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bernoulli's principle

A place to ask general spud cannon related questions.
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Unread postAuthor: jackssmirkingrevenge » Tue Nov 20, 2007 12:03 pm

But the entire point of a hopup is to trade power (kinetic energy, velocity) for accuracy (reproducibility). Same with a rifled barrel, some of the energy in the propellant is used to spin the round, around the axis of flight for a rifled barrel. For a hopup the round is spun perpendicular (usually about the x-axis). In both cases there is less KE in the round, but the round is much more accurate.


The way I understand it, the point of hop-up is to give the projectile an upward spin in order to give it a trajectory that curves upwards, in order to compensate for BB drop - which in the case of airsoft projectiles with poor ballistic coefficients and relatively low velocities, is very pronounced even at short range. This is the reason why some hop-ups can be adjusted, in order to match the amount of upward curve required to match the drop of a particular BB weight at a particular velocity. This is completely different in concept to the spin given by conventional rifling along the axis of flight which gives gyroscopic stability with the additional benefit of preventing elongated projectiles from tipping end over end.

It makes more sense to me to do away with hop-up entirely and just know how much your projectile will drop from your point of aim at a given range, and compensate accordingly.
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Unread postAuthor: al-xg » Tue Nov 20, 2007 1:24 pm

BBs will spin slightly anyway, having some sort of hop up at least makes them spin in the same direction, thats what i found with the srafer: range was very slightly increased but the groupings were alot smaller (especialy at higher pressures and with light BBs).
Without the hop up the BBs would go off course alot sooner.
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Unread postAuthor: jimmy101 » Tue Nov 20, 2007 3:51 pm

Like al-xg said, the hopup does a lot more than just counter the normal drop of the projectile.

All round projectiles exiting a round barrel will have some spin. Spin, in and of itself, is not a problem. What is a problem is that each shot will have a different spin, both in direction and magnitude. That means each round will curve in a different direction.

The drop of the round is really pretty irrelevant to the accuracy of the round. All rounds drop, everything from an airsoft to a 50cal drops. An experienced shooter has zero problems accounting for the drop. An experienced shooter can not correct for varying spin of individual rounds. The best marksman in the world can do absolutely nothing about spread caused by varying spins on his ammo. Hence, the marksman will pick a round and a barrel that gives the exact same spin to every round. For a round round (!?) it doesn't really matter how the round is spun, along the axis of flight or perpendicular to it. As long as the spin is consistent an experienced shooter can hit what they are aiming at.
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Unread postAuthor: ALIHISGREAT » Tue Nov 20, 2007 4:08 pm

jimmy101 wrote:Like al-xg said, the hopup does a lot more than just counter the normal drop of the projectile.

All round projectiles exiting a round barrel will have some spin. Spin, in and of itself, is not a problem. What is a problem is that each shot will have a different spin, both in direction and magnitude. That means each round will curve in a different direction.

The drop of the round is really pretty irrelevant to the accuracy of the round. All rounds drop, everything from an airsoft to a 50cal drops. An experienced shooter has zero problems accounting for the drop. An experienced shooter can not correct for varying spin of individual rounds. The best marksman in the world can do absolutely nothing about spread caused by varying spins on his ammo. Hence, the marksman will pick a round and a barrel that gives the exact same spin to every round. For a round round (!?) it doesn't really matter how the round is spun, along the axis of flight or perpendicular to it. As long as the spin is consistent an experienced shooter can hit what they are aiming at.


i have to disagree the drop of a round is relevent because it becomes more important at range which i'm guessing is what joemama wants or he wouldn't be asking about bernoulli's principle. However i do agree that it helps with accuracy as well for the same reason you stated.

The one problem i can see with trying to incorperate hop-up into a cannon design is tuning, you don't want your projectile flying up a mile when it leaves the barrel or having no power, the one way i can see around this is either to buy a commercial hop-up unit if you are using a 6mm airsoft barrel or to replicate a commercial hop-up unit for a larger bore.

also what are you planning on using hop-up on?
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Unread postAuthor: jackssmirkingrevenge » Tue Nov 20, 2007 10:59 pm

For a round round (!?) it doesn't really matter how the round is spun, along the axis of flight or perpendicular to it.


Fair enough, valid observations - but the concern in airsoft seems more to do with compensating for drop than gyroscopic stability, hence the upward curve.
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Unread postAuthor: jimmy101 » Wed Nov 21, 2007 12:31 pm

jackssmirkingrevenge wrote:
For a round round (!?) it doesn't really matter how the round is spun, along the axis of flight or perpendicular to it.


Fair enough, valid observations - but the concern in airsoft seems more to do with compensating for drop than gyroscopic stability, hence the upward curve.


I don't think so. Think about it, the hopup can be positioned to give spin in any direction perpendicular to the line of flight. Give the round left to right spin and it will curve left (or is it right?). Give it bottom spin and it will sink even more than it would because of gravity alone. Give it back spin and it will "rise" (actually it just falls a slight bit slower). Now, which makes more sense in terms of hitting something? Back spin and front spin both kill the lateral curvature. Back spin increases range a bit (which is good of course) but the key to hitting anything, at any range, is reproducibility of the flght of the ammo.

So, I would say that reproducibility is the goal of the hopup. A slight increase in range is an added benefit but a hopup would still be a good idea if it had zero affect on the range.

YMMV
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