The physics of trim and aerodynamics

[fisherdvm]

Too bad we don't have a good chapter on this matter. Any aerospace/submarine engineers out there?

My belief is that trim is a combination of static forces and dynamic issues. Center of gravity is changed every time you add air to the BC, and depends on the shape of the BC, and where your weight is suspended.

The less air your add to your BC, the less this bubble of air will affect your center of gravity as you dive. So, without adjusting anything else, your trim is best if you use the minimum amount of weight, and wear very little neoprene in your wetsuit. In this way, a trim tropical weather diver might have great trim with his BC set up when diving with swim trunk, and yet have terrible trim with the same BC and 25 lbs of weight with a dry or thick wetsuit (which compresses at depth, forcing the diver to add air to his BC to make up for the lost of buoyancy). So when an instructor tells me on SB that it is better for a new student to be overweighted, I just shudder at the illogical thinking. It is better for he/she to struggle with perfect weighting and learn to manage a small BC bubble, than to struggle with overweighting and the drastic volume change of a large BC bubble.

Kick style seems to affect trim also, as it seems to me that the flutter kick requires alot more weight shifted to the feet area and requiring more weight nearer the feet or away from the head. I really wonder if this is why many people prefer the frog kick.

BC design seems to be a big factor, as the back inflate tends to have equal sized air distribution around a horse shoe or elongated doughnut shape, which does not concentrate air around the waist like some jacket wrap around BC. And second, the back inflate allow you to move your BC forward or backward, to best match your weight distribution, or your kick style.

The perfect trim would be a flat BC with the weight perfectly centered and distributed under it, and a perfect system for keeping the air equally distributed in this BC. I wonder if advanced BC's can be designed with multiple expanding elastic cells which response to volume changes equally over the whole surface area, rather that a bubble that can float to the butt or to the shoulder. As close as we can get to this is a heavy steel backplate under a back inflate BC.

I wish someone well knowledge in this area can write a chapter on the matter. It certainly will help divers more than simply "Get a BP/W set up". As this simply allow one to get away from thinking, and jumping to the solution. It seems that we have so much personal opinion here on SB that somehow, the objectivity gets lost.

 

[Gene_Hobbs]

Well, not for nothing but... HYDRODYNAMIC DRAG

 

[fisherdvm]

Thanks... I think about this every time I see a student swimming at 45 degree into the current, head up. What's worse are the ones floating his feet upward, and slowly ascend as he kicks. The opposite of two extreme, which leads to alot of drag.

THE INFLUENCE OF HYDRODYNAMIC DRAG ON UNDERWATER SWIMMING PERFORMANCE.
Authors: Dobbins, T
Blatherwick, S
Gilbert, M
Evans, M
Samways, S
Keywords: equipment
human
HYDRODYNAMIC DRAG
UNDERWATER SWIMMING
PERFORMANCE
Issue Date: 2001
Abstract: INTRODUCTION: Theoretically the pitch angles that divers adopt whilst swimming, and the equipment that they wear and carry, influence their swimming performance. The aim of this study was to assess the magnitude of these influences on swimming performance at operationally realistic speeds. METHODS: A life-size mannequin, moulded in an underwater swimming posture, was towed through the DERA Haslar (Gosport, UK) Ship Tank. Hydrodynamic drag was measured via a block gauge mounted in the mannequin which was calibrated prior to the trials. The runs were conducted with the mannequin wearing a range of equipment and adopting pitch angles of 0 deg, 10 deg and 20 deg. RESULTS: The effect of diver pitch angle was assessed by comparing the drag at positive pitch angles with drag at zero pitch. It was found that a head up pitch angle of 10 deg increased drag by ~ 30percent, whilst a 20 deg pitch angle increased drag by ~ 130percent. To compare the potential effects of differing breathing apparatus designs on swimming performance, a criterion "sustainable swimming drag" was established. This was equivalent to the drag associated with swimming at a speed of 0.50 m.s-1 (~1 knot) using front mounted breathing apparatus and was found to be 17N. The swimming speeds for back mounted-low profile and back mounted-large breathing apparatus, producing drag equivalent to 17N, were 0.44 (~0.9 knot) and 0.39 m.s-1 (~0.8 knot) respectively, a potential reduction in sustainable swimming speed of 12percent and 22percent. Similarly the effect of carrying equipment (with front mounted breathing apparatus) was compared with breathing apparatus only. Carrying equipment potentially reduces sustainable swimming speed by 0.1 m.s-1 (~ 0.2 knot) or 20percent. CONCLUSION: The design of equipment worn and carried by divers, and the pitch angles adopted when swimming, have the potential to reduce underwater swimming performance. Equipment designers and operational planners should take account of these influences and endeavour to minimise hydrodynamic drag, thus increasing operational effectiveness.
Description: Undersea and Hyperbaric Medical Society, Inc. (UHMS > Home ( DNN 3.2.2 ) )
URI: Rubicon Research Repository: Item 123456789/945
Appears in Collections: UHMS Meeting Abstracts

 

[diver918]

It was found that a head up pitch angle of 10 deg increased drag by ~ 30percent, whilst a 20 deg pitch angle increased drag by ~ 130percent.

