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

In summary, In order to calculate the drag force on an airfoil, you need to know the drag coefficient (Cd), the airspeed (V), and the angle of attack (AoA).
  • #1
kjr
7
0
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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  • #2
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.
 
  • #3
Great, thanks! So once i do that i just put 2) into 1) and that should give me the drag force, right?
 
  • #4
Yes. This is a simple, but powerfull, calculation.
 
  • #5
Good stuff. Thank you!
 
  • #6
what if you don't have a graph?
 
  • #7
F1Guille said:
what if you don't have a graph?

For most common airfoils, the graphs of this are easy to find.
 
  • #8
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?
 
  • #9
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.
 

1. What is drag and why is it important in aerodynamics?

Drag is a force that opposes the motion of an object through a fluid, such as air. In aerodynamics, drag is important because it affects the performance and efficiency of aircraft and other vehicles.

2. What factors affect drag and the coefficient of drag (Cd)?

The factors that affect drag and the coefficient of drag (Cd) include the shape and size of the object, the speed of the object, and the density and viscosity of the fluid through which the object is moving. The surface roughness and angle of attack of the object also play a role in determining the drag and Cd.

3. How is the coefficient of drag (Cd) calculated?

The coefficient of drag (Cd) is calculated by dividing the drag force acting on an object by the product of the dynamic pressure of the fluid, the reference area of the object, and the density of the fluid. This formula is represented as Cd = Drag Force / (Dynamic Pressure x Reference Area x Fluid Density).

4. What is the difference between drag and lift?

Drag and lift are both aerodynamic forces that act on an object moving through a fluid. While drag is a force that acts in the direction of motion and opposes the movement of the object, lift is a force that acts perpendicular to the direction of motion and is responsible for keeping the object aloft.

5. How can the coefficient of drag (Cd) be reduced?

The coefficient of drag (Cd) can be reduced by designing the object to have a streamlined shape, which minimizes the area of contact with the fluid and reduces drag. Additionally, reducing the surface roughness and optimizing the angle of attack can also help reduce Cd. The use of aerodynamic features such as airfoils and winglets can also help in reducing Cd.

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