Quote:
Originally Posted by Talos Woten
Hey aerohead!
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. ;-)
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!
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.
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.
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.
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. 
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.  |
1) we'd be above 20-mph, and at supercritical Reynolds number at all times, so the drag coefficient would be constant throughout the subsonic velocity range, by definition.
We design for SAE standard atmosphere, so air density is also a constant. Kinematic viscosity is a constant. Incompressible. Critical roughness is a constant.
Which implies that the 'shape' is constant, allowing for body-morphing elongation along the lines of the 1930s German designs.
'Sharpness' would only be beneficial in 'transonic' flow, above 250-mph, by definition.
Any added width/depth associated with side mirrors or tires are not typically included in the projected frontal area percentage calculation. They're already included within the percentage used with the gross dimensions.
It's okay to consider a car as the ground reflection of a 'oval' rotated around a central axis, instead of a streamline body, however, the windshield would have to conform to the elliptical form, as far as fineness ratio goes. They behave the same.
The problem is that, if you fabricate a windshield for the 'nose', which satisfies the aerodynamics, it fails 'optically.' The inside of the cockpit is reflected on the inside of the windshield, making it impossible for the driver to see outwards. That cannot happen with the bulbous nose of the streamline half-body, and why Hucho considers the streamline half-body as the ideal.
' (W)ith refinements in aerodynamics progress is towards the (streamline) body of revolution.' Wolf H. Hucho, page 107, 2nd-Ed.
'The smallest values of the coefficient of parasite drag so far observed are those of well-designed (streamline) bodies of revolution.' Ph.D. Richard Von Mises, page 104, Theory of Flight.
' The drag coefficient can be decreased by proper aerodynamic shaping of the body; a major factor is the teardrop taper from front to rear ( and the rear is the predominant factor.' Ph.D. Edward F. Obert, Professor, Mechanical Engineering, University of Wisconsin, Internal Combustion Engines and Air Pollution, Harper & Row.
Streamline bodies are all ichtyoid. They're all 'fishlike.' All their 'noses' represent about 1/3rd of the total body length, leaving 2/3rds for the aft-body, to bring the divided streamlines back together.
Aerodynamically, the Prius already has too much 'nose' and not enough 'tail.'
If you will examine Figure 4.3.16, page 195, of Goro Tamai's, The Leading Edge', you'll discover that your 'nose' modification as described will 'fail.'
That said, extending to accommodate a 'laid-down' radiator and ducting makes it a definite 'GO!'
The thing about aeronautical engineering is that it's all about the 2D 'wing,' ruled by wetted area and surface friction drag Everything else on an aircraft is a drag-increasing excrescence.
Road vehicle aerodynamics is about bluff bodies ruled by flow separation. ' Low drag can only be achieved when separation at the rear is eliminated.' Hucho, 2nd-Ed page 16
About circular streamline half-bodies, Paul Jaray demonstrated his 'pumpkinseed' body, which morphed from semicircular-to-rectangular and back to accommodate passengers and luggage.
An aerospace equivalence is Willi Messerschmitt's famous WW-II, Me 262. Take a gander at the 'square ' bottom of the fuselage transition fore and aft of the low mono-wing.
We can do whatever we want with a body cross-section as long as we respect sectional density, not introducing pressure spikes along the contour.
If I've made incorrect comments, correct me. I'm still a student. I'll take my licks.
The authorities are all the folks who've researched and published since the 1700s. And we're admonished by Hucho to know all of this vast body of work. to draw upon.
Again, aircraft wings operate in two-dimensional flow. They operate within 'flight conditions' above Earth's turbulent boundary layer.
Automobiles operate on the sea-floor of the Troposphere, submerged inside Earth's turbulent boundary layer.
Aeronautical engineers get automotive stuff wrong all the time. One of our members Chris, has a Ph.D., in 'high-speed aerodynamics' (supersonic flight ) aeronautical engineering in Dallas. When it came time to talk automotive aerodynamics to his North Texas Renewable Energy Group, he had me come and do the lecture and power-point. He recognized the divergence the two disciplines had taken many decades ago.
I'll maintain my comment about airfoils. No apologies.
Paul Jaray's 'kombination-form' car had an 'airfoil' roof form. Cribbed from the Zeppelin airship control cars he designed.
Hucho recommends streamline half-bodies, not 'airfoils.' 'Conjoined wingtips' would be acceptable, as they would be streamlined half-bodies.
'Misleading and unhelpful' 'misconclusions' takes effect when it's successfully argued. I've been waiting since 1974.( I'm forbidden to discuss the potential source of such notions )
I'll offer a 'your welcome' in advance of your revelation that airfoils will be of little value.
My dander doesn't get 'up'. I'm just most comfortable with facts. Abraham Lincoln told me that I'm expected to open my big mouth and throw caution flags on the field when I witness potential aerodynamic mischief.
Any criticism from me is made within the context of potentially saving members up to 400 man-hours of wasted time on a modification which might ultimately end up in the landfill unless 'corrected' in the design stage.
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2) I'm completely okay using 'front' in an all-encompassing context. And everyone will be on the same page from the get-go.
And something about mods and testing. Consider having all the mods you're going to test taped over except one. Like 'tear-offs' on the windshield of an Indycar.
