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I wondered about that, although I guess that by the time most cars have acquired enough dents, the performance will have fallen off more than can be offset by aerodynamic improvement. Apparently, the golf ball phenomenon was discovered by the better performance of old, battered balls, so there's hope for some of us.. :-)
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Although the dimples help to keep the boundary layer attached, they are doing so over a small scale. This effect would have ceased to have any effect on a car somehwere between the front bumper and the grille!
Golf balls and motor cars travel at *broadly comparable speeds in the same working fluid. The thing that makes their aerodynamics incomparable is the length scale of the object.
See;
en.wikipedia.org/wiki/Reynolds_number
Note the length scale, D
In short, sorry, no-one has been missing a trick!
* motor cars tend to travel between 0 and, say, 100mph, while golf balls go between 0 and about 200 mph. They aren't orders of magnitude different in terms of speed, but they are orders of magnitude different in size.
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But cars do sometimes have sort of dimples, on a much bigger scale. I'm thinking of those ugly designs that have a kind of groove running round, making them look like air compressors with a slot for winding the pipe round. Or the sills that look as if they have been kicked in or run over something. Do they serve any aerodynamic purpose? Why don't all cars have them?
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I am no expert on aerodynamics but I did read an article recently which found the box fish to have a very efficient body design despite its looks. I believe aircraft designers are now copying similar features to obtain greater efficiency and this involves the use of dimples. So many of our greatest "inventions" are merely copies of what already exists in nature.
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I think fish use exuded slime as their boundary layer...
Aerodynamics has an arcane side. I know a slightly rough finish is preferable to a shiny one at the moderate high speeds attained by cars and some aircraft, but one can't help wondering whether the difference on, say, a very fast and aerodynamic coupe's surfaces would be measurable, given the gross drag caused by things like the wheels and wheel arches. I think one of the advantages of the boundary layer is that it makes the airflow 'unstick' more easily at the trailing edge of whatever the moving object is.
At very high (supersonic) speeds though the compressed air in front of the leading edge of the object breaks through the boundary layer, and friction with the passing air causes heat, at high enough speeds enormous amounts of heat (space vehicles on re-entry).
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>>Aerodynamics has an arcane side.
Yes..., and no.
We know what the equations are;
en.wikipedia.org/wiki/Navier-Stokes_equations
It's just that we can't solve them; certainly not in a general closed form sense, and only clumsily and computer intensively using techniques such as CFD.
en.wikipedia.org/wiki/Computational_fluid_dynamics
So, it's difficult, but, it's definitely not black magic.
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So it's difficult but it's definitely not black magic.
No, but in practice not science either although of course the science has to be right. It's an 'art'. That's why the design of F1 cars is constantly mutating, with the appearance and disappearance of little ears, horns etc. One assumes that with aircraft and missiles etc. the science plays a bigger role, but there is still remarkable variation in shapes.
For example, the lowest drag is supposed to be caused by a round-nosed, long-tailed 'teardrop' shape, a bit like most fish in plan view. But in automobiles a long pointed tail is vulnerable, therefore impractical, and in some very high-speed applications has lacked directional stability. The chopped-off so-called Kamm tail, though, was discovered not to create all that much more drag as a rolling eddy of air simply substituted itself for the long tail with the airflow passing around it. What extra drag there is also helps to make the car directionally stable. The evolution of the Porsche 917 constitutes a clear example.
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>>It's an 'art'.
NO!, it is pure science, but, as in all parts of science, we don't have access to the full picture, and we don't have all the tools in the toolbox.
In attempting to solve what might be described as the forward problem, difficulties in computation are encountered, as described in the links attached in my earlier post. If the forward problem is difficult, then the optimisation problem is currently even further out of reach.
In structural engineering this step is currently being taken, via tools for topology optimisation. For example, this technique has been used to optimise the wing spars of A380. The technique is well described in the books on topology optimisation by Bendsoe.
So, although there's no easily discernible unique route to acheive a unique solution [and in this case, owing to the complexity of the Navier-Stokes equations, unique solutions might not exist], that doesn't stop an endeavour being scientific. As long as hypotheses are being carefully developed and rigorously tested, then, it's science.
It's just one of those things in engineering - once people have one or two successful designs in their portfolio, they tend to mistify the process to protect their continued employment. Phrases such as "it's an art" are dead giveaways. Most largish engineering companies will have a few of these types lurking around.
EDIT: I've just realised that the central point of my post is rather opaque. Hopefully, the extra below might help in clarifying it.
By forward problem, I mean that you already know the geometry of the body, and you know how fast it's going, and you want to work out things like the drag and lift.
The optimisation problem is when you provide regions which are available for the body, and regions which are banned (for example, you would need to make room for the engine!). Between these banned regions, you want to obtain a solution which tells you which body shape has the lowest drag.
Edited by Number_Cruncher on 29/07/2008 at 17:41
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Well, NC, you have taken a lot of space to agree with me that it's important to get the science right.
But you seem curiously resistant to the idea that in the quest (for example) for less drag without loss of stability in race or road cars, different approaches can be taken and some are very clearly more successful than others. No doubt equal care with the science is usually taken in either case, successful or unsuccessful.
That suggests very strongly to me that aerodynamics in practical applications is an art, even if engineers in their gruff way are a bit squeamish about the term, wimpish indeed. Obviously someone poncing around in an engineering shop claiming to be an artist will be looked at askance, unless he comes up with a winning formula of course.
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Still NO!, sorry Lud.
If the changes and addenda that you describe were borne of purely aesthetic sensibilities, then, yes, you could say that there's a strong element of art.
However, there's usually much more logic to it than that. The original CFD model might show a shortcoming which motivates an engineering study to imrove the design. This engineering study will come up with a number of possible design changes borne of the application of the basic principle of fluid dynamics. These designs would then be modelled on a computer, and the best ones tested in the wind tunnel. Shortcomings will be identified.....and the cycle continues. The best F1 teams in terms of engineering will be the ones who can speed up this cycle the most.
This engineering cycle is what can begin to be accelerated by techniques of topology optimisation, and there's no art anywhere near it.
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If the changes and addenda that you describe were borne of purely aesthetic sensibilities then yes you could say that there's a strong element of art.
I have to conclude, slightly sadly, that you don't want to understand what I have been saying. Perhaps it's worth suggesting, though, that any aerodynamic tweak or redesign has to be put into adequate-for-purpose solid form by people, and these processes involve a lot more than computing and equations.
The theoretical scientist with his slide rule, mainframe computer and polystyrene cup of cold tea has his own unassailable status in our advanced society. However only an idiot would trust him to actually do anything. Do I make my frustration with this exchange clear?
:o}
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>>that any aerodynamic tweak or redesign has to be put into adequate-for-purpose solid form by people
Yes, the aerodynamicist will hand the desired shape to a structural engineer, who will determine the required material thickness and required strength of any support devices, and fasteners, again, using nothing more arcane than Hooke's law, and the laws of elasticity.
Having derived both the form, and the required structure, the job will be passed to a CAD operator to produce some detail drawings, which will have to signed off by aerodynamicist and structural engineer before the parts are passed to manufacturing.
There's no art there.
Yes, it is hourses for courses. It would be a real waste allowing a skilled engineer or aerodynamicist to spend his time on a CAD machine, or on a CNC machine, but, it would be a complete disaster to try it the other way round!
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My very emphatic guess would be that a skilled engineer or aerodynamicist who was not just skilled but actually some use at something wouldn't hesitate to 'waste' his time on some piece of measuring equipment, because proper engineers are hands-on in the context we are discussing. Of course any large machine needs cogs, and that is what theoretical engineers are in a racing or car manufacturing machine.
But while an intelligent and experienced grease monkey or mechanic could probably, given time, cobble together a viable safe vehicle, only a few unusually barmy theoretical engineers could even contemplate it. That was what I meant. Number crunching has its value, indeed it is essential. That's why there are people to do it. But in and of itself, it doesn't rule the world or car design or F1 development. It is just part of the scene.
By the way I know a topologist. He was at Oxford at 16 and when I first knew him had not yet graduated and was married with a child. Can't understand a word of what he says about topology though.
Edited by Lud on 29/07/2008 at 22:58
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"cobble together a viable safe vehicle"
As per the most recent 'Scrapheap', where the oily rags (just) beat the academics with their hovercraft. Not a significant sample, I grant you, but I've noticed that those with practical experience of maintaining farm machinery and fixing motorbikes often do better than the 'officer classes' when faced with practical problems and a tight deadline...
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>>because proper engineers are hands-on
I couldn't agree less.
Although I am fairly hands on, being so doesn't automatically make you a good engineer, despite various fairly vague thoughts by non-engineers that it ought to. For example, one of the better structural engineers of my acquantance was defeated by the task of putting up some shelves in his house, but, his ability to solve problems, and sort out the right from the almost correct was unerring.
EDIT - For a quite entertaining reference on this subject, see Sir Stanley Hooper's book, Not Much of an Engineer. Via his mathematical analysis of the airflow in the Merlin engine supercharger, he was largely responsible for its performance increase during the course the war.
>>given time,
Apologies for the obvious and cheap retort, but, monkey, typewriter, Works of Shakespeare!
:-)
>>only a few unusually barmy theoretical engineers...
Yes, that's probably true - it is a question of horses for courses. Having got into professional engineering via an odd route, I have tended to be the one who some of the engineers sheepishly ask about what odd symbols on drawings mean (usually geometrical tolerancing), and practical matters like what sorts of coating can be applied to particular materials, etc, etc; the practical stuff which doesn't fit into a calc!
Can't understand a word of what he says about topology though.
Yes, it's not a subject that I could stand any reasonable scrutiny on, but, the method I mentioned is now largely automated, and is simply a routine which can be called by the analyst.
>>it doesn't rule the world or car design
Yes, that's true. For an example of what happens when engineers do try to do everything, see P6BS. However, without the scientific approach to car design, we would be running around in cars little developed beyond the Morris Minor (yes, the pre-war one!). For examples, see back issues of the Automobile Engineer - even articles going back to the period of the First World War were highly mathematical, vehicles have been engineered using theory in varying degress of sophistication for a long time.
Edited by Number_Cruncher on 29/07/2008 at 23:42
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Eeeeeh.... enough already... I certainly know what you mean, and no doubt you know what I mean. There's no substantial disagreement that I can see.
I say NC, you aren't Aprilia are you? There's something slightly deja vu about some aspects of yr intellectual style. Apologies if this is offensive in any way or wide of the mark.
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No!, I don't have 1% of his experience of vehicle manufacturing practice!
There aren't any multi-name shenanigins going on here - all my posts are in my name, which hasn't changed since I registered. However, I've often wondered what the ratio is between the number of login names and the number of real people on this site!
>>Eeeeeh.... enough already... I certainly know what you mean, and no doubt you know what I mean.
Yes!
I suppose I bang on a bit, because engineering is so often completely misunderstood by the public - in fact possibly only overshadowed in this respect by librarianship. I would hate to think of youngsters going into engineering thinking it was going to be like scrapheap challenge!
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There aren't any multi-name shenanigins going on here - all my posts are in my name which hasn't changed since I registered. However I've often wondered what the ratio is between the number of login names and the number of real people on this site!
>>
There must be some simple software around which lets you paste in two different samples of writing and comes up with a % probability that they have the same author.
I imagine it would look for style similarities, but also for too-obvious deviations that were inconsistent with the bulk of the text.
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"I would hate to think of youngsters going into engineering thinking it was going to be like scrapheap challenge!"
I think anything that raises their awareness, however contrived, is a good thing. I don't remember being disappointed that 'real' engineering wasn't just like Meccano...
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I think anything that raises their awareness however contrived is a good thing. I don't remember being disappointed that 'real' engineering wasn't just like Meccano...
