Aerodynamic Streamlining Template: Part-C
 

Documents,Tables,Charts:

http://ecomodder.com/forum/attachmen...9&d=1296941870

Aerodynamic Streamlining Template: Part-A
 

Link to original post

Link to original post


The above is the intro the the templates which were posted separately by aerohead.

Thanks for the charts and template. Good timing for me because I'm going to start my kamm-back soon. I have several neophyte questions that have not been answered so far (that I could find).

1. Does the optimal fineness ratio change with speed? I am hoping that if I make mine lower, that is to make the slope steeper, but I stay below say 55mph, will the flow stay attached? My aim is to reduce the size of the tail, and I could live with the slower speeds to get the wake and tail as small as possible. I don't think the Egg will ever go "super-critical". Sounds like you would need a flux capacitor for that.

2. Does the kamm-back or boattail have to be closed off to get the best result? How much difference is there between a closed off and an open ended tail?

Keep in mind, the right chart that shows the .25 t/c being optimal is at a reynolds number of 400,000, much much lower than a car traveling at 55mph.

Orangboy, for your vehicle, you want to experiment with shapes at RE of around 11,200,000, considering your vehicle is 15 feet long stock, and i'd expect atleast 5 feet to be added for a boat tail

RE = 9360 * SPEED(in mph)*DIMENSION(in feet)

Using a program called DesignFoil, i have found that the best shape for a vehicle traveling 60mph has the location of minumum pressure located 70% the length of the cord(from front to back). The shape is NACA 67-025.

http://i422.photobucket.com/albums/p...Naca67-025.png

However, this shape has a center of pressure that is out in front of the shape 8% of the chord length. What this means is its very susceptible to cross winds, and will get blown around.

When you move the max width forward a bit, the cd increase slightly, but the center of pressure moves backward drastically, meaning its better in crosswind. For example, going from NACA 67-025 to NACA 66-025 (70% to 60%), the COP is moved back to 30% of the length (IE, its now inside of the shape)

I just thought of some other questions I have not seen answered elsewhere.

Regarding the point of stagnation and the underbelly. Does the air under the car have a lower or higher pressure? I think it's a low pressure area but it's not obvious to me.

Is it better to have a sharply defined, upward sloping air dam parallel to the lowest points on the car or a bumper curved under the car from the stagnation point (like in the template?) the trailing edge of which terminates parallel to the lowest point under the car?

Thanks.

From everything i have read, it seems that the optimal design is to have no airflow underneath the car. The flip side is that this is not really practical in a road car because there needs to be some sort of ground clearance for speed bumps, parking lot entrances (some are not level with road), etc. So the best answer i can give is that the least amount of airflow under the car is ideal.

I would like to hear what aerohead thinks about what i have talked about so far.

:thumbup:

I think you are right regarding the least airflow under a body is best but at some point assuming the same ground clearance are you not increasing frontal area if you block off all under flow. It could be this is an advantage anyhow. If the larger frontal area is offset by reduced drag. I have not heard this spelled out yet. Thoughts?

Well, there has been some discussion about totally covering the front to eliminate all the air going under the car, even if the airdam is lower than the underbody. Its a common practice at the salt flats and in nascar.

http://www.carolinaautomasters.com/I...l/PICT0079.JPG
http://www.saltflats.com/Graphics/JP...%203%20033.jpg

B*tchin' Camaro!

So what you need is:
1) a stiff rubber lip at the front for aero
2) a stiff upper lip to deal with the guff from the unwashed masses.
3) a nascar decal.

Got it.

Thanks for the source postings. The bible I've been going by is "Theory of Wing Sections" by Abbott and Von Doenhoff, which takes up the history from just after that first chart. Being concerned with wings, it makes sense for them to not go beyond 21% thick, as they want to leave something in hand for some angle of attack. That had persuaded me to go to 25% on a strut myself.

