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Old 08-18-2015, 04:19 PM   #11 (permalink)
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Quote:
Originally Posted by ennored View Post
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?
It must?

Aerohead would have to give any exact numbers from his source... But Cd of an infinitely long cylinder moving at low speeds through air is typically given as 1.2. 1/35th of that is .034. That's in the right realm for a streamlined body.

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Old 08-18-2015, 04:39 PM   #12 (permalink)
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Compare this to the old video Darcane posted in this link:
http://ecomodder.com/forum/showthrea...rag-25378.html

From that, at 5:49:
Quote:
A properly streamlined strut can be 10 times thicker than a wire and yet have no more drag.
Ten times. That is quite a bit less than the NACA wing shape, but the wing shape used there did not have the concave tail section.
Would that alone make for such a big difference?
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Old 08-19-2015, 03:58 PM   #13 (permalink)
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cables

Quote:
Originally Posted by elhigh View Post
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.
Here's a selection of shapes for fairing circular sections
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Old 08-19-2015, 04:06 PM   #14 (permalink)
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square

Quote:
Originally Posted by veloman View Post
Change that circle to a square and see how much smaller it'd have to be.

Either way, good lesson.
According to Hoerner,a square section of infinite length,crosswise in the flow would have Cd 0.20.
Korff has data on circular sections,I'll have to get that from home.
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Old 08-19-2015, 04:09 PM   #15 (permalink)
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wing shape

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Originally Posted by RedDevil View Post
Compare this to the old video Darcane posted in this link:
http://ecomodder.com/forum/showthrea...rag-25378.html

From that, at 5:49:

Ten times. That is quite a bit less than the NACA wing shape, but the wing shape used there did not have the concave tail section.
Would that alone make for such a big difference?
Hoerner chose a laminar wing section for the comparison.It's entire boundary layer is laminar under flight conditions,leading to very low surface friction drag.
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Old 08-19-2015, 04:14 PM   #16 (permalink)
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clarify

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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?
These shapes are in 2-dimensional flow and their drag is based upon wetted area.
We'd use the data for wings,struts,tails,rudder,landing gear,pylons,bracing wires,some forms of auxiliary tanks.
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Old 08-19-2015, 04:26 PM   #17 (permalink)
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numbers

Quote:
Originally Posted by ennored View Post
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?
When the chord length of the airfoil section equals 167X the diameter of the wire,the drags are identical.
Bear in mind that we might be considering a length of crude circular wing support on an ultralight aircraft to a length of wing on a Lear Jet.
It's 2-dimensional flow,so not real helpful for automotive applications,however it does illustrate potentialities with respect to streamlining.
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Old 08-19-2015, 04:31 PM   #18 (permalink)
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Quote:
Originally Posted by darcane View Post
It must?

Aerohead would have to give any exact numbers from his source... But Cd of an infinitely long cylinder moving at low speeds through air is typically given as 1.2. 1/35th of that is .034. That's in the right realm for a streamlined body.
Perhaps Abbott and von Doenhoff lists the drag table for this particular airfoil section.
Walter Korff offered a drag value for structural sections as a function of a hundred feet of length at 100-mph.We can probably reverse-engineer some values from that.
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Old 08-20-2015, 03:09 AM   #19 (permalink)
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Quote:
Originally Posted by aerohead View Post
It's 2-dimensional flow,so not real helpful for automotive applications,however it does illustrate potentialities with respect to streamlining.
I already have a wing shaped sleeve over my car's antenna stalk, but it is a bit out of shape.
I'll use this as a template to craft a new one from hard foam and thick alu foil.
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Old 08-20-2015, 03:36 AM   #20 (permalink)
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Maybe some fairings for suspension members (at least what can be covered by a belly pan?

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