TK hop twist barrels, are they worth it?

[Delfi]

The difference in lift isn't as differnt as you'd think. It's just delta P across a body, and in each case it is generated by a change in stream velocity across different surfaces of a body.

 

This is right ... the cross section of an aircraft wing is asymmetric in it's angle of attack. That's what causes lift.

 

In the case of a BB the effect is different ... think table tennis balls.

 

D

[Delfi]

If you could get a BB to backspin while standing still it'd be the same.

 

I don't think that's right ... the forward motion is involved in the lift surely?

[Glenn]

If you could get a BB to backspin while standing still it'd be the same.

It'd generate the same (ignoring that lost to friction, I assume) lift at rest as it would at speed... IF you could control the velocity while still keeping it spinning.

 

 

It doesn't though...you missed my edit:D

[Chimpy]

I don't think that's right ... the forward motion is involved in the lift surely?

 

You're right.

 

Stealth, see the Bernoulli Principle.

 

The spinning ball still needs to be moving relative to the fluid its in. So either the BB has to move through the air or the air has to be moving past the BB. To gain a similar lift with a stationary spinning BB you would need the windspeed to be the same as the velocity of the BB you were trying to match. It would also have to be in the same direction. Hence on windy days you can tilt the gun to the side and negate the effects of a cross-wind somewhat using the lift generated by the hop.

 

Youtube real life demo FTW:

 

[Stealthbomber]

I don't think that's right ... the forward motion is involved in the lift surely?

 

 

You're right.

 

Stealth, see the Bernoulli Principle.

 

Yeah. I didn't think that through properly.

 

Like I say, I don't really know how to explain it but there IS a difference between speed and lift in a wing-form and in a BB.

 

As I say at 1mph or 150mph the difference in air-flow across the BB will always be the same. In the above example, it'll always be 10m/sec. The nett amount of lift is always a function of the 10m/sec differential.

 

In a wing-form the lift will remain proportional but the nett effect increases.

[Chimpy]

Well in the case of a helicopter its moving several wings through the air to generate the lift and the forward motion (in a simple case) has no bearing on the movement of air over the wings as that is being generated by the rotation of them through the air by the engine. In reality this isn't actually the case hence the need to adjust the angle of attack and speed of the blades as the helo changes airspeed. Also depending on the helicopter design part of the lift always has to be sacrificed to accelerating or maintaining velocity. In the case of something like an auto-gyro you can see this in action as the rotor itself isn't powered and the forward motion is generated by a separate propeller. This is due to a phenomena known as autorotation which conventional helos can do in emergencies but they need to maintain a forward airspeed to keep it going.

 

Read up and see you're talking about a different example. Doh!

 

In reference to your wing:

 

The Bernoulli principle is simple. Faster airspeed means lower pressure. Lower pressure on the upper surface causes lift.

 

The shape of the wing generates the faster airspeed over the top of the wing. Moving a wing faster through the air with the same shape will cause an increase in lift.

 

The spin of the BB generates the faster airspeed over the top of the BB. Firing a BB faster through the air with the same amount of spin will cause an increase in lift.

 

If we wanted to get beyond that we need to consider the wing shape in three dimensions and the contributing effect of angle of attack. Which is more physics than I care to remember on a Friday evening. The same is true of the BB as well mind you.

[Chimpy]

Oh yeah and the Newtonian explanation is even easier.

 

Simply the shape of the wing and the spin of the ball force more air downwards and due to Newtons third law push the wing/ball up. Going faster means a greater mass of air travels over the wing per second and thus a greater force is generated and thus more lift.

[Stealthbomber]

The Bernoulli principle is simple. Faster airspeed means lower pressure. Lower pressure on the upper surface causes lift.

Right ho.

 

So, to use the above example, if we have a BB fired at 100m/sec with backspin of 500rpm (thus giving us our approximate 10m/sec differential in air flow) that SAME amount of differential in air flow will create MORE lift if the BB is shot at 200m/sec?

 

If it does then fairy nuff.

 

That was what I was trying to reach a conclusion about.

Somebody pointed out that the differential in air speed across the top and bottom of a BB remains constant regardless of velocity.

If something to do with the speed of air affects the formula for calculating lift then that's fine.

That's what I wanted to establish.

[Chimpy]

Yup, pretty much.

 

The velocity difference may well be the same but the key point in understanding is that there is more air moving over the wing/ball as you get faster. The Newtonian explanation is easier to understand in that regard.

 

On the barrel twist I'm not convinced how much difference it would really make, since the BB doesn't form a perfect seal with the barrel I would expect air escaping round the outside to basically balance it although the spin from the hop might affect this. I imagine is probably does something to help but I doubt it will make you any better in a game.

 

I've read back through the whole thread now and must say it goes from the sublime to the ridiculous. Essentially I think Sale and Stealth nailed it ages ago.

 

My personal take would be that it combines two effects, firstly the barrel is tighter meaning the BB has less ability to move. Secondly the grooves allow air round the outside. These combined allow the stablising effect of of the air cushion that you get in a larger bore barrel along with restricting how much the BB can move. The spiraling of the air channels merely evenly distributes the air around the barrel and thus provides a more stable airflow around the BB. This essentially means the BB is traveling in a straighter line down the barrel and should group better. At a guess that would decrease range and FPS slightly in comparison to a tight bore barrel as more air is escaping around the BB. Longer versus shorter barrels. Again a guess but I imagine the air cushion effect gets less towards the end of the barrel in long barrels as the energy begins to dissipate. The BB then can clip a groove causing things to fly wide.

 

Guess work fueled by physics and beer mind you.

[Sale]

The velocity difference may well be the same but the key point in understanding is that there is more air moving over the wing/ball as you get faster.

This is what I was trying to say. While the difference in air speed over and under the BB is the same, there's a lot more air going over and under the BB when it goes faster.

 

If velocity didn't make a difference, then the BB would float in the air at zero fps, if you just applied enough spin.

 

-Sale