Page 1 of 3 123 LastLast
Results 1 to 6 of 15

Thread: Draw weight, arrow weight, speed & trajectory

  1. #1
    In the White
    Join Date
    May 2009
    Posts
    33
    Thanks
    0
    Thanked 0 Times in 0 Posts

    Draw weight, arrow weight, speed & trajectory

    Hi all

    Just thinking about arrow flight.
    I am wondering.
    For two bows of equal design but different draw weights do you think an arrow will fly the same if it is the same grain weight in proportion to the bow's draw weight ?

    Do you think that a 600grain arrow from a 60# bow will fly the same as a 400grain arrow from a 40# bow ?

    Assuming that we keep other factors, size of fletching, weight of pile, f.o.c balance, etc the same.

  2.  

  3. #2
    It's an X moo-mop's Avatar
    Join Date
    Feb 2006
    Location
    Letchworth
    Posts
    1,583
    Thanks
    0
    Thanked 5 Times in 5 Posts
    No.
    Even if you simplified arrows as a point masses going straight out of a bow with the same drag they would not fly in a way that is proportional to the draw weight of the bow with mass - the amount of energy a bow puts into an arrow is not directly proportional to its weight, but is related to the area under the draw force curve.

    However, it's even more complicated as arrows do not behave simply. The physics of flight actually involves a series of interacting variables, which you can’t keep constant between experiments (e.g. it's difficult to imagine not changing f.o.c. if you tried to keep point weight the same but change the total mass. And you would be varying the arrow spine too if you model real materials; esp if you stick with wood!). The maths is highly tricky and there is no complete model of how a recurve/longbow etc actually behaves published as far as I'm aware.

    Edit: However there are some quite nice basics around to get you started thinking:
    e.g. see http://margo.student.utwente.nl/sagi...s/archghh.html
    "A cow in a sailing boat gently moves which makes its Moo extremely smooooth."
    How to Speak Moo! by Deborah Fajerman

  4. #3
    In the Red
    Join Date
    Jan 2008
    Posts
    324
    Thanks
    0
    Thanked 11 Times in 11 Posts
    As Moomop says, it's very complex but the two bow/arrow combinations you refer to would be expected to produce the same initial arrow speeds if they were both made to the same standard ie working at the same efficiency.

    So a good wooden bow shooting 10gpp arrows and at 28in drawlength might manage 170+fps. The same draw length is key. And scaling down that good bow/arrow should still give 170fps. It's the air resistance that's the tricky bit.
    In practise, there might not be a huge difference.

    David

  5. #4
    In the White
    Join Date
    May 2009
    Posts
    33
    Thanks
    0
    Thanked 0 Times in 0 Posts
    The interaction is obviously complicated but I suppose what I'm getting it is,

    I've swapped bows a couple of times for heavier bows. Each time I have got flatter trajectories and my point on distance has increased. I know that with my current bow (and indeed any bow) if I shoot lighter arrows they fly flatter and go further.

    In trying to understand the physics the basics makes sense. Trajectory would seem to be a function of initial velocity and angle of launch affected by gravity and air resistance. If my understanding of physics is right the weight of the projectile or the amount of kinetic energy it holds doesn't affect the trajectory.

    This all points to two equal design bows getting the same trajectory with arrows suitable for them. Why then do all sorts of people talk of upping their bow weight to get a better trajectory and why do the warbow people talking on the other active thread here have results showing that the heavier bows are shooting further ?

  6. #5
    In the Red alanesq's Avatar
    Join Date
    Jul 2006
    Location
    Nottingham, U.K.
    Posts
    592
    Thanks
    0
    Thanked 0 Times in 0 Posts
    My understanding is:

    A heavier bow will shoot an arrow faster, so it has less time to fall before hitting the target (i.e. flies flatter) - this is why compound bows have a very flat trajectory, they shoot arrow at very high speeds
    - the old story of a bullet shot horizontal will hit the ground at the same time as one just dropped
    If you increase the arrows weight so it flies the same speed then the trajectory will be about the same

    The thing with the warbows is we are trying to shoot a specific arrow so to increase draw weight will increase distance because it shoots the arrow faster
    i.e. launch a livery arrow as far as we can
    We could just use a lighter arrow to get the same distance but then the arrow will have less "stopping power" (i.e. deliver less energy)

    This is what I think people often don't appreciate about warbows - unlike target type archery it is not just about reaching/hitting the target but also (and possibly most) about how much damage the arrow would do when it gets there

    btw - if you are worried about matching the arrow to the bow remember that you can adjust the arrows spine rather than it's weight

  7. #6
    In the Red
    Join Date
    Jan 2008
    Posts
    324
    Thanks
    0
    Thanked 11 Times in 11 Posts
    Jitsuka, There does seem to be a paradox here, and I bet a simple explanation- but I can't think of it.

    One small point is that a heavier arrow with the inertia to resist air resistance will outshoot a lighter arrow, both starting with the same speed, other things being equal, but not by a lot.

    When archers move up in bow weight, they would choose their new arrows according to spine, so arrow weight wouldn't go up in proportion. In fact the same arrows could be stiffened by reducing the pile weight. David

Page 1 of 3 123 LastLast

Similar Threads

  1. What draw weight to aim for ?
    By jitsuka in forum 3D & Field Archery
    Replies: 11
    Last Post: 29-10-09, 10:03 PM
  2. Draw weight?
    By durhamarcher in forum Compound Bow
    Replies: 3
    Last Post: 20-08-09, 10:17 PM
  3. Is draw weight relative to riser weight?
    By Mistake in forum Recurve Bow
    Replies: 27
    Last Post: 14-11-08, 08:55 AM
  4. Replies: 14
    Last Post: 30-11-07, 07:25 PM
  5. Draw weight
    By Cupid in forum Compound Bow
    Replies: 9
    Last Post: 16-06-06, 05:44 AM

Bookmarks

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •