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What's the Most Aerodynamic Shape?

06/13/2008 11:28 AM

If you were to come up with a shape that in wind tunnel testing (real world) would have the least drag, what would it look like? That is what is the most aerodynamic shape?

I have always had it in my mind that a rain drop shape was the most aerodynamic, but is this true? I have never heard any proof stating this, it was just something I was told or heard somewhere.

If it is a rain drop shape, what proof exists?

I do have some constraints, in that the shape must have some volume, it can not be a sheet of paper. The shape should be such that it works best in mid air so no ground effects need apply.

Shapes I commonly think of when I think aerodynamics are car shapes, rocket shapes, and golf ball shapes.

Additionally does size make a difference, is their a shape that is universally the best?

This is just a question for discussion and curiosity, I am not designing anything.

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#1

Re: What's the Most Aerodynamic Shape?

06/13/2008 12:48 PM

I vote for the rain drop - at least as the simplest best shape - since it is in fact shaped by the effects of wind, one assumes that it takes on the form of least resistance.

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#107
In reply to #1

Re: What's the Most Aerodynamic Shape?

01/09/2009 8:12 AM

I understand that the rain drop takes the shape of the air but, wouldn't the rounded part at the very bottom still cause resistance?

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#108
In reply to #107

Re: What's the Most Aerodynamic Shape?

01/09/2009 11:04 AM

The raindrop shape is not aerodynamically efficient.

If by "takes the shape of the air" you mean that it is shaped by the air, you are somewhat correct. The actual raindrop shape would be spherical, due to surface tension, but is flattened on the leading end due to air impact forces. The trailing end is also very rounded, whereas for good aerodynamics it would be long and tapered. As a result, raindrops have a very low terminal speed: they are not the least aerodynamic shape, but they are anything by streamlined.

I am probably repeating what might have been stated above, but the "tear drop" shape is really the shape of a tear as seen on a face (with a long trailing end) and not the shape of a tear as it falls through the air (which is the same shape as a raindrop as it falls through the air.) It is mainly coincidental that a streamlined shape looks like a facial teardrop -- in other words, malleable shapes do not naturally take on a streamlined form. A demonstration of this fact is trail of destruction left by a tornado.

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#109
In reply to #108

Re: What's the Most Aerodynamic Shape?

01/10/2009 12:53 AM

Or to really get proof of this, go to your nearest Radio Shack and purchase one of their $19.99 special strobe lights. Open the back, and play around with adding resistor values over the speed-adjustment potentiometer - get the right range, and the lamp will flash at about 500 times per second with surprising durability.

Next, take it into your bathroom, close the door, start the strobe, turn out all other lights, lower your pants, and let fly (lid up, hopefully). By "CAREFULLY" adjusting the speed knob, the true shape of your, ah stream will be reveled to you in all its aerodynamic and gravitational glory!!!

Oh, adjust without care and you'll learn a very rapid and memorable lesson about Ohm's Law.

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#111
In reply to #1

Re: What's the Most Aerodynamic Shape?

05/19/2009 2:10 PM

How about a Trout's face? It may appear a little odd but I believe it has merit !

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#112
In reply to #1

Re: What's the Most Aerodynamic Shape?

05/29/2009 8:09 PM

The raindrop is not the most aerodynamic shape. This is because, as the water falls, the atoms have some tendency to stick together, causing the overall shape to be more round than something that was "perfectly aerodynamic". To get something more aerodynamic, you'd need it to be more elongated than a raindrop is.

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#2

Re: What's the Most Aerodynamic Shape?

06/13/2008 12:57 PM

The most aerodynamic shape must surely be a cylindrical shape of a diameter approaching zero and with extended front and rear tapered cone shapes with ultra sharp ends. The surface would have to be pure polished teflon coated material to resist air flow turbulence.

This would provide the least amount of air drag on it?

John

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#6
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Re: What's the Most Aerodynamic Shape?

06/13/2008 2:06 PM

For the most drag free aerodynamic body the body must be as small as possible, so as I said the cylindrical shape must tend towards zero diameter - This eventually means it will miss all the air molecules entirely, making it drag free!!

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#11
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Re: What's the Most Aerodynamic Shape?

06/13/2008 5:14 PM

As the cylindrical shape approaches a diameter of zero so does its volume, and when it is zero then it is non existent which sort of defeats the point of having a shape at all.

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#12
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Re: What's the Most Aerodynamic Shape?

06/13/2008 6:02 PM

I have some stuff about the optimum aerodynamics of arrows and crossbow bolts...

they do tend to be slightly barrelled, e.g. tapering at the rear end.

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#26
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Re: What's the Most Aerodynamic Shape?

06/14/2008 10:17 AM

Frank, as I said, the most efficient shape to reduce drag is no shape at all!!!

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#3

Re: What's the Most Aerodynamic Shape?

06/13/2008 1:06 PM

Which raindrop are we referring to? The one that we have in torrential rain or the one we have in drizzle?

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#4

Re: What's the Most Aerodynamic Shape?

06/13/2008 1:53 PM

read Drag

Think Sail fish (fastest fish)

The trailing edge of the shape should be longer then the leading edge:

Instead of the standard way of looking at drag as the force pulling back from the forward direction, think of the external pressures pushing against the sides of the object .

add scales to the outside of the shape. This helps by adding to the inward pressure on the trailing edge as seen in the golf ball dimple effect.

