Models that don't work on real cars

[kach22i]

Originally Posted by bentring

Although I've long been fascinated with automobile aerodynamics, I don't have enough knowledge and interest to speak of the science, so I usually don't bother parroting what's accepted as common knowledge here.

I know there's been prior debate of the subject of the template I haven't read, so if this information has been discussed to death feel free to disregard this post; but I feel like information, pictures, and videos on this page would be relevant to the discussion.

Wow, what a great link.

Thanks!

[kach22i]

Originally Posted by RedDevil

I'm not worried about lift much, at least not at reasonable speed. If anything it slightly lowers rolling resistance, but I don't expect much of that. It will also slightly reduce grip of course, but again not by much.
If lift is the price to pay for less drag I'll take it with both hands.

Not me.

Most of my modifications improved crosswind performance and increased down-force.

I also got an mpg bump, which is nice.

However if mpg were my only goal then an S10 or Tacoma would not be my base vehicle.

UPLIFT has long been an underated issue in this forum in my opinion.

This is a good thread.

[aerohead]

Originally Posted by oil pan 4

When I drove a "new beetle" which is about as close as you can get to the template on a street driven car, that thing generated lift and had horrible cD.
It had to use a spoiler so the back wheels would stay planted to the road surface.
But quoting 2 generations out of date stuff is kind of his thing.

1) I wouldn't associate the New Beetle even remotely with the template.
2) The original Beetle was tested by ROAD & TRACK in 1969. It had no rear lift whatsoever.
3) Too bad you didn't drive the XL1. It IS a 'template' car.

[aerohead]

Originally Posted by JulianEdgar

Aerohead has a belief, that has unfortunately been apparently adopted by many people here, which is quite wrong.

That belief is that The Template (and roughly similar shapes) have zero lift. Of course, these shapes have in fact very high lift.

Aerohead often quotes references to support this incorrect belief, but the clearest and most accessible he has quoted is Figure 2.4, page 51 of Hucho (second edition).

So let's look at that diagram. (Note that I not going to (mis)quote or paraphrase, as Aerohead does so often. Instead, I will reproduce the diagrams and text.)

The diagram Aerohead references:



Now, what does Aerohead say about it? He said:



Note how there are no caveats or qualifications in what Aerohead says - he's accepting the diagram as being applicable to real cars. But look at the caption. The diagram is for inviscid fluid - that is, an imaginary fluid without viscosity!

And does that matter? Let's look at what is written on the next page:



Note from the passage:

On the rear part of the vehicle's upper surface a steep pressure rise occurs, and it is in this region where considerable differences exist between the real flow of a viscous fluid and the inviscid flow shown here.

and

If all X-components of the pressure distribution on the vehicle surface are integrated, the result for the drag will be D=0.

and

In the real, viscous flow there exists a drag force, but it cannot be explained by considering an ideal, inviscid fluid.

None of this is much of a surprise to me - if you measure real stuff on real cars on real roads, you quickly find where people have misunderstood (or mis-applied) theory.

The idea that Aerohead constantly pushes that low drag = low lift is simply rubbish. It's one of many misconceptions he has, most of which are very easily shown by getting away from the keyboard and doing some real-world measurements on the road.

It's not hard - and it might prevent silly mistakes like referencing a diagram for inviscid fluid as if it applies to real cars.

If I can get into my photobucket account some how, I have the schematic Sawatzki came up with for a Jaray car, which has identical flow features as the 2-D image. It appears in Hoerner's AERODYNAMIC DRAG, page 160. The significant lift occurs at the A-pillar.
The car has quite a rear overhang, and with a positive pressure acting over a 23 % body-length moment arm, going beyond the rear axle it generates significant downforce.
Diffusers can also provide a counterbalance with respect to overall lift.
--------------------------------------------------------------------------------------
As to low drag / low lift, don't get obsessive about the upper body pressure profile without investigating under the vehicle.
Spirit generated 22-pounds rear lift at 135-mph. And 30-pounds downforce on the front axle.
Her travel weight was over 4,200 - pounds. My travel speed was 65-mph. Any lift at that velocity wouldn't be on the radar screen at all. Just take the squares of each velocity and compare them as a percentage.

