Spin

Hi Robert,

You are right Robert. The glider is unaware of groundspeed.
Looked several times at the short film of the crash where it is
obvious that the DG500 is flying to slow relative to the fast moving
air rather then to slow relative to the ground while having a lot of
tailwind (which is not very fast either).


I showed the film to one of our airobatic pilots.
His comment was:
- the airflow may have been very turbulent at low altitude due to
several obstructions in the field
- at the last moment the pilot tried to line up the glider with the
runway or his selected landing spot and therefore applied a lot of
right rudder and some right stick input
- when he observed the right wingtip to get rather low he tried to
move it up using left stick input
- so then you had the classic spin inputs: low speed and crossed
controls
- the right wing stalled first because of the aileron deflection
downwards; the full rudder deflection to the right made it worse; a
spin became unavoidable.
- undisciplined glider pilot

Though my calculation of pitch down input during the 180° turn back
curve to the field was based on a wrong supposition (sorry for that)
it would have helped the pilot a lot to speed up his glider which
might have been just enough to make a safe landing. No excuse though.

Karel, NL

Z Goudie wrote in message ...

That old red herring again!

The glider is flying in an airmass which is moving
over the ground at a constant rate. No additional
acceleration is required apart from that normally needed
in a turn to supply the turning force.

There may be some effect caused by descending/putting
the lower wing down through any wind gradient but this
actually improves the situation as the air is moving
'away' from the path of the glider more slowly and
will consequently cause some increase in airspeed.
(You can try the opposite of that effect by pulling
up from a downwind racing finish through a strong wind
gradient; watch the airspeed decay at an alarming rate).

The biggest problem is that the apparent speed over
the ground in say a 15kt wind jumps by 30kts and results
in people trying to reduce the ground rush by raising
the nose with no reference to the ASI.

We have very nice herring in NL, not red however.

Indeed I made a wrong supposition in my calculation of a 10° pitch
angle required during the turn back curve of the DG500 to the
airfield.

A steeper pitch angle then the pilot obviously applied would have
helped him a lot though to keep the glider from stalling and spinning
in.
The comment of one of our aerobatic pilots is that the DG-pilot flew
to slow in the last part of the flight and a full spin with crossed
controls (right rudder and left stick) evolved. The air may have been
very turbulent in the lower layer because of several obstructions on
the field.

Karel

"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 remains 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.

"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

What puzzles me about this discussion is the lack of any appeal to inertial reference frames:

In what follows I am not taking into account any factors relating to windshear - the main assumption is that the windspeed is constant right down to the ground, but the analysis can be extended to take account of that.

What is an inertial reference frame? One in which Newtons laws apply

In this discussion there are two reference frames: one attached to the ground, one attached to the the moving airmass

One frame (the airmass one) is moving linearly (i.e. not accelerated) with the respect to the other (the ground)

The discussions of the particle (glider) motions observed as occurring in the airmass reference frame can be related to the motions observed as occurring in the ground reference frame by the additional of constant equal to the speed of motion of the airmass (e.g. the uniform rate at which the airmass is moving over the ground)

So what is the problem? No accelerations are involved other than that of the the glider due to it's circular motion.

Rgds,

Derrick.s
"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

At 11:00 11 February 2004, Derrick Steed wrote:
So what is the problem? No accelerations are involved
other than that of the the glider due to it's circular
motion.

I think they're suffering from 'Last week I coodn't
spell the word injuneer - now I is one' syndrome.

Z Goudie wrote:
At 11:00 11 February 2004, Derrick Steed wrote:

So what is the problem? No accelerations are involved
other than that of the the glider due to it's circular

motion.

I think they're suffering from 'Last week I coodn't
spell the word injuneer - now I is one' syndrome.

Thats why I said he should talk to his instructor instead of going into
frames of reference. Cuz I be a biolygyst not no train driver ;-)

Shawn