Fins and nosecone to Stabilize Cylinder Descent
Fins and nosecone to Stabilize Cylinder Descent
(OP)
I have a pressure vessel test rig that I need to lower down to about 400 meters in seawater. It is pretty much just a 12 inch diameter pipe that is about 6 feet long. It has end caps, so the whole thing is pretty much cylindrical. There are a few things attached to its exterior, so it has a tendency to rotate when it is lowered into the ocean (in a vertical orientation). We use a swivel to prevent the rotation of the cylinder from twisting the cable onto which it is attached. Now, we need to attach a communication line to this mess, but the electric lines gets wrapped around the cable and fails. I do not want to add a slip ring for the electrical connectors, as those are expensive. So, I thought that putting on fins and maybe a nosecone would help to prevent the rotation. Is anyone aware of any guidelines that I can use to properly size and locate the fins? I am just looking for general guidlines, such as should the fins go on the top, bottom, or middle of the cylinger? How big should they be? Should there be three or four of them?
Thank you.
Oceanman
Thank you.
Oceanman





RE: Fins and nosecone to Stabilize Cylinder Descent
The rule: The center of gravity has to be ahead of the center of pressure. (So that the fins will produce a stabilizing couple when the projectile is not quite aligned with its direction of flight.)
Corollary: The degree of stability is proportional to the distance between the two centers.
Simple measurement: I'll assume that you can figure out how to measure the axial location of the center of gravity. You measure the location of the center of pressure in the same way, except that you measure the CG of a uniformly thick silhouette of the projectile's profile.
I.e., the fins should be on the 'back' end, any heavy components within should be toward the 'front', and four fins are better than three fins of the same size.
You might want to put an encircling tube around the fins to reduce their propensity for fouling cables and stuff.
Mike Halloran
Pembroke Pines, FL, USA
RE: Fins and nosecone to Stabilize Cylinder Descent
The problem here sounds like one of spinning about the axis of travel - and in an application where you have to travel in both directions.
A pair of long fins sticking out more or less anywhere should help to damp the rotation - just needs a bit of care to make sure they don't end up acting like a propellor. If the source of the spin is hydrodynamic, then reducing the winching speed might be helpful.
Instinct says that if you're going to attach a fixed comms cable to the bomb, you should probably remove the swivel in the lowering line - doesn't the swivel concentrate all the twist into one section of the comms lead?
Do you need the cable to be absolutely plumb? I'm toying with a mental image of a vee pair of fins near the nose (and possibly an offset attachment point) to make the cylinder descend in a sort of shallow dive, offsetting the cylinder axis from the cable axis. This might improve stability on the way down, though I can't see it being so helpful when you go to haul back in.
A.
RE: Fins and nosecone to Stabilize Cylinder Descent
Therefore, you might want to look at using a slip ring. Essentially, it's two concentric cylinders with metal sliding contacts on the circumference. If your comm link is very high frequency, you can even get slip-rings with fiberoptic joints.
TTFN
RE: Fins and nosecone to Stabilize Cylinder Descent
How about a radio link then?
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Fins and nosecone to Stabilize Cylinder Descent
RE: Fins and nosecone to Stabilize Cylinder Descent
If not you could go to an active controller - using a compass to detect spin and a gyroscope or trim tabs on the fins to control it.
Or you could use two cables on a wide spacing, but that would be a nightmare.
You could use an ultrasonic coupler rather than radio waves.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Fins and nosecone to Stabilize Cylinder Descent
Maybe terminating the comm cable to a long radial outrigger, and paying out the comm cable from the other end of the boat, while keeping unrelenting tension on the comm cable, would produce enough moment to overcome the swivel's friction and keep the package aligned. If the comm cable can take the tension.
Maybe a compass and servomotor within the test capsule could actively rotate the suspension cable termination to maintain alignment with the Earth. Or, such a device could be added on as an active swivel (de-swivel?), with its own battery and enclosure.
