Constant speed or constant attitude?

I would like to hear what other glider pilots have found to be helpful
in trying to center thermals.

It is common in gliding books to read that a constant diameter circle,
as a product of flying a constant airspeed and a constant bank, is
important in centering thermals. If the airspeed and / or bank is
allowed to vary significantly the thermal circle becomes an inconstant
oval, which can make locating and centering a thermal more difficult.

I understand this, and I think it helps in staying in contact with a
thermal once I have identified the stronger area in a thermal and am
more or less centered in it.

But while I am exploring a thermal, while I am trying to get an idea
of how the lift is varying around the circle I'm flying, while I'm
working my way towards the core of the thermal, I find it more
informative to try to fly a constant attitude and bank and allow the
airspeed to rise and fall as the lift comes and goes, and not lower
and raise the nose in response to the airspeed changes resulting from
the lift changes. I find that flying a constant attitude rather than
a constant airspeed in this task greatly simplifies the task of
locating the stronger lift and moving to it.

If I do try to keep the airspeed constant by raising and lowering the
nose as I fly through the changing lift around the thermal I usually
end up behind the changes in lift and confused about the thermal's
structure.

By keeping a constant attitude while investigating a thermal I seem to
be better able to use the lift-created airspeed changes as markers of
the thermal's structure.

Am I goofing-up my thermalling this way?

"Jim" wrote in message
...
I would like to hear what other glider pilots have found to be helpful
in trying to center thermals.

It is common in gliding books to read that a constant diameter circle,
as a product of flying a constant airspeed and a constant bank, is
important in centering thermals. If the airspeed and / or bank is
allowed to vary significantly the thermal circle becomes an inconstant
oval, which can make locating and centering a thermal more difficult.

I understand this, and I think it helps in staying in contact with a
thermal once I have identified the stronger area in a thermal and am
more or less centered in it.

But while I am exploring a thermal, while I am trying to get an idea
of how the lift is varying around the circle I'm flying, while I'm
working my way towards the core of the thermal, I find it more
informative to try to fly a constant attitude and bank and allow the
airspeed to rise and fall as the lift comes and goes, and not lower
and raise the nose in response to the airspeed changes resulting from
the lift changes. I find that flying a constant attitude rather than
a constant airspeed in this task greatly simplifies the task of
locating the stronger lift and moving to it.

If I do try to keep the airspeed constant by raising and lowering the
nose as I fly through the changing lift around the thermal I usually
end up behind the changes in lift and confused about the thermal's
structure.

By keeping a constant attitude while investigating a thermal I seem to
be better able to use the lift-created airspeed changes as markers of
the thermal's structure.

Am I goofing-up my thermalling this way?

That's pretty much what I do. Keeping a stable attitude does seem to help
visualize the location of the thermal core. However, small speed changes
don't affect the turn diameter nearly as much as small bank changes. See:
http://home.twcny.rr.com/ghernandez/turn_rad.htm

I try to carefully adjust the bank angle to move the center of my turn
towards the stronger lift. Using the typical bank angles and airspeeds, a
15 degree change in bank will either double or halve the turn radius. I
will reduce the bank 15 degrees at the weakest point in the turn and after
an interval of about 8 second have elapsed, steepen the bank 15 degrees.
Plotting this to scale shows that it moves the circle about one diameter
toward the lift. This is not my idea, I got it from someone else.

Bill Daniels

fly the airspeed.

That way you take advantage of the gusts as you fly through stronger or
weaker lift.

Al

"Jim" wrote in message
...
I would like to hear what other glider pilots have found to be helpful
in trying to center thermals.

It is common in gliding books to read that a constant diameter circle,
as a product of flying a constant airspeed and a constant bank, is
important in centering thermals. If the airspeed and / or bank is
allowed to vary significantly the thermal circle becomes an inconstant
oval, which can make locating and centering a thermal more difficult.

I understand this, and I think it helps in staying in contact with a
thermal once I have identified the stronger area in a thermal and am
more or less centered in it.

But while I am exploring a thermal, while I am trying to get an idea
of how the lift is varying around the circle I'm flying, while I'm
working my way towards the core of the thermal, I find it more
informative to try to fly a constant attitude and bank and allow the
airspeed to rise and fall as the lift comes and goes, and not lower
and raise the nose in response to the airspeed changes resulting from
the lift changes. I find that flying a constant attitude rather than
a constant airspeed in this task greatly simplifies the task of
locating the stronger lift and moving to it.

If I do try to keep the airspeed constant by raising and lowering the
nose as I fly through the changing lift around the thermal I usually
end up behind the changes in lift and confused about the thermal's
structure.

By keeping a constant attitude while investigating a thermal I seem to
be better able to use the lift-created airspeed changes as markers of
the thermal's structure.

