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Old 12-05-2012, 09:07 AM   #1 (permalink)
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Why is teardrop the most sreamlined?

I'm really late to coming around to these streamlining discussions, and I understand a lot of work has been done by a lot of people, but why does this shape have to be accepted as being best? Airplanes go through the air well and are not that shape. Rockets are shot into space and are not that shape. Land speed vehicles are usually not that shape, nor are missiles. I think that would prove that the best shape for cutting air would be a pointy cylinder. What am I missing?

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Old 12-05-2012, 10:22 AM   #2 (permalink)
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Aerodynamic drag below ~250mph is different that speeds up to ~750mpg (around the speed of sound). There are three kinds of drag (as I understand it): shape drag, surface drag, and detail drag. Shape drag is dominate for ground vehicles, and the the interaction with the ground is also important.

The wing section of an airplane is essentially a teardrop.

The physics of air is what defines the lowest drag shape. The front of the moving vehicle pushes the air outward from the stagnant point - up and down and to the sides. Then after the apex of that displacement has passed, the rate that the air can *cleanly* close back down together is fixed by two main factors: atmospheric pressure and the localized higher pressure caused by the displacement itself. The latter one changes with the speed of the vehicle (more or less) in a linear fashion up to about 250mph; and so the same shape works well for any speed up to that 250mph limit.

If the surfaces of the vehicle are at the proper angle, this keeps the air "attached" to the vehicle. Too steep an angle and the will be turbulence when the air is no longer "attached". Too shallow an angle, and there is an increase of surface drag.

I hope this helps.
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Old 12-05-2012, 10:23 AM   #3 (permalink)
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the teardrop isn't the best in all situations, yet it is a good way to think about air flow management. one of the big issues for us landlubbers is ground effect, which basically makes 1/2 a teardrop much better than a whole one.
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Old 12-05-2012, 10:30 AM   #4 (permalink)
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Quote:
Originally Posted by minimac View Post
I'm really late to coming around to these streamlining discussions, and I understand a lot of work has been done by a lot of people, but why does this shape have to be accepted as being best? Airplanes go through the air well and are not that shape. Rockets are shot into space and are not that shape. Land speed vehicles are usually not that shape, nor are missiles. I think that would prove that the best shape for cutting air would be a pointy cylinder. What am I missing?
Fluid dynamics is probably the least understood science of them all. There are a few laws (mostly all related to conservation of energy) but there is not one law that can lead to the optimal solution across all scenarios. The main reason being is that fluid (including air) behaves in different states and applications.

Super-sonic jets don't utilize the tear drop because there are different applications such as how sound comes in to play, etc.

When car manufactures make cars they don't apply one law to govern the most aerodynamic shape since there isn't one. They have to use computers to run complicated models to come up with their designs. It is a iterative approach which means the next optimal output depends on the input from the first process. It just happens to be that the tear drop is the most optimal solution for the applications a vehicle will see.

In space there is not fluid so the spacecraft can be any shape it wants to be and still have the same efficiency as another. It is only when it enters the atmosphere does fluid dynamics come into affect. That is the why the space shuttle is not very efficient flyer during return flights but it was design for storage and heat displacement instead.

There are many papers/articles and people can go on and on about aerodynamics but it all boils down to that there isn't one law that encompasses all of aerodynamics.
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Old 12-05-2012, 03:59 PM   #5 (permalink)
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Because it is the most efficient...

Quote:
Originally Posted by minimac View Post
I'm really late to coming around to these streamlining discussions, and I understand a lot of work has been done by a lot of people, but why does this shape have to be accepted as being best? What am I missing?
It has the lowest coefficient of drag and is the most efficient at cutting thru the air--that's why it's the best. Do you not understand math and science?
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Old 12-05-2012, 05:14 PM   #6 (permalink)
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Yes, I do but I learned fluid mechanics in college not high school. A great book for ecomodders is "The Leading Edge" by Goro Tamai. This is mainly about the aerodynamics of solar racers but is very practical for modding passenger cars.
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Old 12-05-2012, 05:44 PM   #7 (permalink)
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Yes, but is the rounded nose of a teardrop the ideal shape? Water has surface tension, which among other reasons means it won't ever develop a pointy nose. Is there something about a rounded nose that is just ideal for lowering air pressure that a pointy nose doesn't have?
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Old 12-05-2012, 08:55 PM   #8 (permalink)
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I too don't quite understand why a teardrop is most optimal. it seems to me that a shape like the image I've attached would be most ideal.
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Old 12-05-2012, 09:25 PM   #9 (permalink)
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Accepted science once stated that the world was flat. It was only by the acts of some pioneering souls, in the face of scorn and ridicule, that it was proven to be otherwise. Could a pointy nose be better? Does it matter if it sloped down to a point at the front(horizontally) or should it be pointed at the center vertically? Is it more efficient to cleve through the air or bend it? Wouldn't a cylinder be better at resisting a side wind than slab sides with a long trailing tail?
I not looking to reinvent the wheel, but just because a shape works, should it be accepted as THE shape?
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Old 12-05-2012, 10:06 PM   #10 (permalink)
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Pointy nose helps at very fast speeds, but a blunt nose is better at speeds below 250mph. The reason as I understand it, is what I wrote in the post above.

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