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Old 08-17-2015, 05:44 PM   #1 (permalink)
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Dramatic drag comparison

The following is an anatomical drag comparison between a circular cylinder section and a NACA laminar wing section.
The two bodies are shown in true size relationship to one another.
The laminar wing section is 167-X longer than the cylinder.
They have identical drag.

(we don't need no stinkin' streamlining)
PS, the table is from 'Boundary Layer Theory,' by Hermann Schlicting,7th-Edition

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Last edited by aerohead; 08-17-2015 at 05:46 PM.. Reason: PS
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Old 08-17-2015, 06:39 PM   #2 (permalink)
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Last two digits for NACA airfoil designations indicate maximum thickness as a percentage of chord.

21% of 167 is 35.1.

So this NACA airfoil has the same drag as the cylinder in spite of having a 35 times larger frontal area.
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Old 08-17-2015, 07:36 PM   #3 (permalink)
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Change that circle to a square and see how much smaller it'd have to be.

Either way, good lesson.
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Old 08-17-2015, 11:08 PM   #4 (permalink)
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Quote:
Originally Posted by darcane View Post
So this NACA airfoil has the same drag as the cylinder in spite of having a 35 times larger frontal area.
Just need to quote this next time the "Can I increase frontal area AND reduce drag?" question comes up.
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Old 08-18-2015, 08:17 AM   #5 (permalink)
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Now I understand, thanks aerohead. CD and frontal area much more dependent on cd, especially when it gets low enough.

regards
mech
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Old 08-18-2015, 11:52 AM   #6 (permalink)
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Clarify for me, by "drag" do we mean CdA and not just Cd? The airfoil has identical CdA? Seems increadible. That's not the case right? This is just identical Cd, yes?
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Old 08-18-2015, 12:15 PM   #7 (permalink)
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Quote:
Originally Posted by California98Civic View Post
Clarify for me, by "drag" do we mean CdA and not just Cd? The airfoil has identical CdA? Seems increadible. That's not the case right? This is just identical Cd, yes?
Drag is the force pushing back on the body as it moves through a fluid. it is directly proportional to CdA as long as fluid density and velocity are held constant.
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Old 08-18-2015, 12:24 PM   #8 (permalink)
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Quote:
Originally Posted by ennored View Post
Just need to quote this next time the "Can I increase frontal area AND reduce drag?" question comes up.
Exactly! My travel trailer build will have a much lower CDa because of this. More can be less, much less
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Old 08-18-2015, 01:04 PM   #9 (permalink)
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That's stunning.

I remember seeing add-on fairings to make the cables on one's ultralight aircraft less of a drag, but they didn't significantly increase the frontal area, they weren't much more than stiff tapes to add to the cable.

This shows that one could replace the cables with, for instance, spars and still have no net gain while picking up lots of strength.
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Old 08-18-2015, 02:22 PM   #10 (permalink)
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A little reality check on the math though....

35 times the frontal area? That means it has 1/35 the drag coefficient, right?

1/35 = .029

IF the coefficient for the round shape was 1 (it must be less, but follow along), the coefficient for the airfoil is .029? Seems too low, even for a section of a chord? Someone has some numbers, right?

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