[Talos Woten]

Hey aerohead!

Originally Posted by aerohead

1) 'Aircraft-type bodies do have optimum fineness ratio values ( for which drag is a minimum ), and our data suggest that road vehicles do also.'
William H. Bettes, Graduate Aeronautical Laboratory California Institute of Technology ( GALCIT ), ' The Aerodynamic Drag of Road Vehicles Past, Present, and Future, Engineering & Science, January 1982, page-8.
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' low drag can only be achieved when the separation at the rear is eliminated.' Hucho, 2nd Ed. page- 16.
' (T)he optimum shape in terms of drag is a ( streamline ) half-body, which forms a complete ( streamline ) body of revolution together with its mirror-image produced through reflection from the roadway.' Hucho, 2nd-Edition, page 15.

1) Actually, we can make stronger statements. The optimum fineness ratio and curve is also determined by the expected cruising speed and the properties of the fluid under cruising conditions (say altitude). We want longer, tapered bodies at higher speeds and shorter, rounder bodies at low speeds; with sharper noses with high viscosity / density fluids and rounder noses with low. Bigger changes in fluid characteristics are induced by subsonic vs supersonic flow, Reynold's number regimes, etc. but for cars that's not an issue. (Man that brings back memories. I've actually been in GALCIT... I took classes in the same buildings. ;-)

Originally Posted by aerohead

' The fineness ratio of...5.53...corresponds to an effective ratio in free air of 2.27. This approaches the drag minimum recognizable... With a greater fineness ratio, the drag would increase again as a result of the increasing friction drag.' Hucho, 2nd-Ed. page 210.
' With a lesser fineness ratio, the drag would increase again as a result of increasing pressure drag.' Aerohead, corollary to the above.
' If we conservatively limit aft-body downward/ inwardly sloping surfaces to respect W. A. Mair's 22-degree limit, then we're limited to streamline bodies of revolution no 'shorter' than 2.5:1 fineness ratio. Cd 0.04.
Hucho, 2nd Ed. Page-61, Table 2.1, 3rd from bottom, ( from S. Hoerner's Fluid-Dynamic Drag, 1965 ).

Oooh now we've got some useful data! The dimensions of the 2009 Toyota Prius are listed as 175″ L x 68″ W x 59″ H, but those include protrusions like the mirrors and tires. The mirrors add 4", and the clearance is around 5.5", so we are really looking at a main body of 175" L x 64" W x 53.5" H. To get Hucho's optimal ratio, that means we are looking for a total length somewhere between 296" and 354".

So cars are nowhere near optimal just in terms of fineness ratio. The widest point chord of an aerofoil is usually 30% of the length:
https://en.wikipedia.org/wiki/NACA_a...NACA0015_a.png.
From the midpoint of the car we are looking at 89"-106" to the nose tip and 207"-248" to the tail end. That means we should add 1.5"-18.5" to the front, as well as make it narrower and sharper than a typical car nose. Sadly, the wheels are in the way, but we should make it as narrow as we can still clearing the wheels.

Thanks for the info, aerohead! I was planning on adding only 6" to the nose, to meet the 1 radiator height inlet duct requirement. But this has convinced me that 9"+ would be better. Hmm... we might be able to estimate a good distance via calculation.

I added on a tail 42" long truncated at 7" high, which means it would have tapered an extra 10.5" to a point. So 87.5"+42"+10.5"= 140 / 248 = 0.564. (Wow, the boattail could be *much* longer.) 0.564 * 18.5" ~ 10.5". So adding that much to the nose gives the same proportions between front and back. Yay!

Originally Posted by aerohead

The more you study, it will become obvious that, it'd a major mistake introducing aeronautical engineering into road vehicle aerodynamics.
Airfoils have no place.

Hmmm... I realize that what you are trying to say with those statements is that the design of airplanes and cars are different. Planes need high lift, produce thrust, are in free stream, etc. while cars have rotating wheels, have multiple airflows, are in ground effect, etc. And I completely agree with you there.

However, claiming that we can't transfer working knowledge and best practices from planes to cars isn't wise. That's ignoring the useful findings of a well established field. Right off the bat, the optimal contour of a car from above is an airfoil shape. If we took an infinite half plane for the ground, and an infinitely tall car, the silhouette shape from above is determined by free space. All that happens near the ground is we make it rotationally symmetric.

In free space, the optimal cross sectional area is a round circle. As we approach the ground the top half remains reasonably stable, but the bottom half becomes "squished". In the limit of 0 ground clearance, we approach Hucho's half airfoil shape from the side, but we always retain the full airfoil shape from above. Your statement "Airfoils have no place" isn't true, and more importantly, is misleading and unhelpful.

Hmmm... I'd like to express my frustration in dealing with you. You provide a very wide range of utility to the reader, ranging from incredibly helpful to condescendingly incendiary. This thread is a perfect example. You've given me very useful data and figures to help guide my car design, and I am thankful and appreciative for the help. You've also made authoritative, inaccurate claims drawn from misconclusions of your sources. That's what kicked off this whole boondoggle of a discussion.

So my question to you is, do you want to seem like the smartest person in the room, or do you want to find the truth together? If it's the former, then I'll cherry pick the useful information you provide and will ignore correcting you when you are wrong. If it's the latter, then it might be beneficial to find an alternate way for you to initiate discussion besides provoking people. We could have held this entire conversation productively without either of us needing to have our dander up.

Originally Posted by aerohead

2) Aerodynamic detail optimization is much more granular than with lumping the nose, cooling system, and air curtains, etc. together in a broad sweep. It's not done.

2) I am beginning to see that you are ludicrously precise with your wording. As a mathematician, I can appreciate that. However, as a mathematician I've also discovered that being pedantic about words in conversation is a trait the vast majority of people find annoying. Just because you don't do it, doesn't mean it's not done. Most people use words imprecisely.

