A drag issue : to bungee wings or not to bungee

[PfcAJ]

AFAIK noone makes an 18# doubles wing anyway. Or a 30# one. Yet every time the amount of lift I should be using comes up, those two numbers get mentioned

So, let's do the numbers - how much lift does LH actually need?

Here's a post earlier in the thread http://www.scubaboard.com/forums/5491273-post124.html 104s are quite a bit larger than hp100s, too.

 

[cool_hardware52]

So was the problem the bungees or the OPV?

Please explain why bungees would have any effect on wing inflation if the inflation gas is ~150 psi (IP) above ambient

At the risk of being labeled an arrogant know it all......

You apparently don't have a clue about how wings work, or why they have an Over Pressure Valve.

Let me educate you. In normal operation a wing (or any BC) will have essentially *Zero* pressure differential between the gas in the bag and the surrounding (ambient) water pressure. Wings / BC's are not pressure vessels, they are not designed for high pressure.

Only when inflated fully, or to the point where it is constrained, will the internal pressure be greater than the ambient.

To ensure the BC / Wing does not burst, fail a seam, or corrugated hose, these inflatables are equipped with an Over Pressure Valve. These are simple spring poppet valves. The cracking pressure will be equal to the spring preload + the ambient pressure.

That prevents the welded bladder seams and other components from being subjected to absolute pressure differentials greater than about 2.5 psi.

And can you also tell litehedded how much lift he needs

LH is using a 55 lbs wing. That's about right based on the assumptions I've made about his configuration.

If you take the time to review one of my previous posts in this very thread I have already provided a simple straight forward means to determine wing capacity.

Look here http://www.scubaboard.com/forums/5491273-post124.html

If there is some part of this explanation you need more help with let me know.

Tobin

 

[Tortuga68]

At the risk of being labeled an arrogant know it all...... You apparently don't have a clue....Let me educate you

Well you've earned the arrogant part

 

[05TurboS2K]

This will be my first post, I've lurked a while though.

I know this is an old thread but..... instead of us all taking classes at a university and then making projections, why don't we just do a simple test?

Take two equal weight/size # bc (with rigs perhaps) and tie a lead weight to them and let em sink, see what falls fastest. It'd be easy enough to keep the proper angle of attack of the bc so I see no issue there. Simply test it empty, 1/2 full and then completely full. Then lets see the data and the gopro vid to go with it. If I'm missing a variable I'm sure it's one that could be solved easily enough. This kind of test would work at reasonable speeds that we divers move at instead of a 120mph wind tunnel for example. It'd also be a test preformed in salter water where we function.

DONE.

-Greg

 

[g1138]

This will be my first post, I've lurked a while though.

I know this is an old thread but..... instead of us all taking classes at a university and then making projections, why don't we just do a simple test?

Take two equal weight/size # bc (with rigs perhaps) and tie a lead weight to them and let em sink, see what falls fastest. It'd be easy enough to keep the proper angle of attack of the bc so I see no issue there. Simply test it empty, 1/2 full and then completely full. Then lets see the data and the gopro vid to go with it. If I'm missing a variable I'm sure it's one that could be solved easily enough. This kind of test would work at reasonable speeds that we divers move at instead of a 120mph wind tunnel for example. It'd also be a test preformed in salter water where we function.

DONE.

-Greg

Air and water are both fluids and will work in generally the same way. The only difference would be in how the vortex trails after it makes contact with the BC, since water will slow/deflect momentum faster than air.

You're still going to see the same results, more turbulence around the bungee wings. Think of it this way, a loose wing will eventually streamline when the water is pushing past it. So too will a bungee wing. The only difference is a bungee wing now has more surface area via the bungee, which will not conform to a streamline profile. It sticks out, creating turbulence and disturbing the streamline.

If you're not convinced, there are water tunnels that will show you the same results.


You can also buy a full octopus from the fish market and trail it behind you as you swim or just spin in a circle. It will conform to a streamline profile. Tie rope around the body and you'll see a lot more turbulence around it.
You can also do the same with a plastic bag if you're not going to eat the octopus afterwards.

 

[D_B]

depending on how the wing is bungeed makes a difference too ... if the bungee is not visible to the slipstream a smaller cross section of the wing might show different results ... see scubapro for different approach to it

 

[dwhthediver]

I dive double HP100s on the plate, and clipped for stages, my 60# Apeks wing works perfect. My single wing is a DR travel EXP, clipped to the plate. As I have said before, my gear doesn't have to be perfect, it has to be perfect for me. So, if bungees wings are perfect for YOU, then dive them, and if not, don't. The only exception, wreck penetration, I believe that the bungees could be an entanglement problem.

 

[Fritz01]

I'm not entirely sure if this was mentioned but people forget what looks streamlined often is not. Case: driving with your truck tailgate up is more gas efficient than driving with it down. While this seems counter intuitive because you look streamlined with it down the eddies created by the tailgate up make it more streamlined (via. Mythbusters). Or professional swimmers keep tiny cracks between their cupped fingers which create a pocket for drag. A rock hard cupped hand isn't as efficient (which is counter intuitive). Also in ballistics it's commonly known that the tail angle has a big effect on drag and making it too steep actually increases drag. ie. a bullet that just stops (like most handguns) is not as streamlined as a bullet that tapers down (think hunting round). I'm not sure if the eddies wouldn't help. Water on water flow has less drag than water on fabric usually.
Just giving the bungee'ers some help.

 

[Nemrod]

There have been various discussions about this I'm sure but I found this on the web and felt like sharing it.

To those who feel offended by this, tough luck



From: Bill Lais
Date: Wed, 11 Mar 1998 14:56:06 EST
To: rickhave@juno.com, cavers@www.geek.com

Subject: Re: Bungee Wings: A Real Drag?

In a message dated 98-03-11 14:42:13 EST, you write:

Recently, myself and several engineering colleagues were performing wind
tunnel tests on a new airfoil, under design within our division at
Grumman Aerospace. Somehow the subject of technical diving and buoyancy
compensation systems came up. We got to talking about the drag profiles
on certain manufacturers brands of BCs and one of the guys suggested that
we run some actual aerodynamics tests. A couple of the engineers are big
time mixed gas wreck divers with lots of experience on deep wrecks such
as the Doria and Republic. They were touting the benefits of the OMS
bungeed wings and I was arguing that the Dive Rite wings provided a
vastly improved knife edge profile with far less drag.

A simple surface area calculation showed that the bungeed wings had an
area of over 12 square feet inflated. This is over 4 times the area of
the Dive Rite wings. The huge box like profile caused immense drag and
horrendous turbulence when we observed it in the wind tunnel. The bungee
cords caused massive vorticial eddies all along the wings. It was very
interesting to observe the colored smoke trails streaming in chaotic
cyclonic patterns around the brightly colored bungees. It was almost
psychedelic. These results were in stark contrast to the Dive Rite wings
which showed very little turbulence and almost hydrodynamic behavior such
that the wings probably generate lift when there is current flow across
them. Needless to say, the extra inflator assembly on the OMS wings
created marked disturbances in the laminar flow field of the trace smoke.
When the wings were in the collapsed state the turbulent patterns were
even more spectacular, clearly discrediting the myth that the bungees
reduce drag by supposedly decreasing the surface area.

After the tests there were no more arguments except about what other uses
these wings might be put to. One of the guys suggested we employ them as
portable cushions for homeless people, another suggestion involved using
them as toxic waste storage containers.

There was much discussion and speculation about the personality and
physical characteristics of the rocket scientist who had designed these
wings. I suspect that the same individual carries a fire extinguisher in
the front seat of his car along with the spare gas can.

Rick

Taken from frogkick.dk, this is NOT my own work.

Next they can do an analysis of the Flying Nun's coif (hat).

Or professional swimmers keep tiny cracks between their cupped fingers which create a pocket for drag. A rock hard cupped hand isn't as efficient (which is counter intuitive)

Do I have to be a "professional" swimmer to know and do this?

I love threads like this, a fellow could make a career on this forum with this pile of stinkiosity. Unfortunately, or fortunately, depending upon one's vantage point, I am just not in the mood, maybe I will book mark it for the future like a squirrel hiding his nuts.

N

 

[Fritz01]

Haha No it's just more persuasive to reference pro's.