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#5
In reply to #4

Re: What's the Most Aerodynamic Shape?

06/13/2008 1:59 PM

GA techno,

This matches what I saw on the Discovery Channel recently.

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#8
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Re: What's the Most Aerodynamic Shape?

06/13/2008 3:00 PM

That's a great point about the symmetrics of the leading edge versus the trailing edge!

The leading edge will easily cleave through the air (or water), but the trailing edge can produce eddies and turbulence because the force at the trailing edge is much lower at drawing the air in toward the surface of the body.

There are also skin effects which create a whole new dimension to the problem.

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#114
In reply to #4

Re: What's the Most Aerodynamic Shape?

11/15/2009 8:21 PM

having a sharp point as a leading edge wont reduce drag because the surface area from the leading edge will be the same as a blunt object, only exception is a square

also governing water and air as the same is a mistake because they have different viscosity levels, which they are governed as the same because they are both viscous fluids

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#7

Re: What's the Most Aerodynamic Shape?

06/13/2008 2:55 PM

A point has the least air resistance. A neutrino has virtually none at all.

Actually, rain, when falling, assumes a spherical shape. The drop oscillates as it falls, but the average shape is a sphere. The teardrop shape that most people assume is the shape of rain, is a misnomer.

The reason that a drop of water assumes a spherical shape is that the water is in a free fall, mostly, and cohesive forces on the skin of the droplet work to shape the water into a shape of lowest energy, which is a sphere.

Under turbulent conditions wind will deform the shape somewhat.

You could take a long cylinder with pointed ends and have a pretty low coefficient of drag, but if the cylinder becomes misaligned in the air stream it will weather vane and increase drag.

Any surface should be kept clean to reduce drag. I remember one auto race where all the cars were highly regulated so that the teams could not modify them in any way. One team washed and waxed their car in a hope that it would give them a slight advantage.

Swimmers and even cyclists sometimes shave hair on their legs for that slight aerodynamic advantage.

Yes, size matters. The size is specified (at least partially) as cross sectional area. It may be easier to visualize this as a hull of a boat through water. You have seen the very long and very slender coxless row boats used in racing where as many as eight rowers sit in a single row. The boats narrow draft and narrow girth provide a minimum of drag.

Compare that to a raft and you will have a huge amount of drag. Look at the back of a raft going through water and you will see swirling eddies of water behind it. That indicates that there is a lot of turbulence and drag. This should also tell you that the optimum shape is a very narrow cylinder with long pointed endpoints. Torpedoes have a similar shape.

Finally, aerodynamics are very different for subsonic versus supersonic aircraft. Subsonic aircraft have a much more blunt nose, which sacrifices some drag for stability in flight. Aerodynamics is a complex subject and you could spend a lifetime learning it.

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#9

Re: What's the Most Aerodynamic Shape?

06/13/2008 4:38 PM

The rain drop shape is a SPHERE !
Lead shot used to be made in a shot tower by dropping molten lead and letting it form droplets.

Maybe you mean tear drop shape?

Drat .. I see Anonymous Hero has beaten me to it...I shall go back to sleep...zzzzz

Del

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#10
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Re: What's the Most Aerodynamic Shape?

06/13/2008 5:10 PM

A tear drop shape is what I meant, but likewise a sphere should be considered also. It just seems to me that I have never seen any analysis of the drag on one shape vs another.

Most people have an idea in their head as to what an aerodynamic low drag shape would be but not much to back it.

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#32
In reply to #10

Re: What's the Most Aerodynamic Shape?

06/15/2008 12:11 AM

It's time to step away from the computer and take a trip to a good library. I suggest a local engineering university. (I used Lehigh) You will find many books on the subject with more explanation and formulas than most of us can comprehend.

In my pursuit of riblet film, I found MUCH, MUCH information on aerodynamics dating back to the mid 1800's. One early book on aerofoil shapes complete with formulas kept me occupied for hours. Unfortunately, riblet film must be too recent to have a lot written about it. I understand the concept but unfortunately can't find a source of it. 3M has manufactured it and supplied it to America's Cup competitors, but it doesn't appear on their product list. It may be a proprietary thing. If anyone knows a source of it, PLEASE let me know.

(I just read post 15. That's the book.)

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#37
In reply to #32

Re: What's the Most Aerodynamic Shape?

06/15/2008 4:23 AM

Fabrication of the 'micro riblet film'

cheers

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#39
In reply to #37

Re: What's the Most Aerodynamic Shape?

06/15/2008 3:53 PM

Thanks! Nice link. The transition from where the riblets reduce drag to where they increase drag is somewhere between Re 103 and Re 104. Generally, appendages on boats (rudders, etc.) operate at low Re numbers (below 103 unless the boat is very large). Re numbers for typical small aircraft are around 106, where you'd expect smooth surfaces to perform better.

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#27
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Re: What's the Most Aerodynamic Shape?

06/14/2008 4:57 PM

Del, would you compromise on a SPHEROID? The fluctuation of the shape of a rain drop as it falls varies due to gravity, air resistance and the cohesive force of surface tension, among other forces. Surface tension would create a sphere if there were no external forces acting thereon.

