wind effect by FPS

[Nihil_75]

I read alot of dicussions about wind's effect on BB's.

The general concensus is the heavier the BB, the less effect wind has on it.

 

But then I though of this- the heavier the BB, the lower the FPS.

and lower FPS means longer flight time to the same range - more time for wind to effect it.

 

Perhaps someone can come up with a calculation of the optimal BB weight while considering the wind effect on slower moving BB's?

[Samm]

There would be so many variables involved in the effects of wind on a BB I think it's fair to say it would be quite impossible to predict in any pertinent situation.

 

Interesting idea but it's hit or miss when it's a gusty day

[Trasher]

The optimal BB weight is somewhere between .30 and .40, IMO...

 

[Nihil_75]

I disagree.

 

you have a 6mm , 0.36G BB flying north at 400 FPS, with a cross wind of 10MPH for 3 seconds.

compare it to a 6mm , 0.25G BB flying north at 500 FPS, with a cross wind of 10MPH for 2 seconds.

These are all the variables, and they are all know.

 

(naturally, the wind in real life isn't so stable, but we can assume it is and get a general idea..)

 

[TDS]

Perhaps someone can come up with a calculation of the optimal BB weight while considering the wind effect on slower moving BB's?

 

It has to do with range, at close to your min engagement lighter bb's are better because they just get there instantly and have nearly no time to be effected by much of anything, at longer ranges heavier bb's are better, but hopup and such stops us from adapting ammo to the situation. with the innate better accuracy and general quality of heavier rounds, its better to just use the heavier rounds all the time than to worry about your hop adjustment if you swap to lighter rounds for closer ranged hits where flight time is very short.

[Nihil_75]

TDS, your going for the easy way out..

 

Theres science to be done!

The one with the calculated answer gets cake.

 

[TDS]

TDS, your going for the easy way out..

Until there's some way to have hop adjusted automatically its irrelevant imho

 

[slu]

I'll try to be more quantitative. This has been done countless times on these boards. In the most simplified case, the most important effect at airsoft velocities is the skin friction drag (the drag we are all most familiar with) which goes as something like velocity squared.

 

Now, assume for now that all BBs exit the muzzle at the same kinetic energy. Consider a low-mass BB of mass m, and a high mass BB twice the mass of the low mass BB, mass 2m.

 

KE_low = KE_high

(1/2)m(v_low)^2 = (1/2)2m(v_high)^2

(1/2)(v_low)^2 = (v_high)^2

(1/sqrt(2))v_low = v_high

 

v_low = (sqrt(2)) v_high

 

Note that sqrt(2) > 1, so we can conclude that the higher mass BB has lower muzzle exit velocity. Note how we said that the skin friction drag force goes as velocity squared. This means, to good approximation, that the more massive BB is experiencing less drag force by about a factor of 1/2 at muzzle exit. Now, suppose both BBs fly the same path to their target. Consider the work done on the BBs by the drag force.

 

Wfric = (integral over the path) Fdrag(ds/dt) ds

 

Assuming a totally straight path, thats just Fdrag(ds/dt = V) * Length of path. You see that drag is a function of velocity, but velocity varies along the path due the drag, so we have some type of coupling, and these is definitely an optimization in mass, which is why we don't choose to use super-massive BBs all the time.

 

Regardless, the concept is, less energy is lost to friction in the heavier BBs case. Also, since the more massive BB is, well, more massive, it is not as effected by disturbances such as wind.

 

As for the answer, given a BB's mass and exit velocity, and given the freestream "wind field" the BB will travel in, it's entirely possible, in fact, not too hard with computers, to predict the trajectory of the BB. So if are optimizing for maximum range, take a few cases where there is no wind at all, some crosswind, headwind, tailwind, etc.

[Nihil_75]

Found a ballistic calculator online.

Unfortunately it won't take an FPS value of less then 500 FPS and a bullet lighter then 0.32 grams.

Regardless, here are the the results, just to prove there is a point to this question.

-I "used" a 5 joule rifle, which produces 500 FPS with 0.43 and 580 FPS with 0.32.

-The wind is 10MPH crosswind. (90 degrees)

 

at 100 yards, a 0.32g 6mm BB traveling at 580 FPS (5J) will deviate 1.2 inch's from its target, after flying 0.520 seconds.

at 100 yards, a 0.43g 6mm BB traveling at 500 FPS (5J) will deviate 1.4 inch's from its target, after flying 0.603 seconds.

 

at 150 yards:

0.32g- 2.6 inch's deviation.

