Moments and forces in different frames of reference

[Marco9518]

Good morning!
I know this may sound a little odd, because there is a theorem regarding it, but i have this question.
Basically, a CFD analysis gives me the value of the forces and the moments, as a function of fuselage's orientation, in a particular frame of reference.
How can i calculate the values in the C.G. frame of reference? Forces have no problem, but moments do.

 

[FactChecker]

Are you looking for the formula for calculating the moment of inertia with respect to a parallel axis? This is a reference.

 

[Marco9518]

Are you looking for the formula for calculating the moment of inertia with respect to a parallel axis? This is a reference.

Thanks, but I was talking about aerodynamic forces.

 

[berkeman]

@Marco9518 -- Is there a different sub-forum where we can move this to where it would be a better fit? Maybe the Aerospace forum or ME forum? Or do you think this General Math forum is still best?

 

[FactChecker]

Thanks, but I was talking about aerodynamic forces.

Aerodynamic forces are in the form of coefficients for the torque in each axis (after proper multiplication by factors). The plane reacts like anything else with the same forces and moments of inertia.

 

[Marco9518]

Are you looking for the formula for calculating the moment of inertia with respect to a parallel axis? This is a reference.

Thanks, but I was talking about aerodynamic forces

@Marco9518 -- Is there a different sub-forum where we can move this to where it would be a better fit? Maybe the Aerospace forum or ME forum? Or do you think this General Math forum is still best?

Hi! i thought about that! I just think that, being this a mathematical/physical problem, would be more helpful to post it here. Do you think it is possible to have it in both sections?

 

[FactChecker]

Thanks, but I was talking about aerodynamic forces

I don't understand. What is it about aerodynamic forces that you think are different from other forces? In my experience, they are treated the same. When you use 6-degree of freedom equations of motion, the aerodynamic forces and moments are combined with other forces like those from the propulsion, gear, gun, gravity, etc.

 

[Marco9518]

I don't understand. What is it about aerodynamic forces that you think are different from other forces? In my experience, they are treated the same. When you use 6-degree of freedom equations of motion, the aerodynamic forces and moments are combined with other forces like those from the propulsion, gear, gun, gravity, etc.

So, basically i need to validate a simulink model that gives me forces and moment acting on the fuselage of an helicopter, given speed, angle of attack and angle of sideslip. There are no rotors, no appendices, nothing but the fuselage. The model needs some coefficients. The CFD analisys gives me forces and moment in a particular frame of references. I need to calculate the same forces and moments in another frame of reference which has its reference pole in the center of gravity.

 

[Marco9518]

Aerodynamic forces are in the form of coefficients for the torque in each axis (after proper multiplication by factors). The plane reacts like anything else with the same forces and moments of inertia.

This forces are not in form of coefficient. What i have is for example: at 0 deg of angle of attack the Fx is 5N, at 15 deg is 20N and so on.

 

[berkeman]

Do you think it is possible to have it in both sections?

No, sorry.

 

[FactChecker]

This forces are not in form of coefficient. What i have is for example: at 0 deg of angle of attack the Fx is 5N, at 15 deg is 20N and so on.

That just removes a slight complication. A force is a force is a force. The only thing that changes is the measurement of the reaction to the force when a different axis of rotation is used. That changes the moment of inertia as described in the link I referenced in post #2.