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

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

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

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

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.

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

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,
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?

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

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----------