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Old 10-27-2009, 10:20 PM   #266 (permalink)
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Originally Posted by NeilBlanchard View Post

I'm a little more "comfortable" with this version, which has a slightly flatter underside than the previous version:

The symmetry may work in free air (on the airplane), but on a car on the ground, I think that the underside needs to be less sloped. It is also slightly longer (nearly 1/2 the length of the car) and the rear fascia is now 12" high vs the 10" of boattail #4.
Hi Neil,

I have been scratching my head for about one week now, since something on the aerocivic tail still does not 'compute' with me, and you have just summed it up quite nicely.

My specific concern is the steepness of the Mike's tail underside from horizontal, and I could not come to grips with what seemed out of place.

The BicycleBob said something that seemed to make sense with me. This is from the aerocivic thread...

Originally Posted by Bicycle Bob View Post
Hi Basjoos, thanks for all the postings, etc. Have you ever done any tuft testing to detect turbulence on your car? I'm particularly curious about the area under the tail, since I have always drawn much shallower angles.
And... from the link on the 'Maximum Angle of Boattail'

Originally Posted by Bicycle Bob View Post
Speed affects the Reynold's number, which has a small effect over the range we are concerned with. The classic answer to afterbody taper is 15 deg. top and sides, and 4 on the bottom. Those are average best numbers, but can be affected by many other factors.
I have worked up a sketch of the aerocivic that hopefully gets my point across, and reinforces your exact feelings as well.

On the front nose of the picture, the stagntion point is referenced.

The BLUE represents the air flowing ABOVE the car.
The RED represents the air flowing UNDER the car.
The ORANGE zone represents the vacuum at the tail of the car, and likely a zone of unattached air flow (high drag).
The PURPLE represents the rough tail shape that would at least try to keep a pressure zone on the tail of the car, and hopefully maintain attached air flow.

Neil, I'm glad to see you are considering keeping the exit angle of the tail relatively shallow, as BicycleBob has been alluding to for some time. He has mentioned keeping the angle close to 4 on several different postings on EcoModder.

It seems that the higher pressure air getting trapped under the car would want to escape to a lower pressure zone, and to make this happen, would squeeze out the sides of the car underbody. When the air finally reaches the tail, the result is even less air to fill the void now. The air that just squeezed out from the underside of the car, must now make a change in direction, and try to fill the large void under the tail.

So the air is being squirted out from under the car, and being sucked back in under the tail. The large void produced by making the tail exit angle large (over 4), is causing an unwanted vacuum there, because the stagnation point at the front of the car is so low to the ground.

Of course, if you make the stagnation point at the front of the higher to compensate for this, the result will be even more air trying to equalize in pressure and squirt out from the car underside. There is a no-win situation here. The fact that the road is close to the underside of the car, changes the rules of normal 'airplane flow lines'.

If this were an airplane body, the stagnation point would be higher (by the prop.), and more air volume would be available to flow on the underside of the fuselage, thus keeping higher pressure there, and thus filling in at the taper of the tail area. The general air flow on the airplane body is unrestricted on the bottom side, since there is no road surface to spike the air pressure in that area.

Just my $.02.

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Last edited by 3-Wheeler; 10-28-2009 at 02:09 PM..
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