Those laws of physics you lot love to..
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Category: Dinghy classes
Forum Name: Dinghy development
Forum Discription: The latest moves in the dinghy market
URL: http://www.yachtsandyachting.com/forum/forum_posts.asp?TID=12866
Printed Date: 05 Jul 25 at 9:40am
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Topic: Those laws of physics you lot love to..
Posted By: iGRF
Subject: Those laws of physics you lot love to..
Date Posted: 27 Sep 17 at 12:17pm
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Accuse me of breaking...
http://www.telegraph.co.uk/news/science/science-news/9035708/Cambridge-scientist-debunks-flying-myth.html" rel="nofollow - Never stop questioning them ------------- https://www.corekite.co.uk/snow-accessories-11-c.asp" rel="nofollow - Snow Equipment Deals https://www.corekite.co.uk" rel="nofollow - New Core Kite website |
Replies:
Posted By: JimC
Date Posted: 27 Sep 17 at 12:34pm
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What he's on about is nothing to do with the laws of physics. I suspect its just an over-simplification - or lie-to-children - that somehow resonated with people and became over egged.
I suppose the classic example is "longer boats are faster than shorter ones", which is a classic law of physics thing. And those laws of physics are still true, but they need to be qualified with "provided its a pure displacement craft operating at speeds where wave making drag is more important than other forms of drag". On the other hand its always good to question "what-everybody-knows" and try and establish the real truth. Look at the old "everyone wants fleet racing" convention for an excellent recent example. |
Posted By: 423zero
Date Posted: 27 Sep 17 at 5:43pm
| This was discussed at some length on CVRDA a couple of years ago, I seem to remember this is same scientist who with one of his colleagues de-bunked Bernoulli, then later in their paper admitted their theory relied on parts of Bernoulli's theory to work. |
Posted By: Rupert
Date Posted: 27 Sep 17 at 5:53pm
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It has always been blatantly obvious that the air particles don't have to reach the back together, and I've never understood why it gets taught that way. Of course, knowing something must be nonsense, and knowing WHY it is are 2 very different things. I'll leave the latter to people in smoke filled labs. ------------- Firefly 2324, Puffin 229, Minisail 3446 Mirror 70686 |
Posted By: Riv
Date Posted: 27 Sep 17 at 8:11pm
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Check out the original paper. http://iopscience.iop.org/article/10.1088/0031-9120/38/6/001/pdf |
Posted By: laser193713
Date Posted: 27 Sep 17 at 10:05pm
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It's one of those badly taught subjects, I don't think anyone who has even the slightest knowledge in physics would have thought that the air molecules have to reach the trailing edge at the same time. Another interesting boat related one is the common misconception that for a boat to be "stable" the centre of gravity has to be below the centre of buoyancy. I was quite shocked at how few people understood this during my naval architecture degree.... moral of the story, never trust an honours degree naval architect, some may never have even been on a boat. This became very clear when one of the people on my course designed a 30 foot cruising yacht as part of a final year project that had no keel, when questioned they asked "What's a keel?". Shocking right? They are now a highly "qualified" naval architect working in the defence industry, go figure! It's always frustrated me that as you progress further and further through education you quickly realise that most of the things that you were taught at the previous stage of your education were basically lies, or rather deliberately wrong to make the core understanding easier to grasp. Take for example the idea of atoms, as a child you are taught that they are the smallest things that exist. Then you reach gcse level and you get taught about protons, neutrons and electrons. Then you learn that electrons are grouped in rings of 2,8,8 etc... oh no, of course when you get to a-level its not that simple, in fact that's barely even true either. I never got further than that in core science but I suspect what I was taught at a-level chemistry/physics is also mostly gibberish. When it comes to aerofoils/wings its simply a case of "steering" the air and harvesting the resultant force of this change in direction. This can be achieved with a flat sheet (paper aeroplane), a curved sheet (sails) or a foil with thickness (aeroplane wing). The intricacy comes from the properties required from the lift, whether it be stability, ultimate low drag etc. Then it is a case of designing or choosing a section that gives the right balance of stability, lift for a given range of angle of attack, and the minimum drag during these conditions. That's where the science (art) comes in!
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Posted By: craiggo
Date Posted: 27 Sep 17 at 10:32pm
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As said, the speed of flow was a simple way to explain the situation to school kids, but the more detailed analysis termed "circulation theory" had been around for a lot longer than the article suggests. ------------- OK 2129 RS200 411 |
Posted By: RS400atC
Date Posted: 28 Sep 17 at 9:27am
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The article bangs on about air flowing over the wing and under it, but in general, the wing is moving through relatively still air. The air under the wing must 'flow under' the wing at sufficient speed such that the wing is not pushing an ever-increasing wodge of air in front of it. If it is slowing locally, it must be getting denser as the air molecules are conserved. Bernoulli's theorem is not wrong, it's just not the whole story. |
Posted By: Cameron Winton
Date Posted: 28 Sep 17 at 2:12pm
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I cant watch the video but I think his comment about sails is utter crap.
The video appears to show an ideal aerofoil at a high incidence of attack. This can mean that laminar airflow has been lost and separation has occurred. This creates turbulence with low mean speed and increases the differential pressure across the aerofoil. The article is very poorly written. In aerodynamics. It has been understood for 100 years that we cannot model directly the behaviour of an aerofoil. The whole "two molecules of air" explanation is about conservation of mass, a fundamental basis for Bernoulli's equation. It still works as an explanation in some circumstances, especially in 2 dimensions (The classic cross section of an aerofoil with streamlines.)It also assumes an ideal fluid which air is not (eg it is compressible) Once in 3-d, the maths becomes utterly horrible and relies on the empirical (The Navier Stokes equations). I cant remember the relationship between Navier Stokes and Circulation but it is the accepted conceptual explanation but cannot be directly proven by maths. A sail is a very non ideal aerofoil operating very much in 3-D. |
Posted By: KazRob
Date Posted: 28 Sep 17 at 3:23pm
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Why would a naval architect need to know about keels on yachts? As I write this I'm sat on a semi-submersible flotel beside a big triangular gas platform on legs. Both work well, both have 'hulls' and none of them have keels, other than perhaps of the structural type and all designed with input from naval architects. Indeed I know one who designs pipelines that get towed out half way between the surface and the sea bed. Buoyancy calcs are important but again nothing to do with keels again.
Most naval architects probably do industrial stuff and I guess you don't need to be able to sail to do most of that ------------- OK 2249 D-1 138 |
Posted By: davidyacht
Date Posted: 28 Sep 17 at 3:43pm
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Metacentric Height might be of more interest ------------- Happily living in the past |
Posted By: Guests
Date Posted: 28 Sep 17 at 8:45pm
This is very wrong. It is not necessary to change the density of the fluid - if that was necessary then keels wouldn't work very well as water is (practically) incompressible. See - I know about keels too! |
Posted By: Guests
Date Posted: 28 Sep 17 at 9:05pm
How do you find the metacentric height of a foiling Moth? ;) |
Posted By: RS400atC
Date Posted: 28 Sep 17 at 9:21pm
My point exactly. Keels, wings, sails, similar things but different operating conditions. Different fluids, different constraints on the speeds either side etc etc. A wing is not exactly like a sail is not exactly like a keel/rudder/daggerboard. The constant things are: The conservation of mass The conservation of energy Hard Sums! If you change the pressure in air, you will change the density. Unless you are changing the temperature instead of course. |
Posted By: 423zero
Date Posted: 28 Sep 17 at 9:31pm
| Maths I believe is only empirical science, so could all change in a couple of decades. |
Posted By: laser193713
Date Posted: 28 Sep 17 at 9:33pm
Well I suppose it depends if you are a marine engineer, a yacht designer, or a naval architect. But to qualify as the latter I'm pretty sure most courses, including the one which I refer to, has a module on sailing yachts. To me the idea of living in a city surrounded by boats in marinas and not to have noticed the big dangly thing hanging out from under them all sat in their cradles is a bit strange. Not to mention we had a module on yacht surveying the year before which was carried out on a 33 foot cruising yacht and one of the main areas of concern on that particular boat was the keel bolts. I don't know a lot about the military side of naval architecture, but I have at least a vague idea what's hidden under the water! My point was rather that there are still a lot of misunderstandings and a lack of real world knowledge in the world of academia. And obviously, as pointed out, the answer to the other misunderstanding is in fact metacentric height. Which, incidentally, doesn't apply to a foiling moth whilst it is actually foiling as it isn't technically buoyant in that state. However, when lowriding I can assure you the answer is pretty damn unstable!  |
Posted By: davidyacht
Date Posted: 28 Sep 17 at 9:58pm
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I guess that the key is to get some real world knowledge in the real world. Having approached a Naval Architectural degree (well actually Yacht and Powerboat design) as a mature student, I found it fascinating to learn the theory after some real world experiences in development classes. ------------- Happily living in the past |
Posted By: Chris 249
Date Posted: 29 Sep 17 at 10:58am
I still only got 4%.  ------------- sailcraftblog.wordpress.com The history and design of the racing dinghy. |
Posted By: 423zero
Date Posted: 29 Sep 17 at 12:27pm
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4% That deserves a merit on it's own. Your report must have been a corker.
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Posted By: Guests
Date Posted: 29 Sep 17 at 6:25pm
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Posted By: 423zero
Date Posted: 29 Sep 17 at 9:34pm
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Mathematics itself is a consistent, logical system and does not require repeated experimentation to affirm something with certainty only approaching, but never reaching 100%. Any valid proof will give you perfect certainty of a mathematical fact, forever. On the other hand, the way we do mathematics is empirical. Take the question of whether we can prove that mathematics is completely internally consistent. This is actually impossible due to the incompleteness theorem of Goedel, but hundreds of years of advanced mathematics hasn't turned up any serious contradictions. To use the appropriate language, we have evidence that mathematics is "true", but we cannot know with absolute certainty. Even checking all proofs is impossible because there are a (countably) infinite number of them. |
Posted By: Rupert
Date Posted: 29 Sep 17 at 10:45pm
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The way I do maths, mainly on my fingers, means pretty much any answer is possible, barring, perhaps, the right one. ------------- Firefly 2324, Puffin 229, Minisail 3446 Mirror 70686 |
Posted By: Riv
Date Posted: 01 Oct 17 at 9:56am
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I keep in mind George Box's (statistician) quote when thinking about lift: "all models are wrong, but some are useful"
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Posted By: zippyRN
Date Posted: 01 Oct 17 at 11:36am
not far off it ... nearly all the science to do with boats and ships is application of the least worst model
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