Thrust line: a no-thought experiment.

It has been put forward that for an aircraft to be in trimmed out
condition with all forces in balance, the thrust line must be pointed at
the drag line.

So let's do a thought experiment...

....wait.

We don't need to do a thought experiment.

We can simply observe a couple of aircraft.

The drag line is the line of the horizontal component of the total
aerodynamic force with its origin through the aerodynamic centre of
pressure, right?

OK. The aerodynamic CoP is pretty much always somewhere in the main wing
slightly behind the centre of mass CoM.

Look at any transport jet with engines mounted beneath the wings.

Where is the thrust line?

Not clear enough?

Let's look at a powered paraglider. The CoP is some 20 *feet* above the
CoM, and thus the drag line is up there too...

....but the engine is on the pilot's back...

....right near the CoM.

Hmmm...

....perhaps a thought experiment is necessary after all.

Imagine a paraglider redesigned as a rigid aircraft. *Why* anyone would
want to, I can't imagine, but go with it. Further imagine that the pilot
is enclosed in a nice aerodynamically slippery nacelle, so that almost
all the drag of the system is in the big fat wing 20 feet above his head.

Now say you want to be able to take this thing off with a rocket to gain
intial altitude and then glide back down, so you put a very lightweight
(so that the impact on the CoM of the system is minimal) rocket motor
somewhere on this bizarre craft:

So you're on the runway with your craft and you switch on the motor.

If you put it up where the drag line is and turn it on, what is going to
happen? Right: disastrous pitch, nose down.

Now put it so that it is aligned with the centre of mass of the system.

What happens? You soar into the air!

Clear enough?

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