Understanding Drag and Coefficient of Drag (Cd) in Aero Engineering

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Discussion Overview

The discussion revolves around understanding drag and the coefficient of drag (Cd) in the context of aero engineering. Participants explore the formulas related to drag, the interpretation of graphs, and the challenges of working with non-standard airfoil designs.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Homework-related

Main Points Raised

  • One participant expresses confusion about the formulas for drag and seeks clarification on the coefficient of drag at zero-lift (Cd,0).
  • Another participant confirms that it is acceptable to read Cd from a graph of coefficient of drag vs angle of attack, but emphasizes the importance of ensuring the graph applies to the specific shape and Reynolds number.
  • A participant suggests that using the second formula in the first can yield the drag force, indicating a straightforward calculation process.
  • Concerns are raised about the lack of a graph for a custom airfoil design, with one participant noting that graphs for common airfoils are readily available.
  • Another participant suggests redesigning the airfoil to match commonly studied sharp leading-edge airfoils if no data is available.
  • Participants discuss potential resources for finding aerodynamic data, including specific books and online tools like "Javafoil" and "XFLR".

Areas of Agreement / Disagreement

Participants generally agree on the methods for determining drag coefficients and the importance of using appropriate graphs, but there is disagreement regarding the availability of data for non-standard airfoil designs. The discussion remains unresolved on how to proceed without existing graphs for custom designs.

Contextual Notes

Limitations include the dependence on specific airfoil shapes and the availability of data for non-standard designs. There is also uncertainty regarding the applicability of general resources to unique airfoil configurations.

kjr
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Hi everyone,

I'm completely new to the aero engineering and have a question regarding drag. The lecturer explained it briefly, threw some formulae at us and now I'm confused trying to make sense of it all.

So, i have these two formulae relating to drag:

1) D = 0.5(rho)(V^2)(A)(Cd) and

2) Cd = Cd,0 + (Cl^2)/(pi)(e)(AR)

From what I've gathered around the internet, the second part in the 2) formula related to induced drag, and the first part is the drag coefficient at zero-lift. What i don't understand is where do i get this Cd,0 from?

I have a graph of coefficient of drag vs angle of attack, do i just read it off the graph?

I'm trying to get the graph of airspeed and drag, so i can find the velocity at minimum drag.

Thank you
 
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kjr said:
I have a graph of coefficient of drag vs angle of attack, do i just read it off the graph?
You can do this. Just make sure the graph applies to your shape and the Reynold's Number of the flow.
 
Great, thanks! So once i do that i just put 2) into 1) and that should give me the drag force, right?
 
Yes. This is a simple, but powerfull, calculation.
 
Good stuff. Thank you!
 
what if you don't have a graph?
 
F1Guille said:
what if you don't have a graph?

For most common airfoils, the graphs of this are easy to find.
 
Yeah, but my airfoil isn't any airfoil hich has been designed before because it isn't rounded in the leading edge, but straight for simplicity for construction. So, if there is no graph?
 
F1Guille said:
Yeah, but my airfoil isn't any airfoil hich has been designed before because it isn't rounded in the leading edge, but straight for simplicity for construction. So, if there is no graph?

Depending on the shape, you may be able to find that as well. That is semi-common in supersonic airfoils. At any rate, my suggestions is to redesign you airfoil so that it matches one of the commonly studied sharp-LE airfoils.
 
  • #10
Okay, then do you know any website where there is data (Cl and Cd at different AoA's) about the airfoil??
 
  • #11
Well see if there is an airfoil you like in the appendices of "Theory of Wing Sections" by Abbott and von Doenhoff or search the internet for wings you think are acceptable.
 
  • #12
F1Guille said:
Okay, then do you know any website where there is data (Cl and Cd at different AoA's) about the airfoil??

Search for an online tool called "Javafoil". Create your custom airfoil then search for a program called "XFLR" All freeware, these tools will get you the aerodynamic data you need.
 

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