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Old 10-18-2020, 02:18 AM   #51 (permalink)
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We are operating at an overall mechanical efficiency of only four percent… Therefore, we find that if we increase the overall mechanical efficiency to only twelve percent we can take care of everybody. That three-fold increase in the overall efficiency can only be accomplished by redesign. – R. Buckminster Fuller

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Old 10-18-2020, 02:26 AM   #52 (permalink)
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But, but, people aren't ideas.
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Old 10-18-2020, 05:29 AM   #53 (permalink)
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Old 10-18-2020, 08:14 AM   #54 (permalink)
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Quote:
Originally Posted by Fat Charlie View Post
Given the general size and shape of the interior of a generic car, the template seems to be a very predictable way of cleaning up the rear end. On any particular car you can get better results, but offering a template is much more constructive than shrugging and telling modders they're on their own and should simply design and test their own prototypes.

While citing a T-100 built to the template is nice, more relevant here is JulianEdgar's false horror at the Insight not meeting the template- and then his addition of a spoiler to his own Insight. Which seems to have brought it closer to the template...

What would be more valuable would be an explanation of how he arrived at that spoiler. What shorter or longer versions didn't do as well? Higher? Lower? How about a good profile pic to put against the template? Simply attacking the Holy Template while seeming to apply it isn't a good method, especially if your position is one of data and testing. Presumably the testing that went into that (beautifully done) spoiler had one experimental version that met the template, or perhaps the final version does. Either one of those facts would have been a much more constructive argument than the ones we got.
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Excellent points. I agree "the template" a great starting point. Not an end all be all for every shape every time. If someone actually built the full 3d hemisphere vehicle I surmise it would be rather low drag.

The point of the T100 is that it is Aerohead's truck. Which Julian chose to put in his book. Before the internet discussions became contentious there was some level of mutual respect or the two would not have agreed for this to occur.

It is no secret that there are many low drag cars that do not meet the template. There are also many that do. Balancing drag, lift/downforce, cargo volume, etcetera must come into play in a design.

Julian goes into a few details in his book on the wing, but the point of the wing/spoiler (as explained by him in his book) is to increase downforce and straight line stability, while increasing drag only a little. This appeared to be his same goal with his full belly pan and diffuser, which as tested slowed his car down on hill decent velocity which he equates to drag, but decreased the ride height which he equates to downforce.

The question in my mind is why the insight needs all these modifications to increase downforce? Is there something about its basic shape that is causing lift? Would it have been better for the designers to pick a different shape, perhaps with higher drag but less lift? Studying various diagrams, local minima for drag typically do not align with local minima for lift. A choice must be made between the two.

One of the reasons I chose to use the AST-2 diagram to plan my modifications is that it is "safe" in that it is not at the bitter edge of the air "letting go" before I want it to. I can put a load in the truck/suv, drop the rear, make all the angles steeper, and it still works. Does that mean I could have made it more aggressive initially? Probably... but what them when I load it?

I do not understand all the drama about a suggested shape that is being offered free of charge on the internet. Not behind a paywall for $$$. Not mandated for new car design. Suggested. For free.
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Old 10-18-2020, 10:23 AM   #55 (permalink)
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Quote:
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Is there something about its basic shape that is causing lift?
Whenever airflow wraps around a curve it creates lower pressure. The same concept as an airplane wing/profile. Since the top of the car is curved to fit the passenger cabin the (negative) pressure on the top is pulling up on the car. If you flipped the car upside down it would make downforce.

That's what I have gathered anyway.
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Old 10-18-2020, 12:36 PM   #56 (permalink)
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Quote:
The same concept as an airplane wing/profile.
A wing in free air is not the same. It's lift in complex. Bernoulli effect is only part of it (camber, angle of attack).

I'd say the ground vehicle is more comparable to a wing tip.
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We are operating at an overall mechanical efficiency of only four percent… Therefore, we find that if we increase the overall mechanical efficiency to only twelve percent we can take care of everybody. That three-fold increase in the overall efficiency can only be accomplished by redesign. – R. Buckminster Fuller

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Old 10-18-2020, 02:42 PM   #57 (permalink)
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Quote:
Originally Posted by freebeard View Post
A wing in free air is not the same. It's lift in complex. Bernoulli effect is only part of it (camber, angle of attack).

I'd say the ground vehicle is more comparable to a wing tip.
Yes. A wing can be considered as a 2D section (this is how the understanding of aeronautics was developed). However, the flow around a car is highly 3-dimensional, which is why every textbook I have cautions against trying to apply an understanding of aeronautics directly to cars.

That said, while a "wing in free air is not the same," the mechanism at work may be the same: airflow "wrapping" around the roof causes an increase in flow speed and reduction in pressure--but of course the ramifications of that are more complex than on a 2D wing section. For example, I found by direct measurement that the flow speed over the Prius' roof is faster toward the middle and slower toward the outside edges, and that this situation is reversed over the rear window.
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Old 10-18-2020, 03:30 PM   #58 (permalink)
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That said, while a "wing in free air is not the same," the mechanism at work may be the same: airflow "wrapping" around the roof causes an increase in flow speed and reduction in pressure
Which is why I compared to the wing-tip.

Central jets are utile in the underbody.
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Old 10-18-2020, 08:54 PM   #59 (permalink)
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There is controversy about why wings produce lift. Many people are aware of Bernoulli's principle. The other component of lift, and maybe the larger one, is that the wing pushes air down as it moves through the air, and that air pushes back, creating lift.
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Old 10-18-2020, 09:38 PM   #60 (permalink)
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The real details of how an object generates lift are very complex and do not lend themselves to simplification. For a gas, we have to simultaneously conserve the mass, momentum, and energy in the flow. Newton's laws of motion are statements concerning the conservation of momentum. Bernoulli's equation is derived by considering conservation of energy. So both of these equations are satisfied in the generation of lift; both are correct. The conservation of mass introduces a lot of complexity into the analysis and understanding of aerodynamic problems. For example, from the conservation of mass, a change in the velocity of a gas in one direction results in a change in the velocity of the gas in a direction perpendicular to the original change. This is very different from the motion of solids, on which we base most of our experiences in physics. The simultaneous conservation of mass, momentum, and energy of a fluid (while neglecting the effects of air viscosity) are called the Euler Equations after Leonard Euler. Euler was a student of Johann Bernoulli, Daniel's father, and for a time had worked with Daniel Bernoulli in St. Petersburg. If we include the effects of viscosity, we have the Navier-Stokes Equations which are named after two independent researchers in France and in England. To truly understand the details of the generation of lift, one has to have a good working knowledge of the Euler Equations.
https://www.grc.nasa.gov/WWW/K-12/airplane/bernnew.html

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