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

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.

Shawn

It may be that the glider tend to accelerate when enters a thermal? If you
mantain a constant AoA and you find yourself into increased lift, the
logical thing (as I see it) would be an airspeed increase, doesn't it? I
usually pull up entering a thermal, even when not transitioning fast...
Maybe the explanation is more esotheric, aerodinamic, or even magical...

"Shawn" sdotcurry@bresnananotherdotnet escribió en el mensaje
...
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.

Shawn

Let me suggest a thought experiment to explain why airspeed increases
when entering a thermal.

For simplicity let's assume that we are flying in trimmed, steady,
unaccelerated flight descending at 2 knots and we fly into a sharp
edged area of rising air. That is, we can continue to climb while
flying straight ahead. What happens?

When the wing encounters the thermal, the angle of attack increases
suddenly. This increase creates more lift, provided we're not stalled.
The weight of the glider hasn't increased so Newton's 2nd law dictates
that the glider accelerates upward (g's in the seat of your pants). I
realize that there are other factors. For example, the pitching moment
of the wing changes with a change in AoA. The pitching moment due to
the distance between the CG and the center of pressure of the wing
changes because lift increases with AoA (assuming the wing isn't
stalled). The downwash on the tail changes with the change in
circulation about the wing. Etc. But these changes will be
overshadowed a fraction of a second later because so far only the wing
is in the thermal.

Next the horizontal tail encounters the gust with twice the area of the
vertical tail with the same moment arm. It's also intended to produce
its lift in the downward direction. (This is exactly analogous to a
glider weather vanning into a cross wind on the ramp except that the
vertical tail isn't producing any lift until the crosswind starts and
gravity isn't going to cause the glider to pick up speed on the ramp.)
The same thermal edge that increased the angle of attack on the wing
decreases the AoA (in this scenario horizontal tail AoA is upside down)
on the tail and hence, down force on the tail. So the glider pitches
nose down.

Pitching forward quickly reduces that angle of attack and the upward
acceleration stops. But the glider has been disturbed from its trimmed
combination of airspeed, angle of attack and weight. Assuming that
the stick is not changed, the glider's airspeed will increase. As the
airspeed over the tail increases it produces more down force and the
glider begins to pitch up again and slow down(static stability).

At some point the glider will slow down and start to climb at the rate
of the thermal minus the original 2 knot descent rate and it will be
back in trim.

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.

--
Bruce | 41.1670S | \ spoken | -+-
Hoult | 174.8263E | /\ here. | ----------O----------

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.

I can see that as a possibility too. Now I've read three plausible
hypotheses. Anyone have any data?

Shawn

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.