Jon Berndt <[EMAIL PROTECTED]> said: > I asked Tony about the aero reference point and the aerodynamic center the > other day. Here is his response: > > > Tony writes: > > The idea of the aerodynamic center is similar to the idea of the center > of gravity. It is the location on the aircraft through which the total > lift and drag can be said to act, just as the center of gravity is the > point through which the total weight acts. > > Each part on the aircraft has its own aerodynamic center. In the > subsonic regime, the aero center of the wing airfoil section is > generally near the 0.25 chord point. But it moves aft as the aircraft > increases speed into the transonic regime, typically as far back as the > 0.5 chord. > > OK, now the aero reference point. Think about all the sources of > aerodynamic pitching moment. The largest of those are the wing and the > horizontal tail. That due to the tail comes largely from the tail lift > multiplied by the tail moment arm. But how do we define the tail moment > arm? The aero center of the wing seems like a natural choice, but > doesn't really work since it moves in flight. So typically a point on > the wing is arbitrarily chosen to be the moment arm zero or reference > point. That's the point that I've dubbed the aero reference point. By > convention, that point is typically along the 0.25 chord line on the > wing. Spanwise, it is typically defined to be at the spanwise location > of the MAC or mean aerodynamic chord. The MAC is often computed using: > cbar/croot=2/3*(1+lambda+lambda*lambda)/(1+lambda) where lambda is the > wing taper ratio, ctip/croot. Once this length is computed, the > spanwise location can be found by finding the point on the wing which > has that chord. > > In the design phase, this point needs to be chosen early and all CFD and > tunnel data reduced using it. >
Interesting. I never thought about that. Where would you reference the 0.25 chord position on a swept wing, for this purpose? > I'll write more about the relationship between the FDM and the 3D model > after dinner and after the kids go to bed. Gimme a few hours. The aero reference data is interesting in helping to come up with some idea of about where to set the origin, in other words, "point the 3D camera" (see my other post). It's got to be a fixed point, and static in relation to the model, as in a fixed offset from the nose. But it doesn't have to be accurate in relation to anything except approximatly what the aircraft looks like it as it is pivoting around in the air. Of course this would change drastically and be visibly incorrect if the tail or a wing broke off, but if we modeled that type of failure I'm sure nobody would be noticing the error on their way to the ground :-) Best, Jim _______________________________________________ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