The NACA tests progressed to "laminar flow shapes" of which the 67-series is the most extreme. These are almost lab curiosities, first achieved in production on the P-51, using bondo over the flush rivets. No such shape can tolerate a blemish, let alone a wheel opening, without tripping the boundary layer into turbulence in a wake spreading 15 deg to each side. The "standard roughness" in these tests, which double the drag, consists of .011" grit, covering 5-10% of the area, just within 8% of the length from the leading edge. (like small bug strikes)

Here are a few selected data points from my book:
The lowest cd shown was for a shape that didn't make a stable wing, so there may be much room for exploration. That wing is the 0010-35, in the old system, 10% thick. At Re from 3 to 9 million, the cd is .003 near-polished, and .009 "rough."

The 2424 shape is similar to the 2412 It gets a cd of .008 smooth and .013 rough, at 24% thickness, while the 12% thick shape gets .0065 smooth and .010 rough.

Moving on to the later laminar shapes, which rely on perfect conditions to maintain a laminar boundary layer back as far as the second term in the identifier, usually the widest part of the shape. A 63-006 shape is a 6-series, (laminar) with 30% laminar flow, no camber for lift, and 6% thick. - ie: 100" long, 6" thick. The 67-025 is 70% laminar, no camber, 25% thick. The 66 and 67s are slipprier than the 63 and 64s, but who can wait for 70% of the way back for the first seam, wheel opening, or other feature?

Shape: cd around 6 mil Re. cd with standard roughness
63-006 .0043 .0087
63-021 .0055 .011
64-009 .0043 .0087
64-015 .0047 .0097
64-021 .0054 .018
66-006 .0032 .0085
66-021 .0041 .013

These all show a clear preference for the slender shape, despite the extra surface drag. The text helps illustrate the trends with increasing speed, and gives examples that improve, such as the 65-418, with less separation as you go faster. This is not uncommon, but the devil is in the details, and these shapes are only a rough indication about designing practical cars.

Re: underflow - it is good if done right, but blocking it off is a cheap fix that gives downforce, which is good for road racing, not cruising. Underflow is not really hard to visualize; it is like a duct, but with the boundary layer on the flat side moving and helping stabilize things. After F1 banned the vacuum cleaner cars, they did quite well with underbody venturi systems.

Great stuff bob! :thumbup:

I've been using a RE of 6million for all of my tests. It seems perfect for a 10foot chord at 65mph.

I have read that the 6 series NACA foils are susceptible to imperfections on the body as well. The reason i am so interested in the shape, while not practical for a 4 wheeled car, is because it seems quite ideal to use for a bicycle and/or motorcycle. Funny thing i say that because there are quite a few high speed HPV's that use a shape very close to the 65,66, and 67.

Example 1: Sam Whittingham's Varna Diablo: the worlds fastest HPV @ 82mph
http://www.varnahandcycles.com/gallery/g12.jpg

Example 2: The Cheetah bike
http://www-eng.lbl.gov/~osborn/cheetah/pop_sci.jpg

This team used the 6 series foil from top to bottom when shaping their fairing, as seen here:
http://www-eng.lbl.gov/~osborn/cheetah/wirefairing.jpg

They went 65mph with this design.

Yes, the NACA 6-series has been very popular with HPV builders, although between the seams, ripples, and vibration, they probably seldom get up to even 20% laminar flow, which makes the shape just a reasonable guess. The Varna is completely done by artist's methods. George sends persistent theorists to argue with me and leave him be, and won't even run a tuft test.

http://ecomodder.com/forum/member-or...y-supplied.jpg

http://ecomodder.com/forum/member-or...5-top-view.jpg

I love how these look. The charm of the hand drawn templates is something I miss with most everything done with computers now.

I think this is a good visualization tool. I have been trying to find something like this for a while.

Now I just have to decide where to cut it off. I may just build a full length detachable cone section aft of the kamm. Why should Basjoos have all the fun?

Somehow, it looks almost identical to the Clark-Y, a very popular wing of the period, so you'd probably want to adjust it nose down a degree or so to kill the lift.

The golden egg is blushing at your inadvertent compliment.:o She weighs a sprightly 3800lbs. I'm fairly certain she won't be lifting off at highway speeds.

Or perhaps you're right and I'm going to be the proud owner of the first ever STOL minivan.:thumbup:

Isn't lift unavoidable when streamlining? The shape is a wing after all. Or are you referring to keeping the underbelly pointing down so as not to create pressure under the car? I suppose added to the lift, that would start to really add up.

The Previa is already nose down because I have removed all the rear seats so if that's the idea, I'm good.

I consider my boattail to be a cargo extension module that just happens to also increase the mileage of the car. The interior cargo space extends to within about a foot of the tip of the boattail.

On that note, thats part of the reason why i am anxious to make a fairing for my Ninja. A streamlined shape will elongate my bike a good foot or 2 in the front and another foot in the rear. I plan on designing some sort of drawer trunk in the front of the fairing (theres going to be a lot of dead space) so as the small weight of luggage i usually carry can be placed over the front of the bike, which makes it more stable.

speed/open-closed
 

(1) The ratio does not vary with speed,as it is fixed by the critical Reynolds number which for cars,occurs at about 20-mph.At 20-mph and above,you've established the turbulent boundary layer and Cd minimum which remains constant up to the beginning of transonic flow,at about 250-mph.--------- If you lengthen the "pumpkin-seed" you will increase friction drag,although friction drag is only about 7% of total drag for a car so it's not a biggie.The only reason I depicted the 2.5:1 ratio,is that below that you run the risk of early separation and high form drag.--------------------------------------------- (2) Sepp has reported that an open shell did not offer as much drag reduction as a closed shell on his Nissan truck.Bob Parson's open boattail for his Vanagon did not return as high a savings as the closed unit on my Transporter but his "lifestyle" dictated an open plan and he was happy to live with the difference.Open units may allow"circulation" which always has to be paid for at the pump.

Phil, your post has reminded me to add "enclose rear of Kammback" to my to-do list. Thanks.

It's easy to do with a piece of cut plexiglass that still preserves my rear view.

thinks
 

theycallmeebryan,good feedback and good eyes.Perhaps you can help out on Marchaj's table for sections.My thought was that at the Reynolds number listed,the relationship between drag and fineness ratios/thickness, would be be maintained across the Reynolds number spectrum,as all airfoils perform as a function of Reynolds number ( all else being equal )and that skin friction is the primary source of drag for sections up to stall,where profile drag would dominate.From my fluids text,for sections in 2-dimensional flow,thickness ratios below 2:1 are dominated by profile drag.Above 4:1 skin friction is the dominant player.--------------------------------------- With respect to the 67-series wing section,I have only data for the 67,1-215 section.It shows a drag minimum at Re 6-million-------------------- Here's the freaky thing about wings.In my Abbott and von Doenhoff' Theory of Wing Sections,on page 28,Chapter-1 it says that"section data are not applicable to wings of low aspect ratio.In fact,an entirely different theory applies to wings of very low aspect ratio." It cites Reference 61,Jones,Robert T.:Properties of Low-aspect Ratio Pointed Wings at Speeds below and above the Speed of Sound.NACA TN No. 1032,1946,as source of data for this situation.------------------- The other thing is that for anything other than zero crosswind,a wing,standing on it's head is experiencing a change in angle of attack,lift is induced,which creates a pitching-moment,and also,friction drag increases due to velocity increase due to lift,and pretty soon your Cd min ( @ zero "angle-of-attack") is right out the window.----------------- Sylph was designed around a NACA 66-section( P-51 Mustang ) and while it tunneled out in 1/4-scale with Cd 0.109 it's still 11% dirtier than GM's pumpkin-seed Sunraycer.--------------------------- And evidently,some wings are chosen for "flight Reynolds number" which would not be experienced in a car until over 200-mph.---------------------- I know BicycleBob and others have a much deeper grounding in struts and wing sections,and the station positions make it great to produce these forms,and I'm not in opposition to their use,however,until I dig a little deeper,it seems like we have to temper our enthusiasm for them when considering the context of their design and limitations imposed by ground-effect,crosswinds,extremely low aspect ratio,spanwise circulation,etc..

The air going under the car takes a straighter path to the rear than the air going around or over the car and according to Bernoulli's theorem,it's pressure would be higher than those other two pathways.---------------------------- With respect to the airdam.if it is no lower than the lowest undercarriage member,then it will not aggravate frontal area,causing a drag increase.Whether the air should go over or around the car is actually something that should be taken on a case specific basis.Currently,airdams are configured to encourage flow around the sides of vehicles.------------------------- With respect to the template,I'd like everyone to ignore the forebody and contemplate the rear aspect of the car.This is where the low-drag challenge resides.

