The Workshops

[Altaïr]

How do you calculate drag?

Drag = 1/2 × ρ × S × Cd × v^2

ρ = 1.225 kg/m^3 : air density. That's at 15°C at sea level (air density varies with temperature and altitude), though you can use this value, it will give you a good idea.

S = π×R^2, where R is the radius of your projectile (it's a sphere right?): cross sectional area

v: the projectile speed: 25 m/s for you.

Cd: drag coefficient. For a sphere in those conditions, you can assume it's 0.5.

 

[Chara-cter]

Drag = 1/2 × ρ × S × Cd × v^2

ρ = 1.225 kg/m^3 : air density. That's at 15°C at sea level (air density varies with temperature and altitude), though you can use this value, it will give you a good idea.

S = π×R^2, where R is the radius of your projectile (it's a sphere right?): cross sectional area

v: the projectile speed: 25 m/s for you.

Cd: drag coefficient. For a sphere in those conditions, you can assume it's 0.5.

so it's 1/2 x 1.225 x πx0.003^2 x 0.5 x 25^2= ~0.005
Did I do it right?

 

[Altaïr]

so it's 1/2 x 1.225 x πx0.003^2 x 0.5 x 25^2= ~0.005
Did I do it right?

Yes, that's it

 

[Chara-cter]

Yes, that's it

0.005 N?

 

[Altaïr]

0.005 N?

Yes. The value seems very low indeed (I suppose that's what bothers you), but if it's applied to an object that weigh 1 gram (so 0.001 kilogram), you get a deceleration of 5 m/s^2, which is noticeable.

 

[Chara-cter]

Yes. The value seems very low indeed (I suppose that's what bothers you), but if it's applied to an object that weigh 1 gram (so 0.001 kilogram), you get a deceleration of 5 m/s^2, which is noticeable.

This object weigh 0.3 gram so I don't think it would be noticeable

 

[Altaïr]

This object weigh 0.3 gram so I don't think it would be noticeable

On the contrary, it has greater impact on a light object. The deceleration from drag is a=F/m. In this case the initial deceleration would be 18 m/s^2. It would decrease quickly though (because drag is proportional to v squared). A heavier object would have more inertia.

 

[Chara-cter]

On the contrary, it has greater impact on a light object. The deceleration from drag is a=F/m. In this case the initial deceleration would be 18 m/s^2. It would decrease quickly though (because drag is proportional to v squared). A heavier object would have more inertia.

hmmm
interesting

 

[Pink]

This object weigh 0.3 gram so I don't think it would be noticeable

Think like this:
Imagine a constant 100 kg (which works out to approx 1000 N) push. That's enough to slowly slow down a small car. But will a semi-truck notice it?

For a given force, it will effect a lighter object more.

 

[Pink]

0.005 N is about 0.5 g of force. Applied to a 0.3 g BB... That's like 50kg of force on a 30kg person.

 

[Chara-cter]

On the contrary, it has greater impact on a light object. The deceleration from drag is a=F/m. In this case the initial deceleration would be 18 m/s^2. It would decrease quickly though (because drag is proportional to v squared). A heavier object would have more inertia.

So how fast should the BB fly to reach 25m? 43m/s?

 

[TtTOtW]

What's the barrel height from ground level? Shoulder height?

 

[Altaïr]

So how fast should the BB fly to reach 25m? 43m/s?

If your only concern is to reach a target at 25 meters then it's fine, it will just need more than 1 second.
Also, just because the initial deceleration is 18 m/s^2 doesn't mean that the speed will drop from 25 to 7 m/s in one second. As the speed decreases, drag decreases as well, so the deceleration decreases too. The distance it can travel in a given time can be calculated, but it's a bit tricky. I can do it for you if you need.

 

[Chara-cter]

If your only concern is to reach a target at 25 meters then it's fine, it will just need more than 1 second.
Also, just because the initial deceleration is 18 m/s^2 doesn't mean that the speed will drop from 25 to 7 m/s in one second. As the speed decreases, drag decreases as well, so the deceleration decreases too. The distance it can travel in a given time can be calculated, but it's a bit tricky. I can do it for you if you need.

can you calculate its ballistic? Like this one

 

[jerome_the_sfs_boss]

why do i not understand any of this?

 

[Chara-cter]

Anyway, this is how I'm gonna make the inside of the gun. It's kinda easy to understand how it works (I guess). If you don't understand I could build a prototype and make a stop-motion gif from it
Blue: Barrel
Orange and red: Mag parts
White: The BB (Sorry if u can't read this lol)
The other white: part of the trigger group
The other blue: Part of the firing pin (that act as a bolt carrier group)
Brown: The firing pin
Green: The old bolt carrier group (Now won't move)
Pink: trigger (the part you pull)
Grey: springs, other parts,...
Darker red: Safety selector

 

[Horus Lupercal]

can you calculate its ballistic? Like this one
View attachment 43792

You can calculate something like this. Though these graphs are made using shot data gathered from physical testing

 

[Altaïr]

can you calculate its ballistic? Like this one
View attachment 43792

I can, but you shouldn't expect it to be very reliable, as your projectile is very light, and it will be very sensitive to variations in atmospheric pressure, wind and so on. I can't guarantee that the result will be accurate.

 

[Chara-cter]

I can, but you shouldn't expect it to be very reliable, as your projectile is very light, and it will be very sensitive to variations in atmospheric pressure, wind and so on. I can't guarantee that the result will be accurate.

It's ok for me

 

[Altaïr]

It's ok for me

This is what I ended with:

Both axis are graduated in meters.

 

[Chara-cter]

This is what I ended with:
View attachment 43950
Both axis are graduated in meters.

Thank you!

 

[Chara-cter]

Yessssssssssssssss
It's finished!

 

[Chara-cter]

First part made out of plastic!

 

[Chara-cter]

Just finished my first prototype
It's as reliable as the L85A1

 

[jerome_the_sfs_boss]

Just finished my first prototype
It's as reliable as the L85A1

nani?