How do you explain why the A/S increases on thermal entry?

The way I look at it is, you're flying at a speed on relatively constant
airmass during cruise. Then when you enter the thermal you're entering a
gust of wind that is moving upward. So that gust of wind is going to
accelerate you, but at first it must gain enough force. While it's gaining
that force it's just blowing extra wind that the instrument translates as
airspeed. I could be wrong but this is how I see it in my mind.
"Denis" wrote in message
...
Fred a écrit :
Just got asked this question, didn't have a quick and easy answer. How
do you explain it?

Does the airspeed really increase on thermal entry ??? I am not
convinced of that.

I think the opposite is true : when the airspeed increases, due to entry
into a thermal, turbulence or any other reason, you
TE-compensated-variometer believes there is a lift !



--
Denis

R. Parce que ça rompt le cours normal de la conversation !!!
Q. Pourquoi ne faut-il pas répondre au-dessus de la question ?



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An 800 pound sailplane flying at a steady 50 knots has all forces
balanced. Lift ~ offsets weight ... i.e. the lift vector is tilted a
bit forward to generate a thrust resultant to offset drag ... let's say
it is ~ 20 pounds.

If you fly into a sharp edged 10 knot thermal at 50 knots ... the wing
'sees' a change in the relative wind equivalent to an AOA increase of ~
11 degrees.

I don't know how much lift the wing would develop ... but lets say the
lift doubles .. and doubles the thrust resultant to 40 pounds. Since
the drag is 20 ... the ship would accelerate ... but 20 pounds of extra
'thrust' on an 800 pound ship would seem to take some time to
translate into velocity. If you made the lift resultant 40 pounds ...
I still don't think you would see the speed increase we all experience.
So is something else also happening ... I think the lift vector also
tilts forward as AOA increases ... so the thrust resultant might be be
much higher. Udo ... where are you when we need you? :-)

I assumed for this example we held the stick steady ... but perhaps we
always tend to push/pressure the stick in these situations to keep the
nose from rising too sharply which effectively puts the ship in dive
....

KK

"Ken Kochanski (KK)" wrote in message
oups.com...
An 800 pound sailplane flying at a steady 50 knots has all forces
balanced. Lift ~ offsets weight ... i.e. the lift vector is tilted a
bit forward to generate a thrust resultant to offset drag ... let's say
it is ~ 20 pounds.

If you fly into a sharp edged 10 knot thermal at 50 knots ... the wing
'sees' a change in the relative wind equivalent to an AOA increase of ~
11 degrees.

I don't know how much lift the wing would develop ... but lets say the
lift doubles .. and doubles the thrust resultant to 40 pounds. Since
the drag is 20 ... the ship would accelerate ... but 20 pounds of extra
'thrust' on an 800 pound ship would seem to take some time to
translate into velocity. If you made the lift resultant 40 pounds ...
I still don't think you would see the speed increase we all experience.
So is something else also happening ... I think the lift vector also
tilts forward as AOA increases ... so the thrust resultant might be be
much higher. Udo ... where are you when we need you? :-)

There are more qualified theoreticians then I.
I will try to rise to this challenge and give some observations.

From a practical view point, watching radio controlled gliders, I was able
to take advantage of that. It would tell me when I entered a thermal, the
fuselage boom would tilt up and when I fell out of one, it would go down.
I noticed that the model glider would stay in an accelerate state after is
stabilized in the thermal In a stick fixed position,
The model would have to be set up similar to a free flight model, so as not
to crash if no control input is given. To take full advantage of the thermal
I would have to use up elevator to maximize the climb and the speed would
be reduced of course, not unlike in the full size gliders.
That would indicate that the relative plane relative to the horizon has
shifted
due to the thermal. The glider wants to fly "more down hill" if no other
input is
given.

The same indicators are use when encountering sink but in reverse.
That is how I see it.
Anybody wants to put some numbers to that.
Udo

All this talk of masses,forces,accelerations,AOA changes etc is
irrelevent. Its simply a change in the apparent wind caused by the
introduction of a new vector (the thermal or sink).

Lets start with a simple example. The glider is just a point fixed in
free space. Introduce a horizontal wind of say X kmh. The glider's ASI
would register X kmh. Now move the airmass vertically (up or down -
doesnt matter) by Y kmh. The glider's ASI will show an *increase* in
speed equal to the vector addition of the X and Y components.

Now since a real glider actually flies down a slight hill this changes
the relative angles of the vectors. The thermal (or sink) is still
vertically oriented (for simplicity) but the glider's vector is tilted.
I never can remember how to set up the vector triangle so I wont try
and describe it here. But the end result is that lift causes a
proportionaly larger increase in ASI. Sink is interesting - for small
sink the ASI drops but for large sink the ASI increases. The anomaly is
dependent on the gradient of the hill.

Peter

From a practical view point, watching radio controlled gliders, I was
able
to take advantage of that. It would tell me when I entered a thermal, the
fuselage boom would tilt up and when I fell out of one, it would go down.
I noticed that the model glider would stay in an accelerate state after is
stabilized in the thermal In a stick fixed position,
The model would have to be set up similar to a free flight model, so as not
to crash if no control input is given. To take full advantage of the thermal
I would have to use up elevator to maximize the climb and the speed would
be reduced of course, not unlike in the full size gliders.
That would indicate that the relative plane relative to the horizon has
shifted due to the thermal. The glider wants to fly "more down hill" if no
other
input is given.

