I am not an aerospace engineer (no, I don't play one on television), and I can't help with the shear center. The aerodynamic center is the point where the sum of all aerodynamic forces acts. If it is forward or aft of the CG, the plane will climb or descend. It is usually slightly above the CG for stability.
Please don't shout. We here you in lower case just fine.
The difference is a design parameter. For a stable airplane, as for passengers or cargo, you'd probably want the center of lift above the center of gravity. Upward angled wings (dihedral) can help. That design can be very stable, like hanging a weight from the ceiling. For maneuverability, as in a fighter, you could lower the center of lift relative to the C/G, perhaps to the point of instability requiring a computerized flight control system.
Again for stability, you might want the center of drag a little behind the center of thrust. Like the wagon behind the horse, not ahead.
Symmetry is important, of course, as is controllability with loss of an engine or in different flight conditions such as transonic or stall.
Shear can be present locally, anywhere in the structure, and can be addressed at that level.
At relatively slow speeds, the torsional stiffness of the wing is enough to counteract the twisting. However, the moment increases as the square of the speed, but the stiffness remains constant. Therefore, at high speeds, the wing fails - sometimes causing it to twist right off of the plane.
The speed at which the wing stiffness can no longer counteract the twisting moment is called the "wing divergence speed".
In order to counteract wing divergence in high-speed aircraft, wings can be designed to minimize the distance between the aerodynamic center and the shear center (on the elastic axis). This is usually accomplished by means of either swept forward or swept back wings.
Swept back wings induce a downward twist in the leading edge of the wing (because the moment arm from the shear center to the point of lift on the aerodynamic center extends toward the back of the plane), which reduces lift, counteracting wing divergence. Most high-speed aircraft have swept-back wings.
Sweep forward, on the other hand, induces an upward twist (because the moment arm, in this case, extends toward the front of the plane). This exacerbates wing divergence - exactly what we don't want. Only a few experimental craft use forward-sweep, presumably in an effort to increase the effect of the ailerons and thus increase maneuverability - but that's a tale for another time.
Here's some bad ascii art, in case it helps you visualize this better. "ac" is the aerodynamic center, "ea" is the elastic axis, "x" marks the point where the lift acts, and the diagonal line that's trying to become an arrow represents the moment arm.
to find shear center , first idealize the given cross section into booms (ie lumped mass) and skin. booms carry bending load or axial load or normal load and the skin carry only shear load. shear center always lye in the axis of symmetry. if the c.s is doubly symmetrical shear center coincide with c.g. if c.s is singly symmetrical say symmetrical about x axis apply vertical shear load at shearcent by assuming that it is acting at a distance of Ex and consider moment balance about any point on the c.s. and viecversa
SHEAR CENTER IN AIRCRAFT
Re: SHEAR CENTER IN AIRCRAFT
