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: 12,316
Thanks: 19,799
Thanked 6,190 Times in 3,818 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: 1,704
Thanks: 95
Thanked 1,376 Times in 961 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: 1,704
Thanks: 95
Thanked 1,376 Times in 961 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: 1,704
Thanks: 95
Thanked 1,376 Times in 961 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
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