Bronze Badge question

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.

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

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

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

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.

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".

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?

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.

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

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