I was disapponted to find that Engineers are not accorded the same status, respect, and salaries as other professionals such as doctors, solicitors, teachers etc etc.
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I was disapponted to find that Engineers are not accorded the same status respect and salaries as other professionals such as doctors solicitors teachers etc etc.
cross out teachers from your list. They get no respect or salary to match.
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"status, respect, and salaries"
Thanks largely to our tabloids, I think respect now only exists among peer groups or those who value celebrity. Status and salary seem to be fairly tightly bound, though - I attended a birthday party at the weekend where the most interesting person was undoubtedly the one who arrived by helicopter.. :-)
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"P6BS"
Wasn't that just a casualty of commercial operations (merger with Leyland)?
AFAIK (which isn't very) it was a promising vehicle. I remember getting pretty excited about it at the time, and then rather disappointed!
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>>Wasn't that just a casualty of commercial operations (merger with Leyland)?
Yes, it happened at about that time, but the reason I mentioned it was that the body wasn't styled (it wasn't considered from an aerodynamic point of view either!), it was put together as a simple engineering structure. There were only engineers available within Alvis at the time - hence, it was as ugly as sin!
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I am consumed with curiosity. What was the P6BS? I can think of a couple of Rover one-offs, racers or prototypes, but is this one of them?
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Lud,
Go to austin-rover.co.uk/
Click on projects/prototypes, and follow Rover P8/P9. There are some pictures and text describing the P6BS about half way down the page.
Note the clear pod behind the rear window, through which the top of the V8's SU carbs can be seen.
It had a mid mounted Rover V8, with a chain drive in the transmission path to help in packaging (I can't remember exactly how/where the chain was fitted)
The design was one of the last projects undertaken by Alvis engineers just after it became part of Leyland, and as mentioned wasn't 'styled' in the usual sense of the word.
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Thanks very much NC. I don't remember noticing that one.
Actually styling-wise it looks pretty good to me, better than a Porsche 914 and quite a lot of other cars that have actually seen production. It was an extravagant beast in concept though and might have been expensive to make. But Rover did have some very good car engineers, didn't it? All swept away or kept firmly in their place, along with the jewel in its crown Issigonis, by the increasingly bloated corporation and its horrible accountant suits, ably aided by the poor misguided productionline monkeys... makes you want to cry.
There's number crunching and then there's bean-counting. I know which I prefer.
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>>Rover did have some very good car engineers,
Yes, the P6 itself was rather special.
Reading some of the underlying motifs in your posts, Lud, I would have imagined that you would be a champion of the bean-counter. Without them, the motor car would certainly have remained a plaything of the rich, out of reach of working calss types.
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Not really. I have to go with the flow because there's only so much to be gained by swimming against the tide, but basically I embody the skewed, contradictory psychology of the car enthusiast to an unusually marked degree. Of course every last one of them is a fruit cake, from Mr Toad to us via Mr Magoo and the Dukes of Hazzard.
I have been one all my life, and have had the moderate good fortune of experience that is wide in some ways. When I grew up a bit and learned something about the way this swamp of tears works I realised that there were things other than the automobile and literature, and that the automobile had been taken up and was being used by the powers that be to do the sort of things and achieve the sort of ends that powers do and achieve. It is part of the once-capitalist, now 'global' system, and an important part (but everything mutates, and nothing lasts for ever). I am certainly sold on western capitalist democracy in all its awfulness as the dominant, and so far the best, 'system' if that's the word, but I personally dislike the end results of the mass diffusion of the automnobile, as I imagine all real enthusiasts do. Nothing to do with the working class, just the numbers and the dumbing-down of the whole thing. Really I am nostalgic for the car as a toy for rich gents, although one of my layers is that of a marxizing left-intello.
Does that make sense? It certainly doesn't to most people.
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>>Does that make sense?
Opacity itself. It puts my meage offerings in the shade, and for that, you have my congratualtions!
Here's the bit I really struggle with;
>>the automobile had been taken up and was being used by the powers that be to do the sort of things and achieve the sort of ends that powers do and achieve.
No-one is forcing anyone to buy them? Would it be better if we were to strive for a decade for a Trabant, and think ourselves priveliged to do so?
What would we doing with the freedom which is implied by your post if we were freed from assisting the powers that be by buying cars?
>>I personally dislike the end results of the mass diffusion of the automnobile
I think almost the opposite. I find it amazing that we can produce quiet, economic, reliable motor cars for so little money. In effect, I think that the products of science and engineering are been given away far too cheaply, and the innovation that goes into them is, therefore, not highly prized.
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It's true the mass diffusion of the automobile, and other things, have led to an evolution of the thing itself that no one could even have comprehended in the early days. As I have always made clear, give or take a couple of faults, I have a mad devotion to the thing itself. And lefty or not, nostalgia for Bugattis and such aside, I am more than happy to take advantage of the way things are to find, for peanuts, a reliable modernish piece of carp that is better in most ways than many of the old rich man's toy cars. But the car is what I like, not the motor industry or thicko 'car culture'. I know better than most people what a car, any car, really is.
You say no one is forcing anyone to buy cars. I think many on reflection would disagree with that, as I do. No one is forcing me to buy a car, but I would try to get hold of one anyway.
As I said, the cheap, highly evolved and even sometimes not entirely rubbish modern jalopy appeals to me very much. Trouble is, the roads are full of people with better ones who don't know what to do with them. It spoils the fun.
Really NC, you must come to terms with the concept of comfortably embodied contradiction.
As for the economic system, we're all in it whether we like it or not. Difficult thing to shift actually. Might as well lie back and think of England, knowImean?
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"the roads are full of people"
That's the problem!
"who don't know what to do with them"
Quite agree. The trouble is that if you limit the purchase/licence of cars to people who can demonstrate some knowledge, skill and appreciation, you still let through a few Chavs who know how to wield a spanner and who can do handbrake turns like Tommi Makkinen! There's also the problem that half of them don't bother with the legal niceties in the first place...
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"hence, it was as ugly as sin!"
I thought that was what you were driving at, NC. I have to say that I disagree, both then and now. A bit utilitarian for a sports car, perhaps, but glassy and crisp, IMHO, especially for a mid-engined coupe. If the body wasn't styled, howcome Spen King gets credited with it? David Bache's P9 version (on the same page) was even better.
I wish some modern cars had pillars that thin, too - there'd be fewer SMIDSYs* with motorcyclists, for sure!
*Sorry mate, I didn't see you (so that's OK then).
Edited by J Bonington Jagworth on 30/07/2008 at 22:47
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>>howcome Spen King gets credited with it?
Not what you might call a notable stylist though?
>>I wish some modern cars had pillars that thin, too
Yes, one of the many attractive features of older large BMWs.
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"notable stylist"
Er, the original Range Rover? Possibly the only acceptable-looking 4x4, ever...
Agree about the BMW's though. I still want a 2002!
Edited by J Bonington Jagworth on 30/07/2008 at 22:54
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>>Er, the original Range Rover?
Design, in terms of broad layout, and engineer, in terms of suspension and chassis, yes, I can readily beleive, but, style? Really?
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"style? Really?"
I've certainly always regarded it as a classic. This from the A-R site, discussing the chief problem for its successor, the P38: "the car's greatest asset was its styling, which looked as good, if not better, in 1990 as it did in 1970"
Of course, beauty is largely in the eye of the beholder, which is probably just as well for some of us.. :-)
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Sorry JBJ, I meant style as a verb.
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"topology optimisation"
With no disrespect to NC, I can see where Lud is coming from. A good aerodynamicist will probably 'know' what works, or at least have a good feel for it, which is human interpretation on top of scientific training. Even physics is subject to an Uncertainty Principle, and the fact that design ideas still have to be confirmed in a wind tunnel does rather suggest that the science is inexact...
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>>science is inexact...
Yes, most science is inexact, but, it's the best structured guide we have. Alas, there is no handbook of nature, and we have to learn the rules as we go along.
>>the fact that design ideas still have to be confirmed in a wind tunnel...
That's actually an inherent part of the scientific method. If the testing confirms the theory, then, OK. If the testing doesn't, that's actually more interesting, and you stand a chance of actually learning something. Learning something that you could never hope to learn without doing both theory and test, and certainly not by any artistic route.
>>A good aerodynamicist will probably 'know' what works
Yes, that's right - is exactly the same with a good engineer, or scientist in any discipline, but any who then go on to produce these "designs" without the backup of theory or test aren't acting like engineers or scientists, and I think that history demonstrates that people engaged in this type of work get further using scientific method than by gut-feel or similar.
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>and the fact that design ideas still have to be confirmed in a wind tunnel does rather suggest that
>the science is inexact...
With something as aerodynamically complex as an F1 car it is still impossible to calculate exactly how slight changes will affect the total package so you still have to confirm your expectations with the old empirical 'suck it and see' method.
The computational tools are improving all the time but they still aren't good enough to give guaranteed results 100% of the time.
Kevin...
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i was just about to say the very same thing........honest
Edited by welshlad on 29/07/2008 at 17:34
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And I've just remembered that Howard Hughes made a name for himself with the help of some flush rivets (to smooth the surface of his aircraft and thus break speed records).
Still, I've also discovered that some expensive cycle helmets now have dimples, but I suspect that uses the placebo effect.. :-)
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And I've just remembered that Howard Hughes made a name for himself with the help of some flush rivets (to smooth the surface of his aircraft and thus break speed records).
Spitfires had the same problem.
Flush rivet was slicker than rounded rivet, so flush rivetted spitfires went about 10mph quicker.
BUT the rivetting took considerably longer in man hours per aircraft.
How did they fix this - they got a flush rivet spitfire, and added split peas to every rivet head with glue!
Eventually, after many peas and much glue, they determined which surfaces should be riveted with flush rivets, and on which surfaces it didn't really matter which rivet you used. If that maskes sense.
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"a slot for winding the pipe round"
You'll have to ask Mr Bangle!
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The jersey Lance Armstrong used for time trials was designed with dimples in certain key areas after wind tunnel tests found it improved airflow over the riders body.
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Its desirable to make golf balls fly. This characteristic is undesirable in cars as Audi found out at Le Mans. No dimples thank you.
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of those ugly designs that have a kind of groove running round making them look like air compressors with a slot for winding the pipe round. Or the sills that look as if they have been kicked in or run over something. Do they serve any aerodynamic purpose? Why don't all cars have them?
I think the term used in the trade for those "grooves" is "swage line". Usually intended to make a car look lower/longer/sleeker than it would without it (a bit like the "cheat line" airlines used to paint on their airliners before it went out of fashion). The strangely shaped sills may have an ostensible aerodynamic purpose (reduce drag around the wheels?), but like air dams and spoilers etc, are probably more cosmetic than functional.
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but like air dams and spoilers etc are probably more cosmetic than functional.
I have a particular distaste for enormous or very obvious tail or roof spoilers, and suspect that most roadgoing cars would be just as safe without them, as well as less loud. However chin spoilers and splitters can make the difference between a car being OK and downright lethal. Or so a friend who used to have one of those rear-engined Renault GTA V6s assured me with great earnestness, having had a horrendous moment at high speed on a windy day and subsequently consulted an aerodynamics guru who fitted a splitter that worked...
Of course the real trouble with those things is that if you drive the car in town these days they will be damaged by the vandalism that local authorities have the damn cheek to call traffic calming measures.
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Thank you, NC, for the info on Reynolds numbers. Fascinating stuff - I hadn't hitherto appreciated that a 13th-scale model aircraft's behaviour in water was roughly equivalent to a full-size one in air!
I have a (totally unscientific) feeling that some textured surface like snakeskin will be found to offer less air resistance, and the first person to use it in F1 will suddenly gain an advantage... :-)
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Without a shadow of doubt, the old Mondeo goes better when it's been washed of its muddy patina :-)
However, this may be due to weight reduction rather than aerodynamic changes.
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