Fins and nosecone to Stabilize Cylinder Descent 3

[oceanman]

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

 

[MikeHalloran]

A long time ago I was involved in building model rockets and in archery. Both kinds of projectiles are fin- stabilized, and there is a simple rule to follow and a simple way to measure what you've got. There's no reason the same stuff shouldn't work in water.

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

 

[zeusfaber]

Mike's guidance is particularly helpful if what you're trying to prevent is tumbling end over end when travelling in one direction. In your calculations, remember that the "centre of pressure" for a tethered object is determined by both the hydrodynamic forces on the fins, and the tension in the suspension line. Your Cp will therefore move toward the cable at low speeds, and towards the fins at high speeds.

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.

 

[IRstuff]

Your goal is to prevent the cable from wrapping itself, not necessarily to stop the rotation, although, that may be required for other reasons.

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

 

[GregLocock]

"I do not want to add a slip ring for the electrical connectors, as those are expensive."

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.

 

[oceanman]

Thanks for the advice so far. I only care about spinning about the the axis, as that seemed to be the problem on previous deployments. with the comm wire wrapping around the com cable. Yes, putting a swivel in one spot concentrates the wrapping close to the pivot point. Maybe something can be done with the location of the pivot point to minimize the comm cable from wrapping as it did before. Yes, I believe a slip ring will work very well, but an underwater version costs lots of money. 2.4 GHz WIFI radio works for only a matter of millimeters in salt water. Other frequencies may work better. Visible and IR are options, but I would still prefer just to somehow stop the rotation, as it is perhaps simpler.

 

[GregLocock]

The problem with fins is that they don't actively control the spin. So the thing may still spin slowly round. That may be acceptable.

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.

 

[MikeHalloran]

He's already got two cables. It appears that the swivel on the suspension cable is not swiveling, so as the tension in that cable untwists it, the swivel rotates the test rig, and that gets the comm cable wrapped around the suspension cable.

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

 

[seatrendgokhan]

oceanman,

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.

 

[oceanman]

GODHAN,

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

 

[btrueblood]

Are you using a "non-rotating lay" cable (I think that's the right term)? All "standard" wire ropes have a tendency to twist & spin the load as they wind off of a drum, but there are special rope/wire braid lays that can reduce this tendency.

 

[colinp]

If you are measuring static pressure rather than line length, don't drop it straight down. Build a glider where the center of lift is offset axially from the center of gravity and the rolling moment always tends to keep the CG down. A vehicle with dihedral wing fins and an inverted vee tail comes to mind.

-Colin

 

[Compositepro]

I cannot see why your cylinder would spin if it is symetrical. It is much more likely that your cable is twisting as btrueblood noted. Your problem may be solved if you do not use any swivel. All cables will torque under tension and that will vary alot depending on cable design and construction and how it was handled in the past.

 

[GregLocock]

colinp

neat!

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[liviebudgie]

With darts and arrows the fins are used to create spin as this increases accuracy,(ballistics)
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.

 

[unclesyd]

You might want to try an offset fin stabilizer. I would put your cylinder on one side of the swivel and on the other side I would offset a fairly large vertical fin with it's area being greater than the aerodynamic profile of the cylinder. Hopefully the fin would tend to keep the whole rig from rotating. It also should help in stabilizing in mid water.

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.

 

[oceanman]

unclesyd,

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.

 

[unclesyd]

I have been in the windmills a little while this morning. Several things have fell out.

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.

http://www.sonardyne.co.uk/Support/Brochures/acoustic_release_brochure.pdf

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.

http://www.specialtydevices.com/Products pages/fairwrap.htm

 

[waross]

As long as you have a swivel the com cable will break. If you put in slip rings the com cable will probably wind around the hoisting cable but will probably not break. You will get your object down with no problems but may have issues retrieving the rig. That is the cables will probably wind together and be difficult to seperate even though the com cable doesn't break.
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