Quote:
Originally Posted by swift
My point is that you will have difficulties to derive reliable coefficients and you seem to agree on this. I do know a little about flight mechanics and trajectory computation (where the 6dof equations intervene) and about coefficient determinations (it is my daily business).
At work we frequently use a simplified tool to calculate the aerodynamic coefficients for subsonic and transsonic flight conditions and I can tell you I would not trust them for applications such as CoD. We use them for different applications where the impact is minor so we can live with it. But CoD would rely heavily on these coefficients and I'd say to obtain something that is halfway close to reality such a tool is not sufficient. And from experts working for years in the aerospace business using modern and highly sophisticated cfd tools I know that using these methods for subsonic regions is far from trivial. And it would take hours to days to calculate just one flight point for one configuration and probably would take longer if one would take into account the viscious terms instead of relying on simplified Euler calculations.
But what we need in CoD does not stop at the determination of lift, drag and lateral force coefficients and the moment coefficients about the three axis. We also need the derivate coefficients to obtain a believable flight model. Up to now the means with which these are "determined" is more than crude and very little reliable.
This gets even more complicated when one considers that each flap, rudder and aileron movement will have an impact on the aerodynamic coefficients (the 6 static coefficients and the derivates). You'd need a database set for several flap, rudder and aileron deflection combination. Then we have the trimmed and untrimmed flight conditions and other aerodynamic control surfaces such as flaps and airbrakes. Now let's talk about canopy open or closed and radiator and oil cooler openings ...
And these are just the coefficients for the airframe. We'd also need reliable data for the propulsion set.
I really do not believe in being able to obtain a full AEDB that will result in a flight performance that will be close to the real thing anyway, provided we even know where the real thing was. I think it is smarter to take the bottom up approach by tweaking the used coefficients in such a way that they fit to the experienced behaviour including test results and, where values are missing, to anecdotical evidence as long as there is a bunch of anecdotes saying the same.
BTW: CoD is definitely using 6dof. What we are disputing is how they come up with the forces and moments they inject into the 6dof equations.
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really good thread so far, and very good and interesting post!