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

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.

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

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

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 a
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/appliedae...atresults.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.