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Old 08-17-2011, 07:42 PM   #191 (permalink)
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A & b

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Originally Posted by jimepting View Post
Phil, I think I remember a thread where you derived the template from referenced material. Perhaps we are all getting a little confused at this point. I searched but could not find the thread that I recall. Can you give us a pointer to the thread.

BTW, I don't think anyone is being critical of the template idea, but it is natural among scientists to revisit the sources. After all we all keep picking up Hucho's book. I for one certainly appreciate your efforts to lead the way.
There's Aerodynamic Streamlining Template Part- A & B which cover the evolution of the illustration which finally emerged in Part-C.
There's also a Part-D.
I'm slowly re-visiting Hucho,I hope to post on that maybe by Friday.I'm doing income tax also and trying to complete Viking.

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Old 08-20-2011, 04:06 PM   #192 (permalink)
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filling

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Originally Posted by Fat Charlie View Post
Even with detached airflow, wouldn't there still be a real benefit to filling in the wake? It wouldn't be optimal, but it looks a lot cleaner than simply leaving a large square hanging out back there to hold you back. I'd expect that "slightly detached" airflow would be a whole lot better than "completely abandoned" airflow. I'm thinking along the lines of "anything that brings you closer to the template helps" with the spoilers.

I'm actually asking a question here (not putting forth an argument)- I'm ignorant enough that I only have aerodynamic questions, not opinions.
In Hucho's 2nd Edition,on page 195 of Chapter 4,he mentions an open convertible in the wind tunnel,and if people or mannequins are placed in the seats,they will 'Fill up' the hollow space behind the windshield and should improve flow,lessen drag.
It's the only place in the book where I can find mention of something I've been calling 'stuffing the wake.'
I started thinking about it after reading about the aero mods to the Chrysler Airflow,DeSoto development vehicle.
An 'imperfect' but better roofline was grafted over the existing body ( just as Volkswagen is doing for the 2012 Beetle )and then an 'imperfect' 'stinger' boat-tail was added as well.
The car never achieved as low a Cd as it might for its length,but at Cd 0.244 in 1934 it was pretty amazing.
So I interpreted that even a non-ideal tail could still cut drag.
This undermines the findings of Fachsenfeld's boss,where Kamm would later come to work,who cut the tail off a wind tunnel model of a car and watched its Cd drop from 0.29,to 0.23.They don't show the car so it's hard to know what they actually did.
Hucho shows a range of Cds for the Golf/Rabbit.The lowest Cd is at the 'Template'.Cd increases as the roof gets steeper in slope,from 0.34,all the way down to 0.42.
Anything 'higher' than VW's production slope would be better even though not ideal.And any of these alternate rooflines would be 'stuffing the wake.'
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Old 08-20-2011, 05:54 PM   #193 (permalink)
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tufts

