Bronze Badge question

[[email protected]]

80) What is the approximate proper airspeed to use when flying between
thermals on a cross-country flight against a headwind?

a) The best lift/drag speed increased by one-half the estimated wind
velocity.
b) The minimum sink speed increased by one-half the estimated wind
velocity.
c) The best lift/drag speed with no regard to wind velocity.

The SAA safety site gives the correct answer as "A". I have alway
heard that when flying between clouds you did not consider the wind in
your calculations. Maybe the tricky part of this question is the word
"Thermals".

[John Cotter]

As you'll see in questions 199 & 200, inter-thermal speed is
independent of wind direction when on a XC flight. Headwind only
impacts speed to fly when flying to a goal on the ground outside the
moving reference frame of the airmass. You are correct and there is a
mistake in the quiz answer.

wrote:
> 80) What is the approximate proper airspeed to use when flying between
> thermals on a cross-country flight against a headwind?
>
> a) The best lift/drag speed increased by one-half the estimated wind
> velocity.
> b) The minimum sink speed increased by one-half the estimated wind
> velocity.
> c) The best lift/drag speed with no regard to wind velocity.
>
> The SAA safety site gives the correct answer as "A". I have alway
> heard that when flying between clouds you did not consider the wind in
> your calculations. Maybe the tricky part of this question is the word
> "Thermals".

[mike]

1/2 Wind velocity should be included.

A better answer though may be "speed to fly' plus 1/2 the wind velocity.

[For Example John Smith]

There's not an answer with "McCready" in it anywhere?
The answer from an XC perspective is to fly the appropriate McCready speed
in order optimize the total distance you can cover in your soaring day.
Answer "A" means you arrive at the next thermal with the maximum possible
altitude retained--not likely a goal of anything but the most conservative,
short XC flight.

> wrote in message
ups.com...
> 80) What is the approximate proper airspeed to use when flying between
> thermals on a cross-country flight against a headwind?
>
> a) The best lift/drag speed increased by one-half the estimated wind
> velocity.
> b) The minimum sink speed increased by one-half the estimated wind
> velocity.
> c) The best lift/drag speed with no regard to wind velocity.
>
> The SAA safety site gives the correct answer as "A". I have alway
> heard that when flying between clouds you did not consider the wind in
> your calculations. Maybe the tricky part of this question is the word
> "Thermals".
>

[jcarlyle]

The key is the phrase "cross-country", because it means that you're
trying to get somewhere. If you want to stay up the longest time, then
answer c) would be the best one of the 3 given. But if you want to
cover a course in the shortest time, then a) is the best answer.

-John

wrote:
> 80) What is the approximate proper airspeed to use when flying between
> thermals on a cross-country flight against a headwind?
>
> a) The best lift/drag speed increased by one-half the estimated wind
> velocity.
> b) The minimum sink speed increased by one-half the estimated wind
> velocity.
> c) The best lift/drag speed with no regard to wind velocity.
>
> The SAA safety site gives the correct answer as "A". I have alway
> heard that when flying between clouds you did not consider the wind in
> your calculations. Maybe the tricky part of this question is the word
> "Thermals".

[John Cotter]

Looking forward to racing against you guys who fly faster into a
headwind while out on course :-) Any additional distance achieved by
doing that will be given up drifting backwards in the next (longer)
climb. For this reason MacCready speed to fly doesn't account for
headwind only estimated strength of next thermal. Best avg XC speed
will be achieved through answer c.

John

[jcarlyle]

You're on, John. In identical aircraft, you fly best L/D, and I'll fly
best L/D plus half the headwind. According to a spreadsheet called
Polar.zip that implements Reichmann's Best Speed equations, in an
ASW-19 with a best L/D of 50 knots, the interthermal flying times and
climbing times are as follows:

50 55 60 65 70 knots
5.92 5.38 4.93 4.55 4.23 minutes, interthermal glide
1.53 1.54 1.61 1.72 1.87 minutes, climbing

As you can see, I'll beat you with any headwind at all. Incidentally,
Speed to Fly for this case is 80 knots.

-John


John Cotter wrote:
> Looking forward to racing against you guys who fly faster into a
> headwind while out on course :-) Any additional distance achieved by
> doing that will be given up drifting backwards in the next (longer)
> climb. For this reason MacCready speed to fly doesn't account for
> headwind only estimated strength of next thermal. Best avg XC speed
> will be achieved through answer c.
>
> John

[chipsoars]

For McCready Theory Reichmann would have you add 50fpm in 15mph, 100 in
25mph and 200fpm at 35mph winds.

A great visual analysis using glider polars with the effects of
headwinds and tailwinds and rising/sinking air can be found in Bob
Wander's Glider Polars and Speed-To-Fly. Bottom line summary by Bob,
add 1/2 to 2/3 of the estimated head wind component to the variometer
speed to fly indication.

And generally, flying a little faster is less of a penalty than slower.
Now maybe if I actually would do that at my next contest, I won't end
up at the bottom of the score sheet ;-}

Chip F.

[Tony Verhulst]

> .... Best avg XC speed
> will be achieved through answer c.

Ok, answer C says "The best lift/drag speed with no regard to wind
velocity.".

So, my best lift/drag speed is 50 knots and I'm flying into a 50 knot
headwind at 50 indicated. And, I'm going ... where? "A" is the correct
answer, IMHO. Answer "A", in the above example, would get you a ground
speed of 25 knots (75 - 50)- you're not going to do much better than
that. The question, though, is poorly worded.

Tony V.

[John Cotter]

To quote page 116 of Reichmann's "Cross-Country Soaring" 7th edition,

"The problem is definitely a different one from that of greatest
distance described above. Then we were optimizing for distance; now we
are optimizing for cruise speed: how fast should we fly from thermal to
thermal to realize the best average speed. Since the best cruise
airspeed will result in the best ground-speed as well, there is no need
to calculate the effects of wind."


Tony Verhulst wrote:
> > .... Best avg XC speed
> > will be achieved through answer c.
>
> Ok, answer C says "The best lift/drag speed with no regard to wind
> velocity.".
>
> So, my best lift/drag speed is 50 knots and I'm flying into a 50 knot
> headwind at 50 indicated. And, I'm going ... where? "A" is the correct
> answer, IMHO. Answer "A", in the above example, would get you a ground
> speed of 25 knots (75 - 50)- you're not going to do much better than
> that. The question, though, is poorly worded.
>
> Tony V.

[Greg Arnold]

Tony Verhulst wrote:
> > .... Best avg XC speed
>> will be achieved through answer c.
>
> Ok, answer C says "The best lift/drag speed with no regard to wind
> velocity.".
>
> So, my best lift/drag speed is 50 knots and I'm flying into a 50 knot
> headwind at 50 indicated. And, I'm going ... where? "A" is the correct
> answer, IMHO. Answer "A", in the above example, would get you a ground
> speed of 25 knots (75 - 50)- you're not going to do much better than
> that. The question, though, is poorly worded.
>
> Tony V.


Ignore the ground, and just focus on the airmass. To get the maximum
distance in that airmass in a given time, you want to fly MacCready. To
the extent the wind is blowing in the wrong direction, your destination
on the ground will be further away in relation to the airmass. But to
get to the destination, you still will want to make the maximum distance
through the airmass. Flying faster than MacC will just reduce your
distance through the airmass, and that will also reduce your distance
toward your destination.

However, this assumes that thermals are fixed with respect to the
airmass. In fact, to stay in a thermal you have to keep moving upwind
in relation to the airmass. This means that thermalling will give you
some extra distance toward your goal. So you want to spend more time
thermalling and less time cruising, which you get by increasing your
speed somewhat. It seems counterintuitive that you want to spend more
time thermalling when you have a headwind, but I believe that is the
correct answer. When you are thermalling, a headwind tends to move you
upwind in relation to the airmass. A headwind does not have the same
effect when you are cruising.

