What's the Most Aerodynamic Shape?

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.

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?

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.

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.

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.

You guys are so stupid. They are not asking for the most aerodynamic shape not not the object so shut up with all this stupid science about water and talk about aerodynamic science because I want to know that. Is this shape aerodynamic?

It depends on who's buying.

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

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!!

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.

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.

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

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

GA techno,

This matches what I saw on the Discovery Channel recently.

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.

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

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.

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

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.

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.)

Fabrication of the 'micro riblet film'

cheers

Thanks! Nice link. The transition from where the riblets reduce drag to where they increase drag is somewhere between Re 10³ and Re 10^4. 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.

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.

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

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.....

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?

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.

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.

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!!

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!!

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.

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.

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.....

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

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

Sherical projectiles in rifled barrels were fine.

...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!!

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

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).

Cool link

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.

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.

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.

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

Nice link, Ken. Thanks!

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

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

The larynx of a politician?

This one seemed to work pretty well.

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

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.

I agree with bubbapebi that seems to work pretty well

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.

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

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.

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

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

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

It's the red part that has me curious!

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

Maybe the point itself was sharp.

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

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.

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.

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

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

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!)

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.

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

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.

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

It's only 'ideal' for one speed.

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

And or only for one direction

A 45 cal.bullet is the right shape.

Tee-hee, and the trajectory of a birdie

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

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.

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)

Del

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

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.