Originally Posted by aerohead (Post 630444)

If you'll revisit Figure 2.4, page 51 of Hucho, you can see how,over the last 14.5% of the body, local pressure rises all the way back to local barometric pressure. Depending on rear overhang, and low pressure under the body, due to a diffuser, rear lift can be zero, like the VSPORT's.

Originally Posted by freebeard (Post 630538)

Thanks OP, I hadn't seen this one before:

https://i.postimg.cc/zG9CGvRy/Fig-2-4.jpg

What do you make of the internal dotted lines? They seem to imply that pressure differentials transmit through a solid body. ???

Originally Posted by M_a_t_t (Post 630526)

The simplest way to look at it:

Rounded surface creates lower pressure. Look at an air foil, the top surface creates lift by lowering the pressure. Overall shape of the car is rounded, thus creates lower pressure on that surface (varying levels). Low pressure on the top half of the car is lift.

Flip a car upside down and you get down force right? :D

Originally Posted by JulianEdgar (Post 630541)

The dotted lines show flow around the car.

Originally Posted by Vman455 (Post 630547)

I don't think this is the case; Hucho is quite clear that this is a two-dimensional section. I believe the dotted lines are meant to represent possible duct paths between intakes and outlets for cooling and cabin ventilation air. The paragraph after the one you posted explains:

Originally Posted by kach22i (Post 630546)

I found the first few sections of the link below informative.


https://www.sciencedirect.com/topics...rodynamic-lift

Originally Posted by bentring (Post 630552)

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.

Originally Posted by Taylor95 (Post 630556)

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

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

Originally Posted by RedDevil (Post 630558)

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.

Originally Posted by JulianEdgar (Post 630557)

Yes I think it makes vastly more sense to trial aerodynamic changes and then test them. Nearly all aero changes can be tested with low cost (often free) changes - about the only one hard to mock up out of cheap materials is a true wing. Otherwise, you use cardboard, tape, stiff sheet plastic - or my favourite, thin marine plywood.

Then we have lot of tools at our disposal to measure nearly every aspect of car aero on the road at low or zero cost. Here's a few:

• locations of separated and attached flow
• panel pressures anywhere on the car
• relative changes in drag
• lift and downforce, both front and rear

That way, you find out what works for your specific car in terms of exactly what you want to achieve.

For car aero, guesswork and blindly following what others have done on different cars in the past is exactly the same as for any other area of car modification. And what's that? It's likely to result in much poorer results than could have been achieved through some simple test and development.

Originally Posted by bentring (Post 630552)

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.

Originally Posted by RedDevil (Post 630558)

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.

Originally Posted by oil pan 4 (Post 630522)

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.

Originally Posted by JulianEdgar (Post 630512)

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:

https://i.postimg.cc/zG9CGvRy/Fig-2-4.jpg

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:

https://i.postimg.cc/QMvhPLwG/second-page.jpg

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.

Originally Posted by bentring (Post 630552)

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.

Originally Posted by JulianEdgar (Post 630554)

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.

Originally Posted by Taylor95 (Post 630556)

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

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

Originally Posted by aerohead

We can better spend our time reinventing everything.

Originally Posted by freebeard (Post 630598)

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

...

Originally Posted by aerohead (Post 630589)

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.

Originally Posted by JulianEdgar (Post 630554)

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.

Originally Posted by bentring (Post 630636)

:facepalm: I've violated the long established axiom of opening my mouth and removing doubt of my foolishness.