[Eric Greenwell]

Greg Arnold wrote:

>
> However, this assumes that thermals are fixed with respect to the
> airmass. In fact, to stay in a thermal you have to keep moving upwind
> in relation to the airmass. This means that thermalling will give you
> some extra distance toward your goal. So you want to spend more time
> thermalling and less time cruising, which you get by increasing your
> speed somewhat.

How much slower than the airmass do you think the thermals are moving? I
don't notice the wind determined by circling to be significantly
different than the wind determined by cruising, according to my
Cambridge 302. And also, why would you get blown out of a thermal? Since
you are circling in it, shouldn't you drift at it's speed, instead of
the wind speed?


--
Change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA

[TTaylor at cc.usu.edu]

This is like asking: What is the air-speed velocity of an unladen
swallow?

There are many more questions to ask before you can give an answer.
All the answers are wrong that are given on the exam. But there are
many more factors that would need to be answered:

1. Is this an intermediate leg or the final leg?
2. How strong is the average thermal?
3. If it is the final leg how close am I to final glide?
4. How strong is the wind?
5. Is there a wind gradient?

If this was meant to assume an intermediate leg, you need to climb back
to altitude at the end of the glide, no appreciable wind gradient, and
the wind is less than speed to fly then the correct answer is the
MacCready speed for the next expected thermal.

Tim

[Jack]

Greg Arnold wrote:

> When you are thermalling, a headwind tends to move you
> upwind in relation to the airmass.

Why then does my GPS record very nice little loops, the centers
of which parallel the movement of the average wind during thermaling
and move DOWNwind?



Jack

[Paul Remde]

Hi,

I agree with Helmut Reichmann on this topic. Thermals move with the wind.
Optimizing speed between them has nothing to do with the wind speed. If you
were flying downwind between thermals would you slow down? No.

Of course it is all different when you are on final glide. At that point
you do want to factor in the effect of wind if you are trying to optimize
distance. All the final glide computers that I am aware of have no way to
tell them than you are on final glide and always give speed to fly commands
for interthermal flying. That is because they are helping you optimize
speed around a task, not distance from a given point. They also work well
to help you determine at what altitude to leave the last thermal. Just set
the MacCready setting on the final glide computer or software to your actual
climb rate in the last thermal. When it says you can make it home you
should leave. That will optimize your final glide time. That is also from
Reichmann's excellent book.

Good Soaring,

Paul Remde
Cumulus Soaring, Inc.
http://www.cumulus-soaring.com

"John Cotter" > wrote in message
ups.com...
> To quote page 116 of Reichmann's "Cross-Country Soaring" 7th edition,
>
> "The problem is definitely a different one from that of greatest
> distance described above. Then we were optimizing for distance; now we
> are optimizing for cruise speed: how fast should we fly from thermal to
> thermal to realize the best average speed. Since the best cruise
> airspeed will result in the best ground-speed as well, there is no need
> to calculate the effects of wind."
>
>
> Tony Verhulst wrote:
>> > .... Best avg XC speed
>> > will be achieved through answer c.
>>
>> Ok, answer C says "The best lift/drag speed with no regard to wind
>> velocity.".
>>
>> So, my best lift/drag speed is 50 knots and I'm flying into a 50 knot
>> headwind at 50 indicated. And, I'm going ... where? "A" is the correct
>> answer, IMHO. Answer "A", in the above example, would get you a ground
>> speed of 25 knots (75 - 50)- you're not going to do much better than
>> that. The question, though, is poorly worded.
>>
>> Tony V.
>

[jettester]

Correct Test Answer is A)

You'd better consider winds throughout flight, as well as inter
thermal..

You'd be smart to use MacCready (correct spelling of Paul's name by the
way) speed estimate plus 1/2 headwind component. If not, I'll beat you
around the course. My Cambridge computer factors in wind component.

