Aerodynamic question for you engineers

On Jan 26, 1:50*pm, Jim Logajan wrote:
Phil J wrote:
Actually as I understand it in stable flight the CL is aft of the CG.
The airplane remains level not because these two are in line, but
because the tail is pressing down to counterbalance the offset of the
CL.

I should have used the term "total lift" so as to avoid confusion with the
lift generated only by the main wings. What you state above appears
internally consistent and correct with the definitions you are using.

After thinking about this question some more, it strikes me that this
situation is equivalent to a lever and fulcrum. *The lever doesn't
rotate around it's CG, it rotates around the fulcrum point. *In an
airplane, this point is the center of lift.

Whether you are talking about center of total lift (that generated by the
main wings, tail or canards, and fuselage) or center of lift of the main
wings, what you state above is _incorrect_.

I know that what you wrote sounds plausible, but the problem is that the
main wings are no more a fulcrum than the tail wings. Suppose the main wing
and the tail wing are very nearly the same size and produce nearly the same
lift and all have elevator controls? Which line is the fulcrum point now
about which the airplane rotates?

Regarding the CL moving around, I think even given that complication
the airplane would still rotate around the CL.

Here's a NASA web link that explains where the rotation point is:

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

Try to find some books on flight mechanics and look for the chapters or
sections that appear to discuss longitudinal static stability of aircraft.
They should all say that the aircraft rotates about the center of gravity.

Ok, I think it see it. There is a difference between the center of
lift and the location of the total lift vector (I guess you could call
this the net lift). In a non-canard airplane, the main wing is
pushing upward and that is the center of lift we have been
discussing. But the stabilizer is pushing downward. The net effect
of these two forces is to move the location of the total lift vector
forward to the CG location, and that results in stable flight. So a
rotational force will rotate the airplane around that point just like
a lever rotates on a fulcrum. The same thing would happen in a
canard, except that the location of the total lift vector would be
between the two wings since they both push upward.

Phil