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

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.

.... 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.

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.