In article ,
Jim Logajan wrote:

Alan Baker wrote:
In article ,
Jim Logajan wrote:

Alan Baker wrote:
The only thing that matter was thrust line.

You've managed to make the same incorrect claim at least twice (that
I've counted) in one thread. Are you sure you aren't overlooking
something?

I'm will to entertain that possibility.

What do you think I'm overlooking?

That the aircraft are moving through a fluid, not a vacuum.

So things like propwash affect a tractor differently than a pusher. In my
own analysis of the two types and their engine on versus engine off
dynamics, the thrust line has not been "the only thing that matter[s]." It
is more complex, and the texts I have handy do not make the situation
appear as simple as you claim.

If thrust line moment arm (relative to the c.g.) were all that mattered,
then the following two bottle rocket firework designs would fly the same
paths, mass, impulse curve, c.g., and thrust moment arm being held the
same:

(A) ----------------+-----+
|=====]

(B) +-----+----------------
|=====]

The attachments to the sticks are made so that the moment arms relative to
the c.g. are otherwise identical, but in (B) the thrust is opposite its
usual direction. Now in a vacuum I would expect them to fly the same paths
and would be surprised if they didn't do that. But in a fluid like the
atmosphere? Would you expect them to fly the same or different paths?
Frankly, I would be surprised if they did fly the same - but as I don't
have any fireworks handy at the moment I haven't gone outside to have fun,
er, run experiments to observe their behavior.

Sorry, but you're proceeding from two wrong assumptions.

1. Your A and B examples would both behave the same in flight if their
centres of mass were both the same. It is the aerodynamic centre of
pressure moving ahead of the centre of mass that would cause B to be
unstable and A stable, if we operate from the assumption that the only
parts involved are the rocket and the stick.

2. In specific, both of the aircraft in this case are stable. They are
both stable because the aerodynamic centre of pressure is behind the
centre of mass and because the centre of lift of the main wing is behind
the centre of gravity. Thus, your assumption that a change in the
engines location matters is based on the erroneous assumption that such
a change is being made in isolation.

--
Alan Baker
Vancouver, British Columbia
http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg