Rear Spoilers vs 'template' contour

[aerohead]

Originally Posted by JulianEdgar

Dr Hucho is alive and well - he wrote me an email just last week.

This idea of 'battle of the templates' is just so bizarre. I would suggest never starting with any template at all! Why on earth would you start with a pre-determined shape and not actually develop the best shape for your own car?

Would you take a Nissan Micra's spring, damper and sway bar rates and apply them to your Mercedes 300SEL? Would you take the engine management map from a naturally aspirated V8 and plug them into your four cylinder turbo's engine management? Or, and this is an even closer parallel, would you state that the air/fuel ratio in your car's engine should always be 14.7:1, because that's 'stoichiometric' - the chemically correct proportions for complete combustion?

The canopy shape looks alright - sure. But why on earth wouldn't you first develop the best shape for that vehicle by doing some testing? For example, first just lay a flat sheet from the roof to the tailgate and see if the airflow stays attached. Even better, do that and measure some pressures. The depicted shape might be best, but it's highly likely it isn't.

I think blindly following a template - any template - is an utterly stupid way of modifying car aero. It seems completely predicated on the idea that testing isn't allowed. Just imagine doing engine management or suspension like that - what are the chances you'd luck-out and get the best results by copying what someone says is best for every car?

* You'd do it because of dimensional analysis data already in the bag.
* If you're going for really low drag, it would be your starting point. Since it's already a known quantity.
* We're talking about Cds below 0.15. You can't get there without something like the template without wasting a lot of material.
* Drag is a function of pressure drag.
* The template doesn't have any, only friction drag.
* And friction drag is at a minimum, by default, since the template was taken from the bottom of the drag bucket to begin with.
* And its a stable platform. Lift is not an issue. Peak suction occurs at 0.075-body length, preceded by full atmospheric pressure ahead of it. Lift is non-existent over the last 8% of the body, on a cantilever behind the rear axle adding a stabilizing torque.
* An underbody diffuser can cancel any positive lift.

[aerohead]

Originally Posted by freebeard

A new record. Seven post in 1/2 hour.

Julian's really busy between when I leave the computer Friday afternoons, and Wednesday morning the next week when I return.

[Vman455]

Originally Posted by aerohead

Would you please provide an example(s) from which I may ponder inaccuracies?

You want me to post links to your threads, wherein you post lists of cars? I would hope you remember writing them and obviate the need.

[aerohead]

Originally Posted by Vman455

You want me to post links to your threads, wherein you post lists of cars? I would hope you remember writing them and obviate the need.

I don't what you're referring to, so it's impossible to know what to look at, look for.
If you can recall somewhere, where you think I went off the rails, the information would really help.

[JulianEdgar]

Originally Posted by aerohead

* Drag is a function of pressure drag.
* The template doesn't have any, only friction drag.

Now unfortunately we're off with the fairies.

[aerohead]

Originally Posted by JulianEdgar

Now unfortunately we're off with the fairies.

Figure 5.13, page 69, 'Frontal drag coefficients of three-dimensional bodies, depending upon their fineness ratio,''AERODYNAMIC DRAG,' Sighard Hoerner, self-published,1951. ( Re. NASA, AVA, DVL , @ supercritical Rn = 10-million )

[JulianEdgar]

Originally Posted by aerohead

Figure 5.13, page 69, 'Frontal drag coefficients of three-dimensional bodies, depending upon their fineness ratio,''AERODYNAMIC DRAG,' Sighard Hoerner, self-published,1951. ( Re. NASA, AVA, DVL , @ supercritical Rn = 10-million )

Means nothing without the diagram.

[freebeard]

Sometimes I try to fill in the blanks. Not always.

[aerohead]

Originally Posted by JulianEdgar

Means nothing without the diagram.

I published that when I published the 'template' material. If you know how to unlock photobucket, I'll be happy to repost it.

[kach22i]

Originally Posted by freebeard

Taylor95 — Way back when, we used to discuss blisters and canopies.........

I've struggled to find real life examples of 1:10 ratio canopies and blisters as advocated by Aerohead's references to text books such as Hucho.

I mean when we see a 2-part body canopy on a solar racer or fighter aircraft and it's 1:5 ratio at best one has got to think that last little bit of theoretical air attachment is an idealized goal and not an actual design recommendation.

Just say'n there is a disconnect.