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Old 04-11-2012, 09:59 AM   #291 (permalink)
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It is a lifting body. It will follow a form that will generate lift, which is just more drag for a rig rolling on the ground. The other thing missing is that pesky ground plane. The lifting body is up in clean air.

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Old 04-11-2012, 10:06 AM   #292 (permalink)
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Quote:
Originally Posted by skyking View Post
The other thing missing is that pesky ground plane.
Is there a pressure wave of some kind hovering over an automobile because of the ground plane interaction?

If so, then this means we need blunter noses to build up attachment and longer bodies (compared to free-air) to maintain it?
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Old 04-11-2012, 10:12 AM   #293 (permalink)
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What I'm saying is, the lifting bodies were designed and tested to generate lift in clean air.
None of that statement applies to vehicle design. It is truly apples and oranges.
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Old 04-11-2012, 03:39 PM   #294 (permalink)
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Quote:
If the top slopes down but the sides don't taper in then lower pressure above tends to suck air up

and over

resulting in vortices.
(emphasis is mine)

This is exactly what I was thinking -- if the sides tapered in, then the air on the side would be more likely to stay on the side and therefore reduce the lifting vortex. Which would mean less drag... Air flow is 3D.
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Old 04-11-2012, 06:56 PM   #295 (permalink)
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ground plane

Quote:
Originally Posted by kach22i View Post
Is there a pressure wave of some kind hovering over an automobile because of the ground plane interaction?

If so, then this means we need blunter noses to build up attachment and longer bodies (compared to free-air) to maintain it?
It was Paul Jaray who discovered that a low drag body in free flow would exhibit a two-fold drag increase when near the ground plane.
A vehicles drag,as far as the atmosphere is concerned,is represented by the vehicle and it's mirror image below the ground.
So your car is only half as long,or twice as high as you observe it,aerodynamically.
Jaray determined that the lowest drag form is half of a streamline body of revolution and its complementing mirror image below the ground.
The 'Template' is derived from a 2.5:1 streamline body of Cd 0.04 which creates a body in ground-effect which is five times as long as it is high of Cd 0.08.
At this 'length' the rear contour just respects Mair's maximum boat tail angle of 22-degrees.
If you build the car shorter,the angles are too steep,you get separation and higher drag.
If you build the car longer your pressure drag isn't any lower and the increased wetted area causes higher overall drag due to the increased friction.
The 2.5:1 ratio is the 'sweet-spot',balancing lowest pressure drag and lowest surface friction.
That's all there is to it.
All pressures around the car will be a function of local flow velocity and vary from location to location.
The bulbous nose of the 'Template' provides gentle uniform flow accelerations,minimum pressure spikes,and vectoring to energetic longitudinal flow to the point of maximum body cross section,after which,the gentle converging aft-body provides the gentle pressure rise necessary to protect the fragile turbulent boundary layer which is operating in an adverse pressure regime,with nothing there to hold it against the body except that gentle contour.
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Old 04-11-2012, 07:04 PM   #296 (permalink)
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exactly

Quote:
Originally Posted by NeilBlanchard View Post
(emphasis is mine)

This is exactly what I was thinking -- if the sides tapered in, then the air on the side would be more likely to stay on the side and therefore reduce the lifting vortex. Which would mean less drag... Air flow is 3D.
Neil,if you had Fachsenfeld's book you'd see Kamm's wind tunnel models doing exactly as you describe it.It is exactly what's going on.
Elsewhere in the book you'd see how Walter Lay 'fixed' it with plan taper.
All else being equal,the difference between straight sides and tapered sides is Cd 0.21 vs Cd 0.12.
This is why conventional 'teardrop' trailers are incapable of truly low drag.And this is also why some messengers make the comment that 'just because it looks streamlined doesn't mean it IS streamlined.'
Good call!
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Old 04-11-2012, 10:53 PM   #297 (permalink)
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Quote:
Originally Posted by aerohead View Post
If you build the car longer your pressure drag isn't any lower and the increased wetted area causes higher overall drag due to the increased friction.
Real world is sometimes different then wind tunnels.

Real world says too short is pretty bad.

Too long is almost exactly the same as "perfect."

Form drag is orders of magnitude worse then skin drag.
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Old 04-12-2012, 12:03 PM   #298 (permalink)
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Wow, plan taper adds as much as 0.09 -- or maybe ~42.5%. That is huge.

If the ideal taper in the side profile is maximum of 22 degrees, then what is the ideal plan taper? This will greatly shorten the length of the back of the body.
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Old 04-12-2012, 12:11 PM   #299 (permalink)
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It was a body of revolution in the tunnel, meaning it theoretically would be the same profile. I think that is excessive, besides it does not follow my needs. Form does follow function sometimes
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Old 04-12-2012, 01:16 PM   #300 (permalink)
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for the sides, the real number is 22 degrees minus side winds.

I don't think 22 degree sides is realistic in the real world IMO.

The top of the car is not affected by side winds.

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