Aerodynamic Streamlining Template: Part-C

[kach22i]

Originally Posted by aerohead

--------------------------------------------------------------------------
Roof contours steeper than Jaray's roofline are what Hucho refers to as 'pseudo-jaray'.These roof contours produce separation and very strong attached longitudinal vortices of remarkably high drag.
--------------------------------------------------------------------------

When you say steeper, I think this is a key term.

Back when I asked if the Cd of a 1/6 scale model was the same as the full scale example, I should have been asking if the airflow was the same - it is from what I currently understand - which is why models are used at all.

For wind tunnel testing the reason the air is accelerated for models is to equal the air pressure (lift in the case of aircraft) to scale, right?

Originally Posted by aerohead

*For a full-scale vehicle the critical Rn will occur at 20 mph.
*1/6-scale 120 mph.

If you think of the scale model with a roof curve being no steeper than the full scale example, then I am on to something.

If you consider the model's roof curve to be steeper than the full scale version then my ideas would appear to be a bust, and therefore scale models hold little value for examining airflow and air attachment.

Is the measure of steepness to be measured horizontally over length? Please, no Calculus, I hate Calculus.

Or is the measure of steepness to be measured in inclination angles (scale model equal to full scale), never exceeding 22 degrees?

I have my answer to the last two questions based on what I see in wind tunnel testing photos (see BigMike's MR-2 site link: Car Wind Tunnel Data), and the airflow over a model (not air pressure) will be similar, the larger the model the more accurate, full scale testing preferred.

What does "steeper" really mean?

Originally Posted by ChazInMT

Your picture here with a line I added shows where you would want to put this line as a base point to work from.........

I agree and described these sketches as following the template's highest point (center). However as Tesla pointed out the distances the arc follows at the sides on a rectangular body will have to be more "aggressive". This is where the call for a new plan view only template stems from.

To be of practical value, one could continue to use the template in profile, but not in plan view. The step to 3D and to fit typical body styles is yet another reach, for another day.

Let's get a working plan view first, and then resolve the transition from square to round.

[ChazInMT]

There is no set way to use the taper in plan view. This has gone around & around in other discussions, mainly here in this thread, post #409 seems to be where the thing started. I’m pretty sure we concluded that the template would be applied pretty well if we use the one from the profile for the top/plan view as well. This is due to the fact that we don’t want the air coming together any faster in one place than the other. Since we know the profile application sets the size of the curve for the top optimally, than we should match that for the sides. Remember the problems really start for us when there are drastic pressure differences in close proximity to each other. So it is better to go too long, than too short when applying the template curve. I still feel putting the template together bottom to bottom as illustrated in post #413 is far too aggressive.

In short, we’ll do pretty dern good if we use the template we generate from the side view as the top view template as well.

Here’s a wrinkle to all this, much like the template application yields diminishing returns as you go further out, I believe that as we get closer to the ideal shape, striving for perfection, or straying from it by some, these small deviations from ideal will have only very minor impact on the achieved Cd. So while we sweat the details as to what is perfection, getting close will still yield some stunningly good results as well.

[kach22i]

Originally Posted by aerohead

-------------------------------------------------------------------------
In light of the fact that so much testing and re-verification testing has been accomplished since 1921,it's extremely unlikely that the aerodynamicists involved have missed anything significant. And so,the 'Template' could pass for off-the-shelf technology,nothing theoretical about it anymore. Just waiting to be exploited.
----------------------------------------------------------------------
And so it's my humble opinion,that we've had enough testing. We've tested the thing to death. And it always delivers the lowest drag (for a car you can walk up to,open a door,and get in or out of,without a bunch of teammates to help).

Not to pick on a point I've labored already on, but all of this theory and testing has involved a half round sectional shape, and not a rectangle which road cars use.

If there is a template based on a square/rectangle in section and plan, then this is the one we should be referencing, and not something off a blimp...........in my humble opinion.

[kach22i]

Originally Posted by ChazInMT

.......because a template is a line.

It is a line based on a half-circle for close proximity to the ground, and the full circle one in free air is much shorter.

What is the line for a rectangle close to the ground?

I bet this line differs for differing cross sectional shapes.

And if it doesn't, I'd like to find out why.

[kach22i]

Originally Posted by ChazInMT

No. The shape was made with the line. The line was based on a foil section that creates the least drag.

You show us what you can do with the line and a rectangle.

A picture is worth a whole lot of words, right?

Industrial Design - Transportation Photos by kach22i | Photobucket
[URL=http://s184.photobucket.com/user/kach22i/media/Industrial%20Design%20-%20Transportation/many-cross-sections-aero-kach22i_zps854abf5d.jpg.html]

This first pass at illustrating the issue goes so far as to accept the semi-circular section's tailing plan (tadpole shaped).

If I were to draw up a more commonly encountered rectangular plan with a wheel at each corner, then a new slew of questions gets raised even higher.

For now imagine these plan conditions would be addressed similar to Rednecks cantilever on his Geo Metro to simplify the matter.

The template line and arc only go with the plan and sectional shape it has been tested on - in my opinion. All other attempts at application to other cross sectional shapes and plans is a nice try, but no cigar.

Template use in this forum is overreach with good intentions but providing unknown and unverifiable results.

[kach22i]

Originally Posted by ChazInMT

Except for maybe......

But what does Aerohead or any of these following people know?

Are you claiming that one of more of these projects follow the template in plan, section and in profile?

The template profile line is married to section and plan in order to work as a 3D shape. To mate that line to anything else is sort of like adultery.

[freebeard]

Where to start? I'm not a Template™ purist.

Naval architects call this lofting.

Consider fineness ratio. If a body is 1/2 as tall as it is wide the taper would be equal in plan and section. If the body is as tall as it is wide , or average, then the fineness ratio in plan is less, and the taper is...greater? Less?

Originally Posted by ChazInMT

You show us what you can do with the line and a rectangle.

Not pickin' on ya, but this ignores the bluff forebody that allows the aft portion to receive a uniform pressure around the profile.

[ChazInMT]

Originally Posted by freebeard

Not pickin' on ya, but this ignores the bluff forebody that allows the aft portion to receive a uniform pressure around the profile.

Yeah, I know, but if Aerohead has said it once he's said it like a gazillion trillion times. Forget the forebody when using Thee Template™, (Cue Gregorian chant music) just pay attention to the back cause that is where the most significant gains can be made if you're gonna modify a car/truck/thingamajig. If you're going to design something from scratch, you'll need help from something other than the template to help you. But from what I understand from reading a bunch in here, most front ends are pretty good to begin with on the modern auto. And Sledge Hammer Girl.

[freebeard]

The phrase that stuck in my head is low hanging fruit.

Are you a Template™ Purist or not?

I should troll this thread with Dymaxion pictures. That pointy tail always bothers me. Seems fragile.

[kach22i]

Originally Posted by ChazInMT

........ it was created with the use of the line.

But a line has no scale if it's not anchored to a particular body platform, making my case for re-scaling the template even stronger (the catch-22).

Originally Posted by freebeard

I should troll this thread with Dymaxion pictures. That pointy tail always bothers me. Seems fragile.

If you think the tail is frail, try sitting in the front when conventional car makes a hit and run on you.

Untitled Document


EDIT:
Template away, I found your car.

http://forum.donanimhaber.com/m_31437284/tm.htm