Talk:Drag coefficient/Archive 1

This is an archive of past discussions about Drag coefficient. Do not edit the contents of this page. If you wish to start a new discussion or revive an old one, please do so on the current talk page.

Archive 1

nomenclature

It would be nice to describe what each variable represents and how it is determined, especially the area "A". —Preceding unsigned comment added by 129.107.97.131 (talk • contribs)

That is explained on the main drag equation page. I don't think we should duplicated it all here, so I have added another link right after the formula. Meggar 01:17, 27 November 2006 (UTC)

Cd vs. Cdmin

It'd be a good idea on the aircraft section to make sure you differentiate between Cd and Cdmin, since the Cd min values given in the reference page are valid only for certain types of flight... besides all that, there's also the question of whether or not it's set up for Cdi or Cdo...

(edit began here, previous segment was unsigned) Cd really needs to be entirely separated in this article. It is very inconsistent in that it is combining Cd and Cd(induced) far too often. It might also be beneficial to state that the Cd itself has no meaning outside of Cd*ref area. That value is the 'equivalent flat plate area' and Cd is initially derived from Flat plate area/reference area, not magically created or tested for.

Mentioning the derivation of Cdo, which is done either using 'wetted area'*'aerodynamic cleanliness' or by individual parts being summed up, and the aerodynamic interference being introduced.

Cdi, induced Cd through lift, is ENTIRELY different, and needs to be clarified as such. This is why a vehicle like an indy car, which has an extremely low aerodynamic cleanliness value, has such a high Cd(a very high negative lift keeps the car on the ground). Lift induces drag via the equation: Cd=Cdo+(Cl^2/pi*AR*e). e is the wing (lifting surface) effiency factor, and AR is the Aspect ratio of the plane, Span^2/wing area. 164.107.199.93 05:54, 24 January 2007 (UTC)Jason Mead

Cars don't have the same induced drag as air vehicles because they are much closer to the ground. The ground effect doesn't allow as much of a vortex to occur, and the induced drag would be lower than expected.

Found comment

I just found this comment in the article —Ben FrantzDale 16:38, 26 April 2007 (UTC):

In the section on drag coefficients, the entries for "hollow semi-sphere opposite stream" and "hollow semi-sphere facing stream" may need to be swapped. In the table entries, "hollow semi-sphere opposite stream" = 1.42 and "hollow semi-sphere facing stream" = 0.38. This is inconsistent with the next two table entries for "hollow semi-cylinder opposite stream" = 1.20 and "hollow semi-cylinder facing stream" = 2.30. So, either the hollow sphere or the hollow cylinder entries need to be swapped.

Peter d. Vangel vangel@stcc.edu

Why is drag equation and coefficient of drag on different pages???

These 2 articles should be merged; I see no reason why the 2 pages are separate. The two topics are about the same thing, except it makes it more inconvenient by having another link to the drag equation page. "drag equation" should be redirected to the "drag coefficient" page Subheight640 23:08, 24 June 2007 (UTC)

They really aren't the same subject, in the same way that Ohm's law,Electrical resistance, and Resistor are different articles. There was some talk about that on the Drag coefficient page. If after some carefull concideration you still believe it should be merged, then some merge suggestion tags will be needed. Meggar 04:54, 25 June 2007 (UTC)

Automobile drag coefficients

I moved a substantial portion of the material that was in this article to a new one entitled Automobile drag coefficients. Rracecarr 18:42, 31 August 2007 (UTC)

The automobile-related talk is also now moved to the new page. Meggar 19:36, 31 August 2007 (UTC)

Accuracy

I think that the article should be clearer that the equation given does not work in all scenarios. The larger the velocity the more the drag becomes linear, and thus deviates from the quadratic dependence of the given equation. —Preceding unsigned comment added by Moto Perpetuo (talk • contribs) 05:41, 17 January 2008 (UTC)

Yes, it is often overlooked that Cd is a variable, not a constant. I've tried to clarify this, but a graph showing Cd as a function of Re would be nice.--Theosch (talk) 16:46, 26 November 2008 (UTC)

Question

Very nice, very well explained, but, how the hell does someone determine the drag coefficient? Where it comes from?

