using semi-rigid airfoil in arrow

[tinyaccel]

I have spend some time designing a new vane for arrow. A wind tunnel is what we have tested the concept and with pretty good result. It really concern me quite a bit since I do not have formal aerodynamic airfoil training. I just do not want to told that I am in nuts to design some thing that just cannot be done. I know that there is no money in it, but I just want to know that I can do it.

Below is what my basic criterias:
1) It has to be quiet at high speed. The vans is meant to be used for hunting, thus some vane design which make a lot of noise is not acceptable for it let the game animal know that the arrow is coming. Sound do travel faster than arrow.
2) With todays arrow able to launch at close to 300+ and up ward 400 ft per second. The traditional feather vanes is just not utilizing what we have learned for the last 100+ years. In the case of modern archery, what I would like to see is the semi-rigid air foil that would allow the arrow NOT to turn at all with speed over 250 fps but be turn at below 250 fps (feet per second) it should turn to gain stability. Currently set at 91 durometer.
3) It has to tolerate as much cross wind as the design would allow. In hunting situation, cross wind is always a big issue. So any design that can minimize it would be great
4) It has to be as light as possible

Any help will be sincerely appreciated.

 

[MikeHalloran]

My limited understanding says that aero stability comes from putting the Center of Pressure behind the Center of Gravity, i.e. making the fletching bigger than the broadhead... but that introduces crosswind sensitivity.

So, if you make the fletching about the same size as the broadhead, you lose aero stability... but applying the fletching in a plane that's not parallel to the shaft axis, so it may be planar, but is effectively twisted, gives you rotation, and with it, stability.

I'm sure that it's possible to produce synthetic fletching with something approaching the aeroelastic behavior you describe, but I'm not sure how it will produce the effect you want.








Mike Halloran
Pembroke Pines, FL, USA

 

[Surestick]

Wouldn't semi-rigid fletching interfere with the bow when the arrow is launched?
I was always under the impression that the fletching is made flexible to allow it to deform as it passes next to the bow.

 

[KENAT]

Mike, you didn't take into account the aerodynamic effect of the shaft itself. The head of the arrow will have its CP at about 25% of its chord (from the front, adjusted for the variation in chord). The shaft will have its CP at about 25% chord. The fletching will have it's CP at about 25% of its chord. The C of G needs to be ahead of the combined CP. Hence the fletching will almost certainly need to be significantly larger than the head. Even with a 'bobbin' or similar head, a relatively large fletching is I believe typically required.

Not sure how it would deal with hitting the bow on the way through as surestick mentions but lattice or grid fins look fun to play with.



KENAT, probably the least qualified checker you'll ever meet...

 

[MikeHalloran]

Natural feather fletching will deform (in an axial direction only) a little without damage, and it's always oriented so that one feather passes through the 'corner' between the bow shank and the arrow rest on the handle. Some arrow rests are raised so that the fletching doesn't have to deform much at all, i.e. there's effectively a 'notch' for it.

Yeah, combined CG has to be ahead of combined CP for aero stability. That's what I inteded to write; make them coincident, and the arrow flops around, unless it's stabilized some other way.

Actually, the dynamics of launch are more difficult than that. The arrow shaft typically buckles a little under compression as the bowstring accelerates it, then oscillates laterally after it leaves the bowstring, sometimes with a couple of different modes. Stand directly behind the archer, and you can see it flex. Even metal arrows do it, though the amplitude is smaller and the frequency is higher.

It used to be de rigeur to select sets of wooden arrows to all wiggle the same way with a given bow. A mismatch would produce effects that you might confuse with crosswind sensitivity.

I suppose that now you could tailor the stiffness of a composite arrow shaft to do all sorts of weird things, maybe even fly straight. The hobby got too expensive for me when metal arrows went into serious production, and they started fitting sights to target bows.

I wonder if it would be 'non-sporting' to fit active tracking fins to the arrow and use a laser designator to hunt?










Mike Halloran
Pembroke Pines, FL, USA

 

[KENAT]

A bit off topic but Myth Busters had an episode where they tried to recreate the opening scene from the Robin Hood film or TV series (the old one) where the second arrow splits the one already in the target.

They had some nice slow mo footage showing the 'wobble'/'wiggle'.

Maybe lattice fins made of a suitable polymer would flex enough and create a lot less 'cross wind' area.

http://en.wikipedia.org/wiki/Grid_fin

KENAT, probably the least qualified checker you'll ever meet...

 

[floattuber]

I'm assuming he's talking about a compound bow which is the only type that can produce those speeds. I've been out of the sport for a while now, but most people use shoot through type arrow rests which minimize or eliminate any contact with the vanes.

One thing I'm confused about is the requirement that the vanes will not induce a spin on the arrow above 250 fps, but will below 250 fps. Spin helps stabilize the arrow much like putting spin on a football.

The arrow's greatest velocity is immediately after release from the string and will only slow down from there. This means the arrow will not be stabilized at the beginning of it's flight which is probably the best time to stabilize it. It will only start spinning after it has slowed down to <250fps.

 

[KENAT]

floattuber, I assumed he was trying to emulate armor piercing discarding sabot type rounds. They use just the fins action for stability, they dont' use the fins to induce rotation for stability.



KENAT, probably the least qualified checker you'll ever meet...

 

[floattuber]

Hmm...but then his #2 requirement says, "...below 250 fps (feet per second) it should turn to gain stability."

I'm taking "turn" to mean spin.

He might be able to come up with some kind of mechanical vane that pops out the vanes using inertia and pulls them back at the desired time. This is similar to mechanical broadheads that pop razor blades out on impact.

 

[KENAT]

float, I know he wants different characteristics at different speeds, I'd thought about the pop out fins to but couldn't think of a good way to make it work.

KENAT, probably the least qualified checker you'll ever meet...