Go Back   EcoModder Forum > EcoModding > Aerodynamics
Register Now
 Register Now
 

Reply  Post New Thread
 
Submit Tools LinkBack Thread Tools
Old 04-07-2021, 06:13 PM   #11 (permalink)
Master EcoModder
 
aerohead's Avatar
 
Join Date: Jan 2008
Location: Sanger,Texas,U.S.A.
Posts: 12,647
Thanks: 20,313
Thanked 6,289 Times in 3,893 Posts
K value

Quote:
Originally Posted by JulianEdgar View Post
I don't understand. The K value doesn't appear to have thing to do with Cd for flat plates.

"Drag coefficients are shown as 'K', which is in lb sq feet (mph)^2"

Why aren't they talking of the frontal area of the car? ie lbs drag per square foot of frontal area * (mph)^2.

I am no mathematician but I would have thought the conversion of that to Cd would be fairly straightforward.
Aeronautical engineers don't typically use projected frontal area-based drag coefficients.
* They may use wetted-area-based coefficients.
* They may use volumetric displacement-based coefficients.
* All components of an aircraft can be calculated individually, then added together, plus an estimated interference drag component.
* Typically, they resolve total aerodynamic drag into that of an equivalent flat plate, flat into the 'wind.'
* Sometimes they've used flat plate area sliding parallel to the streamlines, both sides.
* All calculations are for 'flight conditions,' maybe cruising at 41,000-feet altitude Above Ground Level/ Above Sea Level.
It's a nightmare!

__________________
Photobucket album: http://s1271.photobucket.com/albums/jj622/aerohead2/
  Reply With Quote
Alt Today
Popular topics

Other popular topics in this forum...

   
Old 04-07-2021, 06:15 PM   #12 (permalink)
Master EcoModder
 
JulianEdgar's Avatar
 
Join Date: Nov 2017
Location: Australia
Posts: 1,978
Thanks: 106
Thanked 1,545 Times in 1,097 Posts
Quote:
Originally Posted by aerohead View Post
Aeronautical engineers don't typically use projected frontal area-based drag coefficients.
* They may use wetted-area-based coefficients.
* They may use volumetric displacement-based coefficients.
* All components of an aircraft can be calculated individually, then added together, plus an estimated interference drag component.
* Typically, they resolve total aerodynamic drag into that of an equivalent flat plate, flat into the 'wind.'
* Sometimes they've used flat plate area sliding parallel to the streamlines, both sides.
* All calculations are for 'flight conditions,' maybe cruising at 41,000-feet altitude Above Ground Level/ Above Sea Level.
It's a nightmare!
We're talking about a car, not an aircraft. Why do you assume that Chrysler engineers were using an aircraft approach? There's nothing in the paper to state that, as far as I am aware.
  Reply With Quote
Old 04-07-2021, 06:52 PM   #13 (permalink)
Master EcoModder
 
JulianEdgar's Avatar
 
Join Date: Nov 2017
Location: Australia
Posts: 1,978
Thanks: 106
Thanked 1,545 Times in 1,097 Posts
I have been doing some maths and I realise I made an important mistake earlier.

Looking closely at the Chrysler paper, the equation they use for their K value is in fact:

K = drag force (lb) / [frontal area (square feet) * (mph)^2]

That makes much more sense, as it's the standard drag equation with air density missing (but obviously assumed for a standard atmosphere).

I have now been trying to convert that to drag force (newtons), frontal area (square metres) and speed (metres/second) so that we can convert to our normal Cd values.

Using an air density of 1.2 kg/m^3, and taking into account the 0.5 multiplier on the bottom of the Cd equation, I get a conversion factor of 398.746 (ie multiple Chrysler's K value by that number to get Cd).

But math is not my strong point so it would be great if someone can check.

(Addition: I've been checking that against probable Cd values from the cars shown in the paper, and it seems to largely make sense. I've asked a few of the experts I know to check my calculation, so we'll see how wrong I am!)

Last edited by JulianEdgar; 04-07-2021 at 10:03 PM..
  Reply With Quote
Old 04-08-2021, 04:23 PM   #14 (permalink)
Master EcoModder
 
JulianEdgar's Avatar
 
Join Date: Nov 2017
Location: Australia
Posts: 1,978
Thanks: 106
Thanked 1,545 Times in 1,097 Posts
One of my experts has got back to me.

He used a density of 0.002378 slugs/ft^3, which was the standard density in old imperial units. (Equivalent to 1.225 kg/m^3).

Using that density, his conversion factor is 391.0. That is, multiply the K factor from the Chrysler paper by 391 to get Cd in a modern context.
  Reply With Quote
Old 06-05-2021, 06:59 PM   #15 (permalink)
Cyborg ECU
 
California98Civic's Avatar
 
Join Date: Mar 2011
Location: Coastal Southern California
Posts: 6,150

Black and Green - '98 Honda Civic DX Coupe
Team Honda
90 day: 66.42 mpg (US)

Black and Red - '00 Nashbar Custom built eBike
90 day: 3671.43 mpg (US)
Thanks: 2,328
Thanked 2,062 Times in 1,403 Posts
YouTube just suggested this video to me. It is a historic film from the 1930s created by the Chrysler Corporation to promote the Airflow. Full of details about their attitude and process in designing the car, it is really interesting and really quaint.
__________________
See my car's mod & maintenance thread and my electric bicycle's thread for ongoing projects. I will rebuild Black and Green over decades as parts die, until it becomes a different car of roughly the same shape and color. My minimum fuel economy goal is 55 mpg while averaging posted speed limits. I generally top 60 mpg. See also my Honda manual transmission specs thread.