—Preceding unsigned comment added by 200.144.145.32 (talk) 12:58, 13 March 2006

The nutshell version is that somebody probably saw that certian shaped objects move at different speeds and wanted to figure out why. —Preceding unsigned comment added by Aringarosa (talk • contribs) 16:06, 8 April 2006

Actual drag coefficients are determined through wind tunnel testing, although computer simulation can come very close. — orion eight (talk) 16:50, 27 April 2006 (UTC)

What is the difference between drag coefficient and friction factor in terms of functionality? —Preceding unsigned comment added by Sqdonghao (talk • contribs) 08:45, 16 March 2009

Wikipedia identifies three different applications of friction factor — all three refer to flow inside pipes or ducts. This is sometimes known as internal flow. Drag coefficient is used with the flow of a fluid around a solid object. This is sometimes known as external flow. The mathematical definitions of the two concepts are also different, although both friction factor and drag coefficient are dimensionless. Dolphin51 (talk) 09:50, 16 March 2009 (UTC)

Well, coefficient usually describes a ratio between something and something. Drag coefficient should describe ratio between a body and a _reference shape_ (usually cylinder) of the same frontal area, not just "area". This isnt clarified in the article. 217.174.97.107 (talk) 11:36, 17 March 2009 (UTC)

Capital or Lower-case 'D'?

I've always seen C_D rather than C_d. Why does this article not match this one? --Anthony5429 (talk) 19:28, 24 January 2009 (UTC)

I am an aerospace engineer. The lower case "d" is only used for a 2-dimensional drag coefficient such as of a wing cross-section(airfoil). A capital "D" is used for a 3-dimensional drag coefficient for a whole wing or a vehicle. --Integracer (talk) 19:42, 15 May 2009 (UTC)

more shapes

I like the "measured drag coefficients" diagram.

Please add a couple more shapes to the diagram:

"sideways" short cylinder
"end-on" short cylinder (looking at it from the back, wind in my face, looks like a circle).
"sideways" long cylinder (looking at it from the side looks like a circle; the orientation to the wind of the bracing wires and struts between the wings of a biplane)

I'm pretty sure you already have *one* of the first 2 shapes, but which?

Thank you. --68.0.124.33 (talk) 15:06, 30 July 2009 (UTC)

Add a "how to calculate the drag coefficient" in the page

I think that adding how to calculate the drag coefficient would be important to not only this page but to other page like drag force and ballistic coefficient pages. Here is an example of the NSL equation for the x direction of how the calculation would be done.

F_d=Force of drag (N)
M=mass (kg)
a_x=acceleration in the x-axis $(m/s^{2})$
$\rho$ =density of air $(kg/m^{3})$
v=velocity (units aren't important because you will be making a ratio of final vs initial velocities)
C=coefficient of drag (unitless)
A=cross sectional area $(m^{2})$
$\sum F_{x}=Ma_{x}$
$-F_{d}=Ma_{x};F_{d}=.5\rho \ v^{2}CA$
$-.5\rho \ v^{2}CA=Ma_{x}$
${\frac {-.5\rho \ v^{2}CA}{M}}=a_{x};a_{x}=v{\frac {dv}{dx}}$
${\frac {-.5\rho \ v^{2}CA}{M}}=v{\frac {dv}{dx}}$
$dx={\frac {-2M}{\rho \ vCA}}dv$
$\textstyle \int \limits _{x_{1}}^{x_{2}}\,dx={\frac {-2M}{\rho \ CA}}\textstyle \int \limits _{v_{1}}^{v_{2}}{\frac {1}{v}}\,dv$
$\Delta x={\frac {-2Mln({\frac {v_{2}}{v_{1}}})}{\rho \ CA}}$
$C={\frac {-2Mln({\frac {v_{2}}{v_{1}}})}{\rho \ \Delta xA}}$
Gulielmi2002 (talk) 15:30, 11 December 2009 (UTC)

