flight computer ergonomics and function

Flying in wave conditions with 50kts of wind, I got "low" and the computer indacated a marginal glid home. I dug out until I had 3000 ft over glide (MC 2). Considering the was more than a slight chance of wave sink on the way home, I thought I should crank up my MC as one might do on a thermal flight to insure you are looking at info with some increased margin. Rather than lower my arrival, it increased it to 5000 ft. A fence post case to be sure, but the point for this thread is, you can try fiddling with your MC when flying in wind to see if you can see a difference in arrival height. I had never seen it "reverse" like this in thermal conditions (might happen between 0 and something low) but you can try.

I have always used the rule of thumb for glide into the wind, but in 50kts you can run out of fingers to count on...

On Monday, June 17, 2019 at 5:53:34 PM UTC-6, waremark wrote:
"The S8x manual does not indicate whether current lift/sink is used to modify the instant STF arrows indication, nor whether wind is taken into account. The STF *is* based on the next waypoint, polar, and (I think) MacCready."

Lift/sink does drive the speed up slow down arrows. Wind does not, nor does the next waypoint. As well as polar and Mcready it also considers wing loading. When changing Mcready ballast or bug settings LX devices tell you the zero lift figures for Mcrwady STF and glide ratio.

By the way, if you want to look for a waypoint in a particular direction on your Oudie drag your finger in that direction on the screen - sort on any column heading by touching it, touch the wp you want and goto. But you surely know all this.

Agreed the Push/pull arrows on the S80 do account for lift and sink. I am disappointed that the Speed tape display does not account for lift and sink, it simply displays the calm air speed to fly speed, which is of little value on an active display. The S80 tells me that when I set the MacCready Value.

Brian

On Thursday, June 20, 2019 at 7:31:37 AM UTC-4, RR wrote:
Flying in wave conditions with 50kts of wind, I got "low" and the computer indacated a marginal glid home. I dug out until I had 3000 ft over glide (MC 2). Considering the was more than a slight chance of wave sink on the way home, I thought I should crank up my MC as one might do on a thermal flight to insure you are looking at info with some increased margin. Rather than lower my arrival, it increased it to 5000 ft. A fence post case to be sure, but the point for this thread is, you can try fiddling with your MC when flying in wind to see if you can see a difference in arrival height. I had never seen it "reverse" like this in thermal conditions (might happen between 0 and something low) but you can try.

I have always used the rule of thumb for glide into the wind, but in 50kts you can run out of fingers to count on...

It is convenient to assume that a higher MC setting is "more conservative", but clearly that is not always the case. Presumably the "MC" for the purpose of a final glide just meant that your computer assumed you will glide at the still-air MacCready airspeed for that MC setting. But the computer also took the wind into account when computing your achieved glide ratio and thus arrival altitude. Thus when you set a higher MC the computer read it as a higher airspeed, which was advantageous in this case of flying into a strong headwind. Thus it displayed a higher arrival altitude.

I think in this situation one should first decide on the speed to fly (would be nice if the computer would help you with that choice, but it may not). With a 50-knot headwind obviously you'd pick a rather high speed. Then adjust the MC until the displayed STF fits your choice of speed. Then see what it says for predicted arrival altitude, and add a safety margin to that in feet of arrival altitude, not in MC setting.