Watch that ground plane!
 

Just wanted to mention that the only way the template works,is if the height of the vehicle measured from the road surface is scaled to fit under the curve.

wings on the ground
 

I've been thinking that we should do a separate thread exclusively for wing sections.I've been playing a game of catch up,reading a little deeper into my texts and see the wings with mixed feelings.Here are some random observations:(1) wings operate in free air which is virtually turbulence free.(2) automobiles operate in air which is all turbulence.(3) Virtually all the drag of a wing (unless it stalls) is skin friction.(4) Only about 7% of an automobiles drag is from skin friction,with 55% from profile drag.(5) Wings reach their lowest Cd(min) at aspect ratios of 4-5,and "practical" aspect ratios for wings are in the range of 3-9,and as high as 20 for sailplanes.(6) a wing used for a automobile body could have only a fractional aspect ratio,its tabular data unusable according to Abbott and von Doenhoff.(7)"Flight" Reynolds numbers of 6-million can be achieved at Re 2,000 due to turbulence in ground-effect,and at 20-mph.(8) What would otherwise be a "laminar" wing in free air,will transition to turbulent boundary layer at normal automotive ground clearance and low road velocity.(9) crosswinds would be the same as angle of attack change to a vertical wing section,Cl would quickly climb,as would drag( for example,a Clark-Y in a 17-degree relative wind would see a 180% drag increase),creating a pitching moment about the aerodynamic center.(10)0.0005-inches roughness or less is permissible for at the leading edge of a laminar wing,so dried bug juice and insect remains would be enough to scuttle the low drag of these high performance sections,not to mention free turbulence.----------------------------------------------------- Wings are great and I love what they do.I'm just having a time of it trying to wrap my brain around using their performance criteria in the context of an automotive body.Jaray's form does resemble a Clark-Y from the side,but with all the rounding off of edges on the side and nose,"pumpkin seed" seams a better fit for a descriptor.With the "mirror-image" my brain sees the body of a "pair-of-grins" falcon in full stoop at 250-mph.

"The air going under the car takes a straighter path to the rear than the air going around or over the car and according to Bernoulli's theorem,it's pressure would be higher than those other two pathways.-------------------------"

If the path for the air from the front stagnation point to the low pressure zone at the rear is very smooth, the pressure difference, along with the restriction from the ground, can move the air as quickly as the air going around. This is one of the major techniques to produce downforce for racing - the venturi bottom.

I understand that the NACA 6-series wings, and pretty much all foil shapes, produce the lowest drag while in flight, where air is less turbulent and there is no ground deflection.

Since these shapes are so susceptible to imperfections (bug guts, cracks in shape, roughness in shape, etc), what then would be the optimal shape for a road vehicle? Is it worth even attempting to use a low drag foil shape, like the NACA 6-series, on a road car at that rate? Are you suggesting that a simple shape following the template you have posted would have less drag and more laminar flow than a car designed around a NACA foil, considering every day road conditions?

Would it be safe to say that a shape which has the least average drag from -10 degrees to 10 degrees would be the optimal to select for a road vehicle?

More darned questions....

Some people have reported a loss of mpg after installing belly pans. What are some possible causes for increasing drag with belly pan? For instance: If one inadvertantly created a venturi under a car would that increase drag? Or is that venturi unavoidable? If a van was nose down due to rear seat removal would that increase drag? Is parallel best?

I was trying to get the tail shorter but I guess you can't fool physics.:o (Or aerohead or bicycle bob) Darn.

By the way here is the link to my newly finished roof and sides extensions, soon to be kamm back. I'd be thrilled if you would give it a look. It was mentioned to me to concentrate on the rear end. It's a start.

http://ecomodder.com/forum/showthrea...evia-9437.html

Thanks guys! :thumbup:

This little clip rom Craig Vetters site is telling...

http://www.craigvetter.com/Movies/Streamlined_test.mov

blunt end forward, taper aft is the way to go for low drag.

I'm not sure the narrator, Craig's brother, has got the forces right. He
speaks of air pushing on the front of the car, when I think the real
issue is air pulling on the back as aerohead says.

I'm sure there are also unidentified interactions between the airflow
around the car and the tube due small clearances.

Although it is still technically "vaporware," Mitsubushi's all-electric iMiev
Sport Air concept car seems to be the best embodiment of these concepts:

Autoblog Green

as quickly
 

I'm in town without the aid of my library(as usual).Relying on memory,and referring to Wolf Hucho's text,under vehicle flow will never reach ground speed velocity for typical passenger vehicles with typical ground clearance.He provides some quanta for anyone wishing to run some numbers.-------------------------- In the case of racing vehicles and record cars,sure,you can "idealize" the bottom of the car.Dr.Michael Seal of Western Washington University created the 100-mpg Viking series of cars with high ground clearance and an inverted airfoil underside to provide downforce.The smooth belly cut drag and the profile helped with added stability.-------------------------------- The other school is to lower the car into the weeds,with ground skirts and tunnel effects with diffusers.Whatever the sanctioning body will allow.------------ This second approach requires ground clearance which would not survive the light of day in day to day commuting.

shapes
 

I don't have your wing section,but I spent some time with the NACA 67,1-215.It's drag coefficient(min) occurs between a lift coefficient of 0.175 and 0.375 which from the table,occurs for a range of angle-of-attack between zero and 2-degrees.Here's the kicker,by just changing the angle-of-attack 7-degrees(something quite common in crosswind driving),the Cd rises to 0.0162 a 490% increase.All the 6-series airfoils have a "sag" or "bucket" in their performance curves,and lift (drag) increases rapidly with very small wind direction change.----------------- The other thing to bear in mind is that laminar wings are laminar only up to the first minimum pressure location,and in this adverse pressure gradient,the wing will immediately transition to a turbulent boundary layer.----------- This doesn't mean that flow will separate,it only means that friction drag will increase,bad news in an airplane,but actually good news for automobiles because turbulent boundary layers can sustain attached flow just above the boundary layer,and attached flow is what we need for drag reduction in cars.-------------------------------------- The thing about the template,it is borrowed from a "wing guy"(Jaray"),who,while well acquainted with the tastiness of wings,found the "pumpkin seed" to be more delicious for low drag automobile forms.You will find numerous land speed record cars and solar cars which even today,rely of this form for top honors.-------------- For the template I chose the 2.5:1 fineness ratio as a "minimum" for a 3-dimensional form in 3-dimensional flow,in ground effect.This selection follows a logical linear progression from Jaray's streamline body of revolution completed in it's mirror-image ground reflection,and the German research which isolated the 2.5:1 ratio as having the lest drag.No real Einstein moments there.--------------------- The template respects streamline flow with an economy of structure and literally guarantees attached flow,at least if you don't cheat the minimums.----------- YOU CAN make the body with higher fineness ratio,I just want everyone to understand that there will be a small penalty to extra skin friction.And the good news is that since skin friction plays such a minor role in road vehicle aerodynamics,you won't really nshoot yourself in the foot if you do go longer.--------------------- Getting back to your question:the pumpkin seed will not exhibit drag increases anywhere near the same continent as wing sections in crosswind conditions,the drag will be lower,and while the boundary layer is turbulent,it will be impossible for the flow to separate up to the point you choose to "chop" the body,as Dr.Kamm did on his famous series of cars.

darned questions
 

I have no published reports which suggest that a problem such as this is possible.And not to dis-respect those claiming the degraded performance,I'd have to be there to investigate it myself and do some mutual head-scratching.----------------- I do have a paper from FIAT which experimented with a bagful of vehicle configurations and they did observe that by altering undercar flow,the uppercar flow can definitely be affected.Obviously,if you have the proper tools,you can quickly isolate an area and remedy it.---------------------------- With respect to a venturi,this is something you'd really have to work at creating.If the pan is parallel with the ground until it can begin a continuous upsweep angle for the diffuser everyone should be okay.------------------------- As far as your stainless roof extension,I've been watching it.If you're anywhere near Sanger,Texas,you can drop by and teach me Metal fabrication technique,She's a beauty!------------------ As far as the drag reduction goes,you can't cheat on the curvature,and your drag reduction will be an arithmetic function of the new,smaller wake area the extension creates as compared to the vehicles original wake.--------EXAMPLE: your extension allows for a 10% wake area reduction,that translates to a 10% drag reduction.Simple huh?-------------- A 10% drag drop would net you 5% AND 6% MPG improvement at 55 and 70 mph respectively.-------- So using,say the template,you can engineer your extension on paper and know the results before you ever begin buying materials.--------------- And yes,the rear is where the action is.Since 1922 the rear of the car has been known to be the chief source of drag for ground vehicles,and the only way to reduce it,after attached flow is in the bag for the forebody,is the gently tapering tail as depicted in the template.