Kansas Example:

Winds 40K at altitude / 1-26 sailplane.. L/D of 45K for argument..

c) if I fly at L/D of 45K - I'm going forward at 5K.
b) Min sink of lets say 40K.. plus 1/2 of 40K headwind - I'm going
ahead at 20K.. better
a) L/D speed of 45K plus 1/2 of 40K (20K) equals 65KIAS - I'm going
ahead at 25K .. best.

Best answer for the question.. yet should still use MacCready speed
estimate plus 1/2 headwind component.

Reference FAA Glider Flying Handbook, FAA-H-8083-13; pge 7-34



wrote:
> 80) What is the approximate proper airspeed to use when flying between
> thermals on a cross-country flight against a headwind?
>
> a) The best lift/drag speed increased by one-half the estimated wind
> velocity.
> b) The minimum sink speed increased by one-half the estimated wind
> velocity.
> c) The best lift/drag speed with no regard to wind velocity.
>
> The SAA safety site gives the correct answer as "A". I have alway
> heard that when flying between clouds you did not consider the wind in
> your calculations. Maybe the tricky part of this question is the word
> "Thermals".

[Greg Arnold]

Jack wrote:
> Greg Arnold wrote:
>
>> When you are thermalling, a headwind tends to move you upwind in
>> relation to the airmass.
>
> Why then does my GPS record very nice little loops, the centers
> of which parallel the movement of the average wind during thermaling
> and move DOWNwind?
>
>
>
> Jack

It might be just my imperfect thermalling technique, but it seems to me
that slight upwind adjustments must be made to keep from falling out of
the thermal. This is due to the fact, I believe, that you are not going
up as fast as the air, so you are sinking into a part of the thermal
that has left the ground more recently, and so has not drifted as far
downwind.

But what do I know?

[Greg Arnold]

Eric Greenwell wrote:
> Greg Arnold wrote:
>
>>
>> However, this assumes that thermals are fixed with respect to the
>> airmass. In fact, to stay in a thermal you have to keep moving upwind
>> in relation to the airmass. This means that thermalling will give you
>> some extra distance toward your goal. So you want to spend more time
>> thermalling and less time cruising, which you get by increasing your
>> speed somewhat.
>
> How much slower than the airmass do you think the thermals are moving? I
> don't notice the wind determined by circling to be significantly
> different than the wind determined by cruising, according to my
> Cambridge 302.

I don't think computers are accurate enough to note the difference, and
the difference would be small in any event. Maybe a knot or two. I
don't know if the difference is significant enough to affect the way
that people fly, but my point was that it is not strictly true that we
want to ignore wind speed.

And also, why would you get blown out of a thermal? Since
> you are circling in it, shouldn't you drift at it's speed, instead of
> the wind speed?

No, because you aren't rising as quickly as the air, so you are sinking
into air that has left the ground more recently and that hasn't drifted
as far downwind. Also, see Todd's comment about wind shear.

[Denis]

Eric Greenwell a écrit :

> How much slower than the airmass do you think the thermals are moving? I
> don't notice the wind determined by circling to be significantly
> different than the wind determined by cruising, according to my
> Cambridge 302.

My Lx5000 seems to find a slightly higher wind speed in straight flight than in circling (about 1/4). But it depends on the settings (I use combination of drift and speeds) and there may be an error in Vi as measured by the computer, or in conversion to Vp, that would induce an error in the wind in straight flight, whereas the wind in climb is not subject to these errors.

Anyway, as Todd told it's not worth any correction to Mac Cready setting.

> And also, why would you get blown out of a thermal? Since
> you are circling in it, shouldn't you drift at it's speed, instead of
> the wind speed?

