bernoulli's principle
Posted: Sun Nov 18, 2007 5:22 pm
are projectiles subject to bernoulli's principle if tehy are rounded? if they are isent there a way to make a bullet that has more resistance to projectile moton using lift?
9.85m2 8)VH_man wrote:yes, everything drops at 9.85 meters per second per second, not taking into account drag.
Los Frijoles wrote:It is well known that rifling or putting a roll axis spin on the projectile increases accuracy and sometimes range, but it has been observed that unless the projectile has fins or wings or something, the time from when it is fired to when it hits the ground is equal to the time if it was dropped from the same height it was fired.
So, if you want some lift, put wings on it (wings have a slight tendency to get ripped off during launch if they are not secured very well).
actually that would be backspin, top spin would tend to make the projectile curve towards the ground faster.VH_man wrote:you can also give the projectile topspin if its spherical. this spin will actually work like an airplane wing and give it lift... aka why my airsoft gun can shoot an airsoft pellet 150 feet at 300 FPS...........
Hop-up & Bernoulli's principle
The term Hop-up describes the back-spin put on airsoft pellets and BB's to increase their range and (vertical) accuracy via Bernoulli's principle. Hop-up acts somewhat like the rifling on the barrel of a firearm, but without the increase in horizontal accuracy. Also, being light-weight, airsoft pellets are still affected by wind when fired.
Airsoft hop-up devices apply a backspin to the pellet so that the pressure force acts on the pellet opposite to the direction that gravity is pulling it. This causes the pellet to fall less over a given distance than it would without the spin applied to it.
In airsoft guns this is often implemented as a rubber piece at the rear of the barrel that is thicker at the top of the barrel than the bottom. As the pellet moves past this piece it tends to roll, inducing a backspin. This is occasionally adjustable so that the effect can be tuned to suit the weight or speed of the pellet, and each player's preference.
Bernoulli's principle as applied to an airsoft pellet is as follows. As a spinless spherical pellet flies along its trajectory through the air, the pressures on all sides of the pellet are equal because the air is traveling the same velocity relative to the surface of the pellet. If a spin is applied to the pellet about an axis perpendicular to the velocity vector (for example a backspin) the air will be rushing slower (relative to the pellet surface) on the side that is spinning away from the velocity vector and faster on the side that is spinning towards the velocity vector. Bernoulli's principle says this difference in fluid velocity implies a difference in pressures, which is a force that will cause the pellet to move in a direction perpendicular to the velocity vector.
Another cause of the apparent lift on an airsoft pellet is the Magnus effect. There is a layer of non-moving air on the surface of the pellet (boundary layer). This is why a golf ball has dimples; this layer acts like ball bearings. In the case of a spinning ball, this layer gets thrown off at an angle. Newton's laws say that in order for air to be thrown in one direction, the ball has to move in the other direction. According to the Magnus explanation, the rotating ball would throw air downward and to the rear, thus giving lift. The air on the bottom of the ball is slowed down, so when the separated air comes back together, it is lower than the middle of the ball, appearing like a comet's tail pointing down. This can be verified in wind tunnels and is very well documented in fluid dynamics textbooks.
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.jackssmirkingrevenge wrote:Personally I don't believe in hopup or backspin - what rotational energy is going into spinning the ball is lost from the kinetic energy of the ball's forward motion, which adds up to lack of efficiency which I despise.