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

[Fred]

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

[Robin Birch]

In message . com>, Fred
> writes
>Just got asked this question, didn't have a quick and easy answer. How
>do you explain it?
>
I've always thought of it as a change in the lift drag vector. If your
glider is flying in still air the lift drag vector is pointing up and
towards the tail. If rising air is entered, which effectively increases
the lift vector the new lift/drag vector points slightly more forward
than previously. This reduces the effective drag and the glider
accelerates until everything balances out again.

This may be total rubbish but it is the model I've found easiest to
visualise.

Robin
--
Robin Birch

[Shawn]

Robin Birch wrote:
> In message . com>, Fred
> > writes
>
>> Just got asked this question, didn't have a quick and easy answer. How
>> do you explain it?
>>
> I've always thought of it as a change in the lift drag vector. If your
> glider is flying in still air the lift drag vector is pointing up and
> towards the tail. If rising air is entered, which effectively increases
> the lift vector the new lift/drag vector points slightly more forward
> than previously. This reduces the effective drag and the glider
> accelerates until everything balances out again.
>
> This may be total rubbish but it is the model I've found easiest to
> visualise.

Sounds good to me. Your explanation would seem to require (to me at
least) some pitching down to make everything balance out. I've not
noticed this (maybe too excited that I've found lift). Comments from
someone more observant?

Shawn

[[email protected]]

If you have the stick in a fixed position, this translates to a fixed
AOA. If you move from still or sinking air into lift, your AOA will go
up momentarily. Assuming you do nothing with the stick, the aircraft
will seek and return to its configured AOA, which will result in a
slight pitch down of the nose and a slight increase in speed.

Said another way, the increased angle of attack also affects the
horizontal stabilizer, which mometarily produces more lift, pitching
the nose over slightly, with resulting increase in speed.

[Terry]

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

I'll stick my neck out on this.

In gliding flight, the horizontal component of lift is our "thrust"
that enables an airspeed, while the vertical component is equal to the
weight of the glider.

Once the thermal is entered, there is an increase in the total lift
vector equal to strength of the thermal. This results in an imbalance
of forces which causes the glider to accelerate to the new steady
state.

I flew for years on the east coast of the US and never noticed this
effect until moving out west. Estrella has some strong days were this
effect is very noticable, especially in clean ships. The lowly 233
exhibits the same effect, just not as noticable.

Terry Claussen
Master CFI

[Fred]

Terry: That's the way I explained it too, (& BTW, the phenomenon is
noticeable in the east too). There should be a more elegant (or
simplistic) explanation, don't you think? One that doesn't require
diagrams of lift vectors? Fred

[Edmond Dantes]

Fred,
The thermal is giving you "free lift". Since the wing now doesn't
produce as much lift, induced drag is simultaneously reduced. With
reduced drag, airspeed increases.
Hope this helps,
Brad


On 27 Mar 2005 09:33:53 -0800, "Fred" > wrote:

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

[M B]

Hmmm...
I thought that if the CG is forward, a 'bump' in lift
is behind it (where the wing center of pressure is
maybe) and so the wing is accelerated up and the nose
pitches down.

Try it with drastically different CG. I tried it with
a 240# guy up front. Big difference from the 160#
guy up front.

Anyway, that's my take on it...

At 00:00 28 March 2005, Terry wrote:
>
>Fred wrote:
>> Just got asked this question, didn't have a quick
>>and easy answer.
>How
>> do you explain it?
>================================================== ================
>>========
>
>I'll stick my neck out on this.
>
>In gliding flight, the horizontal component of lift
>is our 'thrust'
>that enables an airspeed, while the vertical component
>is equal to the
>weight of the glider.
>
>Once the thermal is entered, there is an increase in
>the total lift
>vector equal to strength of the thermal. This results
>in an imbalance
>of forces which causes the glider to accelerate to
>the new steady
>state.
>
>I flew for years on the east coast of the US and never
>noticed this
>effect until moving out west. Estrella has some strong
>days were this
>effect is very noticable, especially in clean ships.
> The lowly 233
>exhibits the same effect, just not as noticable.
>
>Terry Claussen
>Master CFI
>
>
Mark J. Boyd

[John Galloway]

It's called the Yates Effect and the mechanism described
by Yates in Gliding magazine in 1951 is basically an
expanded version of what Robin says. Derek Piggot
has an Appendix on the subject in Understanding Gliding.

