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.