Maybe a mechanical solution; to retract the plates and have them spring open and lock on impact.

Gregow,

At the end of the day a field point and a broadhead will fly the same as long as they are the same weight (grain) and most importantly, as long as the bow is properly tuned. If you are seeing a difference in flight it is a tuning problem. A broadhead exacerbates the problem on an out of tune bow as does an overall heavier setup. Lighter is better in archery too from an accuracy perspective in terms of tolerance for a slight out of tune bow. I agree with your steer comment...but only if the broadhead comes off the rest out of alignment in the first place. I think this is what leads guys to falsely conclude that a mechanical broadheads are better. They are just shaped like a fieldpoint so they react the same....even with an out of tune bow. Something else is being compromised in the entire setup. This is why they also misread the signal when they can hit bullseyes all day long at 15 yards and can't understand why its off at 25. The arrow is just oscillating back and forth as the fletch is trying to right it when the flight is interupted by the target. Make sense?

An arrow that comes off the string straight...flies straight...and the fletch is creating drag to keep the arrow oriented properly in flight. If you can spin the arrow with helical fletching all the better. That increase in accuracy is derived the same way barrel rifling imparts spin to a bullet.

Having said that no set-up can be absolutely perfect, but it could be close enough so that you hit a 6 inch circle consistently from any effective bow range.

Just for kicks, try shooting an arrow just before dark using one of those new lighted knocks. You can see if the arrow is fishtailing really easy and make adjustments accordingly to get the arrow to come off straight. I know you know about paper tuning, but I have always found that to be a starting point...not an ending point.

By the way I should mention I'm talking a hunting setup not competition because I recognize some of the advantages of the drop away rests etc.

Anyway, just some of my thoughts as I am trying to be helpful by sharing my experience.

No go hunting...The Rut is here!

Tom

I'm not trying to run you off, I enjoy reading aero stuff. But you might want to talk with the rocket folks too. A similar situation is encountered when adding small fins on the front of a rocket.

Dave

I'm sorry you didn't find that helpful my friend.

If you are convinced that this is an issue regarding non-planing shapes or surfaces; Jim Thomerson provided a solution. As you may know, mechanical broadheads retract and spring open and lock on impact. These are already commercially available and are actively marketed to people that share your view of the problem. You are not the first, you won't be the last.

If you are willing, however, to entertain that this may not have anything to do with non-planing shapes then please look to bow tuning.

Have a nice night and good luck in the woods!

Tom

Would you be so kind as to provide the bowhunting definition of "planing" or "non-planing".

It might help those of us who aren't familiar with bow hunting terminology.

OK, you got a couple of aerodynamic problems.

The chord of an arrowhead's blades is very short. And it's speed is very slow. So the Reynold's numbers are small. So small that the blades really aren't working like an airplane wing, even an average size model airplane. Your blades might be working like park flyer wings do. About any planform wing works for them and about any airfoil.

I'd suggest the blades that're advertised to be non-planing got that way from cut/try engineering. If they actually do anything at all unique, that is.

I think you want a shape that will be stalled quickly and not provide any lift to cause your planing. Your broad-head is the equivalent of a delta wing, the worst shape for you application. The Delta doesn't stall, and makes it's lift by vortex generated lift. It would seam to me that a highly swept thin wing would stall very quickly. If I were to try to make this non-planing head it would be swept back 60*, maybe 1/4" chord on the "wing", and I would airfoil it with a diamond shaped airfoil.

Think about the actual environment the magic blades are going to be working in. Your arrow may accelerate quickly in an archery frame of reference, but it starts out a zero and is pretty slow while the magic is supposed to be keeping those blades from planing. How long does it take for the fletching to stabilize the shaft, and how much does the shaft flex. The magnitude of the flex is going to be displacing the blades. In fact, they're going to be yawing probably faster than they'll be going in the first feet.

Has anyone made blades that "screw around" the point to match the "screwing around" of the fletching? That might be worth an experiment.

The blades actually resist the rotation caused by the fletching. They're going to do it more the faster the rotation. So there should be an enhancement downrange no matter what happens immediately at release. What might be beneficial at release would be the new shape of those helical blades offering less resistance to being driven side against the air. They will now offer a curved side instead of a flat one.

But just brainstorming.......... and trying to cause some thought a bit off the usual path.............

BTW, there might also be an enhanced penetration from them as well. Since lots of hits are through-and-through, the real benefit if there is easier penetration would be from a reduction of blade area.

good luck with your quest..........

Thanks for the replies.

It looks like they're betting the removal of "side area" would reduce the "planing". Planing seems to be nothing more than what vertical fin/rudders do. The only way to get rid of that is to remove them.