[QUOTE=Bicycle Bob;119013]
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Originally Posted by aerohead View Post
---- 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."
Bob,sorry for the lapse in response.
With respect to the 2.5:1 streamlined body of revolution,at Cd 0.04,it's the lowest Cd free-body who's boat-tail is as steep as Mair's 22-degree maximum,but no shorter.
If it's shorter,it will be like an NFL regulation football ( Cd 0.055 )with no way for the air to remain attached.
If it's longer,the flow isn't any better attached,it uses more material,it's heavier.Not as good of engineering.
The 2.5:1 produces a body of Cd 0.08.When wheels are added the drag goes up to Cd 0.13.If wheel fairings are installed the Cd comes back down.
-------------------------------------------------------------------------
If the constant velocity portion of Mair's wind tunnel model is removed it creates a 'template' of about 2.5:1.
In Hucho's 2nd Edition,p.199,Fig.4.117,3rd image down is a 'template' derived from a Cd 0.05 streamline body of revolution which produces Cd 0.15.
In Fig.4.188 below it depicts the 'Template.'
In Fig.4.199,the lowest Cd for ellipsoids.@ above critical Reynolds number falls at 2.5:1 l/d.
In Hoerner's Fig.5.13,Cd min occurs at just above 2:1 but this violates Mair's 22-degree aft-body slope maximum so we can't go that steep.
On pg. 210,Hucho discusses the 1982 Cd 0.15 ARVW .The fineness ratio of the ARVW is l/h= 5.53 which according to Fig.4.118 corresponds to an effective free air fineness ratio of 2.27 "This approaches the drag mimimum recognizable."
On pg. 324,Fig.8.46,Cd vs L/D for geometric shapes,the body#4 ( ellipsoid ) demonstrates the lowest Cd of all bodies depicted.When ground clearance is factored into the dimensions,the drag minimum occurs at L/D= 2.478
On pg.141-142,when discussing the boat-tail on the Mercedes-Benz C-111 III he observes "Qualitatively,the effect of the (Boat-tail) length upon the aerodynamic drag is similar to that of the body of rotation..."
-------------------------------------------------------------------------
As an airship,the 2.5 l/d body would be extremley low drag,separation free,although the crew would suffer motion sickness.
Hucho says "...low drag can only be achieved when the separation at the rear is eliminated." pg 16
"Optimum shapes are 'streamline' bodies having a very slender rear part." pg 61
"Lower drag can only be achieved by extending the length of the vehicle's body." pg 201
"...drag coefficient ....may be plotted against vehicle length,...if the (drag) evaluation is limited to vehicles...developed for the lowest possible drag coefficient,this expected trend is in fact confirmed." pg. 202
-------------------------------------------------------------------------
Hucho cites Dornier's 1920 truncated wing research to justify not boat-tailing.
Dornier's work is shown in Fachsenfeld's book.It is the Deutsch Reichs Patent 394 184 depicting a cambered wing profile with 24% of its aft-section cut away.There are no quanta listed.
For a DVL or Gottingen section of 20.5% thickness,with 24% of the aft-section removed,the drag increases 223% ( 2.23X).
For a 12% d/l DVL smooth fuselage,cutting 50% of the tail away as recommended by Kamm results in a drag increase of 59%.
------------------------------------------------------------------------
Jaray's 1921 pumpkin seed is "Template" out to 32% aft-body,then goes hypo with an exit slope of 26-degrees for Cd 0.13
Lay's pumpkin seed is at "template" and registered Cd 0.12.
Fachsenfeld's Omnibus with inflated boat-tail is very close to the 'Template'.
Other vehicles considered for 'Template' comparisons:
1957 MG EX 181
1957 Grumman E-1 ( WF-2 ) Tracer (radome)
1963 Alfa Romeo TZ
1965 Ford Mk IV
1967 Lotus Europa
1967 Ferrari 330 P4
1967 Chaparral 2F
1968 Mickey Thompson LSR Mustang fastback
1970 Porsche 917 long -tail (D)
1974 VW Golf/Rabbit
1980 Medusa,Ital Design
1981 Flow body,VW Cd 0.15
1981 Drop Shape,VW,Cd 0.16
1981 Blunt Body,VW Cd 0.15
1984 Ferrari Testarossa
1984 Maya,Ital Design
1984 Imperator 108i,Isdera
1987 Express,Chevrolet Cd 0.195
1987 Vesta II,Renault,Cd 0.19
1987 Sunraycer,AeroVironment/GM,Cd 0.089-0.12
1988 Lamborghini Countach 5000
1988 Lotus Esprit Turbo
1991 HX3,GM,Cd 0.258
1991 Ferrari F40
1994 Viper Defender,Dodge
1994 Bugatti EB 110
1994 Saleen S-351 Mustang
1995 Ferrari F355 Berlinetta
2004 Team Corvette C50R
2005 AeroCivic,Cd 0.17
2008 Boxfish (Bionic),Mercedes-Benz,Cd 0.19
2009 Koenigsegg CCXR
2009 1-Liter concept,Volkswagen Cd 0.195
2010 Murcelago,Lamborghini
2010 CTS Wagon,Cadillac
2010 Ford GT
2010 Ferrari 458 Italia
2010 Honda Type R
2010 Ferrari 599XX
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Old 08-21-2011, 12:23 AM   #194 (permalink)
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I have a lot of time to dream and plan while I'm working these last few weeks, so I keep refining my travel trailer design. The template is a thing of beauty to behold, but there are other, more gritty phrases associated with constructing one
I have an angled design going now, with an initial angle of ~9 degrees. I can build a radiused transition section about a foot long and retain my original hairline. Any more curving and it will all end up on the floor.
Does that sound "attachable" ?
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Old 08-21-2011, 08:22 AM   #195 (permalink)
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Quote:
Originally Posted by skyking View Post
I have a lot of time to dream and plan while I'm working these last few weeks, so I keep refining my travel trailer design. The template is a thing of beauty to behold, but there are other, more gritty phrases associated with constructing one
I have an angled design going now, with an initial angle of ~9 degrees. I can build a radiused transition section about a foot long and retain my original hairline. Any more curving and it will all end up on the floor.
Does that sound "attachable" ?
Can you post pix?
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Old 08-21-2011, 10:06 AM   #196 (permalink)
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Bob, I was just getting functional, and thinking about the drawing board. I will draw and scan something. I should make a separate thread for it as well.
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Old 08-21-2011, 10:58 AM   #197 (permalink)
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Quote:
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Bob, I was just getting functional, and thinking about the drawing board. I will draw and scan something. I should make a separate thread for it as well.
Please do.
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Old 08-21-2011, 01:59 PM   #198 (permalink)
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Originally Posted by slowmover View Post
Please do.
http://ecomodder.com/forum/showthrea...ign-18585.html