Am I goofing-up my thermalling this way?

As for this part, Dick Johnson gave an interesting talk about it at a
safety
talk at the Region 9 Championships this year. He gave aerodynamic and
safety reasons to always use a small slip while thermaling. I can't
recall
all the details, but have been doing it ever since then and I like it.

Larry Pardue 2I

I used a little slip ( inner wing forward) before I was flying with
winglets.
Now with winglets it is different. Most winglets only tolerate max + or - 5
deg.
change in angle of attack from optimum. I try to fly now with the string as
closely as possible in the centre.
Udo

When I thermal, there generally seems to be wind,
and since the glider doesn't climb quite as
fast as the thermal, I seem to do better slipping
or changing bank angle to fly a little into the
headwind during each turn (kind of like
turns around a point in power flying).

I started using slips in thermals after I flew
with a competition pilot and noticed he did this
a little when thermalling in steeper banks. I
also sometimes slip a little "into" a thermal.
I suspect this happens mostly because I'm
getting fatigued and my coordination is
getting worse and I'd rather slip into
it than skid. But there may be some
aerodynamic reason for slipping a little.
It seems that at the extreme (in a 90 degree bank),
a slip is better than coordinated flight since it
exposes more fusealage area to the thermal when
C.G. is forward.

There is also some coriolis(SP?) effect, so I notice
on .igc traces of extended thermalling that the
thermal circles a little as it rises.

I also fly by the rule that the center third
altitudes of a thermal often provide the best lift.
Sometimes I fly in the upper third if I expect
to cross a sink area, but the middle third
has been pretty good for me.

I definitely trim for the thermal, and I've
never had a consistent thermal greater than 8 knots,
and I haven't found constant banks greater than 50
degrees useful.

Some of this comes from discussions with Serge Serfaty,
a fellow glider pilot. But he wasn't a big fan of
any uncoordinated flight :P

I've also had to use shallower banks or leave thermals
because I was getting dizzy or tired or hot or couldn't
track a fellow glider. I think these are other
factors that vary based on the pilot.

"Larry Pardue" wrote in message
...


As for this part, Dick Johnson gave an interesting talk about it at a
safety
talk at the Region 9 Championships this year. He gave aerodynamic and
safety reasons to always use a small slip while thermaling. I can't
recall
all the details, but have been doing it ever since then and I like it.

Larry Pardue 2I

Dick's point was that a glider with dihedral has an increased angle of
attack on the leading wing in a slip. (Try bending a #10 envelope to
simulate a wing with dihedral. Look at it from the front and then yaw it as
in a slip - you'll see the AOA difference between the wings.) If you use a
slip instead of top aileron, the wing's airfoil works better without the
deflection of the ailerons. You are also holding top rudder, which in the
event of a stall, would make the glider roll out of the turn - easier to
recover.

Really good talk, Dick.

Bill Daniels

fast as the thermal, I seem to do better slipping
or changing bank angle to fly a little into the
headwind during each turn (kind of like
turns around a point in power flying).

Hmm! Don't understand this and haven't ever done it.

The best way to explain this is to show an extreme example.
Assume a constant wind from the North at 10 knots, and
a stationary source thermal. Also assume that the lift in the
thermal is exactly the same at every altitude, and
the thermal has constant diameter.

The thermal is now a column that tilts south as it rises,
but the column never moves, rather like a tall leaning
tower of Piza (sp?). It remains fixed relative to the
ground.

Now assume that a perfectly centered glider in the thermal
has just enough lift to remain at a given altitude.

If the glider keeps exactly the same bank angle and pitch and
rudder on every 360, the glider will drift with the
wind and exit downwind of the thermal and begin to sink.

So the pilot should extend the upwind time and decrease the
downwind time, to fly a perfect ground reference circle and
remain in the thermal. The ASEL practical test asks pilots to
shallow the bank into the wind and steepen it on
downwind to accomplish this.

I'd guess a lot of competition pilots probably don't
consciously think of this, as their actions to center a thermal
are so subtle that changes in airspeed and direction with
altitude override the primitive assumptions presented here.

Mark Boyd

Your example is not working.

If there is no wind, a perfect thermal would rise vertically and you fly
constant circles to stay in.
If you have a constant wind, the whole airmass - including the thermal -
drifts with the wind. If you stay with your constant circles, you drift at
the same speed as the thermal so you stay perfectly centered.
Just basic vector addition.

Corrections are made because there is no ideal thermal, but corrections are
made into the core, regardless of the direction of wind.
Corrections into the wind are made if the thermal is of orographic, i.e.
rotors.

--
Bert Willing

ASW20 "TW"


"Mark James Boyd" a écrit dans le message de
...
fast as the thermal, I seem to do better slipping
or changing bank angle to fly a little into the
headwind during each turn (kind of like
turns around a point in power flying).

