Jump to content
Vodkaman

Glider theory

Recommended Posts

As I do not build gliders or jerk baits, all that I can do is throw a lot of theory out there, to help you understand how the lure works. Understanding the theory helps the builder to design a lure to take advantage of the forces accordingly. Of course, experienced glider builders will have already figured this stuff out even if they do not know the reasons why their lures work. Experience is a valuable tool, theory only gives you a ‘leg up’ at the start.

As you have already figured out, this is a very complex issue with multiple factors to be taken in to consideration.

The apparent ideal solution for a lure to swim a long distance with efficiency is an arrow with all the weight at the front. But we already know that this would not work in water as it does in air because of the nose down attitude, like I said; multiple factors. Also, the super aerodynamic shape of an arrow is designed NOT to produce vortices.

When we fire an arrow in air for maximum distance, we apply great force and we aim up at 45°, and due to air resistance, the arrow falls at 70°+. Target sports for darts and archery only use the top of the flight arc. Also the arrow is designed not to swing from side to side, a definite requirement of the lure.

As for the lure; we want it to travel in a straight line as far as possible, then on the next pull, we want the same again only in a different/opposite direction (left/right).

So, what causes this desired change of direction? The answer is vortices, my favourite subject.

A waggling lipped lure generates a rapid series of vortices that cause the lure to waggle left and right. The sharp lip causes vortices to be created at a relatively low speed, and the theory of the ‘Kármán vortex street’ causes the vortices to rapidly alternate left/right. But still, the lipped lure requires a minimum speed to operate.

The lipless glider still creates vortices but has a much higher minimum speed to create the vortex. The operation of the lure is to tug or jerk the lure. A single vortex is created and no more as the lure is already slowed below the vortex threshold.

This swirling vortex sucks on the rear half of the lure body causing it to change direction. The next jerk causes the vortex to form across the back of the lure and sucks it in the opposite direction. As the lure slows down, that single vortex is still there, working on the lure, sucking it further around.

This effect can be seen on multiple section swim-baits; a steady, constant retrieve causes alternating vortices that act on the rear of the lure causing that beautiful snake action. BUT, if you jerk the jointed swim-bait, the lure curls around even 90° and beyond. Check out the video, you can almost see the vortex sucking the lure around in the jerk sections with a little imagination.

The above is the basic mechanics of what is going on. Now we have to figure out how to use the mechanics, the theory, to make the glider lure swim how we want.

To start with, I use an analogy that I have talked about many times; Grab a 2 feet length of dowel in the middle in your fist. Rotate your wrist rapidly left and right. The dowel swings fairly easily. Now add ¼ pound of lead at each end of the dowel and repeat. The dowel is much more difficult to swing left and right. Now put the two weights at the center of the dowel and repeat. Once again, the dowel swings easily. This is the effect of inertia.

We want the glider to change direction but we want to resist the continuing change of direction. The solution is to increase resistance to direction change by increasing inertia. By placing weight at the front and rear we increase inertia and resist the change of direction. But as always, design is a compromise. If the inertia of the lure is too great then the change of direction will be minimal or even nonexistent. You may end up with a straight swimming torpedo.

Another feature is the depth of the lure body that the suction of the vortex acts upon. You may think that a deeper body with a larger surface area would resist the side movement, this would be incorrect at least according to theory; the suction force of the vortex acts on the side surface area of the lure, reduce the area and reduce the force.

But yet again, design is a compromise. If you reduce your lure to a torpedo cylinder, no vortex will be created in the first place. If your lure swings excessively as it slows down then consider reducing the body depth. The reduced depth will also reduce resistance to forward motion. Once the glide motion clears the vortex, it will travel aerodynamically like a torpedo. We only require the vortex sucking effect at the initial tug of the lure, if the glider can swim clear of the vortex then it will continue in a straight line for more distance.

If the glide distance is short and the lure continues to turn; reduce body depth and/or extend the weights to front and rear.

If the lure does not change direction then no vortex has been created, you have a torpedo. You can add a flat to the top of the nose to help the vortex form, or increase the depth on the next build. Gliders need to naturally float horizontal, but the rest is a compromise between body depth and ballast distribution.

Dave

 

  • Like 4

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...

×
×
  • Create New...
Top