Craig's brother
 

In the context of a wind tunnel,the air would be impacting the body,in a positive pressure gradient up to the point of maximum cross-section,beyond which,the air would be receding away.With proper contouring,as Craig has already accomplished on his Rifle,etc.,the body could maintain attached flow all the way back,finally breaking away into a relatively turbulence-free wake.-------------- Without the proper contour,the flow would separate,and the base pressure behind the body would be that of the point of separation,acting to "pull the body back."---------- So it's the pressure differential at work here.Separated flow breeds high Delta-P,zero separation yields losses attributable only to friction,internal flow,and induced drag.------------------------------------ The Mitsubishi looks pretty good! I held a protractor up to the computer screen and measured around 22-degrees for the roofline exit profile,just at Mair's ragged-edge for attached flow.I wish they would provide an overhead plan view of the car so we could inspect that aspect of the cars architecture.I wish they'd do that for all cars!----------- Thanks for sharing that!

A lonely messenger
 

[QUOTE=aerohead;119009]---- The template respects streamline flow with an economy of structure and literally guarantees attached flow,at least if you don't cheat the minimums.----------- YOU CAN make the body with higher fineness ratio,I just want everyone to understand that there will be a small penalty to extra skin friction.

I agree that it is good to avoid extra skin friction, but when only one study suggests that flow can stay attached to a shape at 2.5 : 1, and hundreds of tests at a more suitable Reynold's number disagree, I want to see some tuft tests. In the NASA tests for streamlining trucks, you can see a tailcone extension eliminating a Kammback, while increasing the angle a bit, and the tell-tales show separation there, at an angle that is still less than this "ideal."

putting wings to rest
 

This is an addendum to what I presented above in hopes of tempering enthusiasm for symmetrical wing sections,and providing a context in which to contemplate their use:(1) The fundamental assumption for all sub-supercritical -velocity wing theory models used to construct section data is that of 2-dimensional,chord-wise-only flow. (2) The fundamental assumption for all road vehicle aerodynamics is that of 3-dimensional flow,hence,the Navier-Stokes Equations of Spherical Coordinate System,employed in CFD modeling. (3) To presume the "applicability of section data" as presented in "THEORY OF WING SECTIONS" for 3-dimensional flow,in ground-effect and "outside turbulence",violates the fundamental premise under which all the data were conceived,and is stated so by the authors on page#28,paragraph#4,sentences-#2,#3,and #4 of their text.

