by Kisssys » Thu Nov 06, 2014 2:33 pm
The .8 came from just looking at a few frame calculations that I had done with quatostool. It's an approximation. We operated with values off 50 percent for quite a while with good success so their is some leeway in the numbers. The roll and pitch are lower because a couple of the motors will be right on the axis and don't effect the moment.
You can of course do the same thing for Roll and Pitch if you just hang it that way. A flat frame hangs fairly well, if you have gear and gimbal then you need to offset the string to get it to balance. If you needed to add a small weight to keep one boom down it would not effect the moment much as it would be on the very center of the pivot line.
You can add and subtract moments easily. You could find what the moment is of your gimbal for instance and as long as it was positioned the same distance from the COG then you could just add in those moments when you fly with the gimbal. To be honest I've flown hundreds of flights where I add and remove different masses with no noticeable change. The Witch and the Ghost are examples of using moment numbers of just the frame.
You can get carried away and I did at first but I tend to weigh the item and then find the balance point of the object. I enter into Quatostool the point mass at the position that balance point would be reference to COG. Large heavy objects like the battery which are not square need to be properly represented.
A good way to grasp the importance of each parameter is to open a XML file in an editor and Quatostool at the same time. Use a plus configuration and start changing motor position and weights and see how the numbers change. Try a motor boom length of 500 mm and a motor and esc weight of .250 kg and see what you get. Plus makes visualizing the numbers easier without doing trigonometry in your head.
Steve
Kisssys