The inverse is also the explanation for the more important
phenomenon (in terms of thermallling and final turn
stall/spin safety) of the loss of airspeed when we
hit sink

John Galloway

At 21:30 27 March 2005, Robin Birch wrote:
>In message , Fred
> writes
>>Just got asked this question, didn't have a quick and
>>easy answer. How
>>do you explain it?
>>
>I've always thought of it as a change in the lift drag
>vector. If your
>glider is flying in still air the lift drag vector
>is pointing up and
>towards the tail. If rising air is entered, which
>effectively increases
>the lift vector the new lift/drag vector points slightly
>more forward
>than previously. This reduces the effective drag and
>the glider
>accelerates until everything balances out again.
>
>This may be total rubbish but it is the model I've
>found easiest to
>visualise.
>
>Robin
>--
>Robin Birch
>

[Bruce Hoult]

In article >,
Edmond Dantes > wrote:

> Fred,
> The thermal is giving you "free lift". Since the wing now doesn't
> produce as much lift, induced drag is simultaneously reduced. With
> reduced drag, airspeed increases.
> Hope this helps,
> Brad

There is no such thing as "free lift". The wing/tailplane produces lift
-- all of it. If you feel a push upwards, it is the wing doing it. As
you enter the updraft you get an increased angle of atack, increased
lift, increased drag, and upwards acceleration.

As noted by others, if you leave the stick in the same place then the
speed will increase due to stability making the glider pitch down, but
thsi will only be a very temporary effect and will dissappear soon after
the glider's vertical speed has equalized with the updraft -- which is
only a matter of a second or two. Consider that it's pretty common to
feel a half-G surge on entering a strong thermal, that a G is 10 m/s per
second, and that strong thermals are 4 - 7 m/s, and and it's clear that
the glider gains the upwards velocity of the thermal pretty quickly.

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

[Terry]

Fred wrote:
> Terry: That's the way I explained it too, (& BTW, the phenomenon is
> noticeable in the east too). There should be a more elegant (or
> simplistic) explanation, don't you think? One that doesn't require
> diagrams of lift vectors? Fred

================================================== ============
Fred,

I mis-spoke (-typed) and should have said years ago, when I did not
know what I did not know. The phenomenon is just more noticable in the
big liftie out here-and that is where I first noticed it. I thought
you wanted an explanation, not an analogy and thought I did pretty good
without the diagrams and in only 30 words or so.

For talking to someone's hat while in the thermal I use:

Lift is like hitting the gas in the car. This works OK since even 14
year olds have at least a rudimentary idea of what happens in the car.
-or-
For power pilots, lift is an increase in throttle/thrust. So to climb,
we need to hold our speed by pitching up and letting the glider climb.
-or-
For someone who has sailed, I use a tacking analogy. "Take the lift"
with a pitch (or pinch) up--this utilizes another definition for the
same word that may ring the bell for the student.

In every one of these, I will be at the white board at the conclusion
of the flight, or drawing the diagram in the sand between flights if we
are waiting for another tow.

Terry Claussen

[F.L. Whiteley]

Most reasonable explanation and experienced in both weaker UK lift and
strong western US lift. In many cases, ASI response quicker than vario
response.

Frank


"John Galloway" > wrote in message
...
> It's called the Yates Effect and the mechanism described
> by Yates in Gliding magazine in 1951 is basically an
> expanded version of what Robin says. Derek Piggot
> has an Appendix on the subject in Understanding Gliding.
>
> The inverse is also the explanation for the more important
> phenomenon (in terms of thermallling and final turn
> stall/spin safety) of the loss of airspeed when we
> hit sink
>
> John Galloway
>
> At 21:30 27 March 2005, Robin Birch wrote:
> >In message , Fred
> > writes
> >>Just got asked this question, didn't have a quick and
> >>easy answer. How
> >>do you explain it?
> >>
> >I've always thought of it as a change in the lift drag
> >vector. If your
> >glider is flying in still air the lift drag vector
> >is pointing up and
> >towards the tail. If rising air is entered, which
> >effectively increases
> >the lift vector the new lift/drag vector points slightly
> >more forward
> >than previously. This reduces the effective drag and
> >the glider
> >accelerates until everything balances out again.
> >
> >This may be total rubbish but it is the model I've
> >found easiest to
> >visualise.
> >
> >Robin
> >--
> >Robin Birch
> >
>
>

[Edward Lockhart]

At 01:30 28 March 2005, Fred wrote:
>Terry: That's the way I explained it too, (& BTW,
>the phenomenon is
>noticeable in the east too). There should be a more
>elegant (or
>simplistic) explanation, don't you think? One that
>doesn't require
>diagrams of lift vectors? Fred
>
>
If you're flying a child's kite in a steady breeze
and give a quick yank on the string, the kite goes
up.