[aerohead]

Originally Posted by bentring

Although I've long been fascinated with automobile aerodynamics, I don't have enough knowledge and interest to speak of the science, so I usually don't bother parroting what's accepted as common knowledge here.

I know there's been prior debate of the subject of the template I haven't read, so if this information has been discussed to death feel free to disregard this post; but I feel like information, pictures, and videos on this page would be relevant to the discussion.

Online, there's an image of a Schl'o'wagen model being tested under green light, with smoke introduced into the boundary layer if memory serves me. The smoke does not entirely follow the roof contour. Other shapes with a more conservative roofline have measured Cd 0.13.

[aerohead]

Originally Posted by JulianEdgar

With respect, this is likely to just send us back to the BS from which the original garbage descended.

Note: I am not saying that these cars are of no interest, or they they were backward at the time. In fact I am a great fan of them.

But....

The Cd values recorded at the time for these cars are highly unreliable, as has been demonstrated when full-size cars of the past have been tested in modern wind tunnels.

...and...

The designers of the time cared nothing about lift, and so basically all the shapes shown are of such high lift they would never be released by any responsible car company today.

...and...

In general, the designers were not aware of the drag resulting from trailing vortices, and so attached flow was often seen as an end in itself.

More than anything else, these factors explain why no current cars look anything like these cars. For exactly the same reasons, it also explains why using some sort of standardised approach (a template) based on these ideas is usually a pretty bad idea, and using such a template as some sort of model for changing an existing car is an even worse idea.

You're speculating.

[aerohead]

Originally Posted by Taylor95

So it is generally better to not use any kind of template then?

Makes sense given the big differences that exist between different vehicles.

Of course! Even in language. Why even use a 'template' phonetic alphabet when we could make up new novel languages every time we speak or write.
Time isn't money. We can just p... away our lives repeating experiments done millions of times before.
Forget the plumbing code and standardized fittings.
Structural engineering? Who needs it. Skip the rebar, those 'tables' are rubbish.
Medical school. I'll do that C-section for a fraction of the cost.
Wing sections. Logs will do with enough horsepower.There's your real bragging rights!
Just take all the accumulated knowledge of the last 4,000-years and throw it out the window.
We can better spend our time reinventing everything.

[kach22i]

The LFRD method for calculating steel members was replaced with another method a few years after I got out of college.

In other words, this is an example where a standard of science was improved upon and replaced.

Many such examples outside of architecture and structural engineering exist.

In my reading today on this topic I picked up that standard aerodynamic theory and calculations and even CFD can and will be improved upon as the quest for lower drag aircraft continues.

In short, science marches on.

I can see both discarding the past and building on the past as needed.

We in this forum are not in pursuit of that last 2-percent of perfection, so rules of thumbs and templates have a role in my opinion.

I do wish there was a template or more illuminative guide for UPLIFT though.

The extra 50 lbs of down-force I seat of the pants guessed my S10 pickup truck roof wing gives allows me to not use 4WD in heavy rain and still take wide bending curves on Pontiac Trail at 50 mph at night. This means I save gas and get to my destination on time.

50 lbs may not seem like a lot, but it is enough to make a difference in my comfort zone.

[freebeard]

I'm carrying 50lb of downforce I could stand to lose.

Originally Posted by aerohead

We can better spend our time reinventing everything.

True! We've entered the age of adversarial generative AI. Everything is up for grabs.

An off-topic example:
elemental.medium.com: A Supercomputer Analyzed Covid-19 — and an Interesting New Theory Has Emerged
A closer look at the Bradykinin hypothesis

[California98Civic]

Originally Posted by freebeard

I'm carrying 50lb of downforce I could stand to lose.

...

This joke wins a "Frankie" ... a.k.a. the FrankLeePrize, which is represented by the icon (: