Bronze Badge question

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".

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".

1/2 Wind velocity should be included.

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

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".

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".

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

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

.... 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.

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