[beezaur]

Does anyone know if somone has built a wind drift testing device? How do the arrow companies test this?

I am working on a simple free-fall wind drift tester, and don't want to reinvent the wheel any more than I have to.

The device I am proposing is a cord-triggered arrow mount that will hold the arrow at a 45 degree angle, point slanting down. The cord will release the arrow into free-fall toward a soft target 20 feet below. The idea is to measure the divergence between a plumb line and the point's impact. The procedure will simulate a gust of wind.

Ideas or comments?

Scott

 

[I BOW 2]

What about speed?

How will you simulate the arrow speed and shaft rotation in 20 feet? Ken

 

[beezaur]

Luckily, the character of the turbulence that causes drag will be the same as for a full speed arrow. The drag force will just be a lot less.

By dropping the arrow at a 45 degree angle, the wind direction will be the same as for a an arrow with a crosswind velocity equal to its forward velocity. In other words, it would be like shooting a 250 fps arrow into a 250 fps (170 mph) crosswind.

Obviously the drag force on the test arrow will be a lot less, but the arrow will have more time for the force to act too.

It is the same principle that allows the use of scale aircraft models, small scale dams, and other things that can't be immediately prototyped at full size. This is "similitude," one of the stranger aspects of fluid dynamics.

The final proof is in the pudding, as they say, so it will be interesting to see how this all works out.

Scott

 

[PSI-2]

I received a Edmund Scientific's Catalog in the mail a while back. It has all kinds of measuring devices...including a wind measuring device.

http://scientificsonline.com/search.asp?t=ss&ss=skymate

http://scientificsonline.com/product.asp?pn=3082343

You could use it to measure the wind speed from your shooting position and then shoot an arrow at a target 60 or 70 yards away and see how much the arrow drifts. If you have a steady breeze, you could also take readings at various locations between the shooting stake and the target and average them.

On your idea about measuring the distance from "plumb"...won't the arrow pivot from the 45 deg starting angle be based on the FOC of the arrow? Heavier point = quicker pivot/recovery = closer to plumb line?

 

[beezaur]

The problems with me shooting are 1) I am not good enough to tell the difference between two similar arrows, and 2) natural conditions are too variable over the course of 60 yards or so. But then, my wife would love to have an excuse to buy a weather station Cool windmeter.

The plumb line will have to account for the arrow's center of mass, like you say. Theoretically an arrow with zero wind drift, but with drag, will reorient point down and land exactly under where the balance point was. Since the arrows' points will strike first, orientation is a major concern.

I need a good way to either assure that the arows strike when they are vertical, or to be able to record their impact somehow (photography is probably too expensive for me). I am hoping that 20 feet will be enough for the arrows to stabilize and strike far enough away from plumb that any small strike angle will be negligible.

Scott

 

[Ian]

If you can't tell by shooting. What good is testing the drift of the arrow? It won't help you even if you knew.

 

[beezaur]

First of all, I'm curious. There are all these debates, like "are spin wings better than vanes" etc. Rather than sitting around BSing about what works or doesn't and why, I like to study things and find out for myself.

Second, I'm not convinced that it doesn't make a difference. Top shooters win or loose by a hair most times. Like the guy at my archery shop told me the other day, "one of these things may not make a difference in my shooting, but six or seven of these little things do make a difference."

And it will help with arrow building. If you acquire an understanding of just how arrow drag and wind drift work, you can make new things that perform better than what exists now. Bow and arrow design is beginning to get to the point where it is pretty hard to make a major advance by accident. Most of the time you have to develop a good feel for the underlying principles first.

Scott

 

[Joe T]

I know of something similar a while back where a cylinder was dropped into a tank of water at an angle and the resulting flight path measured. (aim was to verify a computer model re the 'aerodynamics' of a flying cylinder). Effectively this corresponds to a bareshaft, zero FOC arrow so not directly applicable. If interested let me know and I'll dig out a reference to a paper on the model and the test results.

Would suggest dropping the arrow from horizontal rather then 45 degrees - probably more consistent release and the arrow deviation will be higher (more sensitive).

Second, I'm not convinced that it doesn't make a difference. Top shooters win or loose by a hair most times.

Not being one I wouldn't really know but there was some discussion on the Sagi board recently about M. Frangelli using ACEs rather then X10's because testing showed them (among other things) to have less wind drift effect.

 

[BILL in PA]

beezaur
You might check Joe T's web site. He has published a great deal of info on arrow fllight.

http://homepage.ntlworld.com/joetapley/

 

[clever_guy]

grass...and guessing...for some I hear it is followed by frustration and a different kind of grass...



-CG

 

[beezaur]

That's a great site, Joe T! You have obviously done a lot with this topic.

My original plan was to drop the arrows horizontally. I may still do that, but the thing I was worried about was arrow rotation (pitch-yaw, not spinning). As you drop an arrow from horizontal, air resistance goes almost completely into rotating the arrow. When the arrow goes through a slanted state, it has rotational velocity which may take away from the effect of lateral force on the arrow. If you start at a slant, a bigger part of the air resistance force is directed sideways. Since the shaft isn't ratating yet, this should allow more lateral motion, or so the theory goes. I'll try both and see which works best.

Scott