Mad strobe?

[vtsteam]

I get it, now!

 

 

Actually, I probably am doing this in a vacuum...... the online bowyer world is reluctant to observe rather than speculate. Which is why I'm talking about it here on Madmodders, where people are mad and mod.

[RussellT]

Hi Steve

You do get involved in some interesting projects.

I was giving this some thought  and I was wondering under what circumstances an arrow could outrun a bowstring.  My initial thought was that it couldn't because you're always applying a force to the arrow so you should always be accelerating the arrow.  However on reflection I decided that didn't have to be true.  As the bow and bowstring accelerates, viscous drag (air resistance), and possibly other resistance forces, will increase so that if the string is moving fast enough the bow may not be able to accelerate the string at all - and if the bows force is sufficiently non linear the string might actually slow down allowing the arrow to outrun the string.

Having said that it seems that doing the experiments in a vacuum might give a misleading result.   You clearly need a pressure, temperature and humidity controlled test chamber.

It also seems that bows where the draw weight decreases as you pull back the string ought to have the advantage of a much smoother acceleration of the arrow - and possibly increased accuracy as a result.

Now, back to the original question.  I think that trying to get some real data is the right approach and I think your strobe approach should work.  I did wonder whether you could approach the strobe the other way around and add flashing LEDs to the bow - or even in lines behind the bow but  I suspect it might be difficult to see the bowstring.  I think that approach is used in some other sports to track players actions.

Russell

[joshagrady]

I worried about how to contain all this in a dark enclosure, but duhhhhhhhhh, just remembered we have this thing here in Vermont called "night"!       So I can just do it outside with my present target setup.

So you've got that night stuff over there now too, huh?  It's amazing how, in this age of the internet, advances made in one part of the world will be adopted just hours later in another.

More critical is firing the bow and the strobe together. I may just make something like a fixed crossbow stock that accepts different sized bows, and has a trigger for both arrow release and the strobe trigger.

I've seen a wrist bracelet/bowstring "trigger" (e.g. this one) that could easily be modified to include the sensor to the Arduino, so that isn't the important part.  The advantage I do see to the crossbow stock is that it will eliminate variation in your pull (or draw, or whatever the technical term is), thus allowing you to more easily perceive change, establish a control/baseline, etc.

[vtsteam]

Thanks Russell! I actually don't know if the string will be outrun. Well actually it must at some point or the arrow would never leave.

Here's just a brief list of conditions that would affect it:

Mass of the arrow varies
Draw weight of the bow varies
Force distance rate varies (ie how much force it takes to draw a certain distance)
Degree of stack varies (stack is a rapid increase in draw weight when near maximum extension -- and yes most bows are non-linear when drawing)
Mass of limbs varies (affecting how fast limbs can accelerate during the release)

Soooooo, we need a picture to see who wins in all of the above.

Just an example of a possibility (not reality) a bow increases in draw weight by 2 pounds for every inch it is drawn past brace height of 6". It stacks to about 5 pounds for the last 2 inches at 28" draw. The arrow is 10 grains per pound of the bow's draw weight of 50 pounds. So the arrow is 500 grains. The limb tips are relatively massive so relatively slow to accelerate on release.

Question, does the arrow outrun the string before the nock reaches brace height, and if so where does it do that and at what velocity?

Josh, yeah I'm curious about that night thing -- did you guys start that or did we?

re. trigger and support for the bow -- support will need to be really solid to get good pics of the string for measurement purposes, so I won't be holding it. Might just try a microswitch in contact with the mechanical arrow release trigger, and then tune the Arduino's timing after seeing the first photos if there's any lag. I could also trigger early and bracket the flashes, if needed. Now there is no economical reason for 8 or 10 flashes only, as there was with the first idea of individual photoflash units. I could do 20.

s.