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

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

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?

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----------

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

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

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

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.

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

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

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 ?