Why is Projected Area constant when varying AoA?

[MD LAT 1492]

For a streamlined and bluff bodies, why is it standard to have the projected area be a fixed reference area, but yet the angles of attack (AoA) vary? If one were to vary the AoA then the projected area would technically change.

The following link discusses that it is a convention to avoid confusion. https://www.researchgate.net/post/What_should_be_the_reference_area_for_calculating_drag_and_lift_coefficients_in_case_of_elliptic_cylinder

My attempt included drawing it up and then projecting it with an arbitrary AoA.
But the projected areas never reduced down to the convention. I appreciate you talking the time to to provide any guidance! Thank you

 

[Benjies]

Hi @MD LAT 1492 , from reading the URL you shared, it appears that the confusion comes from this point: While the drag and coefficient of drag (Cd) of the body will vary with AoA, the Reference Area will not. The responses seem to indicate that you can either vary the projected area, or the drag coefficient, as you change your AoA.

Quoting Aljoscha Sander from the forum. you shared:

"However, If you want to compare angles of attack (for instance in a polar diagram with drag and lift coefficients) length scale and projected area should stay constant at all times."

On the other side of the same coin, for a given body, you could cite the change in projected area and length scale at each drag coefficient. This, would be incredibly confusing, and I believe that's what Sarat C. Praharaj was getting at when he mentioned:

"...any reference area is good so long as you are consistent. Don't vary that with angle of attack, because you will confuse yourself and others by doing that. Just use the major axis for the length scale."

I believe that hey are choosing to use a fixed area and length as reference, to find Cd/Cl at varying AoAs, by convention.

Cheers!

 

[NatSusan]

Hello there! I understand your confusion about the standard for using fixed reference area in streamlined and bluff bodies. Let me try to explain it to you.

First of all, the projected area is a measure of the cross-sectional area of an object when viewed from a certain direction. It is an important factor in calculating drag and lift coefficients, which are essential in determining the aerodynamic characteristics of an object.

Now, the reason why it is standard to have a fixed reference area is to provide a consistent and universal measurement for comparison. This is important in the field of aerodynamics where different objects and shapes are being studied and analyzed. By using a fixed reference area, it eliminates any confusion or discrepancies in the results.

On the other hand, the angle of attack (AoA) is the angle between the object's longitudinal axis and the relative wind direction. This angle can vary depending on the design and purpose of the object. By varying the AoA, it allows for a more comprehensive analysis of the object's aerodynamic performance.

However, as you have mentioned, varying the AoA can also affect the projected area. This is why it is important to use a fixed reference area to maintain consistency and accuracy in the calculations.

In the link you provided, it discusses the convention of using an elliptic cylinder as a reference area for calculating drag and lift coefficients. This is just one of the many conventions used in the field of aerodynamics. Different objects may have different reference areas, but the key is to use a fixed and agreed upon reference area to avoid confusion and ensure accurate results.

I hope this helps clarify the standard for using fixed reference area in streamlined and bluff bodies. Keep on learning and exploring the fascinating world of aerodynamics!