To tie my and Genghis' posts together on the m point, you'll note that the equations for m and b are the same form.
Re the original post, the steps to calculate b are
(i) do the calculate m thing
(ii) pick one of the points, plug in the calculated value for m and the answer is b
B is, indeed, the line's intercept with the vertical axis. In my earlier post, this is the y axis, in Genghis's post, the x axis, and in Tinny's end required equation, the CG axis .. ie the dependant variable axis.
More simply, if
y = m.x + b
put x = 0 and
y = m. 0 + b
ie y = b for x = 0
In the spreadsheet simple conditional tests for forward and aft limits are all that is needed. Be aware that quite a few aircraft have varying aft limits .. indeed some are not straight lines as well ... in which case you may need to run a regression to figure a polynomial for the spreadsheet design.
However, the spreadsheet exercise is no different to any other non-trivial sheet calculation .. plan the logic carefully but do a heap of error testing of the beta before showing it to the world.
Last edited by john_tullamarine; 16th Feb 2005 at 20:55.