Lift Doubt

vignesh_n · December 28, 2015, 00:19

Why do we substitute negative value for lift in Aero foil analysis? Where's positive value of angle of attack is substituted for drag?

fluid23 · December 28, 2015, 15:36

what? I don't follow you...

vignesh_n · December 28, 2015, 15:59

While setting up of solution monitors, why do we give drag coefficient positive whereas lift as negative?

fluid23 · December 28, 2015, 16:16

The picture is blurry so I can't really see what you are talking about, but it doesn't look to me like you are specifying lift and drag here. Unless I am mistaken it is asking for you to define the the X and Y components of the Drag and Lift unit vectors. As to why the X and Y components get flipped and require a sign change between drag and lift, is fairly straight forward. The answer lies in simple Euler rotation of Fx and Fy into L and D.

Hint: cl = cn*cos(AoA)-ca*sin(AoA) and cd = cn*sin(AoA)+ca*cos(AoA)

cn and ca are normal and axial force coefficients, aka Fx and Fy...

vignesh_n · December 29, 2015, 02:15

Thanks a lot MBdonCFD

December 28, 2015, 00:19	Lift Doubt	#1
vignesh_n New Member Vignesh Join Date: Mar 2014 Posts: 22 Rep Power: 12	Why do we substitute negative value for lift in Aero foil analysis? Where's positive value of angle of attack is substituted for drag?

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December 28, 2015, 15:36		#2
fluid23 Senior Member Matt Join Date: Aug 2014 Posts: 947 Rep Power: 17	what? I don't follow you...

December 28, 2015, 16:16		#4
fluid23 Senior Member Matt Join Date: Aug 2014 Posts: 947 Rep Power: 17	The picture is blurry so I can't really see what you are talking about, but it doesn't look to me like you are specifying lift and drag here. Unless I am mistaken it is asking for you to define the the X and Y components of the Drag and Lift unit vectors. As to why the X and Y components get flipped and require a sign change between drag and lift, is fairly straight forward. The answer lies in simple Euler rotation of Fx and Fy into L and D. Hint: cl = cncos(AoA)-casin(AoA) and cd = cnsin(AoA)+cacos(AoA) cn and ca are normal and axial force coefficients, aka Fx and Fy...

December 29, 2015, 02:15		#5
vignesh_n New Member Vignesh Join Date: Mar 2014 Posts: 22 Rep Power: 12	Thanks a lot MBdonCFD