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

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

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

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

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

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 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 ?