# Lift Force of a Rotating Sphere in the Air

• I
• Ruda975
In summary: Hop-up_(airsoft)The loss of linear velocity is not the only concern with the use of a Hop-Up mechanism. The increased backspin often results in increased wear on the BB, which can lead to a decrease in muzzle velocity. I measure this decrease by measuring the muzzle velocity before and after the Hop-Up mechanism is in place.

#### Ruda975

Hello,
I would like to ask one question. What is the equation for the lift force of a rotating sphere when flying through the air:
m = 0.25 g
v = 130 m/s
air density = 1.2292 kg/m^3
air pressure = 101200 Pa
air temperature = 15 °C = 288.15 K
If anyone is interested, it is an airsoft bb. I have also calculated the Reynolds number to be around 49182. I don´t know if that´s correct or not.
I have not yet measured the angular velocity of my airsoft gun, but there is a wikipedia article about hop up (the mechanism that applies backspin to the bb), that says that the backspin can exceed 1000 rpm which is about 105 rad/s, so I am using that value.

What is the direction of that rotation?

Lnewqban said:
What is the direction of that rotation?
https://en.wikipedia.org/wiki/Hop-up_(airsoft)
There is an animation of the rotation of this webpage.
And here is a quick illustration I made.

#### Attachments

• SphereSpin.png
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Lord Jestocost
Ruda975 said:
I have not yet measured the angular velocity of my airsoft gun, but there is a wikipedia article about hop up (the mechanism that applies backspin to the bb), that says that the backspin can exceed 1000 rpm
Do you also increase the CO2 discharge level to compensate for the lost velocity in the barrel? If energy is being put into the backspin of the BB, then linear velocity will be lost in the process, it would seem...

https://en.wikipedia.org/wiki/Hop-up_(airsoft)

Lnewqban
I only consider the behavior of the bb after it leaves the barrel. I do this through measuring the velocity and angular velocity of the bb once it leaves the barrel.

Ruda975 said:
I only consider the behavior of the bb after it leaves the barrel. I do this through measuring the velocity and angular velocity of the bb once it leaves the barrel.
Whate decrease in muzzle velocity do you measure with the Hop-Up mechanism in place?