Easycub and autopilot : Part 1

I had mentioned earlier that I have been working on making an autopilot for my radio controlled airplane, Multiplex Easycub. The aim of the autopilot is of course to make the airplane to fly through a given set of waypoints autonomously without any pilot intervention.
There are readily available autopilot modules like papparazi, ardupilot, etc. However me and and my friend, Yogesh set out to build one ourselves.

Some progress has been made recently in this direction. Before implementing the autopilot using Arduino I thought "Why not simulate the flight dynamics first in order to better understand the effects of the flight control system logic". It would also be a very nice project in itself i.e.flight dynamic simulation of an RC airplane.

I decided that I would use Matlab's simulink to avoid any coding and to shorten the time required for development. The aerospace blockset in simulink has 3DOF and 6DOF blocks for flight simulation. I used the 3DOF block (X, Z & Pitch) instead of the 6DOF block to make things a little easier in the beginning.
The blockset needs the aerodynamic forces in the X, Z (body axes) and moment in the Y direction (body axes).

Yogi suggested an excellent software for obtaining the aerodynamic coefficients: Datcom-Holycows. After going through the datcom manual and other help files, I prepared a .dcm file that contains flight point and geometry details of the easycub. On execution of this input file, I obtained a .out and .ac files among other outputs. The .ac file is for visualization while the .out file contains the aerodynamic coefficients. After playing around with the values, I finally settled down on a geometry which I felt was a close match to the easycub model.

If you are too lazy to setup the .dcm file yourself, you can use mine (you may need to join yahoo's digital_datcom group before you can access the file).