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Old 01-26-2008, 04:19 PM   #1 (permalink)
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Half a billion years of streamlining

Hello all. For years,I've used the work of the Lillianthal Bros.,Gustav Eiffel,Paul Jaray,etc.,as examples of aero-pioneers,when doing public exhibits on aerodynamics and fuel economy. I have also used fish,marine mammals,shark,and birds to demonstrate" Nature's" design solutions for fluid-dynamic challenges.

After reading "More With Less", about Dr.Paul MacCready of Aerovironment, and his teams which gave us man-powered flight and Solaraycer, to name just a few of their accomplishments, I was introduced to the term "biomimicry", a term used by team members to describe how they observed, and looked to nature for clues for how to overcome physical challenges to moving within our atmosphere or waters.

Digging in a little, I've come to learn that our birds date to the Jurassic Period, about 150-million years ago, and fish, to the Devonian Period, about 400-million years ago, and "proto-fish", with all the streamlining efficiency of today's modern fish, to perhaps a half-billion years ago.

The HONDA Dream solar champion ,with Cd0.10, and 400-mpg potential, borrows heavily from Aerovironment's Solaraycer, which itself was modelled after a pumpkin seed.

NASA has been able to create the efficiency of the 150-million-year-old blue-fin tuna,the most streamlined shape in existence.

Peregrine falcons stoop at over 230-mph terminal velocity. Needletail swifts bound through the skies in level flight at over 200-mph. Ocean prey do their best to evade the clutches of fast swimming mako shark, bottlenose dolphin,emperor penguin,leopard seal,and the like.

Atomic submarines have come to embrace the tuna in their exemplery hydrodynamic forms. Canoe and kayak owe their amazing abilities to organic forms.

As carmakers look to the challenges of higher fuel economy,cleaner air,and lower CO2 footprints,they need only look to the heavens or under the waves for the solutions which await them along their line of sight.You as members of ecomodder needn't wait though,as no lenders, committies,shareholder proxies ,nor sales directors,limit your access to nature's forms.You have only the nuts and the bolts of it in your way.

So look up,look down.It's all the same in nature.The forms just keep repeating themselves.And if you're ever over-takin on the road by a four-wheeled tuna,its probably just some nut like me,who bothered to look around . Get you a shop,get out there, tear it up good,be safe,have fun,save fuel. I'll be taking my own advice.

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Old 01-26-2008, 04:44 PM   #2 (permalink)
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I find it interesting that you don't see any examples of the Kammback in nature, just boattails. Makes you wonder how low drag competitive a Kammback is compared to a boattail.
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Old 01-26-2008, 08:03 PM   #3 (permalink)
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Kamm stated when the Kamm tail shape was put forward it was to be a compromise between the ideal theoretical shape and a practical limit for a road going vehicle.

This point is generally missed by a lot of people in discussions on these topics.

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Old 01-26-2008, 08:04 PM   #4 (permalink)
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Aerohead,
Some interesting thoughts there.
Even the 747 looks like a bottlenosed dolphin at the front from some angles.

Pete.
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Old 02-02-2008, 03:09 PM   #5 (permalink)
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Dr.Wunibald Kamm/Kamm-backs/boat-tails