Mike Halloran
Pembroke Pines, FL, USA
RE: Fins and nosecone to Stabilize Cylinder Descent
Why dont you consider to attach adaqute rubber blocks which are capable to float in seawater, to the area which is supposed to stay on top (upright position).
GOKHAN
Naval Arc.& Marine Eng.
RE: Fins and nosecone to Stabilize Cylinder Descent
It does stay vertical in the water column because it is cylindrical and has more weight on the bottom than top. This issues is rotation about the vertical axis. Thanks for the comment.
Oceanman
RE: Fins and nosecone to Stabilize Cylinder Descent
RE: Fins and nosecone to Stabilize Cylinder Descent
-Colin
RE: Fins and nosecone to Stabilize Cylinder Descent
RE: Fins and nosecone to Stabilize Cylinder Descent
neat!
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Fins and nosecone to Stabilize Cylinder Descent
To stop the probe spinning copy what the free divers do and put a weighted cable down and slide the probe on hoops down this.
If the vessel is moving add fins to the weight and probe acting like a weather vane.
RE: Fins and nosecone to Stabilize Cylinder Descent
I have seen such a device, if only I could remember where.
How and where are you going to launch this device?
Somewhat anecdotal:
When we experimenting with very deep water(150-200 fathoms) bottom fishing. We used several methods to prevent twisting of the SS cable and the bait tangling. One was the a variation of a Navy weight, a rain drop or tear drop shape.
The other was a variation of kidney shape lead weight with the largest (heaviest) part inline with the cable. The baits were a the small end and trailed upward.
Another was a 1/4" SS wire triangle (3' equilateral triangle) with the cable and weight (Navy) on the baseline and the bait on apex.
We had to overcome numerous problems getting the baits to the bottom. One problem not mentioned above is the currents, a big factor as the cable diameter increased thus increasing the strumming/shimmering of the cable. We could only attempt to get the gear down when the rotary tides were slack.
RE: Fins and nosecone to Stabilize Cylinder Descent
Thanks a lot. Your idea may be a very good one, based upon our scale modeling of what is going on. It turns out that the cable may be rotating more than the cylinder itself. When you think about this, it sort of makes sense, since the cable has less mass and may have some built up torsional tension. I do not think this explains the whole issue, however. Your idea may stabilize the cable from rotating! I would probably add a fin onto the cylinder too, to make sure it does not rotate. We have been deploying our unit off of San Diego in 400 meters of water. We are using a davit to get the system over the side of a very large fishing boat, and then we use a standard hydraulic reel/cable set up to put it down to the bottom and bring it up. With respect to your comment on ocean currents, the boat drifts at about .5 knots, which always induces a current, even if there is not a natural one present.
RE: Fins and nosecone to Stabilize Cylinder Descent
One of above mentioned non-rotation cables are crane or elevator cables. The cables have some weird construction, (8x25).
One thing we did was hang the weight (rigidly) below the gear. This was to add mass to rig in hopes of dampening the tendency to rotate.
Here is a picture of a device similar, more alaborate, to what I was trying to describe. Looking at it I believe I would put your cylinder in the middle and extend the fin an its counterweight form the cylinder. Note the additional fin and balancing wight on the stabilizing fin in the picture.
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Here is one solution to some cable problems. I believe that this material can be applied at intervals and not required on the overall length.
h
RE: Fins and nosecone to Stabilize Cylinder Descent
If you put in the slip rings you will probably find that the cables still wind together and have the same issues when you try to retrieve the rig.
I will be surprised if you can prevent the cables from twisting together but you can probably prevent the com cable from breaking.
If the hoisting cable is twisting, and it probably is, then it's pointless to try to stabilize the rig.
As I see it you have two issues. One is the com. cable winding up and breaking at the swivel. The other is the cables winding together. If your hoisting cable is twisting rather than the rig rotating then the swivel will make the problem worse.
Suggestions.
Try without the swivel.
Look for a com cable strong enough to lift the rig.
Look for a com cable robust enough to wind up with the hoisting cable.
Look for a lifting cable with internal com conductors.
The cheapest by far if it is usable in your application. Use a recorder and retrieve the data after you hoist the rig.
yours