Spin

"Robert Ehrlich" schreef in bericht
...
"ir. K.P. Termaat" wrote:
...
During standard circling no accelleration forces in the longitudinal
direction of the glider are required to keep the IAS constant when the
glider makes perfect circles relative to the moving layer of air. From
the ground this looks quite different of course. But that is indeed
irrelevant.


You may consider it as irrelevant but it nevertheless complies with the
same laws of dynamics as seen from the air. An observer moving with
the airmass sees a glider with a bank angle generating an horizontal
component of the lift which remains perpendicular to the speed and has
no effect on the magnitude of the speed but only on its direction:
the glider circles. An observer on the ground sees the same horizontal
force but it does not remain perpendicular to the speed and so has an
effect
on its magnitude as well as on its direction. The final resulting effect
is that the glider has increased its speed relative to the ground.
The force needed for this longitudinal acceleration that you were calling
for in your previous post is just the horizontal component of the lift.

I think your reasoning for an observer on the ground is o.k.

However my approach to this would be to add the speedvector Vg(x,y,t) of the
glider in the moving airmass plane (constant in strength with direction
tangent to the circle) to the windvector Vw(x,y) in the groundplane
(constant in strength and direction).

The result would be a trajectory in the ground plane in the shape of open
loops moving in the direction of the wind. This is what the observer on the
ground would see and can be described as a function of time mathematically.
Then one could calculate accellarations of the glider relative to the ground
from this. However, though this is a nice observation I do not see at the
moment an application of this knowledge. So it is a little academic I guess.

All what happens to the glider is controlled by Lift and Drag (aerodynamic
forces) and the Weight of the glider (gravity force). Movements of the
glider as a result of these forces can best be described relative to a
horizontal plane moving with the wind. The glider making coördinated turns
with constant IAS will produce perfect circles as a trajectory on this plane
with a constant radial accelleration in the direction of the center of the
circle and without longitudenal accelleration.

But I guess you know this all already.

Karel