There IS some confusion over Kamm's contribution to aerodynamics.My reading shows that by discharging engine bay air at the base of the windshield,Kamm was able to prevent air from stalling over the windscreen header,allowing attached flow over the roof that would otherwise separate.It was up to the auto designer to chop the body wherever they chose.Airflow would remain attached up to the chop.Kamm's K-car' roof extended to the rear extremity in a continuously descending arc(ie.the "Kamm-back"),and is what people associate with Kamm,not the front-end work that made the attached flow possible in the first place! In nature,we don't see the cropped posterior,as in the K-Car as Nature seems to demand eeking-out the last ounce of efficiency,only found in the long tapered tails we refer to as "boat-tails".It is claimed that truncating the boat-tail ,will produce a "phantom"boat-tail behind the car and its kinda true.I posted a picture of the K-Car in the Volkswagen wind-tunnel over at maxmpg in the Misc.photos.The smoke trace clearly shows the "phantom",however these truncated boat-tails never actually produce the low drag of the long tail.In the movie,"The World's Fastest Indian" we can see Burt Munro's "Munro Special"at speed at Bonneville.The body on the bike takes advantage of the Blue-fin Tuna form for low drag and record-setting speed,however,as is pointed out in the story,it also demonstrates challenges to directional stability associated with center-of-gravity/center-of-pressure domains.BNI will not allow racers to run the course if crosswind speeds get to around 7-8mph,to protect against "weather-vaneing",which could be catastrophic should the vehicles be pushed into a barrel-roll.The T-100 appears to respond favorably to it's truncated boat-tail.I have planned a full 5.5:1 fineness ratio truck/trailer boat-tail,which will allow the vehicle combination to achieve the theoretical ideal form,a full tail extending twelve feet beyond the rear bumper of the T-100.Formulas show a possible 47-mpg at 70-mph.I believe this to be a reach,however it is enticing,and I won't know until I try.As Peter has pointed out,the Kamm-back is a compromise,and in the workaday world avoids the ridiculous lengths I'm going to explore with the trailer.Lower rooflines are the only thing that will allow shorter length and lowdrag together(active-suspension would help).
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Old 02-03-2008, 01:23 PM   #6 (permalink)
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Hi All,

Aerohead has it right. The issue is cross-wind stability. The Kamm-back is a drag compromise to achieve cross-wind stability. Now depending on the vehicles the stability issue will or will not be as much an issue. A pickup truck for example has the CG way forward, and the cap boatail is actually mostly in front of the rear wheels - not much of a problem. A hatchback civic is similar, but the boatail is now behind the rear wheels, but the rear wheels are way back on the car. As other cars have body parts much farther back behind the rear wheels, the boat tail is probably OK on this vehicle too.

Why no Kamm-backs in nature? Because animals can do variable shapes, and twist their tails to allign with the direction of flow. They can also do vectored thrust. Besides animals can do active fluid dynamic control. Ocean going fish also do not need to align with the roadway, and can just go into and then crosswise to currents. And currents in the ocean are very slow compared to tuna speeds. So cross-wind or cross-current has also been designed for over evolutionary time, or are just not as big a consideration.

Last edited by donee; 02-03-2008 at 03:26 PM..
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Old 02-03-2008, 04:09 PM   #7 (permalink)
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If you find this interesting, then you should also look into techniques used to prevent sonic boom on super sonic flights.

It's all a matter of the drag vortex and how you avoid creating one.

CNN News article

I see no reason this cannot be applied to ground vehicles.
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Old 02-03-2008, 08:55 PM   #8 (permalink)
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I think the aero lessons of the DC3 would apply to cars more than those of the Concorde and the F16.

Super sonic (air travelling more quickly than the speed of sound) aero has a totally different set of parameters than sub sonic aerodynamics.
This principally due to compression at Mach One and above which simply does not apply to road vehicles.

An interesting article none the less.

Cheers , Pete.
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Old 02-03-2008, 11:32 PM   #9 (permalink)
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On the subject of nature (very good write up, by the way).... I've yet to find an instance of vertical wing tips in nature Some birds have fanning feathers at their wing tips - but that's physically very different...
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Old 02-04-2008, 12:38 AM   #10 (permalink)
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Very nice post, there is some great insight in there and it was very inspiring.

It always fascinated me how nature evolves over time. I can understand evolution when it does things like, adapt a body to live in a wetter area then it used to or breath a different combination of gases then it is used to. But what always baffled me was aerodynamic adaptations. How does nature know what to do to make itself flow better in the medium in which the creature presides?

I personally can't wait to reconstruct my model wind tunnel that I built in middle school for a science fair project (my dad scrapped it because he needed his leaf blower back) so that I can start testing ideas I have on a scale model of my car and some of my custom ideas I make from scratch, I might actually be able to find a very good way to make a very aerodynamic car for myself someday.

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