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#28
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Re: What's the Most Aerodynamic Shape?

06/14/2008 5:06 PM

I dunno...whatever my opinion is, it won't effect reality... ha maybe I'll believe rain falls in cubes just to see what happens.

hmmm I can imagine it wobbling a bit on the way down.

I just know they used to make lead shot by dropping it from a tower.

Del

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#38
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Re: What's the Most Aerodynamic Shape?

06/15/2008 8:31 AM

I wonder if an oil, that has no surface tension, would make a more accurate shape to copy as a droplet falling though the air......the problem is that water pulls itself back into a ball through surface tension.....the leading part of the ball is probably not a bad shape, but the back end is really wrong and will cause a large eddy.

Many, many years ago, the RN experimented with different shaped noses on torpedoes, from sharply pointed to absolutely flat. The sharply pointed lost out by having a large wetted surface area, but was still the best shape. The rounded (normal Torpedo shape) was next and the last, being flat was naturally the worst shape. But from one to the other, it was only a couple of % difference. It was found out that the water "smoothed" off bad shapes itself to a great degree as well....

But the shape of the "stern" made a huge difference to the speed through the water and fuel efficiency.....I would imagine that air is a similar medium to water in this area.....

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#47
In reply to #38

Re: What's the Most Aerodynamic Shape?

06/15/2008 10:27 PM

I wonder if an oil, that has no surface tension, would make a more accurate shape to copy as a droplet falling though the air......the problem is that water pulls itself back into a ball through surface tension.....the leading part of the ball is probably not a bad shape, but the back end is really wrong and will cause a large eddy.

I think I think the surface tension of water would create a turbulence within the droplet causing and external ripple on the surface. This ripple would cause minor turbulence of the air immediate to the droplet quite less dramatic than the wash from this aeroplane

...what do you think?

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#65
In reply to #38

Re: What's the Most Aerodynamic Shape?

06/17/2008 12:43 PM

I would imagine that air is a similar medium to water in this area

I think the similarities between a gas and a liquid with respect to drag are very limited because of the ability of a gas to be easily compressed whereas a liquid must be displaced. I am just guessing and may be wrong in my thinking.

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#66
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Re: What's the Most Aerodynamic Shape?

06/17/2008 12:59 PM

At speed they are more similar then you expect. At speed a gas would feel more like a liquid due to the momentum of the molecules being displaced. The preassure equalization tendancies of the gas responds very much like a vortice in a liquid.

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#68
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Re: What's the Most Aerodynamic Shape?

06/17/2008 2:02 PM

I had already responded to your first post, now you are almost agreeing with me with your second!

You are allowed to of course, if you wish!!

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#67
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Re: What's the Most Aerodynamic Shape?

06/17/2008 2:01 PM

I believe you are completely right, but I have seen so many "water" tests done over the years for something that has to actually go through the air eventually, but done slower & smaller.....

Unless the water is so confined that that itself causes extra problems of course, I understand that many aspects are similar, except of course water is a bit heavier!!

With respect to a streamlined shape, I do believe that most likely, either can be used, the best shape being either the same or very similar for both mediums....

That is my best guess anyway!!

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#123
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Re: What's the Most Aerodynamic Shape?

05/19/2010 11:51 PM

the compressibility of air is ignored at low speeds (less then 0.3mach) because its is really not relevant. and you can do accurate water tunnel test.

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#73
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Re: What's the Most Aerodynamic Shape?

06/18/2008 10:14 AM

If the sphere has the optimal shape, then ball shaped projectiles of the old muskets would be optimal. Given what Techno posted above, seems like the manufacturers of current day arms might want to consider the shape of the projectiles the make for bullets.

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#74
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Re: What's the Most Aerodynamic Shape?

06/18/2008 10:21 AM

A sphere may be the right shaped head for a projectile, but I seriously doubt that a "round behind!" is good, except on a pretty girl (or boy depending upon your personal persuasion!)

I would imagine that a bullet shape, the same at both ends would be close to optimum.....and sharp at both ends.....

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#75
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Re: What's the Most Aerodynamic Shape?

06/18/2008 1:37 PM

LMAO LOL that's what I said in post number 2...!!!!!!! LOLOLOL

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#79
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Re: What's the Most Aerodynamic Shape?

06/18/2008 3:01 PM

It was the lack of spin on the projectile that made muskets innacurate.

Sherical projectiles in rifled barrels were fine.

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#81
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Re: What's the Most Aerodynamic Shape?

06/18/2008 6:19 PM

...but the back end is wrong, even if the front end could be called acceptable!!!

Its the old (smooth) Golf ball problem all over again!!

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#115
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Re: What's the Most Aerodynamic Shape?

11/15/2009 8:25 PM

a "teardrop" shape is referred to as a drop of water being hung by a needle or sharp object

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#13

Re: What's the Most Aerodynamic Shape?

06/13/2008 7:03 PM

Have a look through some of this stuff from NASA - there are all sorts of bits about aerodynamics (& drag - which is what I think you were getting at).

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#14
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Re: What's the Most Aerodynamic Shape?

06/13/2008 11:16 PM

Cool link

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#15

Re: What's the Most Aerodynamic Shape?