THat's perfectly true if the thermal is vertical, which should be the case if it is not triggered by a ground feature, and with no significant wind shear. In the other case, as you climb slower than the air in the thermal, you will get under it and have to correct upwind to find it again (another solution for us impure pilots is start the engine to climb again in the original part of the thermal without ajust our circling ;-) )

Denis

[Denis]

T o d d P a t t i s t a é c r i t :

> "Paul Remde" > wrote:
>
>> I agree with Helmut Reichmann on this topic. Thermals move with the wind.
>> Optimizing speed between them has nothing to do with the wind speed. If you
>> were flying downwind between thermals would you slow down? No.
>
> I completely agree with Paul on this.
>
>> Of course it is all different when you are on final glide.
>
> And mostly (but not completely) on this point.

+1

The best speed is *always* given by McCready rule. The problem being that if the wind is too strong (or the lift too poor), the best cross country speed available is *negative* !

This appears when the speed-to-fly is lower than the best-glide-speed-with-headwind (which does increase with wind strength) :
- in this case, forget McCready and leave your thermal at best-glide-speed-with-headwind, either to the goal or to a field before it ;-)
- if not, forget the wind and continue climbing until you reach the glide path given by setting you McCready to the current average rate of climb.

Denis

[Eric Greenwell]

Denis wrote:

>> And also, why would you get blown out of a thermal? Since you are
>> circling in it, shouldn't you drift at it's speed, instead of the wind
>> speed?
>
>
> THat's perfectly true if the thermal is vertical, which should be the
> case if it is not triggered by a ground feature, and with no significant
> wind shear. In the other case, as you climb slower than the air in the
> thermal, you will get under it and have to correct upwind to find it
> again (another solution for us impure pilots is start the engine to
> climb again in the original part of the thermal without ajust our
> circling ;-) )

How high do you think this effect (the acceleration of the thermal
until it matches the wind speed) persists? I would expect the thermal
drift to match the wind speed in less than 2000 feet agl, based on
observations of dust devils in our area.


--
Change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA

[bagmaker]

Denis wrote:

And also, why would you get blown out of a thermal? Since you are
circling in it, shouldn't you drift at it's speed, instead of the wind
speed?


THat's perfectly true if the thermal is vertical, which should be the
case if it is not triggered by a ground feature, and with no significant
wind shear. In the other case, as you climb slower than the air in the
thermal, you will get under it and have to correct upwind to find it
again (another solution for us impure pilots is start the engine to
climb again in the original part of the thermal without ajust our
circling ;-) )

How high do you think this effect (the acceleration of the thermal
until it matches the wind speed) persists? I would expect the thermal
drift to match the wind speed in less than 2000 feet agl, based on
observations of dust devils in our area.


--
Change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA
[color=blue]



I beleive the windspeed to be changing through height gradient, sometimes decreasing, usually increasing with altitude, thus a rising thermal mass will always be chasing the speed of the wind, not meeting it.
Thus we can often experience thermic wave, atop our thermal.

Just my 2c.

[Andy]

I'm curious to know your contest record. Also curious to know what
Cambridge computer you use. I thought I had the latest one.

Andy (GY)

[Eric Greenwell]

bagmaker wrote:
> Eric Greenwell Wrote:

>>How high do you think this effect (the acceleration of the thermal
>>until it matches the wind speed) persists? I would expect the thermal
>>drift to match the wind speed in less than 2000 feet agl, based on
>>observations of dust devils in our area.

>
> I beleive the windspeed to be changing through height gradient,
> sometimes decreasing, usually increasing with altitude, thus a rising
> thermal mass will always be chasing the speed of the wind, not meeting
> it.

True, but the variations I see are usually small enough, that I think
the thermal quickly adjusts so the difference is always less than 2
knots. I picked 2 knots because that is the kind of difference I
sometimes see in the wind measurements between circling and cruising
flight. But, perhaps there is a way to measure this:

* the pilot leaves the thermal
* he begins to circle again as soon as he is out of the lift
* after about 4 circles, he continues on his flight
* post-flight, the wind drift in the thermal and out of the thermal can
be compared from the flight record

--
Change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA

[Denis]

