Air Gun Home :: View topic

diipii · Member

In nature surfaces travelling through air or water are less than perfectly polished. E.g. feathers, scales and even golf balls are dimpled.
I think this creates a boundary layer of turbulence that lubricates the surface through the surrounding environment.
Has any one experimented with roughened or dimpled pellets or bullets to achieve better aerodynamics ?
Conversely, has polishing to a fine gloss been found advantageous ?

Alstone · Moderator & Site Supporter

Birds feathers I’m not sure about, but on dimpled golf balls it gives lift to the shot and increases range. Whereas on a pellet you don’t want this, it would increase the trajectory arc thus increasing the flight time, and reducing the impact energy, at least for hunting.

To my way of thinking. Question

BTW Welcome to AirGunHome. Nice to see another Brit around here. Smile

Al

domer_pyle · Senior Member

some pellets have ridges that run along the length of the skirt but i couldn't tell you if they make a difference or not. all the pellets i've ever used have had smooth skirts

rsterne · Moderator

I think you will find that wherever nature uses a textured surfaces.... the reason has to do with Reynold's Number.... which is basically the scaling effect of how a fluid flows around an object due to it's size and speed.... Large objects moving fast have high Rn while small objects moving slow have low Rn.... The Rn has a huge effect on drag.... and there is a critical region at about Rn=50,000 where the flow changes from laminar to turbulent.... A 1/4" long pellet travelling at 375 fps is right at 50,000.... so for most pellets and velocities, the flow is turbulent.... and surface ridges, etc., have little effect.... Shocked

In nature, if the ridges / surface roughness are on a small object travelling slowly.... they are likely there to induce turbulent flow where there isn't any.... because laminar flow at low Rn tends to have a lot of drag.... Once the object gets big enough and moving fast enough that the Rn is over 50,000, small surface imperfections (such as feathers) tend to be below the turbulent boundary layer and simply don't matter.... Cool

The boundary layer gets thinner as Rn increases.... so once you are looking at an airplane at higher speeds (with a wing several feet in chord, Rn in the millions).... then things change again.... Rolling Eyes

Bob