An experiment I would like to see

Most all projectiles I use have a relatively smooth surface. How rough does the surface need to be in order to see benefits?
View attachment 470542
You do not need the entire surface to be rough, you just need something to trip the boundary layer before it gets to the base, and preferably before it gets to the boat tail. It is possible that the marks in the sides of the slug from the rifling could be enough or any small band of slightly rougher material. It is standard practice on wind tunnel models to use boundary layer trip bands on the models to produce a turbulent boundary layer. Also remember that what looks like minor roughness to you will look like a cliff to an air molecule.
 
But look at some real world experiments on this, and you'll see the affect for sure. Almost like air or fluid sticks more to a smooth surface than it does to a bit rougher one.
Real world experience shows that turbulent boundary layers are much better at following curves than laminar ones when it comes to objects flying through the air. Many world war two fighters which had laminar flow wing sections had some form of boundary layer trip to ensure the air would remain attached to the rear of the wing surface, particularly when manoeuvring. Spheres can have their drag hugely reduced by tripping the boundary layer before the air reaches the base. It is one of the first things you are taught on aerodynamics lectures.
 
You do not need the entire surface to be rough, you just need something to trip the boundary layer before it gets to the base, and preferably before it gets to the boat tail. It is possible that the marks in the sides of the slug from the rifling could be enough or any small band of slightly rougher material. It is standard practice on wind tunnel models to use boundary layer trip bands on the models to produce a turbulent boundary layer. Also remember that what looks like minor roughness to you will look like a cliff to an air molecule.
As in a driving band just in front of the boat tail, with land engravings? Or a slight rebate before the boat tail? Both of those being common features of some of the boat tail air gun slugs.

IMG_2984.jpeg
 
  • Like
Reactions: 357cal
As in a driving band just in front of the boat tail, with land engravings? Or a slight rebate before the boat tail? Both of those being common features of some of the boat tail air gun slugs.

View attachment 470875
A driving band will definitely cause turbulence, it may deflect the airflow a bit as well, which may be good or bad. Driving bands do produce a small increase in drag, but that may be offset if the boat tail is not correct and giving high base drag. A rebate no necessarily so, as the air may re-attach and stay laminar. I think many projectile surfaces are rough enough already, though the transition point may change from one to the other without some purpose made band. Large wind tunnel models use a very small band of very fine sand to trip boundary layers, but that wouldn't do your barrel much good.

That is a very well-designed boat tail in your picture. ;)
 
A driving band will definitely cause turbulence, it may deflect the airflow a bit as well, which may be good or bad. Driving bands do produce a small increase in drag, but that may be offset if the boat tail is not correct and giving high base drag. A rebate no necessarily so, as the air may re-attach and stay laminar. I think many projectile surfaces are rough enough already, though the transition point may change from one to the other without some purpose made band. Large wind tunnel models use a very small band of very fine sand to trip boundary layers, but that wouldn't do your barrel much good.

That is a very well-designed boat tail in your picture. ;)
That slug is a NOE 7mm, 106gr (2.5 calibers long). One of Bob Sterne’s latest designs. I know that it incorporates updates based on some of your recommendations. In my opinion, those boat tail slugs using 2 to 3 caliber for length are the best compromise for ELR airgunning. Though I’m beginning to experiment with longer slugs, specifically .308 in 3 to 4 calibers long.
 
  • Like
Reactions: dgeesaman
Unfortunately, in this case, your simple science is misguiding you. Boat tails produce destabilizing aerodynamic moments about the CG due to giving negative lift when yawed to the air flow, so you require higher twist rates for the same stability factor. Coupled with this is the fact that the boat tail also increases the boundary layer thickness, which will increase Magnus moments, leading to a reduction in dynamic stability.

As for drag, the largest single component of drag for a slug with a reasonably shaped nose is the base drag. It is a well proven fact that turbulent boundary layers at the base of a projectile reduce the base drag compared to that which is produced by a laminar boundary layer. The turbulent boundary layer does this by increasing the mixing of the air in the low pressure area behind the base, increasing base pressure and thus reducing base drag. So, while you may obtain a small reduction in aerodynamic skin friction drag by keeping a laminar boundary layer, you will create a much larger increase in base drag with a laminar boundary layer compared to the turbulent boundary layer. This is basic aerodynamics, as shown by the tests on sphere drag, amongst other things. Slugs and airgun projectile complicate things further by flying at relatively low Reynolds numbers, which can cause the complications above.

from www.alcobullets.com replied to your message, What type of finish are your pcp bullets?​

The bullets are smooth finish
09 Jun, 2024​