Eric Greenwell a écrit :
> Denis wrote:
>
>>> And also, why would you get blown out of a thermal? Since you are
>>> circling in it, shouldn't you drift at it's speed, instead of the
>>> wind speed?
>>
>>
>> THat's perfectly true if the thermal is vertical, which should be the
>> case if it is not triggered by a ground feature, and with no
>> significant wind shear. In the other case, as you climb slower than
>> the air in the thermal, you will get under it and have to correct
>> upwind to find it again (another solution for us impure pilots is
>> start the engine to climb again in the original part of the thermal
>> without ajust our circling ;-) )
>
> How high do you think this effect (the acceleration of the thermal until
> it matches the wind speed) persists? I would expect the thermal drift to
> match the wind speed in less than 2000 feet agl, based on observations
> of dust devils in our area.

I don't really know. I found up to 10 kmh difference but it might be measuring errors.

But even if the thermal moves at wind speed, if it is continuously climbing from the same ground point, it will be oblique (downwind from its triggering point) hence the need to correct upwind from time to time...

Denis

[Eric Greenwell]

Denis wrote:
> Eric Greenwell a écrit :
>
>> Denis wrote:
>>
>>>> And also, why would you get blown out of a thermal? Since you are
>>>> circling in it, shouldn't you drift at it's speed, instead of the
>>>> wind speed?
>>>
>>>
>>>
>>> THat's perfectly true if the thermal is vertical, which should be the
>>> case if it is not triggered by a ground feature, and with no
>>> significant wind shear. In the other case, as you climb slower than
>>> the air in the thermal, you will get under it and have to correct
>>> upwind to find it again (another solution for us impure pilots is
>>> start the engine to climb again in the original part of the thermal
>>> without ajust our circling ;-) )
>>
>>
>> How high do you think this effect (the acceleration of the thermal
>> until it matches the wind speed) persists? I would expect the thermal
>> drift to match the wind speed in less than 2000 feet agl, based on
>> observations of dust devils in our area.
>
>
> I don't really know. I found up to 10 kmh difference but it might be
> measuring errors.
>
> But even if the thermal moves at wind speed, if it is continuously
> climbing from the same ground point, it will be oblique (downwind from
> its triggering point) hence the need to correct upwind from time to time...

It would be oblique with respect to the airmass until it had the same
speed; then, it would be vertical. It would always look oblique to
someone looking at the thermal (if it had dust or gliders in it, for
example) while standing on the ground. Once the thermal and the airmass
are moving at the same speed, there would be no need to correct upwind.

What I'm not sure about is how long it takes for the thermal to speed up
to the airmass speed. What do you think of the idea of circling for a
few turns after leaving a thermal? I think this might let us measure the
speed difference, if any, between a thermal and the airmass.


--
Change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA

[Jack]

T o d d P a t t i s t wrote:

> Imagine a 10 knot thermal....

What's "a ten-knot thermal"?


Jack
(in N. IL)

[mike]

They are weak thermals we have in New Mexico.

[Eric Greenwell]

T o d d P a t t i s t wrote:
> Eric Greenwell > wrote:
>
>
>>Once the thermal and the airmass
>>are moving at the same speed, there would be no need to correct upwind.
>
>
> Imagine a 10 knot thermal being continuously generated from
> a quarry or other warm spot on the ground. Assume a 10 knot
> steady breeze with no speed change with altitude (no wind
> shear). In one tenth of an hour (6 minutes) the thermal
> will have risen to one nautical mile high (6,000') and its
> top will have drifted one nautical mile downwind of the
> quarry.
>
> Now imagine a glider at 6,000' that began to circle (in
> sink) directly over the quarry when the thermal started.
> The glider has a 2.5 knot descent rate when turning. In the
> absence of the thermal, in the same six minutes, the glider
> would be circling about 1500' lower and have drifted the
> same one nautical mile downwind of the quarry. Clearly, the
> descending downwind angled path of the glider (dropping from
> 6000' to 4500') and the rising downwind track of the thermal
> (rising from 0' to 6000') must cross, so what happens at
> that point?
>
> The answer is simply that the glider begins to rise as it
> descends into the rising air. However, it does not rise as
> fast as the thermal. It's still descending at the 2.5 knot
> descent rate relative to the rising 10 knot thermal. Each
> instant that the glider is in the rising air, it is
> descending slightly in the thermal, and each bit of descent
> takes it into air that left the ground later and was
> slightly farther upwind relative to where the glider
> started.

