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Yes, this is true......but to me it is better to keep the vectors
simple. If you apply a component along the line of the fuselage (aft
vector) then you have to add in the other component too. What
direction? Remember aft is parallel to the glider, not the flight
path of the glider.

We could in fact break any vector up into any number of
components......but eventually you have to combine them again.

To me, using the Earth (as horizontal and vertical reference) is
best. Then we can easily see the climb angle of the glider, the
direction of flight if you will, and the speed. We can also easily
see the angle of attack. With this reference we need to apply only 4
vectors (forces) lift, drag, weight, thrust. IF we use the glider
itself, longitudinal axis as reference, we right away have 8 vectors
to contend with. On tow, if we know any three forces, we can
calculate the forth. In gliding flight its only three forces (thrust
=3D 0) so its even easier.

Ultimately, if in "steady flight" there is in fact no force acting on
the glider......because the sum of all of the components =3D 0.

I like your explaination of climbing aircraft above. Another way to
look at it: (speed kept constant) IF thrust is greater than drag, the
aircraft will climb, IF thrust =3D drag, the aircraft will fly level
with the Earth. IF thrust is less than drag, the aircraft will
descend. If thrust =3D 0 the aircraft will descent at its L/D angle.
(assuming thrust is applied along the direction of flight)

Oh yeah...yet another factor from the earlier version of this
discussion. The force of the tow rope(thrust) does not necessarily
act through the glider's center of gravity. Neither does the drag
vector. This can cause a pitching moment, which will require elevator
input to counteract. Another factor that can give a different "feel"
on tow.


Cookie


There are a multiplicity of possible axes systems that are used in flight
dynamics - which one you use (and what you call it) depends on where you
are (US, UK, rest of world), and which one makes the sums simpler!

The equations of motion for climb and descent in free-flight are exactly
the same - just some terms disappear, or change sign. On tow is another
matter, since the tow rope angle introduces an additional force, and a
constraint on the motion.

DLR, the German aero reseach instititute, did do some work in 1999 on the
longitudinal dynamics of aerotowing, looking at the effect of downwash,
rope forces and hook position on pitch stability ... at least I think they
did - I have the report, but need to get it translated! Doesn't look to
contain anything concrete on lateral stability though.