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Old 02-28-2010, 11:23 AM   #1 (permalink)
Cd
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Let's talk about Fineness Ratios ( pointy front ends )

So what can you guys tell me about finess ratios and their importance to achieving a low Cd ?

I have seen a lots of sports cars that look fast and have sloping front ends, but for the most part, all the low drag concept cars that I have seen have rather bulbous front ends.

From Wikipedia : Fineness ratio - Wikipedia, the free encyclopedia

"Aircraft that spend time at supersonic speeds generally have high fineness ratios, a canonical example being Concorde.

At speeds below critical mach, one of the primary forms of drag is skin friction. As the name implies, this is drag caused by the interaction of the airflow with the aircraft's skin. To minimize this drag, the aircraft should be designed to minimize the exposed skin area, or "wetted surface", which generally implies the fuselage should be somewhat "egg shaped", with a fineness ratio about 4.5. A good example of such a design is the Questair Venture."

Since we aren't breaking the sound barrier with our cars, it would seem that the ideal front end shape would be that of an egg versus a spike.



So with much respect to those of you that have done it, how does adding a pointed front end to our cars help to lower drag ?

( I'm not 'dissin' you guys that have done it, I'm just not smart enough to grasp the concept. )

Here is my other thread which asks the very same question ( I'm starting a new thread because that one is a mess. This one is dedicated solely to fineness ratios )

http://ecomodder.com/forum/showthrea...s-2914-20.html

And Frank ... I can already see your eyes rolling.

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Old 02-28-2010, 12:43 PM   #2 (permalink)
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So with much respect to those of you that have done it, how does adding a pointed front end to our cars help to lower drag ?
My guess is (I don't know if this came up in the other thread, as I didn't get through it) that the 'wedge nose' is to lower the stagnation point and reduce the amount of airflow under the car. The slight increase in skin friction is outweighed by the decrease in undercar drag. Whether this works (and how well) with an almost perfect bellypan - I don't know.
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Old 02-28-2010, 01:19 PM   #3 (permalink)
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...airplanes should "lift" to fly, cars, however, shouldn't...hence, the "downward" wings on Formula 1 cars.
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Old 02-28-2010, 01:35 PM   #4 (permalink)
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I don't believe the argument that skin friction is so important. Look at how much skin an aero bed cover or a boat tail adds. Also, some on this forum have said that a blunt front end creates a virtual pointy nose. Well that virtual pointy nose is also experiencing friction, from the air flowing around it, and that force gets transmitted to the vehicle. You don't get a free pass just because your nose is "virtual".
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Old 02-28-2010, 01:42 PM   #5 (permalink)
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Also, I think the aerofoil shape is misapplied to cars. The reason a wing has a front end that's rounded is because it has to work through a wide angle-of-attack range. A plane creates lift by tilting the wing. A sharp front edge on a wing would create vortices when the wing is angled relative to airflow. A car doesn't have to worry about this, only an average airflow vector within some yaw angle hitting it from the front (30 degrees?)

I die a little every time I see a teardrop template overlaid on a car. For the above reason, plus the fact that folks always align the bottom of the teardrop with the ground, as if a vehicle with more ground clearance somehow needs shallower curves up top.
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Old 02-28-2010, 01:52 PM   #6 (permalink)
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Also The reason a wing has a front end that's rounded is because it has to work through a wide angle-of-attack range.
...exactly, because high-speed jet wings have almost razor-sharp leading edges, not the smoothly rounded edges seen on a Piper Cub, etc.
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Old 02-28-2010, 02:37 PM   #7 (permalink)
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Originally Posted by winkosmosis View Post
Also, I think the aerofoil shape is misapplied to cars. The reason a wing has a front end that's rounded is because it has to work through a wide angle-of-attack range. A plane creates lift by tilting the wing. A sharp front edge on a wing would create vortices when the wing is angled relative to airflow. A car doesn't have to worry about this, only an average airflow vector within some yaw angle hitting it from the front (30 degrees?)
Cars are also travelling at sub-sub-sub Mach speeds, ergo, the shape simply need not be like a plane that's Mach one, two, or three capable, to be efficient. Or, more aptly: efficient enough.

In addition to that, not having a huge schnoz on a car that makes it efficient for straight ahead travel, but a tad inconvenient for turning corners, or parking in a standard parking lot space.

Both aspects of this are most certainly compromises. Die a little on the inside if you so choose, but people trying to sort out optimum shapes are deciding that one somewhat inconvenient extension of their car is better than two very inconvenient extensions.
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Old 02-28-2010, 02:43 PM   #8 (permalink)
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Originally Posted by ChrstphrR View Post
Cars are also travelling at sub-sub-sub Mach speeds, ergo, the shape simply need not be like a plane that's Mach one, two, or three capable, to be efficient. Or, more aptly: efficient enough.

In addition to that, not having a huge schnoz on a car that makes it efficient for straight ahead travel, but a tad inconvenient for turning corners, or parking in a standard parking lot space.

Both aspects of this are most certainly compromises. Die a little on the inside if you so choose, but people trying to sort out optimum shapes are deciding that one somewhat inconvenient extension of their car is better than two very inconvenient extensions.
What makes me die is that they use the teardrop template to guide what they do on the rear, when it's completely arbitrary how the curve falls on the photo. For example, the way people use it here, a lifted truck supposedly has a shallower optimal aerocap angle than a stock one.
The template also has no way of knowing what the actual airflow over the car is like, or the speed that it's travelling at.
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Old 02-28-2010, 02:46 PM   #9 (permalink)
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What makes me die is that they use the teardrop template to guide what they do on the rear, when it's completely arbitrary how the curve falls on the photo.
...darn! where's the MythBusters when you really need them? A drop of falling water is round, not teardrop shaped.
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Old 02-28-2010, 03:01 PM   #10 (permalink)
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Quote:
Originally Posted by winkosmosis View Post
What makes me die is that they use the teardrop template to guide what they do on the rear, when it's completely arbitrary how the curve falls on the photo. For example, the way people use it here, a lifted truck supposedly has a shallower optimal aerocap angle than a stock one.
The template also has no way of knowing what the actual airflow over the car is like, or the speed that it's travelling at.
People misusing the template is a big problem, and I think you're misunderstanding how to use it, too. The template, correctly used, is fully constrained:

Align the ground planes, scale the template to the same height as the roof of the car, then slide the template forward or backward to align the highest point on the roof with the tallest point on the template.

The ideal half-teardrop has no ground clearance. A streamlined car is only an approximation of that ideal.

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