Another FlightSIM related example is the following question and answer extracted from our support knowledge base. In this case, it shows how we make different parallel calculations of calculations and velocities based on different theorems.
Q: In the FLSIM_SYSTEM_EOM structure, there is a substructure called "body" with relative and absolute accelerations and velocities (i.e., a_abs_cm, a_rel_cm, v_abs_cm, and v_rel_cm). What is the difference between the absolute and relative value?
A: "F = ma" is a simplified version of Newton's law which specifies that the force is given by the derivative of the linear momentum. However, that derivative must be done with respect to a fixed frame (meaning a non-rotating frame) which in our case is the ECC frame.
Alternatively, the Coriolis theorem specifies that the derivative of a vector with respect to a fixed frame is equal to the derivative of that vector with respect to a rotating frame (in our case the body frame) to which is added the cross product of the angular velocity vector between the rotating and fixed frames with that vector.
With these two equations in mind, our values "_abs" represent the derivatives in the fixed frame and "_rel" represent the derivatives in the rotating frame.
For instance, if you specify that the earth is not rotating then you will see that v_abs_cm and v_rel_cm are the same. Aside from what is explained in the EoM System from Chapter 4 of the FlightSIMRef.pdf, The "eom->body.a_abs_cm" represents the absolute (meaning with respect to
the fixed ECC frame) acceleration vector of the reference center of mass and expressed in the body frame while "eom->body.a_rel_cm" represents the relative (meaning with respect to the rotating body frame) acceleration vector of the reference center of mass and expressed in the body frame.
You can find more frequently asked questions, along with detailed How-Tos and other interesting topics in the Presagis Knowledge Base, available under the Customer Portal. I will feature more interesting topics from this repository over time.