I'm not suggesting an attempt to create rotational stability with the arrowhead's shape, I'm suggesting shaping the sucker to reduce it's side area and that area's interaction with the air. As each flat blade of of the arrowhead is rotated the air it encounters works on the blade almost exactly as air works on a propeller blade. There will be an AOA (angle of attack) created that would be a source of lift and drag. The idea is to shape the arrowhead blades to reduce that AOA. Rotational stability has nothing to do with the idea.

The planform shape of each blade really isn't going to matter much at the terribly low Reynolds numbers they're seeing. There really aren't much "aerodynamics" in play there. The shapes aren't going to matter much. For examples why, look at the discussions about the electric foamys designed for 3D flight, where the model operates most of the time at very slow airspeeds. They use flat foam sheets with square leading and trailing edges. Nothing really "aerodynamic" there at all.

It appears that what happens to an arrow in the planing envelope isn't something that could be very aerodynamic. The design feature that matters most with the 3D foamies is area and location. You can't change location.

But truth is, experimentation is fun. So try something "aerodynamic" and let us know how it works.

I think there's more than you imagine. 3D foamies are like a fly swimming in molasses, the Reynolds numbers are low, but the speeds are high so there's a lot of force being applied, I agree that area is important.

What is wrong with the head shown on the right hand picture?

The arrow speed is not much at all during the time the planing is supposed to be happening. Also, the planing is supposedly done by the time the arrow is up to speed.

The arrow is at maximum speed as it leaves the bowstring,gregow said planing happens in flight, not on the bow. From slow motion pictures I've seen of arrows in flight the shaft oscillates about 3", probably at about 50Hz.

gregow

What you drew is close to what I tried to describe, the rotating of the blades should help stop the planing also.

So there is a problem even though it worked?

Seriously though, in reference to avoiding side area interaction, I used to bow hunt quite a lot and used what I think were called "Razorbacks". They seemed to work on that principle. They were long skinny blades attached to a cylinder that freely rotated on a shaft with a point, that allowed the arrow shaft to spin at will without interference of the blades. They were the most accurate broad head I had used at the time.

ok</p>

The only non-planning shapes are a sphere, a semi-sphere and a cylinder or rod perpendicular to the air stream.
Any other flat shape will introduce an aerodynamic force IF it faces the air stream with some angle of incidence.

I have seen that arrow shafts bend substantially during fly; hence, any plane perfectly aligned with the axis of the arrow will eventually present an incidence angle to the air stream.

The force created by that surface will change directions if the arrow shaft also rotates.

However, the CG of an arrow follows a projectile trajectory rather than fly.
That CG is located by the mid section of the shaft.
Any perpendicular force that appears at the head or tail will only rotate the whole arrow in pitch around the CG.

Besides playing with shapes at the head, I suggest increasing the stability margin (or distance between the CG and the center of drag) as much as possible.

That will give less mechanical advantage to any force at the head and more to the forces at the tail.

There are two ways to achieve a big stability margin:
1) Moving the CG forward (which increases the total weight of the arrow and reduces acceleration at launch).
2) Increasing the tail surfaces.

ok</p>

I believe yours is an excellent idea from the Aerodynamical point of view.

However, I have no clue about how it will work for the arrow's mission.

Keep us updated, it is quite interesting!

ok

In the rocket world, you always want fins at the back and not the front. Fins move the Center of Pressure back to create stability. The fletching on the arrow does the same thing.

In the missile world they often have small fins up front, frequently called turn fins. These can be deflected to cause the missile to turn but also add a destabilizing effect by moving the CP forward. For stability the CP must be behind the CG. Your planing blades are creating the same effect as turn fins on a missile, I would think mostly due to the "turn fin" effect, not due to destabilization.

The amount of force generated by a turn fin is governed by the side area of the fin and the angle of attack. Aspect ratio does enter into the equation but to a much lesser extent than the side area. Of course, this is what the non-planing head you show is designed to do, reduce the side area.

My take on it would be that for your application, most any shape you want would be acceptable and that the amount of planing would be mostly governed by the side area of the head.

Your rotating head idea brings up an interesting point, if you'll pardon the pun. How loose with the blades be? Would it be loose enough to weather vane? If so that could eliminate the planing. Of course at the same time, the springing of the arrow would also wiggle the blades from side to side and could affect accuracy.

I would also be concerned about the steep angles you have on the blades affecting penetration. As long as you're looking at movable blades, why not make them replacable. Then it's no problem if you wind up bending one.

There are rocketry forums at http://www.rocketryonline.com/ and http://www.rocketryforum.com/ . I'm sure they'd love to get in on the discussion.

Dave