flight computer ergonomics and function

On Tuesday, June 18, 2019 at 5:34:40 PM UTC-4, Mike the Strike wrote:
On Tuesday, June 18, 2019 at 7:43:16 AM UTC-7, krasw wrote:
Simple table that tells you how much to increase MC at certain headwind component would be enough to solve this issue.

Unfortunately not.

The problem of optimum MacCready speed in wind relates to both wind shear and the relative horizontal velocity of the thermals. With a uniform atmosphere in which the wind speed is constant with height and in which thermals move at the same speed as the wind, standard MacCready theory applies. In these ideal conditions, there is no advantage in changing speed upwind or downwind.

In the case where thermals move more slowly than the wind (the more general case) or where they don't move at all (wave), the optimal speed will generally be faster upwind and slower downwind. This is what many experienced cross-country pilots have intuitively figured out in the real world.

Mathematical analyses of these have been done by several folks, including John Cochrane and Branko Stojkovic. Long enough ago to be forgotten.

The solution of speed to fly with wind is not easy, since the effects of wind shear and thermal speed are variable and may not be known well enough for a satisfactory general mathematical solution.

Mike

Thank you Mike. But a saving grace is that the achieved XC speed for some range around the optimum STF does not vary very much, exactly because it is an optimum: it is the top of a roundish hump in the curve. In that sense a "simple table", or even a rough guess, of the correction, if any, for the wind, is "enough". Lacking precise data on the local shear etc, fancier methods wouldn't do any better.

And, to paraphrase UH, the left-and-right controls are far more important (for task speed) than the up-and-down. Deviate to where the air is half a knot sweeter and you'll get there faster, even if your STF is 10 knots off.

Indeed, the wind question is mostly academic as we rarely fly long cross-country tasks in wind speeds that make a big difference. Identifying convergence (and divergence) lines and dealing with long distances between thermals are more useful problems to solve. MacCready theory optimises your speed between thermals but doesn’t tell you what to do when you get low or can't find a thermal (John Cochrane wrote a paper on this too). Perhaps this is something that smarter flight computers could address?

Mike