Aerodynamic question for you engineers

On Jan 26, 5:19 am, Stealth Pilot
wrote:
On Fri, 25 Jan 2008 17:03:04 -0800 (PST),
wrote:

And if the CG is moving horizontally at 100 knots, where
is the rotation point now? As soon as rotation starts, the aircraft
begins to change its flight path, and any determination of rotation
point, whether it's the CG or CP or any other point, becomes very hard
to determine and might be irrelevant. I would prefer to think of the
fixed end of the flight path radius (which is also changing) as the
airplane rotates, just like one of those complicated cabinet door
hinges that has two arms and four pivot points. Where is the rotation
point of that door? There is no fixed point.

Dan

clueless. you'd never have understood the propeller like the wrights
did because you cant simplify situations until the problem becomes
solveable. finding a solution to a problem is often a matter of
thinking about it with just the relevant factors at play.

the answer to your door problem lies in understanding that there are
multiple hinge points that each act in a simple manner.

the frame of reference is moving at 100 knots. the aircraft rotates
about it's centre of gravity (centre of mass).

You are assuming that the aircraft rotates about its CG as it
would in space. But we're NOT in space, and lift and drag and various
vectors all come into play here. As the aircraft rotates nose-up, CL
(coefficient of lift) increases; CP shifts forward; the relationship
between CG and CP changes because of the CP shift, and especially so
if the CG is well above or below the CP; the tail's downforce
increases, adding another vector to the whole thing. Then we have
thrust and drag trying to rotate the airplane around some other point.
We can't consider mass alone if we're trying to figure a rotational
point. If mass is all you're concerned with, then the CG is good
enough. I'd rather have more than mass acting on my airplane; it flies
better that way.
Kerschner likely has something to say on it. I'll have a look.

Dan