one study
 

[QUOTE=Bicycle Bob;119013]I have been unable to post any attachments since I posted the template.I've asked for help each computer session since and so far no help,so I'm deadlocked as far as that goes.---------------- I had additional documents scanned and attempted to attach with the other 5-documents but EcoModder message came up saying I was "maxed-out".------------------------------- The DVL research was conducted at Re 3,000,000.The work is included in Fig.5.13,page 69,Aerodynamic Drag,by Hoerner,1951.Why Wolf Hucho chose to include it in his table would be a matter of conjecture on my part.The form does exhibit one of the lowest Cds for a 3-dimensional form and is of lower drag than airship bodies of greater fineness ratio,of which it shares great resemblance.The airship Los Angeles,with fineness ratio of 7.23:1 only increases to Cd 0.045,when fins,gondola,and engine nacelles are taken into account),a 12.5% increase,however a 25% increase in ground-effect.----------------- The study does not suggest that this is the only form that will support attached flow.It does claim that a body of revolution with a fineness ratio this small CAN have full attached flow.So I chose it as a minimum,as with smaller fineness ratio,the exit architecture exceeds Mair's 22-degree maximum for a boat tail,suffering separation and attendant rise in profile drag.----------------------------- Since it is Hucho that says only a long tapering tail can prevent separation,and Jaray says that a half-body of a streamline body of revolution is the lowest 3-dimensional form for a vehicle body,my pea-brain seized on The DVL 2.5:1 as the logical minimum.Just a starting place.Obviously,this isn't a one-size-fits-all solution.The data suggests that it could be a no-brainer go-no-go,or template for which to contemplate potential fuel savings from aft body modifications,which Hucho suggests is our only path for truly significant drag reduction.He also states that in the context of cars which are designed specifically for low drag,the science is good and you can lean hard on it.------------------------------------------------ With respect to the NASA van,I think the exit angle off the boat tail is around 20-degrees,same as for my VW.If this is the same photo you're referring to,then I can tell you that it works and got me an extra 4-mpg.The important thing is the gentle curvature leading into the tail as you will see in Mair's boat tail research.Without that,the flow immediately separates,with little gain.------------------------------------------- I have a tuft-test photograph of the boat-tailed and streamlined T-100.There is very little perturbation of the airstream and all tufts are straight back at the terminus.The trailer will be a follow-on to the boat tail,providing,through the use of the gap-fillers,a continuous boat tail taper the a point about 10-feet aft of the tailgate.She's a little longer than needs to be,but that's where I'll be "living" on my way to and from Bonneville and Battle Mountain.The rig comes in at about 5.3:1 on the ground and 2.65:1 in "mirror" so she's a little over my minimum but I'll welcome the extra space inside.-------------------------- If you can post the NASA image I'd appreciate it,and be able to compare to what I have.Mines the truncated tail,only a couple of feet,maybe a meter in length.Thanks!

aerohead,

here's the best I can come up with for an overhead shot of the iMiev Sport
Air concept car.

http://www.youtube.com/watch?v=6PcvEQVR1mY&NR=1

http://www.youtube.com/watch?v=LlIoP...eature=related

Not very good, but you can see that in essence it has a rectangular plan-
form at all levels with verry little side-to-side taper aft of the max cross
section.

I notice that the prototype has nowhere near the swoopy lines that the
artist's drawing did... Reality, that is salability and day-to-day survivability,
rears its ugly head. :p

Hope this helps.

Aerohead,

I PM'ed you but perhaps you didn't get it. I would be happy to post your images and files. PM me with your email address and I'll send you mine.

If I'm ever in the area, I'd be glad to show you how I work. In the meantime, I have thought of doing more detailed posts here if people were interested. I could do little videos, I suppose.

Thanks again for all the advice.

You can see some boat tailing of the roof structure although the body has less camber.Thanks for the link!

PM
 

Thanks orange4boy,I left work early today to check PMs and had heard back from Darin and SVOboy.They think they have me fixed up with more capacity and this Saturday I'll try again with images.------------------------------ Your video is a great idea.I learn so much from observing and its a powerful teaching tool.At the build forum,such a "clinic" could get a lot of mileage from the members and lurkers worldwide.Nothing like class-A tin bashing!

low fineness-ratio tuft study
 

Al helped me export a photo of the T-100 undergoing a tuft study to illustrate attached flow at low fineness ratio,something the template suggests as a "minimum" roofline curvature which still guarantees no separation.---------------------- The photo was taken by friends a couple years ago,on LOOP 288,Eastbound from FM 2164 at 60-mph.----------------------------- As you can see from the image,while there IS some lateral flow along the flanks,the flow is clean as the flow exits the body.Kamm's car had this "spanwise" lateral flow,which while not ideal,is nevertheless attached,and further aft,can deliver energy to the wake for pressure recovery.NOTE: the boattail becomes the basis for the articulated gap-fillers with the trailer project,providing for continuous flow to the trailer tail.

What are the signs of separation in a tell tale. I was testing my kamm back and got flat tell-tales that scooted side to side a bit. Do they have to be straight and motionless or is a bit of side to side ok?

What is a good angle to start with for a kamm back. Profiles are nice but my printer ain't that big.