That increase is pretty crazy...thanks for sharing.

 

[Blackwood]

My belief is that trim is a combination of static forces and dynamic issues. Center of gravity is changed every time you add air to the BC, and depends on the shape of the BC, and where your weight is suspended.

Almost correct. Your C_G will hardly change when you add or remove air. Rather, the center of LIFT may change.

For a diver, trim while in equilibrium is the major concern. With the exception of drag, I think we can discount dynamic forces. Do we create some hydrodynamic lift or downforce every time we move. Probably. Is it predictable or significant? Probably not. So let's say trim while in vertical static equilibrium is the major concern.

If we only consider two point loads (weight at mass center and lift at lift center), you'll always have a moment unless the mass center is at the lift center. The further apart they are, the more significant the moment is.



That said, the lift center is in one way a function of trim. If you are head up, the air in your BC and thus the C_L will move forward. Vice versa for head down. However, your Mass Center is only a function of you. Few people move weights around while they're diving, and the tank (variable mass) should be more or less equally distributed around the mass center. The main thing that allows us to alter our C_G is leg position.

If I'm in horizontal trim and want to pitch up, I can straighten legs. That moves my C_G aft of my C_L and the moment pitches me up. If I want to pitch down, I can pull my legs in towards my butt for the opposite effect.

 

[Papa_Bear]

Isn't it the old Archimedes's principle thing? Displacement creates more drag, so if you put air in your BC it tends to go to the top of it tipping the diver up as well! Less efficient more mass and more drag! Neutral buoyancy means your displacing just enough water volume to stay at a given depth in the water column! So where is your mass and where is your wight placed? Because we are all built differently it is an individual trial and error situation!

 

[ClayJar]

When speaking about horizontal trim, it is necessary to consider the concepts of stable and unstable equilibrium. For those of us a few years removed from thinking about high school physics, a little refresher is in order.

As you can probably guess, "equilibrium" is the term science uses for a state in which everything balances out. For the physics of trim, we're talking about the forces of weight, buoyancy, thrust, and drag. We'll limit the discussion to hovering or moving at speeds where drag can be ignored, as that's a reasonable assumption which will simplify the examples.

I mentioned stable and unstable equilibrium. Now, what do those terms mean? The easiest way to think about it is by example. Consider the chandelier over a dining room table. It isn't moving, so it must be in equilibrium. Now, give it a little push. It starts swinging back and forth around its previous equilibrium position, and eventually it'll slow all the way down and settle right back where it started. That equilibrium position is said to be "stable" -- after a small disturbance, the system will return to that equilibrium point.

On the other hand, consider a pencil balanced on its point. If you balance it *perfectly*, it would stay vertical, but with any tiny deviation from perfectly balanced, it will fall over. The balance point is still a state of equilibrium, but that equilibrium is said to be "unstable" -- after a small disturbance, the system will *not* return to the equilibrium point.

(I thought about linking to a physics page, but I didn't want to scare anyone away. )​

Now, where does buoyancy underwater fall? Well, consider a small medicine ball which is filled with sand and some air. At the surface, it floats, although not by much. Once you get it below about 10 feet, the air inside is compressed enough that it sinks (as it is the same weight but displacing less water). Somewhere in between, there is a depth at which the air inside has compressed enough that it will be precisely neutrally buoyant. So, let's take our imaginary medicine ball and go to that depth in our imaginary pool.

Here we are, then, with our neutrally-buoyant medicine ball about, say, nine feet deep. We let go of it, and just as it should, it basically just hovers. It seems, however, that when we let it go, we pulled it ever so slightly deeper (maybe by the water movement when we pulled our hands away). As it's slightly deeper, the pressure's slightly more, which compresses the air slightly more, which reduces the buoyancy ever so slightly, which makes it "heavier", which makes it sink faster, and so on all the way to the bottom. Let's give it another try.

Okay, so we're back here at nine feet with our neutrally-buoyant medicine ball. We let it go, this time being careful not to pull it down... but it seems we over corrected. It ends up just a millimeter above the depth it would be neutral. That means the pressure's just ever so slightly more, which means it's positively buoyant... yep, all the way to the surface.

Obviously, being neutrally buoyant is an equilibrium state. (If it floats at eight feet deep and sinks at ten feet deep, somewhere in the middle, there has to be a spot where it neither floats nor sinks. That's a mathematical fact.) As even a slight perturbation from that precise spot is enough to set off a chain of events driving it ever further from equilibrium, we know that it is an unstable equilibrium.

(By the way, it's an 8-pound purple rubber-jacketed medicine ball that I bought from Walmart. :biggrin​

So, our buoyancy underwater is an unstable equilibrium. That doesn't usually bother us, as we tend to like to *breathe* anyway, and breathing changes our buoyancy as well. Divers use their BC to get close enough to neutral buoyancy, and then they make small changes to their breathing to keep "balanced" on the unstable equilibrium of neutral buoyancy. Where the fact that buoyancy is an unstable equilibrium really shows up, however, is trim.

