Modelica: use of der() in a custom class/model
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I'm trying to learn Modelica and I have constructed a very simple model using the multibody library. the model consists of a world object and a body (mass) connected to to beams which are then connected to 2 extended PartialOneFrame_a classes (see below) which I modified to create a constant force in one axis. essentially all this group of objects does is fall under gravity and spin around due to the two forces acting at a longituidnal offset from the body center creating a couple about the cg.

        model Constant_Force
          extends Modelica.Mechanics.MultiBody.Interfaces.PartialOneFrame_a;
          parameter Real force = 1.0;
        
        equation 
          frame_a.f = {0.0,0.0,force};
          frame_a.t = {0.0,0.0,0.0};
        end Constant_Force;

I next wanted see if I could create a very simple aerodynamic force component which I would connect to the end of one the rotating 'arms'. My idea was to follow the example of the Constant_force model above and for my first simple cut generate forces based on the local frame velocities. This where my problem arose - I tried to compute the velocity using der(frame_a.r_0) which I was then going to transform to local frame using resolve2 function but adding the der(...) line caused the model to not work properly - it would 'successfully' simulate (using OpenModelica) but the v11b vector (see below) would be all zeros, so would der(frame_a.r_0) that appeared for plot plotting - not only that, all the other component behaviors were now also just zero constantly - (frame_a.r_0, w_a etc of the body).

        model Aerosurf
          extends Modelica.Mechanics.MultiBody.Interfaces.PartialOneFrame_a;
          import Modelica.Mechanics.MultiBody.Frames;
          import Modelica.SIunits;
          //Real[3] v11b;
          SIunits.Velocity v11b[3];
        //initial equation
        //  v11b={0.0,0.0,0.0};
        algorithm
          //v11b:=der(frame_a.r_0);
        equation 
          v11b=der(frame_a.r_0);
          frame_a.f = {0.0,0.0,0.0};
          frame_a.t = {0.0,0.0,0.0};
        end Aerosurf;

I tried a number of ways just to simply compute the velocities (you will see from the commented lines) so i could plot to check correct behavior but to no avail. I used algorithm or equation approach - I did acheive some different (but also incorrect behaviours) with the different approaches.

Any tips? I must be missing something fundamental here, it seems the frame component does not inherently carry the velocity vector so I must have to compute it??