Confused about great circle navigation

OK, I'll give it a shot.

First, remember that a Lambert Conformal projection only *approximates* a sphere
for the latitudes it covers. You can't, actually, have such a chart that covers
the entire northern hemisphere because the North Pole would be the apex on the
cone of the conformal projection.

Because the chart only approximates a sphere, any line drawn between two points
only approximates a great circle. If you have access to the AOPA flight
planner, plan a route between NYC and LA. You'll see that there is a
considerable "curve" to the route plotted on the screen. That's the curve you
would have to follow, constantly changing you heading to do so.

Now use DeLorme Street Atlas or some other land-based program to draw a straight
line between the same two points. You'll see that the straight line doesn't
match up very well with the line drawn by flight planner.

To perfectly fly a great circle between two points you do have to constantly
change heading. The idea behind a rhumb line is "segment" the circle so that
you can fly a constant heading between the points that define the rhumb line.
This will make the rhumb line course slightly longer. How much longer depends
on the number of points: more points equals a closer approximation of the circle
but is harder to fly.

Unless you're flying very long distances it makes very little sense to worry
about the difference between a great circle and a straight line on a chart. For
example, the great circle between my home base here in MA and my mother's home
in SoCal would take me over terrain I don't want to fly over anyway, so the GC
is meaningless. On shorter trips the delta is so small as to be insignificant.
IOW, don't sweat it.

Dave Reinhart


xerj wrote:

Not that it matters terribly much, but there's a few things I don't get.

On a WAC, which is a Lambert chart, a straight line is more or less a great
circle, right? However, to fly a great circle, you have to constantly adjust
your heading. I still can't conceptually work out why, I must say. Any
pointers?

If you had a WAC chart that displayed the entire Northern Hemisphere on one
chart, you could draw a straight line from Los Angeles to New York. Wouldn't
this be a great circle? And if it is, why couldn't you just fly the single
heading of that line? Is it that because of the fact that a chart that big
would have different magnetic north references at different meridians, and
what you would actually be drawing is a rhumb line?

And speaking of rhumb lines, if you fly one by keeping a constant magnetic
heading between two points, does that mean you actually describe a curve
over the earth's surface?

Thanks in advance, because I'm pretty confused.