# Lift force given by simulation not correspont to theory value

Hi everyone,
I have realised a simulation where the lift coefficient is equal to 0.466, pressure force y equal to 577.48kN and viscous force y equal to -99.98N. Y axe is the lift axe.
If I use the equation from theory(1), I get a lift force of : 1.90MN.

L=Cl·S·V^2*1/2·rho; rho=0.365kg/m^3, S=527.21m^2, Cl=0.466 and V=206 (1)

How can I obtain the theory value of simulation in order to validate?

Hey there,

V = 206 m/s and A = 527 m^2 looks like really big speed (supersonic) and area. Remember that this area should be the perpendicular-to-flow-direction projected transversal area.

Can you please provide more details? Such as project link, model description, etc.

The initial conditions of simulation are h=36000ft (11000m) and M=0.7. Respect to area, the model CAD is a blended wing body aircraft. I’ve noticed that I configurated the “result control” with the frontal area, so the plot must be showing the correct values for drag, in my case x-axis.
The following link is my project:
https://www.simscale.com/workbench/?pid=2064516406883889884&mi=spec%3A394a15f7-29b2-47b7-a16a-725e1742ab21%2Cservice%3ASIMULATION%2Cstrategy%3A151

In this particular case: Compressible Central>Simulation Runs>BWB 5º Central Ruder 206m/s

The area parameter you mention above does not match what you specified for the coefficient computation. Please review your calculations!

Sorry, the surface I was referring to was the wet area. In the simulation, the surface is the front surface, that is 58.83.
As I can see from the article, (How To Analyze the Pitch, Lift, and Drag Coefficients | SimScale), the frontal area is often used as a reference area to calculate the coefficient of drag. , then if I compare the theoretical drag force with the simulated drag:

D=Cd·S·V^2·rho·0.5=0,03728·58,83·206^2·0,365·0,5=16,985kN
D_simulation=F_viscous+F_pressure=40430+6104=46,535kN

I would like to understand what I do wrong.

In the other hand, I would like to compute the force moment produce for the deflection rudder. Could I take the value given by plots “Force and Moments”?

By the way, the speed equal to 206m/s at 36000ft, it is not a super speed, since the sound speed at this altitude is 295 m/s, so, how I have explained, the simulation’s mach is equal to 0.7, under sound speed.

Hi,

Just a comment: the reference area depends on the application. The rule of thumb is:

• For applications where drag is the parameter of interest, then the area of reference will be the frontal area (e.g. for cars).
• If lift is the parameter of interest, then the reference area is based on a top view (wings go here).

Note that the same area of interest (top projection) is used for both lift and drag, in case of wings.

Cheers

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Thanks for your comment @RicardoParis, my simulations had not sense, since my aircraft, with different angles of attack, had different areas, and that’s meant the wings has a different area in each angle of attack (different frontal area), despite of the aircraft is the same.

Now, I have changed the reference area for the top projection, and my last doubt is the force moment given by “Force and Moments”, what is the point where it is this moment? I understand the reference point is the mass center, but the force moment will be different at 1 meter of center than a 15 meter, no?

Hi @acatalbache,

Well, the base point of the momentum could be any, but you can define the one which would be the most representative for your case. Just to give you an example, maybe you are interested in the moment that lift/drag forces on a wing generate where it connects to the fuselage of the aircraft. It’s not mandatory to set it on the center of mass

In any case, if you are interested in the center of mass, it should be possible to obtain it in some CAD tools, by checking the properties of the parts.

Cheers

Hello again,
In my CFD analysis, I am interested in the force moment produced by the deflection of the rudder. In my case, I set the center of mass in “Forces and moment coefficients” as center of rotation, so I want to study the moment at vertical axes (yaw moment).
Consequently, the plots of the simulation give me the force moment made by the deflection of rudder in respect to the center of mass? (That’s that I want to get)