Wind drift - theory vs reality

[davidjoe]

Forget about moving with the air or "cutting through it". The interaction of the air with the bullet is the source of the drag force. So you can think about it as if the air isn't there - it's been replaced by a force that is both slowing the bullet down along it's flight path, and pushing it to the side.

And for anyone wondering why we still see round holes with the bullet tipped to the side like that, it's because the angle is tiny - less than 1 degree.

Agree, maybe we all do see this the same. I only think of wind as pressure. Bronze archer talked about the lateral drag component last week (factors that gets rounded down to zero perhaps prematurely he and I wondered) in my case regarding the the profile. My remaining snag has only been with the proposition that when the bullet turns in to face the wind, it achieves “exact” lateral pressure equilibrium. Because it doesn’t drift in lockstep with the airmass.

 

[BronzeArcher]

A stable bullet cannot change direction without an applied torque, because angular momentum must be conserved. So if there is a torque somehow applied by the crosswind, then there must also be an applied force - an applied lateral force.

Every time that bullet changes direction as the wind speed and direction change along its flight path - to maintain pointing into the airflow, there must be another applied torque, because conservation of angular momentum would keep that bullet pointing in the same direction if no external torque were applied. But to apply that external torque, there must also be a lateral force - that is a force acting in some direction other than rearward.

The laws of physics that demand this lateral force are as solid as Newton's laws.

 

[davidjoe]

A stable bullet cannot change direction without an applied torque, because angular momentum must be conserved. So if there is a torque somehow applied by the crosswind, then there must also be an applied force - an applied lateral force.

Every time that bullet changes direction as the wind speed and direction change along its flight path - to maintain pointing into the airflow, there must be another applied torque, because conservation of angular momentum would keep that bullet pointing in the same direction if no external torque were applied. But to apply that external torque, there must also be a lateral force - that is a force acting in some direction other than rearward.

The laws of physics that demand this lateral force are as solid as Newton's laws.

...yes, and what presses against a bullet is necessarily moving faster than the bullet, from that direction. The bullet is losing ground relative to lateral air pressure exerted, or there would not have been pressure in the first place.

“Achieving” alignment with the total wind vector is either a mere close approximation of a bullets’ behavior, or something else has to explain why bullets don’t match the lateral distance of the crosswind mass they are traveling within.

My theory is that this a “limit” math problem. The bullet starts out very much unaligned with a total wind vector, and gets exponentially closer and closer to actual alignment every successive section of its flight examined. In the meantime diminishing lateral pressure persists, meaning crosswind outpaces it, until eventually the bullet loses all the energy imparted by the gun, then would become completely adrift, exactly as you’d see with an arrow shot skyward, coin thrown off a building, or raft thrust into a slow current.

 

[dcali]

If you were to have a change in wind then yes the bullet would have to reorient into the new airflow. But just as is the case with the launch of the bullet that happens very quickly and without substantial net lift.

The force responsible for wind deflection is not a torque at all. The bullet doesn’t steer along its path - it just deflects because that’s the way the drag force is pushing it. Changes in the wind bector do have small impacts on trajectory. (There’s another thread on aerodynamic jump somewhere that gets into this). But it doesn’t change the nature of the primary driver of wind deflection.

 

[dcali]

My theory is that this a “limit” math problem. The bullet starts out very much unaligned with a total wind vector, and gets exponentially closer and closer to actual alignment every successive section of its flight examined.

This sort of happens. Sometimes. On well behaved bullets this happens quickly and is never a large misalignment. The net effect of this lift is negligible.

But! Even a well behaved bullet will not settle down to point directly into the airflow. Because of the downward arc of the trajectory imposed by gravity combined with a gyroscopic effect, the nose of the bullet winds up pointing on average ever so slightly in the direction of the twist. That net yaw (called the yaw of repose) creates a lifting force that pushes the bullet in the direction of the yaw. (Confusingly, lift does not mean up. It means sideways). That drift is not canceled out by the bullet’s spin, and is a small but significant factor at long range. It’s known as spin drift.

And another but! Some bullets damp out better than others. You cannot assume a bullet’s yaw will eventually damp out.

 

[Ballisticboy]

A stable bullet cannot change direction without an applied torque, because angular momentum must be conserved. So if there is a torque somehow applied by the crosswind, then there must also be an applied force - an applied lateral force.

