Originally Posted by
Sundog
I've never even heard the term fuselage loading, although, in the way they are referencing it to the X-3, I think they would be talking about the aerodynamic loads as the inertial roll coupling diverges and the aerodynamic loads increase on the fuselage. As the aerodynamic loading along the fuselage isn't trivial given it has a lot of area ahead of and behind the center of mass and that greatly affects the stability derivatives.
I only encountered this type of loading in stability and control. For instance, when an aircraft yaws, many people just consider the area of the vertical tail, when what you really want is the control volume of the vertical tail (control volume of the vertical tail is vertical tail area multiplied by the longitudinal distance between the tail's aerodynamic center and the center of mass, which is a measure of control power). However, when you think about an aircraft yawing and remember that an aircraft rotates about it's center of mass, the fuselage is starting to turn sideways to the airflow. So the air hitting the side of the aircraft into the relative wind (The airflow coming at the aircraft in an aircraft frame of reference) is a dynamic pressure load on the fuselage, whereas on the leeward side, opposite the relative wind, you can develop suction loads.
The part of the fuselage ahead of the center mass wants to destabilize the aircraft; it wants to turn it away from the direction it is moving. The part of the fuselage behind the center of mass wants to stabilize the aircraft, make it weather vane into the direction it is moving. However, it is also in the wake of the wing and in a more turbulent region, so it may not develop loads as powerful as those at the nose; it depends on the design. As noted above, with a fuselage as long as the X-3's, the loads aren't trivial, especially when taken into account with the mass distribution and the small aerodynamic surfaces (small dampers).
So my guess is they were looking at the aerodynamic load distribution down the fuselage as it diverged more and more away from the velocity vector. I can see it in my head, but I'm not sure my words are conveying it so well. let me know and I can probably make some drawings this week to show what I'm talking about.
Bookmarks