On Thu, 24 Jan 2008 21:22:23 -0900, Pete Brown wrote in
:


If a conventional aircraft is in stable level flight and the stick is
pulled back, all of the texts I have read indicate that the aircraft
pitches up, rotating through the CG.

Is this exactly correct or is it a very useful approximation good for
all practical purposes?

Most aircraft have the CG located slightly forward of the center of
pressure ( CP or center of lift) for positive pitch stability. I was
wondering if the actual point of rotation is displaced somewhat aft of
the CG, someplace close to the CG but in fact some small distance
towards the CP.

When the aircraft is subject only to the force of gravity, any
displacement will cause it to rotate around the cg but in flight its
subject to gravity as well as the aerodynamic forces which act through
the CP, suggesting to me that the point of rotation is not quite on the CG.



That's an interesting question. I hadn't thought about it before.

First let me say, that I'm not an aeronautical engineer, but
intuitively I figure it this way.

In stable, level flight lift (acting through the center of pressure) =
weight (acting through the center of gravity), so it would seem that a
downward force on the tail would cause the aircraft to rotate on its
lateral axis through a point midway between the center of
lift/pressure and center of gravity. But that's a guess, and it
doesn't consider the displacement of the center of pressure forward
with the increase in angle of attack.