06/13/2008 11:46 PM

Airfoils, as used on airplanes, are designed to create a lot of lift and very little drag. Given that, you could assume that they would all look pretty similar in cross section, and in a way, they do. But the details are quite different from one to the next: thickness as a percentage of chord length, location of maximum thickness, location of maximum camber (if the section is cambered), leading edge radius, etc.

There are probably 100 different airfoil shapes (and loads of sizes scaled up and down from these basic shapes) used on current airplanes and sailplanes. The NACA numbering system provides for practically unlimited shapes, and a book like Abbott and Doenhoff's Theory of Wing Sections lists test data for about 100 section shapes, I'd guess. (If I hadn't lost my copy in my last move, I could do an actual count.)

The common elements are a rounded leading edge, a sharp trailing edge, (often) a maximum thickness at about 25-30% of chord length (the length from the leading edge to the trailing edge.) Many section shapes can be described fairly simply mathematically (and the curve is "fair," without bumps).

If an airfoil operated with zero angle of attack all the time, then there could be some advantage to a sharp leading edge. But even if all you are doing is streamlining, (and not intentionally generating lift), the angle of attack is often non-zero, due to crosswinds, etc. So in practice, streamlining shapes look just like airfoil shapes.

How thick? Well, suppose you want a person inside your device. At the fattest part, you'd want the shape to be just slightly greater than person width. Then, do you make the shape 10 times that long, 6 times that long, or 4 times that long? If you look at a 10% section (10 times as long as wide) it looks very streamlined. If you look at a 25% section (4 times as long as wide) it looks too stubby. (It is easy to imagine that a thin shape doesn't have to bash that air around so vigorously.) However, the very long thin section also has more surface area, so there is more skin friction drag. As it happens shapes like this have already been in wind tunnels, and a 25% section is a good compromise, if you want to streamline something and fit something inside.

So take a NACA 0025 section and spin it around in a lathe to make a teardrop shape, and you have about as streamlined a shape as you can reasonably obtain, if you want something to fit inside.

In cars, the back portions of the car that are remotely close to vertical (actually anything that is more than about 20 degrees from horizontal) is called base area. The flow behind the base area is fully detached, and pretty fully turbulent. The size of the base area (as a percentage of frontal area) is critical to the cars coefficient of drag. SUVs have very large base area, and high Cd, as a result. They are also big, so the combination of size and rotten aerodynamics can mean they require twice the HP to push air out of the way at a given speed, as compared to a sedan. Sometimes the rear end of a car is chopped off (Kamm tail) because the flow near the back end is already fully detached, so a fully streamlined tail just ads weight. Often the rear end is trimmed for practicality: a 6' wide car would be 24' long, if shaped like a 0025 section, and the last 5-6' would be hard to use for practical purposes.

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#30
In reply to #15

Re: What's the Most Aerodynamic Shape?

06/14/2008 6:22 PM

Angle of attack is, essentially, the only reason an aircraft flies.

People are taught in school about airfoils and how they generate lift. That information is wrong. I have had a long discourse on that in an older post.

The other thing is that lift and drag are not inversely proportional. The more lift a wing produces the higher the drag. Wing shape has more to do with stability than lift.

As I said before, go look at an aerobatic aircraft wing and you will see that it is symmetrical in shape, not a classic airfoil at all.

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#35
In reply to #30

Re: What's the Most Aerodynamic Shape?

06/15/2008 1:37 AM

Angle of attack is, essentially, the only reason an aircraft flies.

Sort of. There are many wing profiles which will produce, at 0 degrees angle of attack, sufficient lift to make an airplane fly. The 0025 section I mentioned above is symmetrical, but a cambered version, such as the 2425, produces enough lift for cruise flight, given an adequately sized wing. (I'm stretching the point here a little -- any cambered wing will generate some lift at 0 degrees angle of attack. If it will generate some lift, then the rest is just getting enough area and speed so that lift equals weight.)

People are taught in school about airfoils and how they generate lift. That information is wrong. I have had a long discourse on that in an older post.

Agreed, sort of. It depends on what you mean by "school". I know some people at Stanford, Cornell, and Georgia Tech who can spin a pretty accurate tale re lift generation. That older post may have been in a thread I started a while back on the "lies" involved in describing lift generation. Your post in that thread looks like you might have thought I was serious in contending that the "classic" Bernoulli effect explanation adequately describes lift generation -- anyone who did the math would find it only explains about 2% of the observed lift.

The other thing is that lift and drag are not inversely proportional. The more lift a wing produces the higher the drag. Wing shape has more to do with stability than lift.

I don't think anyone would claim that lift and drag are inversely proportional. Of course the more lift produced, the higher the drag (and, in fact, drag increases much faster than lift, when talking about 3D wings). Wing profile has essentially nothing to do with stability from a design standpoint, and everything to do with L/D at a chosen design coefficient of lift. It is for this reason that there are hundreds of "standard" NACA 4, 5, and 6 digit sections plus zillions of non standard NACA sections, plus a ton of sections that fall entirely outside the NACA numbering system. Virtually every wing is inherently unstable, but making the aircraft stable has much more to do with weight distribution and placement of the other airfoils (horizontal stabilizer, etc) than the particular moments generated by the wing.

As I said before, go look at an aerobatic aircraft wing and you will see that it is symmetrical in shape, not a classic airfoil at all.

