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.

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.

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

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 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

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