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Old 10-25-2009, 04:14 PM   #261 (permalink)
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Thanks for the suggestion Mike,

Here's what the boattail might look like based on the airplane profile you posted:



My first drawing was fairly close, but the top was too steep at the back. I also just truncated it about where it would have to for taillights, etc.

I just had a thought: the video cameras I am using for side mirrors could be used to video a tuft test of the underside of the boattail! They cost ~$22 and they could be recorded on a laptop with a basic analog video input plug.

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Old 10-25-2009, 11:16 PM   #262 (permalink)
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Hi basjoos,

You are correct about the NACA airfoils. When I was viewing these carefully about two years ago, I was specifically looking for those that were close to symmetrical in shape as possible.

As you pointed out, we are not looking for shapes that create lift, but rather shapes that allow a low Cd.

Mike, I'm still amazed that the Questair Venture can still keep the air attached at 350 mph with that sharp of an angle. Even the Bonneville Salt Flats land speed record motorcycles are much longer in shape, and they also get up to about 350 mph or so. In fact, if I recall correctly, the old Piper Cub airplanes have a longer tail than the Questar, and my Dad's cruised at about 65 knots.

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Old 10-26-2009, 06:15 PM   #263 (permalink)
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Quote:
Originally Posted by 3-Wheeler View Post
Mike, I'm still amazed that the Questair Venture can still keep the air attached at 350 mph with that sharp of an angle. Even the Bonneville Salt Flats land speed record motorcycles are much longer in shape, and they also get up to about 350 mph or so. In fact, if I recall correctly, the old Piper Cub airplanes have a longer tail than the Questar, and my Dad's cruised at about 65 knots.

Jim.
The tail cone on the back of the space shuttle has about the same angles as the Questair Venture tail cone. There are also some WWII aircraft drop tanks and JATO units that terminate in similar angles. If the Venture didn't have attached air flow, it couldn't go as fast as it does on its engine power output. As I mentioned earlier, there are factors in addition to attached airflow that aircraft designers are considering when determining the length of an aircraft's tail. The Venture's designers decided to go for the shortest tailcone with a larger than usual vertical stabilizer.
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Old 10-26-2009, 11:41 PM   #264 (permalink)
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Mike,

All interesting stuff! Thanks for the input.

Cd,

Thanks for the pictures of the Space Shuttle.

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Old 10-27-2009, 09:02 AM   #265 (permalink)
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boattail #5

Hi,

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.
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Old 10-27-2009, 11:20 PM   #266 (permalink)
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Quote:
Originally Posted by NeilBlanchard View Post
Hi,

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...

http://ecomodder.com/forum/showthrea...0-a-290-9.html

Quote:
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'

http://ecomodder.com/forum/showthrea...tail-8927.html

Quote:
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.

Jim.
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Old 10-28-2009, 12:56 PM   #267 (permalink)
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Does the fact that Basjoos has installed inner and outer side skirts (pontoon style) between the front and back tires and a totally smooth belly pan allow him to keep more airflow under the car? Would this allow him to cheat the 4 degree limit on the bottom of the boat tail? If so, this would be helpful for a daily driven vehicle that has to negotiate steep driveways.
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The power needed to push an object through a fluid increases as the cube of the velocity. Mechanical friction increases as the square, so increasing speed requires progressively more power.
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Old 10-28-2009, 03:04 PM   #268 (permalink)
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Quote:
Originally Posted by COcyclist View Post
Does the fact that Basjoos has installed inner and outer side skirts (pontoon style) between the front and back tires and a totally smooth belly pan allow him to keep more airflow under the car? Would this allow him to cheat the 4 degree limit on the bottom of the boat tail? If so, this would be helpful for a daily driven vehicle that has to negotiate steep driveways.
Hi COcyclist,

I think the pontoons can certainly help to keep some of the air under the car, but the extra volume of air needed to fill that expanse under the tail, means that the extra air has to come from somewhere.

One could put side ducts by the rear tires to force air into the space, but then there's more drag because of the ducts.

At this point in time, it seems the only option is to limit the angle of the boat tail underside to some volume in which the air coming into the underside of the car at the front, can easily fill at the rear.

To keep the air attached to surfaces on the car body, there appears to be the requirement of 'pressure'. That is, if one were to attach tiny manometers to the surface of various panels on the car, the air is still attached when the pressure is at or slightly above atmospheric.

If the air is allowed to try and fill too large of a void or pocket with more than is available, then the pressure will drop below atmospheric and the air will then be allowed to become detached from the body surface.

The ideal boat tail exit angle may not be possible in situations where it needs clearance over steep inclines and such.

Jim.
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Old 10-28-2009, 03:41 PM   #269 (permalink)
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How does the side taper of the boatail affect the 4 degree limit? The red/blue drawing above shows a 2 dimensional vertical taper, but a boattail that tapers both vertically and horizontally is going to require less airflow to fill the void under the boattail than a boattail that only tapers vertically (as shown in the drawing).
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Old 10-28-2009, 08:54 PM   #270 (permalink)
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Right, we cannot forget the 3-dimensionality of the situation. I remember reading in Hucho about the Schlör "Pillbug", that they predicted a Cd of 0.18 theoretically based on the 2D profile, but after they built it they measured 0.13 (they had to tweak things like the height).

My best guess is that the air flow around the sides is how it ended up being better than the 2D profile had predicted. BTW, here's the drawing from the Hucho book of the Schlör "Pillbug". I think the lofting lines are not completely accurate, as I started to make a 3D CAD model by tracing them, and it does not look right.



The other thing that makes me think this is only approximate is the layout of the windshield glass -- the car had a center section of glass, whereas this drawing shows a pillar in the center of the windshield. And I'm fairly sure that the 1275mm height is for the body of the car; not it's height above the ground.

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