A glider is designed to convert the vertical pull of
gravity into nearly horizontal motion. The uprush of
air (change in AoA) on entering a thermal has an effect
similar to a sudden increase in gravity (if that were
possible) pulling the glider downwards through the
air. The glider converts that to an increase in forward
motion.

You are, in effect, getting a brief winch launch every
time you enter a thermal.

Ed.

[[email protected]]

One simple explanation and easy to draw for your students: The CG of
the glider is typically ahead of the "center of lift". An "up" gust
will cause nose to pitch down around the CG. Refer to "Aerodynamics
for Naval Aviators" for more details.

Be "one" with your glider. Feeling (and hearing) the slight airspeed
change is usually a faster indication of lift than your vario. So look
outside the cockpit to clear, pull and turn into the thermal core!

Burt
Marfa Gliders, west Texas USA
www.flygliders.com

[[email protected]]

I had the delightful opportunity to fly with Derek Piggott last year
before the Senior Nationals and specifically asked him about this
observation. It seems to me that a thermal is a vertical gust that
would be noticed by the wing as increase in AOA and hence the airspeed
should decrease..but it doesn't? He said, (as best as I can recollect)
that "we used to call this the 'Yates Effect' but that this has pretty
much fallen from favor. A thermal with an accelerating core creates
somewhat of a venturi that will entrain surrounding air and will
manifest itself with horizontal gusts as well as the vertical as you
enter the thermal". If I took the trouble to graft it out, I would not
be able to illustrate this on paper as it would look like a tailwind
pushing me into the thermal rather than a frontal gust, but from the
cockpit of a G103 with the Jedi-Master it seemed a perfectly simple
explanation.

Gene

[nafod40]

It's useful to look at extremes here.

If a plane was going straight down (extreme case) and flew into a 10
knot thermal, it's speed would increase by 10 knots instantaneously.

If a plane was going straight and level, and flew into a thermal, it's
speed would increase by zero knots. no increase.

For any rate of descent, the plane's speed would increase upon entry
into a thermal by some value between 0 and 10 knots, varying based on
it's rate of descent/angle when it penetrates the thermal.

[Shawn]

nafod40 wrote:
> It's useful to look at extremes here.
>
> If a plane was going straight down (extreme case) and flew into a 10
> knot thermal, it's speed would increase by 10 knots instantaneously.
>
> If a plane was going straight and level, and flew into a thermal, it's
> speed would increase by zero knots. no increase.
>
> For any rate of descent, the plane's speed would increase upon entry
> into a thermal by some value between 0 and 10 knots, varying based on
> it's rate of descent/angle when it penetrates the thermal.
>
As I understand what you're saying, the portion of AS increase results
from the increase in relative wind due to the component of the thermal
in line with the direction of flight, since the glider is descending at
an angle. I don't buy it, and here's why (It's been a long time since I
did trig but here goes.):
38:1 glider has a glide slope of about 1.5 degrees in still air flies
into a 10 kt thermal with an IAS of 50 kts.
The component of the thermals upward velocity in the direction of flight
is sin1.5x10kts=0.26 kts or 50.26 kts IAS.
I see a lot bigger jump than this (like my ASI would show a quarter knot
dif!).
OK, you math profs can tear me to shreds now.

Shawn

[Brian]

I think you are onto the right answer here.

Try thinking of it this way. with the glider sitting the ground in
flight attitude (for still air, best glide angle) the wing will be
angle down somewhat. If you put a fan directly under the wing blowing
straight up(the Thermal) it would deflect a portion of the air back
creating thrust.

In the air this only happens initually as the glider accerates upward
it is also generating addtional thrust from the thermal. For example a
glider descending at 2kts encounters a thermal going up a 6kts. before
hitting the thermal the air is going vertically past the glider at
2kts. Upon entering the thermal the vertical air is going past the
glider at 6kts. The glider will accelerate both upward and forward
until the glider is as a climb rate of 4kts and the vertical air going
past the glider is back to 2kts.

Brian
CFIIG/ASEL

[Fred]

Brian:

I'm not comfortable with that explanation. The glider goes "up" only
in relation to the earth, not in relation to the airmass. In fact, the
glider is steady in the airmass -- descending at 2 kts (more or less)
-- and we only seek out the rising airmass because it puts us higher
relative to the earth. Your explanation makes it sound as if the
glider is staying steady relative to the earth while the airmass
accelerates from 2 to 6 kts. (If this is so, it is only very
momentary.)

I think Terry and Burt have the explanation that is both
aerodynamically accurate and something a student can grasp relatively
quickly and easily.

Thanks to all. Fred

[Denis]

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 ?

[Slick]

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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[Ken Kochanski (KK)]

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

[Udo Rumpf]

"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

[[email protected]]

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

[Shawn]

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

[J.A.M.]

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

[[email protected]]

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.

[Bruce Hoult]

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----------

[Shawn]

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

[Roger Worden]

> 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.

[Dave Houlton]

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.
>

[F.L. Whiteley]

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.
>>

[[email protected]]

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.

[Shawn]

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.]

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

[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

[F.L. Whiteley]

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

[Steve]

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.

[J.A.M.]

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.

[F.L. Whiteley]

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.

[[email protected]]

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

This ain't rocket science it is basic aerodynamics. Experimentation
is worthless without a basic understanding of aerodynamic force. You
will only propagate the narrow minded misinformation found in some real
science books. Its takes life long conspiracy to get pilots (private
and commercial) so ignorant about aerodynamic force that they do not
know the simple single difference between the definition of lift and
drag. You have to start when they are young and gullible and hope they
do not practice much original thought or apply what you tell them with
actual occurrence.

When you introduce them to one of the many different inaccurate
definitions like drag is a resistance force hope they do not realize
that the major use of lift in aeronautics is to resist gravity. When
you talk about the dynamics of a balloon preoccupy them with the
aerostatic lift that it produces and do not mention the fact that it
has circumnavigated the earth using drag exclusively for horizontal
acceleration. When you tell them that lift is the aerodynamic force
that supports the weight of an airplane in flight hope that they
don't realize that if drag opposes motion and if that motion is
downward drag is in an upward direction. Explain terminal velocity of
a dropped object on another day maybe they won't put two and two
together.

Why if you were to try something novel like telling them the truth the
hole truth and nothing but the truth so help you god maybe when they
are thinking about an aerodynamic force that impedes the forward motion
or an aircraft they will have an open mind and not just jump to the
inaccurate conclusion that it is always drag. When they are thinking
about an aerodynamic force that accelerates a glider upward they will
not jump to the inaccurate conclusion that it is always lift especially
since the glider is in lift.

It is impossible for the air speed of a glider in normal flight to not
be increased by a sudden thermal. The only way that the glider will not
be affected is if it had no inertia. If the glider moved readily with
the thermal there would be no increase in motion of the glider in
relation with the upward airflow. If experimentation is needed to
figure this out how are you going to be able to figure out the
experiment?

I hate to brag but I am a walking talking aerodynamic experiment. Why
even in my sleep I possess the ingredients for aerodynamic force. I am
a solid object that is influenced by a relative airflow 24-7 as a
result of respiration among other things. Not to mention other fluid
flows like blood and urine. And I have been known to emit a little
swamp gas (what you refer to as "bad air"). To the people that I am
related to this is referred to as relative wind.

[nimbusgb]

Steve wrote:
> 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

Anyone can get outclimbed by a garbage bag. Its when the full cement
bags go by that you need to look at your thermalling techniques.

:-)

Ian

[Shawn]

wrote:
> 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
>
>
> This ain't rocket science it is basic aerodynamics. Experimentation
> is worthless without a basic understanding of aerodynamic force. You
> will only propagate the narrow minded misinformation found in some real
> science books. Its takes life long conspiracy to get pilots (private
> and commercial) so ignorant about aerodynamic force that they do not
> know the simple single difference between the definition of lift and
> drag. You have to start when they are young and gullible and hope they
> do not practice much original thought or apply what you tell them with
> actual occurrence.

snip

> I hate to brag but I am a walking talking aerodynamic experiment. Why
> even in my sleep I possess the ingredients for aerodynamic force. I am
> a solid object that is influenced by a relative airflow 24-7 as a
> result of respiration among other things. Not to mention other fluid
> flows like blood and urine. And I have been known to emit a little
> swamp gas (what you refer to as "bad air"). To the people that I am
> related to this is referred to as relative wind.

