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April 25, 2012, 14:29 
Finding Lift/Drag/Moment Coefficients on 3D model aircraft

#1 
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Eric
Join Date: Jan 2012
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This may be a simple question but it is tripping me up. I am solving for the coefficient of lift/drag/moment on a 3D model of a Boeing x48B and was curious which reference area and reference length to use? I understand the lift direction to be the direction perpendicular to the aircraft's top surface and the drag direction straight into the nose of the aircraft.
For the reference area, am I to use only the area of one wing or both? Since the X48B is a blended wing body, if I select the area of both wings then that will essentially be the entire aircraft body area. Also, do I only find the area of the bottom surface of the wing, or the entire surface area top and bottom? Is this reference area the same for drag as lift? As far as the reference length to be used for the moment coefficient where do I measure from? The X48B has swept wings which are blended into the body of the aircraft so this confuses me as well. I attached an image where I labelled 3 possible lengths. Which one should I use? (A,B, or C) I am using the following function: functions { forces { type forceCoeffs; functionObjectLibs ( "libforces.so" ); outputControl timeStep; outputInterval 1; patches ( plane ); pName p; UName U; rhoName rhoInf; log true; rhoInf 1; CofR ( 0 0 0 ); liftDir ( 0 1 0 ); dragDir ( 0 0 1 ); pitchAxis ( 1 0 0 ); magUInf 10; lRef 1; Aref 1; } } 

April 25, 2012, 18:02 

#2 
Senior Member

You can use whatever you want. For instance you could you your medium aerodynamic chord times your span (c in you figure) but it won't make any different… You can use half of it or you can use 1, the drag and the lift will still be the same, only your coefficients will change according to what you choose as a reference area…


April 25, 2012, 18:58 

#3 
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Eric
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So I could just use a reference area and length of 1 to make it simple and then when I solve for Lift it will really be Lift per unit area? So to get the total lift on the aircraft I would have to multiply by the surface area of the aircraft?
Also I want to be sure I am correct with my thinking here. Is the lift direction perpendicular to the wing surface or is it always the vertical axis regardless of the angle of attack? I am doing tests at multiple angles of attack so I want to be certain I have these directions sorted out. 

April 26, 2012, 02:54 

#4  
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Quote:
Drag force => D=0.5*rho*Sref*V^2*Cd. So, If you have Cd=0.2 with Sref=2 (m^2) or if you have Cd=0.4 with Sref=1 (m^2), then you will have the same results. For exemple : rho=1.225 (kg.m^3), V=10 (m.s^1), Cd=0.2, Sref=2 (m^2) ====> D=24.5 N rho=1.225 (kg.m^3), V=10 (m.s^1), Cd=0.4, Sref=1 (m^2) ====> D=24.5 N Quote:


April 26, 2012, 04:56 

#5 
Senior Member

Actually the lift is defined as the aerodynamic force acting perpendicular to the flow direction and the drag is the force that has the same direction as the flow. So, if you rotate your plane around the pitch axis the lift and drag directions won't change.


April 26, 2012, 05:21 

#6 
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I think we say the same thing.
We have two cases : If you change the components of the velocity (and don't rotate the geometry), the lift and drag direction must change. If you rotate the geometry, the lift and drag direction don't change. With his question, I think that he wants to change the angle of attack like the first case. 

April 26, 2012, 06:03 

#7  
Senior Member

Yes, I thought of that actually… You are right, we are saying the same thing, that's why I said:
Quote:


April 27, 2012, 10:40 

#8 
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Eric
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Thanks so much for your help guys! You all cleared up any doubts I had. Thanks again!


August 11, 2012, 05:33 
Re

#9 
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nauman hashmi
Join Date: Jun 2012
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sir can i have the 3d model of x48B i want to study the aerodynamic loads on it. if you can send me the 3d model at naumaneshanhashmi@gmail.com. i would extremely thankful to you.
regards 

July 20, 2013, 05:09 

#10  
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Quote:
I write the function like this, but the openfoam just output the flap's coefficient not the total wing. so what should I do? thank you! functions { forces { type forceCoeffs; functionObjectLibs ( "libforces.so" ); outputControl timeStep; outputInterval 1; patches ( wing_main wing_flap ); rhoName rhoInf; log true; rhoInf 1; CofR ( 0 0 0 ); liftDir (0 1 0 ); dragDir ( 1 0 0 ); pitchAxis ( 0 0 1 ); magUInf 10; lRef 1.0; Aref 1.0; } 

July 20, 2013, 17:01 

#11 
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reza sadeghi
Join Date: May 2013
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Hi,
I'm not sure. Instead of: Code:
wing_main wing_flap Code:
"wing.*" Reza 

July 27, 2013, 14:03 

#12 
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August 7, 2013, 16:22 

#13 
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reza sadeghi
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Hi wei,
Try this: Code:
"wing_*" or "wing*" the point is that where there are some walls with different names and the part of their names are same, we can refere to the part which is same and * symbole will include every words of the rest. 

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