Some purpose-built aerobatic aircraft have symmetrical airfoils*. My old aerobatic Beech, however, did not -- its wing was quite asymmetric, so when flying upside down, it required a much higher angle of attack than when flying right side up. The Citabrias (airbatic spelled backwards) had highly cambered sections with the Decathlon having a less cambered but still asymmetric section. But even the symmetrical section wings are classic airfoils -- in fact the most classic airfoil is arguably the 0012 from which the 2412 is developed, and the Pitts Special uses the 0012 at the tip. There is nothing un-classic about a symmetrical airfoil. Take this classic shape, and add camber, and you get the 2412 which loads of Cessnas have used.

*Although this is true mainly of aerobatic planes built for air shows, and fun flying. Fighters are highly aerobatic but few fighters use symmetrical sections.

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#53
In reply to #35

Re: What's the Most Aerodynamic Shape?

06/16/2008 8:01 AM

Help me, what do you mean by camber in a wing?

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#54
In reply to #53

Re: What's the Most Aerodynamic Shape?

06/16/2008 12:05 PM

Here's an application which lets you play around with NACA 4 digit wing profile shapes. The four digit shapes are the most often used "standard" airfoil shapes. The four digit numbering system works like this: first digit: amount of camber in percent of the chord length, second digit: location of the peak in the camber line, in tenths of chord from the leading edge -- basically the point around which the wing is "bent" to give it camber. Third and forth digit: the thickness in percent of chord. So a 0012 section is symmetrical (no camber, ie the "mean camber line" is a straight line); there is no camber, so the second digit is meaningless; the wing is 12% as thick as its dimension from leading edge to trailing edge (the chord).

This application defaults to a 0012 section. If you then move the top two sliders both to 4, you will generate a 4412 section, which is considered a fairly heavily cambered section. If you then plotted a curve that was equidistant from the top and bottom surfaces, that curve would be called the "mean camber line" and, in this case, the height of its peak would be 4% of the chord length. So camber is the amount by which the section shape is bent to make the mean camber line a curve (the short answer to your question). The chord line is the line from the leading edge to the trailing edge, so in an uncambered section, the chord line and the mean camber line are coincident.

This application draws the chord line horizontal, meaning that the angle of attack (the angle of the relative airflow with the chord line) is drawn at zero too, if you assume the airflow is horizontal and from left to right. If you look at the 4412 section, it looks almost like it is flying downhill slightly, but is actually at 0 degrees angle of attack. At that angle of attack the coefficient of lift would be about .4 -- enough to provide adequate lift at cruise speed, typically, on the plane to which the wing is fitted. (That's an oversimplification, but in general, in cruise flight the best L/D for most wings will be at the "design coefficient of lift" and that design Cl usually occurs pretty close to 0 degrees angle of attack. In 6 and 7 (laminar) series profiles, there is a distinct dip in the drag curve on near the design Cl, so a designer would aim to have the airplane operate (at cruise) near the center of that dip.

By fiddling around with different profile shapes, wing loadings, aspect ratios (especially), etc., the L/D (in practice) of the complete plane can go from almost as low as 6 to as high as 60 (with biplanes at one end, and sailplanes at the other). L/D and glide slope are effectively the same, so a biplane a mile up with a dead engine had best look around pretty quick for a place to land, whereas a glider can have a range of about 60 miles from that altitude in still air.

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#55
In reply to #54

Re: What's the Most Aerodynamic Shape?

06/16/2008 12:33 PM

Nice link, Ken. Thanks!

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#56
In reply to #54

Re: What's the Most Aerodynamic Shape?

06/16/2008 1:51 PM

GA GA GA, but they only let me vote once!!! Spoilsports!!

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#16

Re: What's the Most Aerodynamic Shape?

06/13/2008 11:47 PM

General Motors has done some has done lots of research in this area and they have concluded that the Hummer is not an efficient shape, unless your driving one in vacuum.

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#20
In reply to #16

Re: What's the Most Aerodynamic Shape?

06/14/2008 3:23 AM

Yes, that is the ideal environment for Hummer drivers...

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#17

Re: What's the Most Aerodynamic Shape?

06/13/2008 11:57 PM

For a practical application

Look to the hydrodynamic shapes - nuc subs

Rain and lead drops have a pivotal issue - they are rotating randomly and as such become spheres...if you want a projectile which does not go through random rotation and since they are free of ground effect go to subs, bows of super tankers, keels of sail boats...whales, porpoises, and dolphins and many of the deep sea pelgaic fish.

Oh by the way assuming you are not going faster than the speed of sound and you are in air, friction and turbulence is the problems...whales have two additional issue solved by one measure - cold of sea water and turbulence are solved by a FLEXIBLE skin with thickness based on the turbulence vortices created by water/air moving over the surface.

TOM

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#18

Re: What's the Most Aerodynamic Shape?

06/14/2008 2:20 AM

The larynx of a politician?

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#19

Re: What's the Most Aerodynamic Shape?

06/14/2008 3:15 AM

This one seemed to work pretty well.


http://en.wikipedia.org/wiki/Tallboy_bomb

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#31
In reply to #19

Re: What's the Most Aerodynamic Shape?

06/14/2008 6:41 PM

A picture's worth a thousand words. I mentioned (in my thousand words above) taking a NACA 0025 section and spinning it in a lathe. That would get you within fractions of an inch of the Tallboy shape: maximum thickness at 30% of chord, 25% thickness ratio, etc. If you want to streamline some mass, this shape is hard to beat.

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#118
In reply to #19

Re: What's the Most Aerodynamic Shape?

12/01/2009 11:58 AM

I agree with bubbapebi that seems to work pretty well

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#121
In reply to #19

Re: What's the Most Aerodynamic Shape?

04/25/2010 11:53 PM

This is an old discussion, but I was asking myself the same question just now.

Bubbapebi is correct.

Ironically, you don't need to be a rocket scientist to know the answer. You don't even need to be smart because the answer is "dumb". Dumb bombs, or iron bombs, are really only designed to do one thing, and that's fall as fast as possible, in a predictable path, preferably straight down, and explode (those are all one thing). They don't contain anything except the explosive device therefore they don't compromise on shape due to engineering constraints.

Because bombers dropping iron bombs are already traveling at a certain velocity, so are the bombs when they leave the aircraft. Their trajectory is ballistic. Air resistance slows them down, and they accelerate along the vertical axis due to gravity. Bombers have a computer that works out something called CCIP - Continously Calculated Impact Point for this reason.

There are iron bombs which have drag chutes which open up upon release to bleed off forward velocity so they'll drop straight down. As I understand it, this is for low level bombing, where the pilot would prefer the bombs didn't follow him on his way out when they explode.

On a side note, someone did ask what would happen if you dropped a liquid which had less molecular cohesion, like oil or alcohol. Is this force so strong that wind resistance will never overcome it?

What about air bubbles rising to the surface of a lake? Don't they take on a different shape? I guess it's a matter of perspective. The air is less dense so the lake is trying to fall underneath it. When a rain drop falls, it's the less dense fluid, atmosphere, trying to get above it.

With that in mind (I'm just guessing), a spherical object traveling at speed will experience lower pressure behind it, thus accelerating it backwards.

Golf balls are dimpled to create turbulance behind the ball and raise the pressure. Race cars will follow each other around a track because the pressure is lower behind the lead car, thus allowing the trailing car to save gas. The wax in lava lamps prefers to remain spherical as it rises.

Again, just guessing, back to the example of the raindrop, the denser fluid will resist acceleration more than the less dense fluid, correct? The air behind the matter rushes in as best it can, since air accelerates more easily, being less dense, thus creating the spherical shape of the raindrop. In other words the raindrop punches a hole in the air, and something has to fill it, and it's more likely to be air. Since the air can't rush in fast enough, it creates a low pressure zone, and the water's too busy trying to fall. In the case of the dumb bomb, the engineers did all the work ahead of time and filled the low pressure zone behind the bomb... with more bomb.

There seems to only be a few constants here when dealing with fluids in free fall. The physical properties of the surrounding fluid, like density, or mass, or whatever (I'm not that learned) and the density or mass of the surrounded fluid. I haven't really experimented with this but I'm just assuming that any liquid in free fall will form a sphere.

I can't really think of any case where air resistance causes another fluid to stretch out into a thin line to reduce its footprint. I don't know what the friction is between certain other liquids and air, but as I recall, the spherical shape reduces the amount of surface area, right?

So we have a shape with the least surface area (less friction) whose weight under gravity exceeds the force applied by the pressure differential.

Infact I remember it being mentioned that raindrops aren't completely spherical. Do liquids exist where this is more evident?

What would happen if you took a material that was less cohesive and dropped it, like a ball of magnetic bits held together by a variable level of magnetism? I'd imagine that bits would try to fly off the back end.

How about a candle flame? The flame is just super heated air and the colder air around it trying to get underneath it. Is this a case of aerodynamic shape determined by two fluids of similar density? The shape seems to resemble a dumb bomb, again.

Maybe someone can explain this in simple terms: if teardrops are the most efficient shape below the speed of sound, then why aren't cars, especially formula 1 race cars not designed to be teardrop shaped?

Maybe abducted farmers in the midwestern US claiming to have seen cigar shaped UFOs know...

The end of my brainstorm/reply.

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#122
In reply to #121

Re: What's the Most Aerodynamic Shape?

04/26/2010 12:28 AM

Just to add to that, both African and European swallows are very aerodynamic considering they weigh 5oz, that is, provided they are not carrying 1lb coconuts, although 1lb coconuts are also very aerodynamic provided they fall husk first.

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#127
In reply to #121

Re: What's the Most Aerodynamic Shape?

09/18/2010 12:18 AM

F1 car is more like a boundary layer device - they are designed to be held down by air flow and induced drag.

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#21

Re: What's the Most Aerodynamic Shape?

06/14/2008 3:27 AM

Of course it depends on what you are trying to make aerodynamic.

The optimum designs for an arrow and a double decker bus will be somewhat different.

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#22

Re: What's the Most Aerodynamic Shape?

06/14/2008 3:46 AM

i wouldent go for the rain drop because it falls at a rate of 22fps

once it hits its maximum speed it does not go any faster or slower the reson i knoe this i have been out in the rain with a radar and it shown 22 f.p.s (feet per second ) for all those who does not know what that means it does not mean that the rain drop shape is not aerodynamic because its not is slow. in the way aerodynamic is in the air aka jets birds rockets and others

its made to fall and once it hits 22 fps it will not go any faster or slower i dont think its a very good shape

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#23
In reply to #22

Re: What's the Most Aerodynamic Shape?

06/14/2008 3:52 AM

What the hell are you doing in your avatar?! Getting ready to harpoon a whale?!

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#36
In reply to #23

Re: What's the Most Aerodynamic Shape?

06/15/2008 4:09 AM

you were asking about what i was doing

we were on a navy ship and no that is not a harpoon in my hands its a wepon i was on a task which we had to defend the ship from attackers and i am navy here

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#50
In reply to #36

Re: What's the Most Aerodynamic Shape?

06/16/2008 12:22 AM

It's the red part that has me curious!

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#130
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Re: What's the Most Aerodynamic Shape?

12/22/2010 5:47 AM

Goodest day. Wot navi you are?

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#29
In reply to #22

Re: What's the Most Aerodynamic Shape?

06/14/2008 5:13 PM

You have a point but it's not very sharp or did I miss something?

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#129
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Re: What's the Most Aerodynamic Shape?

12/22/2010 5:45 AM

Maybe the point itself was sharp.

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#24

Re: What's the Most Aerodynamic Shape?

06/14/2008 4:41 AM

Whatever shape you finally decide on, give it a Sharkskin surface, or at least the dimples of a golf ball......it reduces friction considerably......!

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#25
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Re: What's the Most Aerodynamic Shape?

06/14/2008 5:15 AM

Hi Andy,

That was a great idea, if we covered the surface of an aircraft with small dimples it might help with the fuel economy? I understand the principle of the golf ball as the dimples reduce the drag (Reynolds number), and I think that it would be worth trying on an aircraft?

Spencer.

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#33
In reply to #25

Re: What's the Most Aerodynamic Shape?

06/15/2008 12:36 AM

Applying riblet film to the leading edge of aircraft wings has been tested and found to reduce drag by 8-10%. I think the problem with it is that it accumulates dirt over time and the effort to keep it clean exceeds the benefit cost-wise. Next time you have a chance to study an aircraft wing surface with some time on it, notice the dirt streaks around rivets, winglets and panel edges.

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#34
In reply to #25

Re: What's the Most Aerodynamic Shape?

06/15/2008 12:42 AM

I'll loan you my ball pine hammer if you take it to the nearest airport... Just give me time to get my camera!

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#41
In reply to #24

Re: What's the Most Aerodynamic Shape?

06/15/2008 4:08 PM

Those 'dimples on the golf ball only work because it's rotating.

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#42
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Re: What's the Most Aerodynamic Shape?

06/15/2008 4:36 PM

I do not believe that.

The same effect (with smaller dimples) is seen with "Shark skin" swimming costumes, and that is not rotating.....neither does a shark or a Dolphin's skin rotate, nor the feathers on a Penguin (that is a very interestingly effective shape by the way, I almost forgot!)

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#43
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Re: What's the Most Aerodynamic Shape?

06/15/2008 5:04 PM

I do not believe that..

Well try harder..try clenching your fists and repeating it a few times..you'll soon get the hang of it...

Del

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#44
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Re: What's the Most Aerodynamic Shape?

06/15/2008 5:35 PM

Psssssssh! Back to your corner!

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#45
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Re: What's the Most Aerodynamic Shape?

06/15/2008 5:44 PM

mroooow ftzzz <skulks off>

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#52
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Re: What's the Most Aerodynamic Shape?

06/16/2008 2:11 AM

Now do the "ftzzz" with your mouth ONLY!!!

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#51
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Re: What's the Most Aerodynamic Shape?

06/16/2008 2:10 AM

I did really try, promise!!

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#62
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Re: What's the Most Aerodynamic Shape?

06/16/2008 3:35 PM

It's been years since I took any fluid mechanics but I think I remember why the dimples on a golf ball reduces the aerodynamic drag. The dimples help change the airflow from laminar to turbulent at least by the time the air reaches the trailing side of the ball (the leading side is probably still laminar). This change to turbulent flow on the trailing side as opposed to laminar flow on the trailing side of the ball makes the low pressure zone on the trailing side not as low pressure as if it were laminar. Therefor there is less drag on the trailing side because it doesn't "hold back" as much.

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#69
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Re: What's the Most Aerodynamic Shape?

06/17/2008 11:50 PM

And now you expect me to believe those dimples just evolved by accident?!

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#70
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Re: What's the Most Aerodynamic Shape?

06/18/2008 2:37 AM

Of course they did... One day this bloke was making a golf ball and it fell into a bucket of dimples...the rest is history.

Same thing happend to Shirley Temple when she was a baby .

Del

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#72
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Re: What's the Most Aerodynamic Shape?

06/18/2008 6:07 AM

The question is, did she also fly further when thrown than an un-dimpled small girl of the same weight?

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#86
In reply to #70

Re: What's the Most Aerodynamic Shape?

06/18/2008 9:23 PM

...SHIRLEY TEMPLE?!

Only an Englishman would have that munchkin close enough to consciousness to use her in a joke!!! In the States, we're all given electroshock therapy at age six to expunge her from our memories for ever more or until someone like YOU has to go and say her name!!!

Well, guess I better dig out my old mouth piece... Hope it still fits!

