2 ms·

> As the bullet pushes through the air, the air pressure above and below it is increased (you can actually see that in the photo in the post) That's just a 2d

by rtkwe 2y ago

> As the bullet pushes through the air, the air pressure above and below it is increased (you can actually see that in the photo in the post) That's just a 2d slice/projection of the air density, the shock wave you see is actually conical and symmetric around the whole bullet it's just not possible to capture with schlieren photography. As far as I know the pressure above and below are basically identical the bullets are symmetric and until they're well subsonic (the transition between super and sub sonic is a lot of what defines the effective accurate range of a round because the transition is chaotic so it's hard to predict and account for it's path after transitioning) they're traveling so fast forward there's effectively no angle of attack to generate a differential on either side of the bullet. If it does generate a negligible amount of lift it's completely dominated by the force of gravity pulling it down.

tempestn 2y ago
Yes, of course the shock wave is conical; I just highlighted above and below because they're the only directions likely to influence how quickly the bullet falls. I don't think the effect would generate lift per se, assuming a horizontally fired bullet (since, again, it's symmetrical). What it does seem to me though is that it would effectively increase the density of air local to the bullet (in all directions except for behind), and so it would essentially be falling through denser air than it would be if just dropped from stationary. That would result in a small but non-zero increase in the time taken to fall the same distance. Again though, that's just what intuition tells me; I certainly haven't tried to model it or anything.
- rtkwe 2y ago
  The shock wave doesn't actually touch the bullet on the sides though so it doesn't provide meaningful lift. Look closely at the schlieren the high pressure region isn't against the sides of the bullet at all. Air is already quite dense at sea level equivalent to ~194 dB sound pressure so the sonic boom pictured which is 130-160 dB is not a significant increase in the density/pressure of the air.

by rtkwe 2y ago

tempestn 2y ago

Yes, of course the shock wave is conical; I just highlighted above and below because they're the only directions likely to influence how quickly the bullet falls. I don't think the effect would generate lift per se, assuming a horizontally fired bullet (since, again, it's symmetrical). What it does seem to me though is that it would effectively increase the density of air local to the bullet (in all directions except for behind), and so it would essentially be falling through denser air than it would be if just dropped from stationary. That would result in a small but non-zero increase in the time taken to fall the same distance. Again though, that's just what intuition tells me; I certainly haven't tried to model it or anything.

rtkwe 2y ago
The shock wave doesn't actually touch the bullet on the sides though so it doesn't provide meaningful lift. Look closely at the schlieren the high pressure region isn't against the sides of the bullet at all. Air is already quite dense at sea level equivalent to ~194 dB sound pressure so the sonic boom pictured which is 130-160 dB is not a significant increase in the density/pressure of the air.