# Aerodynamic forces and moments

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Thomas on 30 Apr 2024
Commented: Paul on 3 May 2024
In the "Aerodynamic Forces and Moments" block, are the x/y/z forces assumed to act at the CG (such that the resulting moment from the forces acting at another point would need to be included in the moment coefficient about each axis)?

Paul on 30 Apr 2024
The doc page Aerodynamic Forces and Moments states that the outputs of the block are:
Port_1 Aerodynamic forces
Aerodynamic forces (in the chosen output axes), returned as three-element vector, at the center of gravity in x-, y-, and z-axes.
Port_2 Aerodynamic moments
Aerodynamic moments (in the chosen output axes), returned as three-element vector, at the center of gravity in x-, y-, and z-axes.
I'm not sure if that answers the question because I don't fully understand the parenthetical.
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Thomas on 3 May 2024
My question is essentially whether the moment calculation assumes the forces (specified by the axial/normal/side force coefficients) are acting at the CG or at the CP. If they are assumed to act at the CG, then the moment coefficients supplied would need to include the contribution from the forces acting at the CP; however, if the forces are assumed to act at the CP, their effects would already be included in the moment calculation without needing to specify those effects in the moment coefficients.
Paul on 3 May 2024
On input, the three force coeficients define the force acting through the center-of-pressure, which is the fourth input port to the block, which "can also be taken as any general moment reference point as long as the rest of the model reflects the use of the moment reference point."
The calculations inside the block add r1 x F to the moment defined by the three moment coeficients, where r1 is the vector from the cg to the cp, though it's really implemented as F x r2 where r2 is the vector from the cp to the cg. You can look under the block mask to see the implementation and open the HL-20 example on the doc page to see how the block is used.
On output, the aerodynamic force should treated as if it is acting through the center-of-gravity and the aerodynamic moment is the result of all aerodynamic forces acting around the center-of-gravity.
At least that's how it all looks to me based on the doc, the block implementation, and the HL-20 example.

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