RH Barnard on templates

[Vman455]

I hope he doesn't mind my excerpting his book here, but this quote, from Chapter 4 "The Aerodynamic Design of Family Cars" in Road Vehicle Aerodynamic Design: An Introduction, is salient to the discussion surrounding the 'template' on this site in recent months:

As was explained in Chapter 2, due to the proximity of the ground, the 'ideal' form for a road vehicle is neither an axisymmetric teardrop nor a half teardrop, but a cambered version, slightly flattened on the underside, with the optimum geometry being dependent on the ground clearance. A further complication arises from the fact that the cross-section will need to contain at least two people seated side by side, so a circular or semicircular section is unsuitable. Also, if excessive frontal area is to be avoided, the surrounding shell needs to fit the occupants quite closely. With these conditions imposed, there is no longer a single 'optimum', and the best shape for low drag will depend on the ground clearance and the degree of tailoring required to accommodate the occupants and internal components.

A few comments:

We know from the literature on car aerodynamics and direct measurement that the range of variation of shapes that can support attached flow (that is, that are streamlined) is large; I posted a quote from a 2010 paper by Hucho on another thread here yesterday regarding the leeway available to designers to change fastback backlight angle without increasing drag, for example, and tuft test images posted here by a handful of people show attached flow over a range of shapes.

This excerpt considers the impact of practical factors, such as occupant room, and physical parameters, such as ground clearance, on the 'ideal' form. Even if we were building cars from scratch--which most of us aren't--slavishly following a 'template' will not by default obtain the best results.

Now, consider that most of us are modifying already-existing production cars, cars that are not half-bodies of revolution, cars that may have air dams or limited underbody paneling or extensive underbody paneling from the factory, cars with narrow tires or wide tires, cars with high ground clearance or low ground clearance, cars with varying rear-body shapes that translate to varying flow fields and pressure profiles, cars with all sorts of differently-optimized (or not-optimized!) aerodynamic and styling details.

I used to think, when I didn't know any better, that extending along a 'template' was the solution--based on what I read on this site. I now realize that this an incredibly simplistic view, and one that does not take into account the facts that 1) there is no single optimum 'template', and 2) the variation in shape and thus flow over the cars we modify is in no way consistent, so a "one size fits all" approach simply cannot produce the best results possible.

For example, a few weeks ago I measured surface panel pressures and found that the flow speed down the center of the roof of a Prius is faster than at the outside edges, and that over the rear window the opposite is true. The 'template' assumes uniform pressure and thus uniform flow speed over its circular section. Given that this is not the case on a Prius, why would a person assume that extending it with a half-circular tail at a specific angle is automatically the best solution to decrease drag? Maybe it isn't. Perhaps the optimum solution doesn't involve fitting a tail at all, if doing so would increase difficulty of parking or not being able to fit in a standard garage, for instance, but looking at other areas of the car to decrease drag. Perhaps a tail that does not have a half-circular cross section would reduce drag more than one that does. How on earth would one know without testing? Without measuring, I would never have known that the flow speed over the rear body is not constant side-to-side in the first place!

'Templates' are like security blankets: someone guarantees results with a minimum of thought or effort. But after trying it, testing is infinitely more rewarding, not very expensive, and not that difficult; the hardest part is really just finding the time to do it, and that's not a huge hurdle. More importantly, testing will reveal what the air is actually doing over your car--no guessing or predicting necessary. Go try it!

[Piotrsko]

Ummmm, that is kinda a summation of the Edgar/ aerohead driver debate. Nicely put.

[freebeard]

the 'ideal' form for a road vehicle is neither an axisymmetric teardrop nor a half teardrop, but a cambered version, slightly flattened on the underside, with the optimum geometry being dependent on the ground clearance.

My octahedral geodesic aeroform isn't a half teardrop, more of a Dymaxion. It's asymmetrical top to bottom and front to back. Supporting examples would be the fuselage of the ME-262 or Aeron26. The only changes I'd make today would be a concave truncation/boxed cavity or the Magic B*tt Trumpet.



In this case shown on a [mathematically definable] superelliptic cross-section rather than geodesic.

[JulianEdgar]

Originally Posted by Vman455

We know from the literature on car aerodynamics and direct measurement that the range of variation of shapes that can support attached flow (that is, that are streamlined) is large; I posted a quote from a 2010 paper by Hucho on another thread here yesterday regarding the leeway available to designers to change fastback backlight angle without increasing drag, for example, and tuft test images posted here by a handful of people show attached flow over a range of shapes.

This excerpt considers the impact of practical factors, such as occupant room, and physical parameters, such as ground clearance, on the 'ideal' form. Even if we were building cars from scratch--which most of us aren't--slavishly following a 'template' will not by default obtain the best results.

Now, consider that most of us are modifying already-existing production cars, cars that are not half-bodies of revolution, cars that may have air dams or limited underbody paneling or extensive underbody paneling from the factory, cars with narrow tires or wide tires, cars with high ground clearance or low ground clearance, cars with varying rear-body shapes that translate to varying flow fields and pressure profiles, cars with all sorts of differently-optimized (or not-optimized!) aerodynamic and styling details.

I used to think, when I didn't know any better, that extending along a 'template' was the solution--based on what I read on this site. I now realize that this an incredibly simplistic view, and one that does not take into account the facts that 1) there is no single optimum 'template', and 2) the variation in shape and thus flow over the cars we modify is in no way consistent, so a "one size fits all" approach simply cannot produce the best results possible.

For example, a few weeks ago I measured surface panel pressures and found that the flow speed down the center of the roof of a Prius is faster than at the outside edges, and that over the rear window the opposite is true. The 'template' assumes uniform pressure and thus uniform flow speed over its circular section. Given that this is not the case on a Prius, why would a person assume that extending it with a half-circular tail at a specific angle is automatically the best solution to decrease drag? Maybe it isn't. Perhaps the optimum solution doesn't involve fitting a tail at all, if doing so would increase difficulty of parking or not being able to fit in a standard garage, for instance, but looking at other areas of the car to decrease drag. Perhaps a tail that does not have a half-circular cross section would reduce drag more than one that does. How on earth would one know without testing? Without measuring, I would never have known that the flow speed over the rear body is not constant side-to-side in the first place!

'Templates' are like security blankets: someone guarantees results with a minimum of thought or effort. But after trying it, testing is infinitely more rewarding, not very expensive, and not that difficult; the hardest part is really just finding the time to do it, and that's not a huge hurdle. More importantly, testing will reveal what the air is actually doing over your car--no guessing or predicting necessary. Go try it!

Well said.

And before Aerohead arrives next week with reams of material, or his acolytes rush to his defence, note these three key points that Vman455 has made:

1. There is no single best aerodynamic shape for a road car.

2. Following the shape of a 'mythical best shape' (ie The Template) when making modifications to existing cars cannot produce the best possible results. (Or if it did, it would be just complete coincidence.)

3. If you want to achieve best aerodynamic modification of your car, literally nothing beats testing.

Kudos also to Vman455 for acknowledging how he was previously misled by material that has been posted here. Unfortunately, over a very long period of constant misinformation, a lot of major misunderstandings have developed here about car aerodynamics.

[freebeard]

There is no single best aerodynamic shape for a road car.

What do you think about this for off-road car?


exploder ii concept 02 by criarpo d5b2ct2

[JulianEdgar]

Originally Posted by freebeard

What do you think about this for off-road car?


exploder ii concept 02 by criarpo d5b2ct2

Sorry, I think looking at shapes and then guessing is pretty pointless.

Moreover, I think that approach is part of the problem with information on this site, where people apparently believe that such guesswork has credibility.

Ten minutes of testing is worth 100 hours of guesswork.

[Fat Charlie]

Originally Posted by JulianEdgar

1. There is no single best aerodynamic shape for a road car.

Build it to task, then make its shape as clean as you can.

That's why I'm such a fan of the template- it tries to be universal, and it gives a guide for improvement. Only real testing can say for sure, but the template is a good staring place. It's not the best goal for modding any particular platform, but it's a good benchmark: if the first version of your boattail isn't an attempt to reach the template, an explanation why might be in order. If your first version was, and your third (or fourth) and final version isn't, then I'll be very interested to learn about why. That's a great way to move forward.

[freebeard]

Being opinionated....
I suspect is is as viable a starting point as any for the hours of fabrication leading to the ten solid minutes of testing.

Longitudinal arc on the underbody, lateral arc on the upper, room for your head to knock back and forth.

Fat Charlie — Little Template blisters on bigger Template blisters. With Art Deco scallops.

[JulianEdgar]

Originally Posted by Fat Charlie

Build it to task, then make its shape as clean as you can.

That's why I'm such a fan of the template- it tries to be universal, and it gives a guide for improvement. Only real testing can say for sure, but the template is a good staring place. It's not the best goal for modding any particular platform, but it's a good benchmark: if the first version of your boattail isn't an attempt to reach the template, an explanation why might be in order. If your first version was, and your third (or fourth) and final version isn't, then I'll be very interested to learn about why. That's a great way to move forward.

I am sorry, but you don't seem to have taken anything on board from the first post in this thread.

The Template gives absolutely zero valid guidance to making modifications on cars. It is not a benchmark; it is quite irrelevant.

Read any SAE paper on the aero development of a specific car. The use of a standardised shape is never even mentioned.

Read any current textbook on car aerodynamics. Standardised, ideal shapes are given a few pages out of perhaps 500.

Seriously, in what you have written, you've quite lost the wood for the trees.

[JulianEdgar]

Originally Posted by freebeard

Being opinionated....
I suspect is is as viable a starting point as any for the hours of fabrication leading to the ten solid minutes of testing.

Longitudinal arc on the underbody, lateral arc on the upper, room for your head to knock back and forth.

Fat Charlie — Little Template blisters on bigger Template blisters. With Art Deco scallops.

When you can point to the aerodynamic testing you've done, and be able to compare that to your guesswork, we can have a discussion about the relative merits. Otherwise...