Done.
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Old 08-24-2011, 01:45 AM   #199 (permalink)
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This is a fascinating thread but I've been reading it and thinking most of the way along...

hey!, they're missing something.

Now maybe someone's dismissed this point already and I've missed it, maybe not, but my point seems to be illustrated by several car designs which appear to have steeper tails but claim good drag numbers.

The curves discussed in this thread are typically derived from NACA wing sections, but we're not dealing with a wing. A wing is very long compared to any other dimension, so it can to all intents and purposes be analysed as a 2D structure, each front to back section taken anywhere from the root to the tip of the wing can be thought of as being indistinguishable from the section adjacent to it.

But a car is inherently a 3D structure, the air over the centre line is very different to the air directly over the driver... (the best aero cars taper not just top to bottom, but side to side)

If we take a small but finite section of wing, all the air molecules at it's top edge will need to get back to the very tip of the trailing edge as the wing passes.

However in the simplest 3D case, a tapered dowl, not all the molecules around the fattest section will need to get back to the trailing point, only one need do this, the rest can fill a small tube around this, repeat this for each layer of molecules and the vast majority will not have to rebound terribly far.... in effect the air needs to rebound less in the 3D case than in the 2D thus one would expect higher angles in the idealised curve for a 3D object

A: have I got this all wrong?
B: has someone else already pointed this out?

Clearly if one is generating a simple curved kammback with flat sides then the wing section approach makes sense, but what if we're using complex compound curves.. can we get better performance in a shorter space?

Derek
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Old 08-24-2011, 10:39 AM   #200 (permalink)
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Good points.

Short answer is, not 2D, this takes into account 3D, which ever view you have of a car, top or side, it should match the aero template. The current models of the VW Scirocco and Volvo C30 bring home how this works in top view the best.

Bottom line is this is the shape that offers the lowest Cd potential if a vehicle were to be shaped like it. Obviously, they're not. This template is intended to be a guide for someone who wants to add something to the back end of their vehicle to improve the Cd. You overlay the outline and cut to the dotted line, what ever you build to fill in the blank, should then be an optimized improvement to the previous shape you had.

Here is a recent 88 B2000 Mazda P/U I did this overlay on.




So if a guy were to build a cap on his cargo bed shaped to match this line, it would be the best compromise between too steep and too shallow of an angle along the top.

Yer welcome to try and explain why a different shape would be better, this one is the product of a very keen persons lifetime of work and thought in the realm of aerodynamics. He may be wrong. But I'd bet real money and body parts that he's really close, and in this game, really close will get you the result you're after when it comes to trying to improve the aerodynamics of a bluff body in ground effect, deviate from it at your own peril.

Look around you at most of the current European cars as you're out and about, you'll see their roof lines in profile match this template almost universally, funny that.

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