08-17-2015, 06:44 PM
|
#1 (permalink)
|
|
Master EcoModder
Join Date: Jan 2008
Location: Sanger,Texas,U.S.A.
Posts: 16,335
Thanks: 24,453
Thanked 7,394 Times in 4,789 Posts
|
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
__________________
Photobucket album: http://s1271.photobucket.com/albums/jj622/aerohead2/
Last edited by aerohead; 08-17-2015 at 06:46 PM..
Reason: PS
|
|
|
|
|
The Following 14 Users Say Thank You to aerohead For This Useful Post:
|
BamZipPow (08-18-2015), California98Civic (08-18-2015), darcane (08-17-2015), Ecky (08-19-2015), elhigh (08-18-2015), JRMichler (08-18-2015), oldtamiyaphile (08-18-2015), P-hack (08-20-2015), RedDevil (08-20-2015), sendler (08-19-2015), The donkey CRX (08-18-2015), UFO (08-18-2015), user removed (08-18-2015), wdb (08-18-2015) |
 Today
|
|
|
|
Other popular topics in this forum...
|
|
|
|
|
08-17-2015, 07:39 PM
|
#2 (permalink)
|
|
.........................
Join Date: Aug 2009
Location: Buckley, WA
Posts: 1,597
Thanks: 391
Thanked 488 Times in 316 Posts
|
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.
|
|
|
|
|
The Following User Says Thank You to darcane For This Useful Post:
|
|
|
08-17-2015, 08:36 PM
|
#3 (permalink)
|
|
EcoModding Lurker
Join Date: Nov 2014
Location: Austin TX
Posts: 72
Thanks: 19
Thanked 16 Times in 12 Posts
|
Change that circle to a square and see how much smaller it'd have to be.
Either way, good lesson.
|
|
|
|
|
08-18-2015, 12:08 AM
|
#4 (permalink)
|
|
Master EcoModder
Join Date: Sep 2012
Location: Motor City
Posts: 281
Thanks: 0
Thanked 223 Times in 138 Posts
|
Quote:
Originally Posted by darcane
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. |
|
|
|
|
08-18-2015, 09:17 AM
|
#5 (permalink)
|
|
Master EcoModder
Join Date: Sep 2009
Posts: 5,927
Thanks: 877
Thanked 2,024 Times in 1,304 Posts
|
Now I understand, thanks aerohead. CD and frontal area much more dependent on cd, especially when it gets low enough.
regards
mech
|
|
|
|
|
08-18-2015, 12:52 PM
|
#6 (permalink)
|
|
Cyborg ECU
Join Date: Mar 2011
Location: Coastal Southern California
Posts: 6,299
Thanks: 2,373
Thanked 2,174 Times in 1,470 Posts
|
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?
__________________
See my car's mod & maintenance thread and my electric bicycle's thread for ongoing projects. I will rebuild Black and Green over decades as parts die, until it becomes a different car of roughly the same shape and color. My minimum fuel economy goal is 55 mpg while averaging posted speed limits. I generally top 60 mpg. See also my Honda manual transmission specs thread.
|
|
|
|
|
08-18-2015, 01:15 PM
|
#7 (permalink)
|
|
.........................
Join Date: Aug 2009
Location: Buckley, WA
Posts: 1,597
Thanks: 391
Thanked 488 Times in 316 Posts
|
Quote:
Originally Posted by California98Civic
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. |
|
|
|
|
The Following 2 Users Say Thank You to darcane For This Useful Post:
|
|
|
08-18-2015, 01:24 PM
|
#8 (permalink)
|
|
Master EcoModder
Join Date: Apr 2011
Location: Tacoma WA
Posts: 1,399
Thanks: 743
Thanked 528 Times in 344 Posts
|
Quote:
Originally Posted by ennored
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 
__________________
2007 Dodge Ram 3500 SRW 4x4 with 6MT
2003 TDI Beetle
2002 TDI Beetle
currently parked - 1996 Dodge 2500 Cummins Turbodiesel
Custom cab, auto, 3.55 gears
|
|
|
|
|
08-18-2015, 02:04 PM
|
#9 (permalink)
|
|
Master Novice
Join Date: Jan 2008
Location: SE USA - East Tennessee
Posts: 2,314
Thanks: 427
Thanked 616 Times in 450 Posts
|
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.
__________________
Lead or follow. Either is fine.
|
|
|
|
|
The Following User Says Thank You to elhigh For This Useful Post:
|
|
|
08-18-2015, 03:22 PM
|
#10 (permalink)
|
|
Master EcoModder
Join Date: Sep 2012
Location: Motor City
Posts: 281
Thanks: 0
Thanked 223 Times in 138 Posts
|
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?
|
|
|
|
|