Every time that bullet changes direction as the wind speed and direction change along its flight path - to maintain pointing into the airflow, there must be another applied torque, because conservation of angular momentum would keep that bullet pointing in the same direction if no external torque were applied. But to apply that external torque, there must also be a lateral force - that is a force acting in some direction other than rearward.

The laws of physics that demand this lateral force are as solid as Newton's laws.

The aerodynamic moment is not applied by the cross wind it is produced by the small angle change between the total air flow and the axis of the bullet if the cross wind changes speed or direction. The resulting lift induced moment, produced by the change in the pressure distribution over the whole of the bullet but predominantly by the sections of the bullet where there is a change in cross sectional area i.e. nose and boat tail, will be produced in exactly the same way as every other lift induced moment on a bullet as described by aerodynamic laws. In reality there is no single lateral force or drag force, we only create those and the corresponding aerodynamic centre for our own convenience.

 

[davidjoe]

There’s an old saying that a bullet is just a faster rock.

Some common principles would be:

1) any object at all you put into or hold against a current is going to orient with its highest drag area down stream. This is why anchors are on the front of ships, and ships don’t stray over their chains.

2) a bullet’s conical nose does not keep it pointed forward absent spin, nor require it to face into the wind vector. A thrown rock will also face into the total wind vector. An arrow too. What’s really happening is that the highest drag area of each catches more of the flow, wanting to go more slowly than the rest of the object.

3) the side of every object moving relative to a current is “seen.” A bullet’s sides are seen. The indentations of rifling are seen.

4) nothing put into a cross current escapes pressure on its upstream side. The motion AND physical traits of the object determine how it interacts with the current.

5) nothing thrust upwind a current moves as far or fast downwind/downstream as the current does until pressure 100% equalizes. Pressure doesn’t equalize while the object retains original kinetic energy from being thrust into the current.

 

[davidjoe]

In reality there is no single lateral force or drag force, we only create those and the corresponding aerodynamic centre for our own convenience.

Yay, I think.

 

[Ray Gross]

…but predominantly by the sections of the bullet where there is a change in cross sectional area...

Why predominately?

 

[JPeelen]

In an earlier message in this thread the question of experimental verification was raised.
In the mid-seventies, the Swiss did exactly that with their anti-tank Raketenrohr (a bazooka). The weapon was mounted on a carriage running on straight rails, firing 90 degrees sideways. The crosswind was simulated by using a Range Rover automobile to accelerate the carriage. In other words, the weapon moved sideways with a defined speed to simulate a crosswind of that speed. The details were of course much more complicated.
But the result confirmed the agreement of computed and measured crosswind effect.

 

[BronzeArcher]

In an earlier message in this thread the question of experimental verification was raised.
In the mid-seventies, the Swiss did exactly that with their anti-tank Raketenrohr (a bazooka). The weapon was mounted on a carriage running on straight rails, firing 90 degrees sideways. The crosswind was simulated by using a Range Rover automobile to accelerate the carriage. In other words, the weapon moved sideways with a defined speed to simulate a crosswind of that speed. The details were of course much more complicated.
But the result confirmed the agreement of computed and measured crosswind effect.

Claims of validation without giving the accuracy of that validation and a published reference is scientifically useless. I never believe claims of scientific validation from anonymous internet sources without proper references so I verify it.

 

[JPeelen]

I was under the impression this is a shooter's forum, not a scientific one. I saw no point in citing foreign language sources here. Gruppe für Rüstungsdienste (today armasuisse) ran the tests in 1979 and Beat Kneubühl was in charge of them.

 

[mysticplayer]

Interesting opinion.

I appreciate your input.

The question was whether anyone has or is doing real-world testing to validate the BC to wind drift relationship?

YES... email if you want to chat. Steve is likely to come down in May.. I am not sure if I can make it down. Going to try for June. Will be a great chat over Mexican food.....

Jerry Teo
info@mysticprecision.com

 

[Curmudgeon]

I shoot nra and nbrsa matches. We put up the yellow and red nylon flags people call wind flags. I will now call them direction flags. They fly at 90 degrees to the ground in 10 mph wind and 20 mph wind. Useless for judging wind speed. Put up anemometers at the benches from down range distances and the we will at least have useful info for our adjustments. This is probably posted in the wrong place but without proper wind info all the ballistic programs are so much hot air.