D Ramapriya wrote:
On Jan 26, 5:31 am, Jim Logajan wrote:

As I understand it, the force of the tail plane's elevators typically
moves the center of lift forward and backward along the airplane's
axis as the elevators are moved up and down (as well as changing the
lift magnitude a little - though that is secondary). One presumably
enters stable flight when the center of lift is moved to coincide
with the center of gravity.


Since the CL can be altered by the wing configuration - deployment/
retraction of flaps for a given pitch, e.g., I'm not sure that the CG
and CL need to necessarily coincide for stable flight. Also, for a
body such as an aircraft, I think the CG would theoretically be
somewhere within it while the CL is a point on the fuselage, so their
coincidence may even be an impossibility.

If the total lift vector does not pass through the center of gravity, then
the resulting moment will rotate the aircraft. That is not considered a
stable situation. Here's what I hope is considered an authoritative web
site that discusses this issue:

http://www.grc.nasa.gov/WWW/K-12/airplane/acg.html

See also any text on flight mechanics and aerodynamics that has sections on
the subject of longitudinal static stability.