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

This seems like a lot of effort to explain how the vertical component of
a thermal may be seen as a horizontal component on your ASI. Isn't it
simpler just to assume that the thermal *has* a horizontal component,
and that's what your ASI reports, momentarily, as you enter it? In
every thermal I've actually seen (i.e. every dust-devil), the apparent
horizontal component of the air movement is at least as great as the
vertical component.

Of course, if this hypothesis were true, you'd be as likely to see your
ASI drop as rise, as you enter...

Bruce Hoult wrote:
In article ,
Shawn sdotcurry@bresnananotherdotnet wrote:


wrote:

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.

Check one of my earlier posts in this thread for the math. A 10 kt
thermal will change the IAS of a 38:1 glider by about 1/4 kt. Something
else is going on.


Your calculation took into account only the fact that the glider is
going slightly downhill, so the vertical gust increases the airsped in
the direction the glider is travelling very slightly. He's talking
about something else -- basically that your airspeed indicator doesnt'
in fact measure the speed of the glider in a direction parallel to the
fuselage centerline. It in fact registers *any* airflow that comes
more-or-less from the front, even if it is at a reasonable angle to the
fuselage centerline. Airspeed indicators are designed that way on
purpose so that changes in AOA or small slip angles don't cause the
airspeed indicator to read differently.

Suppose you're flying at 50 knots in a glider with infinite L/D and hit
a 10 knot thermal. After a second or two the glider will have
accelerated upwards and come to equilibrium with the thermal, but the
instantaneous effect is that the total wind is now a little stronger --
sqrt(50^2 + 10^2) = 50.9902 knots -- at an angle to the fuselage
centerline of arctan(10/50) or about 10 degres.

If you have less than infinite L/D then the increase will be a little
more.

Hi Dave,

Most thermals are not dust devils, but plumes with little or no rotation.
Coriolis effect does not apply at this scale either.

Frank

Dave Houlton wrote:

This seems like a lot of effort to explain how the vertical component of
a thermal may be seen as a horizontal component on your ASI. Isn't it
simpler just to assume that the thermal *has* a horizontal component,
and that's what your ASI reports, momentarily, as you enter it? In
every thermal I've actually seen (i.e. every dust-devil), the apparent
horizontal component of the air movement is at least as great as the
vertical component.

Of course, if this hypothesis were true, you'd be as likely to see your
ASI drop as rise, as you enter...

Bruce Hoult wrote:
In article ,
Shawn sdotcurry@bresnananotherdotnet wrote:


wrote:

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.

Check one of my earlier posts in this thread for the math. A 10 kt
thermal will change the IAS of a 38:1 glider by about 1/4 kt. Something
else is going on.


Your calculation took into account only the fact that the glider is
going slightly downhill, so the vertical gust increases the airsped in
the direction the glider is travelling very slightly. He's talking
about something else -- basically that your airspeed indicator doesnt'
in fact measure the speed of the glider in a direction parallel to the
fuselage centerline. It in fact registers *any* airflow that comes
more-or-less from the front, even if it is at a reasonable angle to the
fuselage centerline. Airspeed indicators are designed that way on
purpose so that changes in AOA or small slip angles don't cause the
airspeed indicator to read differently.

Suppose you're flying at 50 knots in a glider with infinite L/D and hit
a 10 knot thermal. After a second or two the glider will have
accelerated upwards and come to equilibrium with the thermal, but the
instantaneous effect is that the total wind is now a little stronger --
sqrt(50^2 + 10^2) = 50.9902 knots -- at an angle to the fuselage
centerline of arctan(10/50) or about 10 degres.

If you have less than infinite L/D then the increase will be a little
more.

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

On 5 Apr 2005 20:11:57 -0700, wrote:

Very Nicely said...

That would explain why I get outclimbed by a garbage bag that got
sucked up in a thermal.

Steve


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.

Not always... the technique is sutile and differs from pilot to pilot. Neve
been able to summarize it and in the end always ends the same way, if you
want to learn how to thermal... fly a lot of thermals and try a lot of
approaches to it.
One thing for sure. Turning away from the lift won't make it.

"F.L. Whiteley" escribió en el mensaje
...
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.

Actually, one notable pilot gave a demo at the SSA convention once that
turning away from the rising wing is exactly how to center more quickly.
He made a convincing argument for the technique.

Frank

J.A.M. wrote:

Not always... the technique is sutile and differs from pilot to pilot.
Neve been able to summarize it and in the end always ends the same way, if
you want to learn how to thermal... fly a lot of thermals and try a lot of
approaches to it.
One thing for sure. Turning away from the lift won't make it.

"F.L. Whiteley" escribió en el mensaje
...
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.