Suppose I built a small wind tunnel for an arrow sized object that I would blow compressed air through at about 180 miles per hour plus or minus. The arrow would be suspended at the front by a very small string that fed back through the tunnel and attached to a scale. The tunnel would be set verticaly to minimise the effects of gravity.
Could this type of simple rig be useful in comparing arrow shapes and surfaces or would it be useless because it did not dupicate the exact circumstances an arrow might be flying under.
The proper setup would almost certainly be useful for comparisons. If you can show that a particular arrow shape or fletching material yields a lower drag in this wind tunnel, it would be entirely reasonable to expect something similar in the real world.
This approach has a fruitful history, dating back to the Wright Brothers
Isn’t the design of an arrow that the feathers are mounted slightly spiral to impart spin and stability?
I suppose lack of spin (or a short amount, until the string twists into a knot) might yield some meaningful data. I can’t see spin being as relevant to reesistance, except that fighting the spin tendency would add to the resistance of the arrow.
Also, breaking wind (so to speak) is a key piece of air resistance. the attach point and string are already parting the flow.
The usual mounting system would be a stiff thing rod from the back, and/or a stiff rod from below. This is attached to a strain gauge to measure the force with minimal “give”.
I realize the string would affect the flow right off the bat, but I was concerned that a rear mount would not be able to keep the arrow pointed into the wind, the fletching on a flight arrow are straight and very small compared to a target arrow. The drag at the rear of the arrow is what keeps the point facing into the wind. I can’t figure out how to do it without making some sacrifice somewhere.
You’re always going to be making approximations in any activity in science. But it looks like here the approximations you’re making would be relatively small. I would just make sure that the string is very low torsion, to allow the arrow to spin freely.
As long as you don’t let it get too wound up, you’ll probably get more swivelability from a length of ordinary thread than you would from the fisherman’s swivel
The actual test shouldn’t have to run more than about 10 seconds. I actually want the wind velocity starting at about 250 fps and slowing down to about 150 fps. I have some fine spider wire thread I can use and don’t expect much spinning anyway.
How big a tunnel? I suspect the biggest source of error would be the uneven air flow - i.e. faster airflow at the middle of the tunnel than near the walls. You can minimize this error by using a tunnel much larger than the width of the arrow.
I was concerned about this, I had planned on using about a 1 1/4" clear plastic tube as supplying enough air to something larger might be a bit of a problem for me. I am not sure how much air flow I could generate with a vacuume cleaner, hopefully I can get enough to be of some value, I was thinking one shop vac blowing air in and one sucking air out the rear to hopefull level out the air flow. I have no idea how two sources of air like that would effect it but thought I would give it a try.
For a flying object I think rather than hanging from a string, it would be more typical to support it on swivel from the balance point (Like a weathervane). Any changes you make with regard to wind resistance are going to affect stability and you won’t be able to see that if its hanging from its nose.
I am concerned with measuring actual drag resistance. I may use a very light rubber band in line with the string and just see how far it stretches with various desings.
A real wind tunnel will be have a diameter between 2 & 10 times the span of the thing being tested. So for testing an arrowhead an inch across you would want to build your tunnel from 5" pipe or so. A too-small tunnel produces very unrealistic numbers.
In a real tunnel the engineers are trying to accurately measure free-stream relevant values they can scale to the real thing. I suspect your goal is to do simple A-B testing between two full-scale examples to see which is lower drag or more stable.
If that is your goal, using a smaller diameter tunnel, say 2x your wingspan, will work OK enough.
You also want to put a good long hose, 5-10’ between your airflow source and your test chamber. The air exiting the airflow source’s fan is swirling like mad. That swirl will introduce all sorts of variables & chaos that can totally mask which design is more stable.
A long supply hose will help to damp the swirl before it arrives at the test chamber. In a real tunnel getting totally straight uniform airflow is a big deal. For you, avoiding gross vorticity is close enough.
Good and helpful sugestions. My arrow is a very small flight arrow, 23" long, 1/4" diameter, the point is just wood rounded over. The fletching are 1/4" high and 1" long. So the widest point will be at the fletching at just under 3/4" wide.
If the differences you are going to be looking for are very small,the the better the wind tunnel & instrumentation needs to be.
I got to spend several hours at the wind tunnel in College Station Texas while they were doing actual tests. Most impressive. The important things I was told that might apply to your problem was that the fan power was the first thing after the test area which fed into a enlarged tube that turned & fed into a huge expansion chamber so the air could still, cool from compression, remove turbulence.
They were running test at 500 MPH wind speed and you could barley feel the air move in the middle of the expansion chamber, then came a filter IIRC with the final turn to the run in to the test area which was way smaller of course and had a ‘venturi’ shape that was the actual test area. The area of the model had clear panels to watch through.
The test area needs to be a clear tube so an open area does not influence the test.
Need to really access your rubegoldburg talents to get the best you can.
Another thing you have not mentioned is the arrow flexing during flight.
Watched some high speed videos of arrows in flight and right out of the bow and all the way to the target the arrows would flex significantly. ( like an oscillation )This won’t be incorporated in a tunnel test or could you make it so?
Gunspot, the ocillation you are reffering to is another issue I am dealing with. The flexing is actually caused by the rear of the arrow accelerating the front of the arrow. We rate the spine of the arrow by measuring deflexion with a specified weight. Shooting flight arrows rather than target arrows we tend to lean toward stiffer arrows with less flex but most importantly they have to come out of the bow clean and straighten up fast or else they loose all thier velocity. The point weight on these arrows is non existent besides the weight of the shaft so we can achieve higher dynamic spine even with a skinnier weaker arrow.