Discussion in 'General Gun Discussions' started by JellyJar, Jul 19, 2016.
I'm scratching my head here,,,
I'm scratching my head here,,,
How can a symmetrically shaped object generate lift.
There has to be a difference in air-speed over the surface,,,
To create an air pressure difference (Bernouli effect).
I'm not saying it can't,,,
But with my small knowledge of kinematics,,,
I just can't see how it could possibly generate any lift.
And, it's rotating fairly quickly,,,
There would be no single force vector for direction of lift.
My SWAG would be for a "no" answer.
Now, to look up that Magnus Effect.
I don't think Magnus Effect applies,,,
The spin of a bullet is not at an angle to the direction to it's movement.
I don't think it applies.
I wonder if this may happen to some bullets.
Edit: pic was too big check here and you can see examples of symmetrical airfoils.
Problem with a bullet is that it is spinning so the "top" is only the "top" for a very tiny part of a second.
What can change is the ballistic coefficient as velocity changes.
Note, I corrected a lot of the spelling in the above quote, so I don't particularly trust it much. So the Magnus Effect is basically saying that the force exerted on a the air around a spinning object is going to create opposing forces on that spinning object, which can stabilize or destabilize a bullet, and thus change it's path slightly. That makes sense. But if this is occurring under a bullet, it would be occurring above the bullet as well, effectively nullifying any sort of aerodynamic lift effect. Essentially it could effect the path of the bullet ever so slightly, but gravity, the speed of the bullet, microscopic interactions between metal surface imperfections and air molecules, as well as temperature differentials in air, and the limits of optics accuracy would make this difficult to measure. Either way, I don't think calling it "lift" is appropriate. Given the right circumstances, it could send a slug into the ground just as easily as cause it to land a few feet further than intended.
I'm with Aarond
However, I'm just thinking of the logistics here, and am in no way a physicist or engineer.
Well, not exactly. Even for well stabilized projectiles, there is some yaw involved at times which does create an angle between the axis of spin and direction of the projectile's path. Yes, the angle is small, but it is there.
Again, not exactly. And once again the difference is very small, but it exists. Depending on the wind that the bullet is flying through, the sum of the spin related airflow and the relative wind airflow will not be the same on the top and bottom of the bullet. It will be faster on top than on the bottom, or vice versa. This airflow speed difference will result in a pressure differential which will cause positive or negative lift depending on wind direction relative to the bullet's path.
Here is a classic discussion of "How Bullets Fly" that explains both affects much better than my feeble attempts.
I have experienced this effect first hand, and it can be quite significant, but I am not sure if it can technically considered "lift" as the reason for the jump (or negative jump depending on projectile rotation and relative wind) is the impact of the wind from the side of the projectile manifesting 90 degrees later in rotation due to gyroscopic precession. Lift is a result of a pressure differential unless there are other definitions. Enough of my babble. Here is the excerpt.
"3) Projectile jump (vertical plane gyroscopic effect).
(a) When a crew fires a weapon from a helicopter in flight and the weapon's muzzle is pointing in any direction other than into the helicopter's relative wind, the projectile will experience projectile jump. Projectile jump begins when the projectile experiences an initial yaw as it leaves the muzzle. The yaw is in the same direction as the projectile's direction of rotation. The jump occurs because of the precession (change in axis of rotation) induced by crosswind.
(b) The amount a projectile jumps is proportional to its initial yaw. Firing to the right produces a downward jump; firing to the left produces an upward jump. To compensate the gunner must aim slightly above a target on the right of a helicopter and slightly below a target on the left. The amount of compensation required increases as helicopter speed and angular deflection of the weapon increase. Compensation for projectile jump is not required when firing from a hover."
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platform that the gun is being fired from. A helicopter has multiple movement variations to consider.
Uh, no. What is the difference between a helicopter moving forward through the air while it's gun is aimed and firing off-axis or a sniper firing with a crosswind?
Wind effect is wind effect. The bullet doesn't know the difference. The dynamic forces acting upon it while it's spinning are the same.
All bullets on typical trajectories will be in the air longer than if they were to free fall, thanks to the green arrow (vertical component of drag force).
And in addition they do experience some very small lift.
I tried giving some a dose of viagra, but in testing it didn't seem to work all that well.
Again the dynamic effect of the wind, or more specifically wind drift, affects the round regardless of firing while on the ground or in the air.
Four types of ballistics affect a spin stabilized round fired from a helicopter; interior, exterior, aerial, and terminal. Wind drift is an external ballistic effect. Trajectory shift and Projectile jump are 2 of 3 types of aerial ballistics affecting that round.
Trajectory shift is the trajectory change of a round caused by transference of the helicopter's forward motion during an off axis shot. In turn, you end up leading or trailing the target depending on the direction you're firing (right or left of the centerline axis).
Projectile jump, on the other hand, is a result of the round initially yawing into a relative wind moving in a direction across the spinning bullet's flight path, similar to a crosswind. Whether the bullet jumps up or down, depends on if the round is impacted by the wind from the left or the right of its flight path.
As USAav8r posted, any force imparted on a spinning object is manifested 90 degrees from that point in the rotation. If you fire a long range shot, while on the ground, with a high crosswind nearer to the target, that projectile will react by climbing or descending due to projectile jump. These ballistic effects aren't lift.
A bullet doesn't produce lift. It simply reacts to the various ballistic forces affecting it from firing to impact.
eta: In order for a bullet to create lift you must have a pressure differential with a lower pressure area above it and a higher pressure area below it. This combined with the forward velocity will create lift. If these pressure areas are reversed, you'll create negative lift.
Read my second post. That momentum is called trajectory shift.
Yes, bullets develop lift.
That is the reason they have to be spun to stabilize them. The location of the center of pressure (the point where the 'lift' acts) is in front of the center of gravity, so you have an overturning moment.
However the amount of lift relative to the weight is so small it does not "keep them in the air longer".
And yes, bullets do see asymmetric air flow in both the X axis (direction of flight), and the radial axis. They yaw considerably initially up to 6 or 7 degrees depending on many factors and the precession of the nose never fully damps out. Some are much better and damping out the yaw than others. And even if they totally do damp out the yaw, there is the velocity vector shown in the image posted by Goosey a few post above.
Yaw - 224" dia, 55 gr, 5.5 cal ogive, 9 degree BT:
Yaw - .308" dia, 147 gr, 10 cal secant ogive, 9 degree BT:
The Magnus effect is classed as a "lift" force, and it is caused by asymmetric airflow.
The tendency for the nose of the bullet to remain at (or near) the launch angle is a phenomenon known as "tractability" and can occur if the spin is too high, although normally only a problem at high launch angles.
If you are truly interested in the physics of exterior ballistics, you should read all of this
Let's pretend we have an airplane wing covered with fabric. We'll also pretend that for no good reason the designer made is so the fabric could move around the long axis of the wing. You could spin said fabric as much as you liked and the wing would still produce lift even if the top was there for a fraction of a second.
Bullets aren't spun to counteract the effects of lift. The location of the center of pressure under and slightly to the rear of the leading edge of a symetrical airfoil (like a helicopter blade) is what produces lift. Also it's the center of lift and not the center of gravity that would determine in which direction an "overturning moment" would be.
So drag will keep a bullet in the air longer? LOL. And, pray tell, in which direction is the lift relative to the ground? Up? Down? Sideways?
An airplane's wing doesn't generate lift by making air travel faster on top than below. It does it by forcing air at the back of the wing to flow downward (Newton' "equal and opposite" reaction, rather than Bernoulli's principle).
Draw what conclusions you may.
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