Have all your pressure taps in place. All your lines run. Datalogger hooked up.
Do your back-to-back run, then tear off a cover and go again, repeating until you've captured all your data. Weather can change during testing and this will narrow that window, aiding data reduction afterwards.
I did this at the wind tunnel. We could capture data every 7-minutes, until the entire suite of modifications were bagged. Lots of cardboard and duct tape.
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3) Perhaps a thermistor could send a variable signal to the analog input gate of a A to D converter, with an MPGUINO 'looking' for a preset threshold to 'trigger' your active cooling elements through an output gate, automatically controlling the devices as you focus on driving.
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4) I couldn't link to your address, I'll try for Thursday and keep going.
Hucho admonished that we gain a command of all material, from the 'vast' body of empirical science since day one. So yes, some of my material is older than dirt, however, it sets the foundational basis for everything that follows as what Hucho refers to as the 'ground rules for fluid mechanics.'
We're supposed to know all of this.
And I respectfully disagree that any leaps and bounds have been made since 1922. 1910 for Eiffel. 1860s Lillianthal Brothers describe the essentials for modern aviation. Pierre Julienne flew the first longitudinally-asymmetrical streamline body of revolution in the 1700s. How old are 257-mph peregrine falcons?
We had the technological means for Cd 0.09 in 1986.
Modern testing must be taken within the context of cheating out any drag reduction which can be had from cars that should never make it to production. We're still dominated by Paris Dressmakers.
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5) What I'm going to test would cost many hundreds of thousands of dollars to make OEM quality, like someone would be comfortable with. You don't want to invest that until the aerodynamics are 'proven.'
So the tunnel allows me to investigate the shape, maybe modify in situ. Without endangering the public in traffic. I already have a blockage-ratio calibration model rom Toyota's full-scale tunnel to sort out my numbers. I'll be done in two hours. All I'll have are numbers from that tunnel and what other calibration cars measured at that tunnel and two others. It is what it is.
If you can keep Champrius away from hills and really curvy roads, and out of hybrid assist, the onboard display may be accurate enough to see a showing for the mods, especially if you've got a bunch ganged on.
At a constant fuel rail pressure, pulse-width is controlling injector open time, and the ECU ought to be able to calculate fuel consumption to a high degree of accuracy, combined with all the other sensor data. I've compared 'tank' mpg to onboard readout on my friend's 2008 model. I have a high degree of confidence in the electronics.
The SAE HANDBOOK has their official coastdown test protocol and all the mathematics necessary to reduce the data.
If you're feeling rich, CAR and DRIVER might be coaxed into helping for a fee. Their offices are close to the Chrysler Proving Grounds in East Chelsea, Michigan, where they tested my Honda. They did top speed also, and threw in skidpad for free.
I'm out of time. I'll catch up Thursday.
Okay, I'm back
* As to the coastdown testing, all I can recommend is that you print off the SAE protocols, study it very hard, and determine if you have the resources and fortitude to take it on.
A video would require 1/10th second resolution time stamping of the deceleration profile, with simultaneous meteorological data transposed directly alongside your velocities.
If you have an advanced degree in the mathematics of special engineering problems I suppose you'll comprehend all the protocols. It involves partial differential equations for simultaneous multiple unknowns for a dynamic histogram of the events. You must perform ten, simultaneous back-to-back runs, from 70-mph down to 20-mph for each aerodynamic configuration of interest.
Csaba Csere, CAR and DRIVER's MIT graduate engineer, and technical editor wrote a custom software program to reduce the data we collected at Chrysler.
The late, Don Schroeder did all the testing, and his assistant manned the weather station for the duration. Computer runs were done back at their headquarters. They gave me a 'good-guy' price of $500 ( 1991-dollars ) for the entire project.
From the Bonneville data, Wolf Heinrich Hucho's metrics, Chrysler, and tank mileage, Glen Scharpf of General Motors Aerodynamic Laboratory helped me nail down the Cd 0.235 over the telephone. Consider getting help on this one.
As to your testing methodology, just bear in mind that your results will be considered within the context of how the data were ascertained.
Technically, you'd be required to normalize all data to standard SAE conditions. Record as much data as you can stand and include it in your presentations.
I have 345,000-miles of tank mpg data on Spindletop.
I have 412,000-miles of tank mpg on SPIRIT.
You're looking for 'trends' and they'll show up by default over time as a consequence of the physics. Been there, done that.
Keep a journal. Try and use the same gas station, the same pump, parked at the pump in the same direction. Ask about fuel storage tank temperatures. Or insert a 6-inch stainless steel stem thermometer in the filler neck while refueling. Local airports have barometers. Get to know the FBO. A phone call can get you raw barometric data. Air density needs to be known as best you can. Watch windmills and flags for wind clues. Your GPS should have an 'altimeter' function. Camprius should already have a display for outside air temp. Without 'crashing' , try to record temp on a timely basis. Or travel with a friend, designated as 'recorder in chief.' Especially during night driving. All magazine road tests are done with at least two people onboard. Be safe!
When you get the 'itch', we could pass the hat around and do a crowd funding for the A2. It's $500 / hour, with a two-hour minimum.
'Rolling-road' is around $ 4,000 / hour.