Please check

In the table near the end of the artcle, please check the values for spheres. I'm not certain, but I think they're off by a factor of greater than 10. ike9898 (talk) 19:58, 9 February 2010 (UTC)

"associated particular surface area"

In the intro paragraph the article states: "The drag coefficient is always associated with a particular surface area". But then the article goes on to list lots and lots of drag coefficient values for various shapes - and never mentions an associated area. How is one to interpret this? Xmrs (talk) 21:55, 9 February 2010 (UTC)

Notice that the coefficients for rough sphere and smooth sphere are exchanged. It should be 0,1 for rough sphere and 0,4 for smooth sphere. —Preceding unsigned comment added by 85.243.31.73 (talk) 14:46, 11 February 2010 (UTC)

Notice that the coefficients for rough sphere and smooth sphere are exchanged. It should be 0,1 for rough sphere and 0,4 for smooth sphere.

Car Advert

I seriously doubt the Merc E-Class counts in good science.

I would also expect it is not the lowest Cd of all time.

A table perhaps? —Preceding unsigned comment added by 81.174.174.31 (talk) 09:11, 4 March 2010 (UTC)

Abelcarvalho (talk) 14:50, 11 February 2010 (UTC)Abelcarvalho

To Do

will someone please check if the following numerical info is available in the article: [1] , [2] —Preceding unsigned comment added by Saeed.Veradi (talk • contribs) 05:20, 5 June 2010 (UTC)

It might be helpful to condense or clarify the table data on flat plates. I feel that it is misleading, since I am finding many different values for drag coefficient when considering the length-width ratio. Some of the values I've found have been 1.15-1.17 for L/d=1 [3], to 1.28 [4], and 1.98 for infinitely long flat plates.

Cd for a cone at different sharpnesses

The table at the top of the article gives cone's a Cd of 0.50, without specifying the sharpness. That's misleading (it mislead me).

According to: http://www.aerospaceweb.org/question/aerodynamics/q0231.shtml a nose cone's Cd can be approximated (sub-mach speeds) by Cd = 0.0112 * (half-vertex angle) + 0.162, thus can span a considerable range. A cone with a 10 degree angle would have a Cd = 0.274, very different from 0.50.

The current table should be amended to specify that the cone pictured is for a half-vertex angle of 30 degrees, and the above approximation should be added into the article somewhere. —Preceding unsigned comment added by 122.57.68.104 (talk) 01:21, 9 March 2011 (UTC)

Why use bold if it's a scaler

The drag coefficient C_d is a scalar, or as the article puts it "a dimensionless quantity". If that is the case, then why is bold face used, i.e. why is it denoted by C_d in the first line? Bold face is reserved for vectors. — Fly by Night (talk) 13:23, 8 May 2011 (UTC)

i removed Cd=16720/Rel^1/2 this is nonsense ,this relation is incorrect form of Cd=1.382/Rel^(1/2) which is only valid for laminar flow past a plate — Preceding unsigned comment added by 93.172.111.55 (talk) 15:42, 31 May 2011 (UTC)

Cw

I just added that Cw is another way to write drag coefficient, based on http://www.vectorbd.com/peugeot/archive/General/2003/Mar/0063.html, the only source I could find. Correct me if I'm wrong. DirkvdM 07:48, 13 September 2006 (UTC)

Cw is just German, W is for "Widerstand" = resistance. --Theosch (talk) 16:43, 26 November 2008 (UTC)

Several decades ago, my aerodynamics professor explained the existence of both Cd and Cw as follows: Cd and Cw are two different types of aero drag coefficients which are not freely interchangeable. Cd is used in the case of long slender bodies or streamlined bodies, where the boundary layer of the flow does not separate from the object. The reference area A is the total "wet" area. Cw is used for short, relatively blunt objects, where there is a considerable amount of flow separation, such as cars, golf balls etc. In this case, it makes more sense to use the frontal area as the reference area A. So Cd refers to wetted area and Cw refers to frontal area.Koyovis (talk) 10:14, 6 April 2013 (UTC)