wrote


D: Interesting that the above article does not address thrust acting
through centre of mass. Not once. Thrust opposes drag, and acts on
mass only during acceleration or deceleration.
*********************

J: Once again, I did not have the 's pop onto this reply. Every once in a
while that happens, and I have no idea why. I'll use D: for the post I am
replying to,
and J: for my posts.

J: Yes, that (Thrust opposes drag, and acts on mass only during
acceleration or deceleration) is the key point that Alan could not seem to
understand. The center of mas is ONLY an important factor in understanding
where a rotating body rotates, ONLY while it is actually rotating-never mind
what forces caused the rotation. Once the rotation ceases, nobody needs to
think about where the center of mass actually is, other than to keep it as a
factor in the calculations to make sure ballanced flight is taking place,
and that the plane behaves in an expected way.

J: While the plane changes speed due to a change in thrust, if the thrust
line is not somewhat through the center of mass (wherever it is at that
moment) the plane would rotate for a couple seconds, _ _ if_ _ the pilot did
nothing to oppose it with control surface changes, until the plane settles
on its new speed.
*********************

D: The thrust line does not *have* to be aligned anywhere near the CG.
**********************

J: That should be very obvious for anyone to see, if they only think about
a few different designs, for just a few seconds.
***********************

D: Too much of this discussion has ignored the stabilizer. There's a
thing called decalage, which is the difference in angles of incidence
between the wing and the stab/elevator. The stab is usually angled
down a degree or two relative to the wing's chord line and has the
effect of stabilizing the rotational couple imparted by the CG being
ahead of the CP. This downward alignment also has the necessary effect
of lifting the nose when power is applied so that the nose will fall
into a glide all by itself if the power fails or is reduced. This
aspect of the airplane is mandated by design standards, and is the
reason lifting tails were abandoned early in WWI.
(In lifting-tail airplanes a power reduction would lead to a pitch-
up and stall, and usually an unrecoverable spin. And it still happens
once n a while when some dude overloads his airplane with all that
beer in the back, CG way aft of the limit, and has to hold the yoke
forward to keep the nose from pitching up. Death often follows,
especially on approach when power is low and the tail can't be lifted
anymore.)
*********************

J: You are correct, but many things have not been mentioned, and perhaps it
is because the whole subject of aeronautics is such a complex subject, with
so many things being interrelated to each of the other factors, that whole
books sometimes fail to address all of the different related factors. Your
mention of the factors above is right on, but should not be necessary to
understand the "thrust line" discussion taking place, but perhaps it will
help some people see what is happening.

D: So thrust lines that aren't or can't be aligned near the CG are
not a big deal. The stab incidence is adjusted so that the desired
pitching occurs. Just look at the Lake amphibians, with their very
high thrust lines; The decalage is quite pronounced, and even at that
there's some odd behavior when the throttle is opened.
*********************

J: EXACTLY true; it rotates _ _around the center of mass_ _
while and only while the plane is _ _actually rotating_ _. Once
the new speed is established, and the plane is in ballanced flight, the
thrust line (wherever it relates to incidences, and everything you have
mentioned) can be anywhere and it only serves to keep everything ballanced,
and the plane behaving properly.

D: But it still glides like it should when the power's taken off.
http://www.seaplanes.org/graphics/members/la4200.jpg
Note that the thrust is pointed right at the stab to help control
pitching; it's not pointed through the CG at all. THAT would be
impossible.
See the decalage he http://mars.ark.com/~dcf/takeoff2.jpg
And he http://www.seabee.info/images/teal/N6595K-03-640.jpg

Now look at the decalage on various "ordinary" airplanes that have
their props on the nose, and see that there's very little compared to
those amphibians.

Decalage: http://en.wikipedia.org/wiki/Decalage
******************************

J: Excellent post. You explained things well. The changed thrust line's
relation to the center of mass was a "blue herring", I think, in the whole
discussion. While it is true that if the thrust line is not through the
center of mass that the plane will tend to rotate about the center of mass,
that rotation attempt will be compesated for, and be prevented by control
movements. Those movements opposing the rotation are only necessaary until
rotation ceases; if it ever did even rotate. Once speed is stabilized, and
no rotation is taking place or trying to take place, the thrust line has no
relation to the center of mass. Only to all of the aerodynamic forces being
in ballance in trimmed flight.

J: I hope I did not confuse anyone with my response to your fine post. I
hope the subject is now fully explained and understood, and it can now be
put to bed.
--
Jim in NC