with less friction. This allows for longer barrels to be used. So does increasing the tank pressure, because it helps overcome friction and means there is less pressure drop throughout the barrel. This might apply to potato cannons where it's snug in the barrel but that's about it.
We want to run it again at some point with a better projectile and different types of valves to get more realistic results.

[@jonathanmillner]

Barrel is too small. There is a lot of air power being wasted with just not having a long enough barrel. Built one for my senior high school project. Had a 6" reservoir that was about 30" long attached to a 12 foot barrel. Our barrel was 1" diameter instead of 1.5". We sawed the end off of a 1 inch bolt you can by in any store. All of a sudden we had a metal projectile that weighed over a pound, that fit perfectly in the barrel. We fired it off at home plate on the baseball diamond and it went over the fence about another 150 feet. I suspect it left the barrel at about 300mph. It was pretty impressive. If shot at the front of a pickup, at near point blank range, it'd definitely have destroyed it. Anyways, longer barrel. Before we had a 12 foot, we tried an 6 foot, but instead of being able to fire 500 feet, we only got about 100 feet out of it. It's not anywhere near the same. So much of it is about the barrel and using every last bit of energy. For what it's worth...

[@ConHathy]

You’re right for most canons, the main reason our optimal ratio is so low is the amount of friction. It was a very tight fitting rubber stopper so you can’t have a very long barrel without the pressure being overcome by friction. This is just a side effect trying to make a valveless cannon with variable barrel length

[@jonathanmillner]

@@ConHathy Ya, it looked like you might have a munitions issue. Things that are cut very precisely... many metal things... wood things... they make great ammo because they fit perfect without a lot of friction problems. None the less, cool, right?