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Jim Lux · May 16th 08, 12:23 AM posted to rec.radio.amateur.antenna

wrote:
On May 15, 11:46 am, "R.Scott" wrote:

I'm trying to restring my Dipole up in a Pine (the tree has grown
about 10 ft, and the rope needs replacing) so I got out my trusty
Surgical tubing wrist rocket and a 1oz egg weight tied to my 15# test
fishing rod/reel (only using the bottom eye).

No matter how far I pulled back I got about 2/3s of the way up. Im
estimating that is about the 35 ft level.

Shouldn't a wrister go farther ???? need bigger weight ?

Suggestions.

Sounds like a bigger weight would help.

An interesting question, and a fairly good answer within limits. I used
to do a lot of this sort of thing in the special effects industry (why
yes, you CAN launch a refrigerator with bungee cords).

Here are the physics things to think about.

The amount of energy you can put into the projectile is determined by:
a) How far you pull back
b) How hard you had to pull (i.e. spring constant of the tubes)

To a first order, Q = 1/2 * Peak Force * distance pulled back

That energy gets transferred to the projectile as kinetic energy.

To a first order, Q = 1/2 * Mass * V^2

Then, the energy gets converted back to potential energy (height) and
lost through air drag and pulling the line out. For slow speeds and
small things, the line drag is probably dominant.

By the way, if the pull is about 10 lbs and you pulled back 18", with no
air or line drag, a one ounce projectile will go to about 36 meters (120 ft)

A little bit of line drag makes a huge difference. a one ounce drag,
essentially doubles the weight of your projectile. For the example,
that would change your max height to about 19m (60ft)

If you had a 2 ounce projectile, on the other hand...with no drag, you
get to 19m, but the 1 ounce drag only brings you back to 12.4 m (40ft)

Air drag is a smaller order effect.

force in lbs = area in square feet * v(mi/hr)^2/391.. Your one ounce
weight is moving about 60 mi/hr at launch (9 lb/sf), but is less than a
square inch, so it's about an ounce of drag, but it drops off as the
square, so as the projectile slows, the drag drops off..

A heavier projectile goes slower, so that helps on the airdrag too.

Going back to 1 oz, no line drag, but counting air drag for 1" diameter,
27m or so height (about 2 seconds after launch, as a cross check)
1 oz, 1 oz line drag, air drag, 15.5 m
2 oz, no line, air drag, 1" diam, 18.8 m
2 oz, 1 oz line, air drag, 11.6 m

So this should give you a feel for the relative effects of line drag and
air drag and projectile mass. I did the calculations with a simple
spreadsheet in Excel that numerically integrates the physics equations
with very short time steps.