  Reply With Quote
The Following 2 Users Say Thank You to California98Civic For This Useful Post:
freebeard (06-05-2021), jakobnev (06-07-2021)
Old 06-05-2021, 08:17 PM   #16 (permalink)
Master EcoModder
 
freebeard's Avatar
 
Join Date: Aug 2012
Location: northwest of normal
Posts: 19,316
Thanks: 5,415
Thanked 6,365 Times in 5,106 Posts
What should one call that frame, a birdcage? I thought Nash was first with unit construction:

Quote:
For the 1941 and 1942 model years ....[t]he Nash Ambassador 600, built on a 112-inch (2,845 mm) wheelbase, became the first popular domestic automobile to be built using the single-welded "unibody" type of monocoque construction that Nash called "Unitized", rather than body-on-frame. From 1941 through 1948, Nash Ambassador models placed this unibody structure on top of a conventional frame,[15] thus creating a solid and sturdy automobile. It was also one of the first in the "low-priced" market segment with coil spring suspension in front and back, "giving it the best ride in its class."
The Chrysler Brothers were consummate engineers, but some of it never made it to market. I never saw Airflow on a used car lot in the 1950s, but I wish they'd been full of Star Cars:


www.hemmings.com: SIA Flashback - Mopar's Star Cars

It looks like the original article 404s, but here's something squirreled way in my albums:



Front-wheel drive 5-cylinder radial.
__________________
.


If I leave here tomorrow, would you still remember me?
Lynyrd Skynrd -- Free Bird
_________________
  Reply With Quote
The Following User Says Thank You to freebeard For This Useful Post:
California98Civic (06-06-2021)
Old 06-06-2021, 10:47 AM   #17 (permalink)
Cyborg ECU
 
California98Civic's Avatar
 
Join Date: Mar 2011
Location: Coastal Southern California
Posts: 6,150

Black and Green - '98 Honda Civic DX Coupe
Team Honda
90 day: 66.42 mpg (US)

Black and Red - '00 Nashbar Custom built eBike
90 day: 3671.43 mpg (US)
Thanks: 2,328
Thanked 2,062 Times in 1,403 Posts
That long decked rumbleseat coupe is a beauty. Here is another video of an Airflow on today's roads. Great panning shots of the dash. And the guy seems to daily drive this super rare coupe version. Must have bank!

__________________
See my car's mod & maintenance thread and my electric bicycle's thread for ongoing projects. I will rebuild Black and Green over decades as parts die, until it becomes a different car of roughly the same shape and color. My minimum fuel economy goal is 55 mpg while averaging posted speed limits. I generally top 60 mpg. See also my Honda manual transmission specs thread.



  Reply With Quote
Old 06-07-2021, 04:20 AM   #18 (permalink)
Master EcoModder
 
JulianEdgar's Avatar
 
Join Date: Nov 2017
Location: Australia
Posts: 1,978
Thanks: 106
Thanked 1,545 Times in 1,097 Posts
Quote:
Originally Posted by California98Civic View Post
YouTube just suggested this video to me. It is a historic film from the 1930s created by the Chrysler Corporation to promote the Airflow. Full of details about their attitude and process in designing the car, it is really interesting and really quaint.
There's another video (also on YouTube) that shows all their wind tunnel test models. Again it's interesting, in part because you quickly realise that they had little idea of what Paul Jaray had been doing in Germany about a decade earlier.
  Reply With Quote
Old 06-07-2021, 09:56 AM   #19 (permalink)
Long time lurker
 
AeroMcAeroFace's Avatar
 
Join Date: May 2019
Location: Uk
Posts: 191
Thanks: 96
Thanked 144 Times in 111 Posts
Click image for larger version

Name:	Capture.JPG
Views:	23
Size:	23.8 KB
ID:	30808

I think that is an optimistic flow attachment angle
  Reply With Quote
Old 06-07-2021, 10:29 AM   #20 (permalink)
Cyborg ECU
 
California98Civic's Avatar
 
Join Date: Mar 2011
Location: Coastal Southern California
Posts: 6,150

Black and Green - '98 Honda Civic DX Coupe
Team Honda
90 day: 66.42 mpg (US)

Black and Red - '00 Nashbar Custom built eBike
90 day: 3671.43 mpg (US)
Thanks: 2,328
Thanked 2,062 Times in 1,403 Posts
Quote:
Originally Posted by AeroMcAeroFace View Post
Attachment 30808

I think that is an optimistic flow attachment angle
Me too. I thought it was interesting that they sorta started from scratch in wind tunnel design and that they claimed to have worked with one of the Wright brothers (among other people). It show at once how serious they were and how little they yet knew.

__________________
See my car's mod & maintenance thread and my electric bicycle's thread for ongoing projects. I will rebuild Black and Green over decades as parts die, until it becomes a different car of roughly the same shape and color. My minimum fuel economy goal is 55 mpg while averaging posted speed limits. I generally top 60 mpg. See also my Honda manual transmission specs thread.



  Reply With Quote
Reply  Post New Thread


Thread Tools




Powered by vBulletin® Version 3.8.11
Copyright ©2000 - 2021, vBulletin Solutions Inc.
Content Relevant URLs by vBSEO 3.5.2
All content copyright EcoModder.com