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

[kjr]

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

 

[edgepflow]

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.

 

[kjr]

Great, thanks! So once i do that i just put 2) into 1) and that should give me the drag force, right?

 

[edgepflow]

Yes. This is a simple, but powerfull, calculation.

 

[kjr]

Good stuff. Thank you!

 

[F1Guille]

what if you don't have a graph?

 

[boneh3ad]

what if you don't have a graph?

For most common airfoils, the graphs of this are easy to find.

 

[F1Guille]

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?

 

[boneh3ad]

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.

 

[F1Guille]

Okay, then do you know any website where there is data (Cl and Cd at different AoA's) about the airfoil??

 

[boneh3ad]

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.

 

[viscousflow]

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.