Howdy ScatterShot,
That's a very interesting comment you made in pointing out the connection between spine and arrow length.
When a 32" arrow's spine is measured at 28", the resulting spine(?) (bendiness) will increase. So, yes, we can adjust arrow flex by changing the length.
Also noted, was the connection between arrow tip weight and spine.
To change the weight of your glued-on arrow tip will affect the flex (spine) of the arrow when it is shot.
I know of two other interesting, related factors that affect flight; first is the overall arrow weight. Bear Archery published an article stating that a 600 grain arrow has a much higher penetration than a 500 grain arrow. (so, to increase the arrow length will increase its weight - increase its penetration).
Secondly, to adjust the weight of the glued-on arrow tip affects, again, the overall arrow weight and the Balance Point of the arrow. See, we can measure (with a ruler) the Center Point of an arrow. (It's half of the total length). But the Balance Point is where, on the arrow, there is equal weight on eather side of that point - (like a seesaw). Balance Point can be adjusted by changing the weight of the arrow tip. And, when the Balance Point is forward of the Center Point, you'll get improved flight. Furthermore, when your set of arrows all have the same Balance Point, their flight will be very consistant.
So, we have Spine, Arrow Length, Arrow Weight, and Balance Point of the arrow to conceder.
(Also, I think, the more red stripes you put on your arrow, the faster it goes - I know this is true for race cars.)
Really, that's more than I know about arrows. But I've been doing some reading and thinkin' about this stuff.
Now, I'm going outside to get started on building that Spine Tester.
Thanks for listening and for your comments,
Canoe