Is the glider simply weathervaning around the lateral (pitch) axis due to
upward pressure on the horizontal stabilizer? Once in the thermal the lift
is affecting both the wing and fuselage and tail, but might the arm of the
horiz stab cause the whole glider to rotate around the CG, i.e. nosing down?
We would feel that as more than just an upward kick in the seat, also as a
slight tilt forward. If the tilt persists, the airspeed increases. You
report seeing the boom tilt but don't see which way... I seem to recall my
R/C glider nosing up first, as the wing enters the thermal first, but maybe
the tail overcomes it a second later - I don't remember.

Fred wrote:
Just got asked this question, didn't have a quick and easy answer.
How
do you explain it?

Airspeed is not defined as the speed of the glider nor is it the speed
of the wind moving over and around it. Airspeed is the speed of an
aircraft relative to the air in which it is flying. It is the speed of
the relative airflow that is aerodynamically influencing the aircraft.

When the glider is flying in still air all of its airspeed is generated
by its motion thru the air. To move thru the air it must over come
drag. When the glider in slightly downward flight experiences a thermal
part of its airspeed is now caused by the motion of air around the
glider. To remain still in moving air you must overcome drag.

The only thing the glider has to overcome the drag caused by moving air
is its reluctance to change speed (inertia). The glider is being held
in the upward airflow by inertia. As drag overcomes the gliders inertia
it starts to accelerate upward with the airflow. This reduces the
effect of the thermal on the airspeed and aerodynamics because the
glider is now moving with the upward flow and produces no relative
motion between the two.

The glider in downward flight in still air is supported by an
aerodynamic resistance force that opposes gravity called lift. Also
like lift in particular circumstances drag can resist motion and if
that motion is downward drag is upward also resisting gravity. Lift and
drag slows its descent in still air and cause its assent while in a
thermal.

The upward acceleration of the flying glider in a thermal entry is
caused 100 percent by the component of the relative airflow caused by
the thermal. It requires a force to accelerate the glider upward. Lets
see what aerodynamic force is most accurately defined as the
aerodynamic force that is in the direction of the relative airflow that
caused it? That's right drag.

It is true that angle of attack goes up causing more lift but as far as
accelerating the glider upward this extra lift is negated by the fact
that the direction of this lift moves farther away from the upward
direction. This extra lift comes with extra drag and its direction is
more in the upward direction as a result of the thermal. The thermal
not only increases the airspeed it changes its direction.

wrote:
Fred wrote:

Just got asked this question, didn't have a quick and easy answer.

How

do you explain it?

This thread reminds me of the original explanation for malaria "bad
air". Everyone knew that he disease came from bad air wafting up from
hot swamps. It took some actual research to determine that it came from
a mosquito borne parasite.
Clearly there are lots of arm-chair (desk-chair?) explanations for why
IAS increases upon entering a thermal, but nobody *really* knows because
no experimentation has been done to figure it out. Some wise old sages
out there are certain of their explanation, and maybe they're right, or
maybe it's just bad air.
This seems like it would be a good youth-in-soaring sort of question to
solve with real science.

Shawn

I don't think it's just that important.
If you're fliying into a thermal, a wing rises, the IAS increases, etc...
just pull back to the original speed, turn into the rising wing, an FLY.
Complex aerodinamic explanations are good for winter, but look out of the
window... and go flying.

"Shawn" sdotcurry@bresnananotherdotnet escribió en el mensaje
...
wrote:
Fred wrote:

Just got asked this question, didn't have a quick and easy answer.

How

do you explain it?

This thread reminds me of the original explanation for malaria "bad
air". Everyone knew that he disease came from bad air wafting up from
hot swamps. It took some actual research to determine that it came from
a mosquito borne parasite.
Clearly there are lots of arm-chair (desk-chair?) explanations for why
IAS increases upon entering a thermal, but nobody *really* knows because
no experimentation has been done to figure it out. Some wise old sages
out there are certain of their explanation, and maybe they're right, or
maybe it's just bad air.
This seems like it would be a good youth-in-soaring sort of question to
solve with real science.

Shawn

J.A.M. wrote:
I don't think it's just that important.
If you're fliying into a thermal, a wing rises, the IAS increases, etc...
just pull back to the original speed, turn into the rising wing, an FLY.
Complex aerodinamic explanations are good for winter, but look out of the
window... and go flying.

Don't get me wrong. I don't think it's vital that this be explained.
However, insight into the dynamics of flying, and soaring in particular
might be gained by a well designed research program. A few experiments
run by some undergrad Aero E students would give them an interesting
project and answer a common question for the rest of us.
That, and the forecast for the weekend is crappy. :-)

Shawn

Are you sure you center more quickly if you turn toward the rising wing?

Frank

J.A.M. wrote:

I don't think it's just that important.
If you're fliying into a thermal, a wing rises, the IAS increases, etc...
just pull back to the original speed, turn into the rising wing, an FLY.
Complex aerodinamic explanations are good for winter, but look out of the
window... and go flying.

"Shawn" sdotcurry@bresnananotherdotnet escribió en el mensaje
...
wrote:
Fred wrote:

Just got asked this question, didn't have a quick and easy answer.

How

do you explain it?

This thread reminds me of the original explanation for malaria "bad
air". Everyone knew that he disease came from bad air wafting up from
hot swamps. It took some actual research to determine that it came from
a mosquito borne parasite.
Clearly there are lots of arm-chair (desk-chair?) explanations for why
IAS increases upon entering a thermal, but nobody *really* knows because
no experimentation has been done to figure it out. Some wise old sages
out there are certain of their explanation, and maybe they're right, or
maybe it's just bad air.
This seems like it would be a good youth-in-soaring sort of question to
solve with real science.

Shawn