So I'd like to propose a new word closer to what I meant, which is "front". I'll use "front of the car" as an umbrella term to encompass the nose, wheels, radiator, etc. Is that agreeable? And yes, I'm going to do lots of granular work on the front, each of which is an individual, well defined piece with it's own terminology.

Let me explain even further. Treating each granular modification as a separate entity has advantages. It's basically the discretizing a product, which reduces complexity and increases understanding of what each piece does. I'd like to propose that there are also advantages to treating the whole car aero as an integrated unit. Mods upstream impact mods downstream, and how the whole airflow is shaped / routed affects drag / fuel economy.

So my using "nose" as "front" is actually a reflection of how I'm thinking about holistic design now. I'm trying to create a front as a single piece that includes all the individual elements integrated in. Sort of like how air curtains are now effectively included in bumpers, except I want to aero the whole front at the same time.

Originally Posted by aerohead

3) If the pump volume is to remain constant, and the object is to increase residence time within the heat exchanger, then we're talking about adding more core, or enlarging passages. Just engineer for the worse-case scenario.
If you add core, your now increasing the porosity of the car, kind of a backwards move, aerodynamically speaking.

3) The Toyota engineers have already done the worst case scenario. That's a 330 sq in core with a 130 sq in inlet (split into two sections). I'm trying to do the reverse minimization problem, which is what is the smallest amount of inlet surface area I can safely get away with for 95% of my driving? Then I have some kind of variable intake adjustment for the 5% extreme cases where I need the full radiator power / original airflow. That's better engineering, to only use the airflow we need at any given time, which is why active grilles have become so popular.

Originally Posted by aerohead

4) We weren't talking about 'prediction.' You spoke as if referring to a bird-in-hand, prima facie evidence.
* there's little probability that reshaping the nose will net 3%. 14% was the limit, and you were already there.
* The BMW air-curtain is attributed with a 0.01% drag reduction if my notes are correct.
* If you removed the cooling drag completely, you're only looking at delta- Cd -.025. That would put you at Cd 0.235. Only a 9.6% drag reduction, and maybe a 3% mpg improvement.

4) Ahhh I understand something else. The reason you cite so many references is because you want evidence for things, as opposed to unsubstantiated claims. That's reasonable. I retract my earlier statement, since all I have to refer to are anecdotal secondhand claims I've read on racing forums and the like.

Here's the calculation that makes me believe why there's lots of gains to be had at the front of the car:
https://www.instagram.com/p/CeeL6xyFp-k/
You are right. I don't have proof or evidence. Just faith in mathematics and the predictive power of science.

By the way, as a word of caution, I've noticed that most of the books you are citing are decades old. The techniques and understanding of car engineering has made leaps and bounds since then. I'm not knocking your sources, and I think it's great that you are making the effort to cite them. But as a consultant I saw this problem occur many times, where people would rely on information that became out of date, and was inaccurate enough to cause decisional error. So be aware that testing on cars, as they are, today, is more relevant than what testing was done on cars, a half century ago. Ur, unless we are driving classic cars or somesuch.

Originally Posted by aerohead

5) A wager is problematic, as your testing protocol is also problematic. The hybrid system makes nonsense of conventional methodologies. Regen compounds it further. Any 'hypermiling' throws it completely into outer space.
I don't know how we'd ever sort it out into anything definitive.
This is why we have wind tunnels. And my science background dictates that this is the only appropriate venue in which to adjudicate the exercise.$$$$$$
A little salsa makes crow a fine meal! I have no reservation about the contest. We'd all learn something. It's the 'measuring' and 'quantifying' that would be the challenge.
The fellas did a crowd-funding for me back in 2014, to help defray some of the wind tunnel costs. I'm indebted to them all. In 2017 I paid for another member's wind tunnel time. I'm no longer in a position to do that.
Maybe we could pass the hat another time, and help get Champrius into the A2 Wind Tunnel. They tested the 2013 Prius and we know something about intra-tunnel blockage-ratio data variability compared to Toyota's wind tunnel. Something to think about.

5) Hmmm... that's a good point. I don't use hypermiling either driving or testing, but in the past when I experimented with it I could easily get +10-25% fuel economy improvement. That could easily skew the numbers. That's to say nothing of inherent variability in the environmental conditions themselves. Those can also easily account for a similar swing in either direction.

I'm assuming from the wind tunnel that you mean to measure the drag coefficient? Would videos of a coast down test (to infer the drag coefficient) be acceptable to you? I'm aiming for a 0.185 Cd with a new front.

Just so you know, I'm actually after different figures. It seems you want to use a wind tunnel because it gives controlled, repeatable data about the car, which is good scientific laboratory methodology. I want to gather data about how the mods perform in the real world, including all the horrible variability, which is good engineering use case practice. That seems like the (less precise but) more relevant figure drivers are interested in, myself included. Most consumers aren't interested in EPA, they are care about what they fill up at the pump. They only use the former to estimate the latter.

Anyhoo, because of that, my gold standard figure is a full tank of gas. Then to reduce fillup error we take that averaged over many tanks. Basically, I'm reducing error from variability by taking lots of data points. The best cases are loops; that at least eliminates height variation. My best tank of gas with Champrius 3.0 was 69 mpg... but it was over a 2,400 ft drop from start to finish.

So along those lines, would several tanks over 5k miles be a reasonable proof positive for you? That's around what it would take for me personally to conclude that a mod actually makes a real difference and wasn't testing error.

By the way, I'm also down with wind tunnel testing. I think it would be cool to see Champrius inside one. But if it's expensive, I wouldn't be in a position to pay for it either.