Constant speed or constant attitude?

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.