Hmm! Don't understand this and haven't ever done it.

The best way to explain this is to show an extreme example.
Assume a constant wind from the North at 10 knots, and
a stationary source thermal. Also assume that the lift in the
thermal is exactly the same at every altitude, and
the thermal has constant diameter.

The thermal is now a column that tilts south as it rises,
but the column never moves, rather like a tall leaning
tower of Piza (sp?). It remains fixed relative to the
ground.

Now assume that a perfectly centered glider in the thermal
has just enough lift to remain at a given altitude.

If the glider keeps exactly the same bank angle and pitch and
rudder on every 360, the glider will drift with the
wind and exit downwind of the thermal and begin to sink.

So the pilot should extend the upwind time and decrease the
downwind time, to fly a perfect ground reference circle and
remain in the thermal. The ASEL practical test asks pilots to
shallow the bank into the wind and steepen it on
downwind to accomplish this.

I'd guess a lot of competition pilots probably don't
consciously think of this, as their actions to center a thermal
are so subtle that changes in airspeed and direction with
altitude override the primitive assumptions presented here.

Mark Boyd

If you have a constant wind, the whole airmass - including the thermal -
drifts with the wind.

Depends
Ground disconnected bubbles may drift.
Ground connected weaker thermals may have a slight tilt.
Ground related strong thermals can be rock steady, with
the wind blowing around it
Chris
Melbourne



"Bert Willing" wrote in
message ...
Your example is not working.

If there is no wind, a perfect thermal would rise vertically and you fly
constant circles to stay in.
If you have a constant wind, the whole airmass - including the thermal -
drifts with the wind. If you stay with your constant circles, you drift at
the same speed as the thermal so you stay perfectly centered.
Just basic vector addition.

Corrections are made because there is no ideal thermal, but corrections
are
made into the core, regardless of the direction of wind.
Corrections into the wind are made if the thermal is of orographic, i.e.
rotors.

--
Bert Willing

ASW20 "TW"


"Mark James Boyd" a écrit dans le message de
...
fast as the thermal, I seem to do better slipping
or changing bank angle to fly a little into the
headwind during each turn (kind of like
turns around a point in power flying).

Hmm! Don't understand this and haven't ever done it.

The best way to explain this is to show an extreme example.
Assume a constant wind from the North at 10 knots, and
a stationary source thermal. Also assume that the lift in the
thermal is exactly the same at every altitude, and
the thermal has constant diameter.

The thermal is now a column that tilts south as it rises,
but the column never moves, rather like a tall leaning
tower of Piza (sp?). It remains fixed relative to the
ground.

Now assume that a perfectly centered glider in the thermal
has just enough lift to remain at a given altitude.

If the glider keeps exactly the same bank angle and pitch and
rudder on every 360, the glider will drift with the
wind and exit downwind of the thermal and begin to sink.

So the pilot should extend the upwind time and decrease the
downwind time, to fly a perfect ground reference circle and
remain in the thermal. The ASEL practical test asks pilots to
shallow the bank into the wind and steepen it on
downwind to accomplish this.

I'd guess a lot of competition pilots probably don't
consciously think of this, as their actions to center a thermal
are so subtle that changes in airspeed and direction with
altitude override the primitive assumptions presented here.

Mark Boyd

Bert Willing wrote:

Your example is not working.

If there is no wind, a perfect thermal would rise vertically and you fly
constant circles to stay in.
If you have a constant wind, the whole airmass - including the thermal -
drifts with the wind. If you stay with your constant circles, you drift at
the same speed as the thermal so you stay perfectly centered.
Just basic vector addition.

Corrections are made because there is no ideal thermal, but corrections are
made into the core, regardless of the direction of wind.
Corrections into the wind are made if the thermal is of orographic, i.e.
rotors.


Nevertheless Helmut Reichman in his famous book says the same thing
as Mark James Boyd. Except he mentions also a case where you have to
do the opposite. You are right that the thermal drifts with the wind
but the glider sinks in the thermal. You may either figure the thermal
as an oblique column of rising or a sequence of bubbles rising while
drifting downwind and so connected by an oblique line. In both cases
sinking in the thermal will bring you below it, and in order to get back
into the column or the next bubble, you have to move upwind. The case
above mentionned where you have to do the opposite is the case of a
continuous column with a strong maximum, below which the lift is weaker
and converging, as well a weaker and diverging above it. In this case,
despite your sink in the thermal, it will bring the glider in the upper
part of weaker lift, and at this height the stronger lift is downwind.

A thing that Mark James Boyd made me discover is that the needed upwind
move is higher with lower climb speed, up to completely cancelling the
drift when the climb speed is zero.