This is where this model is wrong. What you describe is true near the
ground, where the airmass speed exceeds the thermal source (the ground
point) speed by 10 knots. At 1000', the airmass speed is still 10 knots,
but the thermal speed is now (for example) 5 knots because the the wind
has accelerated it; i.e., the airmass above 1000' is being fed by a
_moving_ source, not a stationary one.

At some point (I suggest 2000') the thermal has accelerated to the same
horizontal speed as the air mass. At that point, the airmass above 2000'
is being fed by a thermal source (the airmass at 2000') that is moving
at the same speed it is.

> Eventually the glider drops out the bottom of the
> angled downwind, rising, path of the thermal (provided the
> glider makes no centering corrections) and it continues its
> downward and downwind drifting path, having been delayed as
> its descending path crossed the thermal's rising path.

As long as the glider enters the thermal above 2000' (in this case), it
will not drop out of thermal, since the thermal is moving at the wind
speed. In fact, this is usually the case I encounter, because most of my
thermals do not require an upwind correction.

So far, no one has commented on my suggestion we measure the difference
in the wind speed and the thermal drift by circling a few times after we
leave a thermal, then comparing the drift from the flight trace later.
Does anyone have a better idea?

--
Change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA

[Bruce]

Jack wrote:
> T o d d P a t t i s t wrote:
>
>> Imagine a 10 knot thermal....
>
>
> What's "a ten-knot thermal"?
>
>
> Jack
> (in N. IL)
Fair, but not booming...

Best anyone in our club has achieved was 10m/s - on the averager - At Gariep.
That's booming.

--
Bruce Greeff
Std Cirrus #57
I'm no-T at the address above.

[Eric Greenwell]

I'm going to retract these comments about the thermal position relative
to the glider. The comments about how often I have to correct upwind are
still right, but that perhaps it's because where I usually fly might
have most of the thermals moving over the ground, rather than
originating at point sources. I don't think the comments apply to the
case Todd described, though they do seem to apply to what I encounter
where I normally fly, and to the shape of some dust devils.

These dust devils are moving over the ground, though more slowly than
the wind aloft. The bottom 1000 feet or so appears to go straight up,
then it bends over quickly until it's about a 10-30 degree angle with
the ground, has a relatively straight portion, then it bends up and is
straight again. I'm not sure of this, but perhaps the straight, bent
over portion is elongating as it speeds up to match the wind speed, then
begins to feed the upper air mass as I described below. The effect would
be a thermal source that travels with the wind, but being fed by a
slower moving source on the ground.

Eric Greenwell wrote:

