"Jim Logajan" wrote in message ...
Phil J wrote:
Imagine
that you had a couple of tall jack stands that you could place under
the wings to elevate the airplane a foot or so off the ground. Let's
say you place the stands under the wings just back from the CG such
that you have to press down on the tail to keep the nosewheel off the
ground. This is similar to the condition of flight since the center
of lift is aft of the center of gravity. Now if you push down on the
tail, the airplane will rotate about the center of lift. Wouldn't it
work the same way in the air?

They aren't equivalent situations, mechanically speaking.

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.



You are in stable flight when the forces are balanced. You do not want the CG and CP to coincide if you want positive
stability on the airframe. With a conventional aircraft, the CG is forward of the CP and the horiz tail provides a down
force to hold the nose up. The elevator is a wing adjuster and basically sets the AOA. At a high AOA the CP moves
forward and at a low AOA the CP moves back. The rotation still occurs around the CG. This is why the nose pitches down
during a 'stall'. The C/L deteriorates in the stalled condition so it is no longer holding the nose up, and the up force
from the horiz tail pulls it up and the nose rotates down...

When the CG and CL coincide, the elevator forces are very light and the airplane is at best neutrally stable. Because
the CP moves with AOA (therefore C/L) if a gust upsets the aircraft and pitches it up for instance, the CP moves
forward, the plane rotates around the CG as the tail goes down, and there is no self stabilizing force to bring the tail
back up. If left uncorrected the wing will stall. This movement of the CP is just one of the Wright brothers many
insights that allowed them to make an airplane that could be maneuvered.

Now, if you want to be efficient, you load the airplane so the CG is at or just forward of the aft cg limit (and make
sure it will be there as the flight progresses). This way the down force is minimized ands you can fly at a lower AOA,
lower C/L, and therefore drag. I think some of the airliners put fuel in the horiz tail so the CG can be tweaked while
in flight to keep the CG back and maintain higher efficiency.

A canard design is just the opposite. The CG is behind the CP, and when the canard stalls, the nose drops because their
is no longer any lift to hold it up. This is why canards must always stall the front wing first.