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#87
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Re: What's the Most Aerodynamic Shape?

06/18/2008 9:40 PM
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#88
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Re: What's the Most Aerodynamic Shape?

06/18/2008 9:49 PM

For God sake, man!!! In the name of John Agar, stop this before it's too late!!!

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#90
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Re: What's the Most Aerodynamic Shape?

06/19/2008 2:45 AM

There, there old chap..we'll send you for an rest and recouperation cruise.
.
..
...

On the Good Ship Lollipop <Cackles off wildly monitor left>

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#98
In reply to #88

Re: What's the Most Aerodynamic Shape?

06/19/2008 11:33 PM

If you drop Shirley Temple from a height of 10', she will always land dimple side up.

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#89
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Re: What's the Most Aerodynamic Shape?

06/19/2008 1:31 AM

What mouthpiece?

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#71
In reply to #69

Re: What's the Most Aerodynamic Shape?

06/18/2008 6:06 AM

Actually they did sort of evolve by accident. Many years ago, golf balls were just small hard round balls with no dimples and golfers found that older balls with cuts and damage flew significantly further than the then (smooth) balls.....

You will find a good explanation of eventually how the ball was made/evolved and the different sorts of dimples here.

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#40

Re: What's the Most Aerodynamic Shape?

06/15/2008 4:01 PM

It's a 'tear-drop` for sub-sonic speeds, but the aspect ratio is speed dependent.

It's only 'ideal' for one speed.

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#48
In reply to #40

Re: What's the Most Aerodynamic Shape?

06/15/2008 10:30 PM

"It's only 'ideal' for one speed."

And or only for one direction

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#46

Re: What's the Most Aerodynamic Shape?

06/15/2008 8:38 PM

A 45 cal.bullet is the right shape.

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#49
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Re: What's the Most Aerodynamic Shape?

06/15/2008 10:33 PM

Tee-hee, and the trajectory of a birdie

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#57

Re: What's the Most Aerodynamic Shape?

06/16/2008 1:57 PM

A sphere ? Seems like a sphere (planet, etc.) would be the most aerodynamic entity anywhere.

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#58
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Re: What's the Most Aerodynamic Shape?

06/16/2008 2:15 PM

Try taking a large round coffee can and sweep the bottom of the can through some water (i.e., a pool or pond). You will notice the wake is full of turbulence and that means there is going to be drag.

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#59
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Re: What's the Most Aerodynamic Shape?

06/16/2008 2:23 PM

If we are thinking water ..think Shark, Seal or fish..

If we think of air think Swallow Sparrow hawk...nature has had millennia to evolve aerodynamic/hydrodynamic shapes. (Unless you are a Creationist in which case the Deity of your choice conjured the shapes out of thin air and got 'em pretty good)

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#60
In reply to #59

Re: What's the Most Aerodynamic Shape?

06/16/2008 2:42 PM

I was thinking more of beer and my throat lining!!

The first pint goes down so smoothly and quickly it must be the most aero...errrmmm hydrod..... errrmmm

alcoholdynamic surface ever?!! and its taken all this time to evolve and perfect this surface.

John

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#61
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Re: What's the Most Aerodynamic Shape?

06/16/2008 3:25 PM

Over here the closest thing we get to smooth is Boddington's ale.

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#63
In reply to #61

Re: What's the Most Aerodynamic Shape?

06/16/2008 5:04 PM

Boddie's int bad (if there's nothing better around!). I guess you get it all fizzed up - which is the only way I've found it in Reading & other southern parts (of th UK).

Prob'ly a pretty good pint 'from the wood' (if such a thing exists).

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#64
In reply to #63

Re: What's the Most Aerodynamic Shape?

06/16/2008 9:00 PM

Comes in a pint can with a widget inside. Nice head, very, very smooth. Best beer we've had in the States so far.

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#85
In reply to #59

Re: What's the Most Aerodynamic Shape?

06/18/2008 9:06 PM

I suppose from wind shaped rock one may garner a hint or two?

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#132
In reply to #57

Re: What's the Most Aerodynamic Shape?

02/17/2011 5:31 PM

A sphere is highly aerodynamic in laminar flow. As soon as higher velocities are present and flow becomes turbulent a sphere is no longer aerodynamically efficient. Aerodynamics is too complex for one model to represent the "best" shape under all circumstances.

However, in general, a sphere is very efficient at low velocities and elongated tear drop shapes are efficient at higher velocities.

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#76

Re: What's the Most Aerodynamic Shape?

06/18/2008 1:52 PM

Most of these postings are flawed because they cite examples of engineered shapes that have functional requirements beyond minimizing aerodynamic drag.

Golf balls have to be a shape that can be struck by a golf club and meet the limitations of golf rules. Row boats have to carry rowers. Airplanes have to lift engines and payloads and they have to be stable and maneuverable in flight. Bombs have to carry explosives and a trigger inside. All of these shapes compromise drag to some degree to perform their primary functions.

Even the example in nature, the rain drop, is a shape that is influenced by cohesive forces and other physical properties.

The original question asked which shape minimized drag in an ariflow, and had no other requirements (except to have a measurable volume). So forget the engineered examples. The shape that answers his question can be determined by aerodynamic theory and the design should start on a blank sheet of paper.

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