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Old 12-09-2012, 12:14 PM   #21 (permalink)
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1. Optimum aero is not the only (or primary) design criterion in transportation, even air transport. The fuselage of a commercial airliner looks like a rounded barrell because that is one of the easiest shapes to pressurize without fatigue stresses downing the airliner after a few thousand "cycles."

(Also notice that "new" designs often have extended fuselages based on previous designs, due to the fact that the previous design has already gone through certification, and a modified design can be put into production more cheaply and quickly. Example: DC-9 and MD-80.)

2. Pointy noses are best when supersonic, as the air ahead of the vehicle "doesn't know what's coming" and slams into the leading edge. Subsonic, the air starts getting out of the way before the object ever gets there.

(A pointy leading edge isn't all that bad, subsonic, provided you never, ever, have a crosswind. Pointed leading edges are very intolerant of wind hitting it sideways.)

3. A teardrop, of defined "fineness ratio," is the optimum shape to enclose a given volume. Usually, that isn't the primary concern: the P-51 was designed to have a frontal area just large enough to encapsulate the pilot. This made for a plane that was "too skinny," in terms of fineness area, for optimal aero, but it wouldn't make sense to make the plane "fatter" just to meet the teardrop ideal--humans aren't fluids, and you can't "make up" for a deficiency in one dimension by making a surplus in another!

(Now, a auxillary fuel tank encloses a fluid, and thus makes an excellent candidate for a teardrop shape.)

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Old 01-08-2013, 12:53 PM   #22 (permalink)
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Quote:
Originally Posted by meanjoe75fan View Post
.....
(A pointy leading edge isn't all that bad, subsonic, provided you never, ever, have a crosswind. Pointed leading edges are very intolerant of wind hitting it sideways.)

3. A teardrop, of defined "fineness ratio," is the optimum shape to enclose a given volume. Usually, that isn't the primary concern: the P-51 was designed to have a frontal area just large enough to encapsulate the pilot. This made for a plane that was "too skinny," in terms of fineness area, for optimal aero, but it wouldn't make sense to make the plane "fatter" just to meet the teardrop ideal--humans aren't fluids, and you can't "make up" for a deficiency in one dimension by making a surplus in another!
So, as I now understand it, a blunted rounded front pushes a "cushion" of air in front of it, which bends the air around the following shape reducing frictional or surface drag. A pointy nose cuts through the air easier, but results in an increase in this surface/frictional drag-while being more unstable encountering cross winds.
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Old 01-09-2013, 05:09 PM   #23 (permalink)
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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?
your pretty much right,just cut the cylinder in half,we don't need all that extra surface area.Generally speaking in the front, it can be narrowed for a good low cd, coefficient of drag[keep the front to a reasonably small amount of surface area] and that long tail lets the air run clean and doesn't create any any bad air to swirl around and come back,creating drag].The idea is to keep the air running clean,undisturbed by mirrors,sharp edges,short corner's etc.The worlds fastest bicycle goes 82 mph,on a non air-streamed bike,90 percent of your pedal power,goes to overcoming air resistance.So you see how important it is,done right you can double the mileage of a motorcycle,but it's not easy to do.Banning it on early motogp bikes,because of crashes and the high speeds they saw,didn't help the learning curve.The manufactures weren't to concerned about aerodynamics,more about packaging and keeping up with all the other square boxes with rounded corner's,though it seems to be changing a little.I'm really surprised BMW hasn't put into a limited production,their concept the Simple,which is a showcase of good technology,that can be built right now.Hope this helps a little,putting all the comments together should answer your question pretty good.
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Old 01-09-2013, 05:52 PM   #24 (permalink)
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One of the things that I remember reading about the leading edge of wings is that they could be angled with a hard edge if they would only be flown at a particular angle against the wind. The blunt/rounded (teardrop) shape lets them move through the wind at a wide range of angles without generating turbulence.

In a car, we drive forward, but we encounter x-winds; a hard/pointy edge on the front might work for head on, but it would generate an eddy in a x-wind.

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