0.43g- 3.1 inch's deviation.

 

here is the calc if you want to fiddle with it yourself.

http://www.eskimo.com/~jbm/calculations/traj/traj.html

note this calculator probably assumes a bullet shaped projectile, not a BB.

Also, no hop is applied and its effect is unknown.

 

 

[Nihil_75]

Regardless, the concept is, less energy is lost to friction in the heavier BBs case.

 

According to the calc, the 0.32g BB was travelling at 570.3 fps at 100 yards (left barrel at 584.9)

the 0.43 BB was travelling at 491.4 FPS at 100 yards. (left barrel at 504.3).

 

So the heavier BB DID maintain its speed better.

computer says "yes".

[scar527]

If you look at the airsoft trajectory project, you will actually see that on distant targets the heavier bb actually gets there first. That is because it maintains the energy for much longer . For example, if a .3g bb left the barrel a 500 fps, then at 200 feet it will more than likely be traveling at 300-400fps, because a light projectile will lose speed/momentum because of air resistance. Now take a heaver .43 g bb, that leaves the barrel at 500fps, at 200 feet it will most likely be traveling at 450fps to 500 fps. While the air resistance is still the same, the heavier weight will mainain its momentum for much longer.

[Nihil_75]

If you look at the airsoft trajectory project, you will actually see that on distant targets the heavier bb actually gets there first. That is because it maintains the energy for much longer . For example, if a .3g bb left the barrel a 500 fps, then at 200 feet it will more than likely be traveling at 300-400fps, because a light projectile will lose speed/momentum because of air resistance. Now take a heaver .43 g bb, that leaves the barrel at 500fps, at 200 feet it will most likely be traveling at 450fps to 500 fps. While the air resistance is still the same, the heavier weight will mainain its momentum for much longer.

 

5J rifle

0.32g, left barrel at 580.5 FPS, struck target at 200 feet at 551.7 FPS after a 1.061 second flyght, deviated 4.7 inch due to 10 MPH crosswind, dropped 85.8 (!) inches below target (no hop).

 

0.43g, left barrel at 500.4 FPS, struck target at 200 feet at 475.1 FPS after a 1.232 second flyght, deviated 5.6 inch due to 10 MPH crosswind, dropped 12 inches below target (no hop).

 

The lighter BB got to 200 yards first.

The heavier BB lost less energy due to drag = more range.

but still, longer flight time = more wind deflection.

 

I think these tests conclude that yes, a faster moving BB will be less effected by wind,

but we are talknig about a 1 inch difference, while the range gained by heavier BBs makes a dramatic change of 70 inchs of drop at 200 feet.

 

I'd stick with heavier BB's . the heavier the better.

(I wish I could run the same tests on BB's lighter the 0.32, but the calc cant hack it.)

[scar527]

I was going off of what the A.T.P. said. Did you caulculate those #'s? You should try both in your rifle in the field and see which one is better. Sometimes our caulculations are proved false in the field, but that is some pretty impressive math there nihl.

[Nihil_75]

Found a ballistic calculator online.

here is the calc if you want to fiddle with it yourself.

http://www.eskimo.com/~jbm/calculations/traj/traj.html

 

 

Nihil + math = DOES NOT COMPUTE!!!

[juppe]

http://www.tarkkailija.net/wp-content/uplo...06/09/rata1.zip

 

Here is one calculator made for airsoft guns, help files are in finnish though but it shouldn't be too hard to figure out how to use it.

[Libris]

 

Nihil + math = DOES NOT COMPUTE!!!

 

I'd agree with that

 

I calculate a velocity drop of about 20% and 15% for 0.32 and 0.43g bbs after the respective flight times.

 

As for the drop in height, the bb's will fall at approximately the same rate and will have dropped roughly 5 and 6 metres for the 0.32 and 0.43 respectively if fired from a horizontal barrel, For a bb to only have dropped 12" means it has only been in flight for about quarter of a second.

 

As for wind deflection although the heavier bb is in flight for 16% longer in time it has almost a third more cross sectional density so won't actually be deflected as much as the lighter one.

[Nihil_75]

http://www.tarkkailija.net/wp-content/uplo...06/09/rata1.zip

 

Here is one calculator made for airsoft guns, help files are in finnish though but it shouldn't be too hard to figure out how to use it.

 

Thank you very much juppe!!! this calculator is greate!

I translated the instructions using google-translate and its very helpfull!

 

I really recommend this calc as it lets you realize the max hop your gun is capable of generating and calculating the optimal BB weight for your needs.

 

Thanks again!