flyingcheesehead
Touchdown! Greaser!
lancefisher said:It's been a while but I ran some calculatons to see what effect the winds had on the minimum altitude necessary to remain within gliding distance and if memory serves (a depressingly rare occurance these days) the wind has virtually no influence on that. It certainly changes the decision point but I think that's about it. To get accurate abou this you have to consider the changes in best glide speed with wind (which shifts the origin of the L/D curve on which best glide is based). If I get bored I might try to run through the math again and see if my memory is good or bad.
Lance,
I was using a fairly simple groundspeed/sink calculation. Best glide for me is something like 80 kt and 1000fpm sink (not looking at actual numbers here). With a 50kt wind out of the west, that means 30kt westbound and I'd have to be within 6.25 miles of shore to make it into the wind from 13,000 MSL (roughly 12,500 AWL) (though this could be increased by flying faster than "best" glide).
However, eastbound would be 130kt for 12.5 min or about 27nm. So, that removes most of the exposure (33nm out of about 40-45 depending on the crossing location). Again, minimum sink airspeed would be a better choice here.
Doesn't work for this particular example, but it actually did for the numbers I had at the time. I was also compensating for the varying winds aloft at the various altitudes.
I guess the extreme example would be with a wind speed approaching Vne (It'd be somewhat less due to the downward angle of the airplane at Vne.) Your decision point would be at the western shore, and if your engine failed right there you'd have to point your nose down and go really fast (airspeed) to get zero groundspeed and land right on the shore. However, you'd be doing something like 275kt at minimum sink and be able to glide for 57nm eastbound, plenty to make it across the lake.
You mention shifting the origin of the L/D curve on which best glide is based, I assume you are talking about the effects of headwind/tailwind component on the actual over-ground best glide distance (Higher Vg for headwind, lower for tailwind)?