LOL

Guess you're right Spock pun intended ;-) , the current fashion is to
eschew science in favor of "common" sense.

Shawn

[Robert Ehrlich]

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

Although this thread is now pretty old, I would like to add my
contribution, for 2 reasons. The first one is that nobody among the
contributors made a clear distinction between two effects involved in
this process. The second one is to try to share with all the readers
of this newsgroup a quantitative estimate of one of these two effects.

The two effects among which I want to make a distiction are:
1) the pitch stability of the glider tend to keep it at the same
angle relatively to the air airstream, as long as the pilot doesn't
move any control that would change that. The thermal make a change
in the direction of the airstream, the stability of the glider make
it follow this change by pitching down. This is a cause of long term
airspeed increase.
2) even in the absence of the first effect, e.g. if the pilot reacts
to the pitch down tendancy (that's what I teach to my students: don't
let the thermal accelerate your glider, otherwise it will throw you
outside by increasing your turn radius), there is a change in the
aerodynamic force (vector sum of lift + drag) which causes an
immediate acceleration.

Now let's have a closer look on the 2nd effect. In order to make a
rough estimate of this effect, I want to do some first order
approximations. i.e. neglecting what I consider as second order
quantities. More precisely I will consider some quantities as "small"
compared to others, and second order quantities are those which are
"small" compared to some one which is already "small". As primary
small things I will consider that drag is small compared to lift,
and the vertical speed of the thermal (increase) is small compared
to the airspeed.

The first thing caused by the thermal is to change the relative wind
in force and direction. As the thermal velocity is almost perpendicular
to the initial velocity, the change in force is a second order change,
so I will only consider the change in direction.

The change in direction causes an identical change in angle of attack.
This change causes a change in the aerodynamic force. As before the
thermal the aerodynamic force was exactly opposite to the weight, so
that the sum of all forces was zero, after the change, the net resulting
force causing an acceleration is just the (vector) difference beteween
the new aerodynamic force and the previous one. For a quantitative
estimate, as we usually count (and feel) acceleration in "g" rather
than in m/s² (or ft/s² for metrically challenged people), what is
interesting is the relative change in this force, this gives directly
the accelaration in g.

There is a change both in direction and in intensity. Both changes are
small, so the new direction is close to the previous one. So the change
in direction provides mainly an horizontal component of the differential
force an the change in intensity mainly a vertical componemt, mainly
being understood as: the difference with the real change is a second
order quantity and may be neglected.

I assume that the glider was near its best L/D speed. In this case the
change of L/D when there is a small change of angle of attack is a
second order change. So we neglect it, i.e. we consider that the
angle between the aerodynamic force and the direction of the relative
wind doesn't change. So the change in direction of the aerodynamic force
is the same as the change in the direction of the relative wind. This
change in radians is the ratio Vz/V of the thermal velocity to the
airspeed, so this is equal to the first order to the horizontal
relative change in force dFh/F and to the horizontal accelaration in
"g".

Now what is the vertical accelaration? To the first order the relative
change dFv/F is equal to the relative change of the lift coefficient
dCl/Cl. The lift coefficient Cl is near 1 at best L/D, so we may focus
on the absolute change dCl. The aerodynamic litterature says that for
a thin plate the theorical value of dCl/da (da being the change of angle
of attack) is 2pi, and "The result, that CL changes by 2pi per radian
change of angle of attack (.1096/deg) is not far from the measured slope
for many airfoils"
(http://www.desktopaero.com/appliedaero/airfoils1/tatresults.html). This
is for an infinite aspect ratio, there is a correction factor of
AR/(AR+2) for finite aspect ratio AR which is so close to 1 for the
usual aspect ratio of most gliders that we can assumme it is 1,
especially if we approximate 2pi by 6.

So the vertical acceleration is roughly 6 times Vz/V and 6 times
the horizontal acceleration. This explains (if such an explanation was
needed :-) that we mainly feel the vertical acceleration. A glider
flying at 50 knots and encoutering a 1 knot thermal (again for
metrically challenged people) will roughly accelarate by .02 g
horizontally and .12 g vertically.