Let's consider three divers: Tom, Rick, and Harry. Tom is a skilled diver diving a drysuit, backplate, and wing. Rick dives a wetsuit and a jacket BC, but his skills are also quite well honed. Harry, on the other hand, is doing his first ever dive. He also has a jacket BC, but he thinks he may need a lift bag to help when he does the weight belt removal and replacement. Still, he read somewhere about trim, so he'd like to have some. So, let's consider their trim, one diver at a time.

Tom has it easy. He lets a little air get into each leg, and he arches his back ever so slightly so that the air in his wing will move toward his shoulders. He's basically hanging from a tripod, with three major points of buoyancy and his center of weight below and between. Buoyancy is still an unstable equilibrium, of course, but he has several ways to manage that.

The center of buoyancy of his lungs is slightly forward of the his center of weight. If he starts to pitch forward, he can adjust his breathing for a moment to cancel that out. If he starts to pitch backward, on the other hand, he can adjust the arch of his back slightly. As he bends slightly forward, the center of buoyancy of the air in his wing moves slightly aft. (This gives him a tremendous range of pitch adjustment, as the center of buoyancy of air in his wing can move all the way from right at his shoulders to down at his waist, or anywhere in between, with nothing more than a slight change in posture.) The equilibrium is still unstable, but he has considerable control available to help him maintain that equilibrium.

Rick doesn't have it *quite* as easy. His jacket BC's design does allow him to vary its center of buoyancy via posture changes, but its design is such that the range available is less. It simply behaves less like two long tubes. The shoulder area of his BC may not have sufficient volume for all the air to go there, and the shape of the air cell (as it is on him while being worn) is somewhat less conducive to posture-based control. (The jacket, however, has some other features that he absolutely adores.)

Of course, that certainly doesn't mean he can't maintain his equilibrium in horizontal trim. He holds his level hover like a pro. He doesn't have a drysuit, so he can't easily change the buoyancy of his legs during a dive, and without as much range of control over the center of his BC's buoyancy, he simply has to be more precise at trimming himself out. While Tom only has to be in the ballpark in order to easily reach and maintain the unstable equilibrium of horizontal trim, Rick has to get his weight placement quite close to ideal in order to maintain his unstable equilibrium.

(Interesting tidbit: When I'm in my drysuit, I have a much larger air volume. Buoyancy control takes more work, but I have much more control over trim.
When I'm in my wetsuit, I have much less air volume. Buoyancy control is much less work, but I have significantly less control over trim.
Neither is a problem for buoyancy or trim, but it's fun to note how they mirror each other. )​

Now, Harry, he's another story entirely. He is most likely carrying too much weight, and he has it all in a weight belt. He has no trim weight (not even an ankle weight on the valve or a couple hard weights on a spare cam band at the shoulder of his tank), and he certainly has some air in his BC. That puts his center of weight well aft of his center of buoyancy. Without finning, he cannot hope even to *reach* an unstable equilibrium, much less to maintain it.

Obviously, since he can't get to equilibrium without finning, if he stops, he'll pitch backward into a head-up position. Still, he knows that he wants to be horizontal in the water, so he just keeps swimming. He's horizontal, so it doesn't really *look* like he's wasting energy by swimming "uphill" all dive, but since he's using thrust from his fins to keep himself from pitching backward, he is in fact carrying at least a portion of his weight (instead of letting his buoyancy do the lifting).

The further Harry is from equilibrium (where the centers of buoyancy and weight match up), the more work he has to do to stay horizontal. While even Tom and Rick have to do a little work (i.e. the balancing act to maintain the unstable equilibrium), you could say that they are only maintaining the equilibrium, not actively contributing to it.

(Compare setting a (rental, hehe) tank down on its valve and keeping it from tipping over, versus picking up the tank and carrying it. )​

So, what does it all boil down to? Horizontal trim is an unstable equilibrium. To reach this equilibrium, a diver may need to move weight (or buoyancy) around. Diving horizontally is possible without reaching equilibrium, but while it looks the same, it's a lot more work. The ease of maintaining the unstable equilibrium of horizontal trim has quite a bit to do with the gear you're diving, but while you may need to be more precise in a wetsuit and jacket BC, you can still do it. And finally, even if you don't actually reach the equilibrium state, the closer you are, the less work your diving will be.

 

[PhilEllis]

What he said.....I think.

Phil Ellis

 

[roakey]

Almost correct. Your C_G will hardly change when you add or remove air. Rather, the center of LIFT may change.

AKA "Center of Buoyancy"

Not that we haven't covered static versus dynamic trim over and over for approaching a decade...

http://www.scubaboard.com/forums/basic-scuba-discussions/4911-getting-perfect-buoyancy.html (this is pretty good and a short thread)
http://www.scubaboard.com/forums/technical-diving-specialties/11998-dirf-sobering-experience.html (starting about reply 55)

Roak

 

[JeffG]

What he said.....I think.

Phil Ellis

You read the whole thing? Yeeeesh