> T o d d P a t t i s t wrote:
>
>> Eric Greenwell > wrote:
>>
>>
>>> Once the thermal and the airmass are moving at the same speed, there
>>> would be no need to correct upwind.
>>
>>
>>
>> Imagine a 10 knot thermal being continuously generated from
>> a quarry or other warm spot on the ground. Assume a 10 knot
>> steady breeze with no speed change with altitude (no wind
>> shear). In one tenth of an hour (6 minutes) the thermal
>> will have risen to one nautical mile high (6,000') and its
>> top will have drifted one nautical mile downwind of the
>> quarry.
>>
>> Now imagine a glider at 6,000' that began to circle (in
>> sink) directly over the quarry when the thermal started.
>> The glider has a 2.5 knot descent rate when turning. In the
>> absence of the thermal, in the same six minutes, the glider
>> would be circling about 1500' lower and have drifted the
>> same one nautical mile downwind of the quarry. Clearly, the
>> descending downwind angled path of the glider (dropping from
>> 6000' to 4500') and the rising downwind track of the thermal
>> (rising from 0' to 6000') must cross, so what happens at
>> that point?
>>
>> The answer is simply that the glider begins to rise as it
>> descends into the rising air. However, it does not rise as
>> fast as the thermal. It's still descending at the 2.5 knot
>> descent rate relative to the rising 10 knot thermal. Each
>> instant that the glider is in the rising air, it is
>> descending slightly in the thermal, and each bit of descent
>> takes it into air that left the ground later and was
>> slightly farther upwind relative to where the glider
>> started.
>
>
> This is where this model is wrong. What you describe is true near the
> ground, where the airmass speed exceeds the thermal source (the ground
> point) speed by 10 knots. At 1000', the airmass speed is still 10 knots,
> but the thermal speed is now (for example) 5 knots because the the wind
> has accelerated it; i.e., the airmass above 1000' is being fed by a
> _moving_ source, not a stationary one.
>
> At some point (I suggest 2000') the thermal has accelerated to the same
> horizontal speed as the air mass. At that point, the airmass above 2000'
> is being fed by a thermal source (the airmass at 2000') that is moving
> at the same speed it is.
>
>> Eventually the glider drops out the bottom of the
>> angled downwind, rising, path of the thermal (provided the
>> glider makes no centering corrections) and it continues its
>> downward and downwind drifting path, having been delayed as
>> its descending path crossed the thermal's rising path.
>
>
> As long as the glider enters the thermal above 2000' (in this case), it
> will not drop out of thermal, since the thermal is moving at the wind
> speed. In fact, this is usually the case I encounter, because most of my
> thermals do not require an upwind correction.
>
> So far, no one has commented on my suggestion we measure the difference
> in the wind speed and the thermal drift by circling a few times after we
> leave a thermal, then comparing the drift from the flight trace later.
> Does anyone have a better idea?
>


--
Change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA

[jb92563]

Here is the clue guys......."BRONZE BADGE"...... meaning, I just
learned how to fly a glider and Im trying to get to the destination
airport.

Lets start with best L/D because Im trying to not land out and time is
not an issue, and then add 1/2 the windspeed, since most gliders will
do this and still be on a reasonably decent part of the polar in terms
of sink...and get some better penetration.

I say "A" is the correct answer for a Bronze Badge level pilot since
they base their knowledge on a lot of generalizations about performance
to make inflight calculations relatively easy...

...of course the contest level pilot will have a much more refined
answer with his vast knowledge of his own glider and intimacy with its
performance specs, along with his computer instruments that he can
program for optimum results.

Ray

[Eric Greenwell]

T o d d P a t t i s t wrote:

> Eric Greenwell > wrote:
>
>
>>I'm going to retract these comments about the thermal position relative
>>to the glider.
>
>
> I was going to comment on your earlier post, but saw that
> you made this second one and decided to read it first. After
> reading it, I'm still not sure whether we agree or disagree,
> but unless you tell me otherwise, I'm going to assume we
> agree that even with a constant wind speed, no shear with
> altitude and instantaneous acceleration of the thermal mass
> to equal wind speed, the glider can still drop out of the
> bottom of a thermal that starts from a fixed ground
> location.

Yes, I do agree.
>
> As to thermals like the dust devil you describe, I can only
> dream about thermals strong enough to produce dust devils.

You have to have the dust, too! Come out West sometime.

I'm beginning to think that "many" thermals might act like the dust
devil I described, even ones from a fixed ground point. These thermals,
above a certain altitude, would be vertical in the air mass. Perhaps, as
the bent over portion stretches, it eventually breaks and starts a new
thermal closer to the origin point. The upper portion would drift off
with the wind and eventually dissipate, since it's no longer connected
to it's feed source.

Pilots would interpret this breaking and restarting as "bubbles" or the
thermal source dying, even though the fixed origin may be pumping out a
continuous thermal for hours.

--
Change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA