On Tuesday, March 1, 2016 at 3:04:06 PM UTC-8, wrote:

The Price You Pay for McCready Speeds: http://www.betsybyars.com/guy/soarin...a/72price.html

Oldie but goodie. Speeds tend to be a bit higher now with modern gliders having a knee in the polar that is at a bit higher speed, plus more water capacity.

There's nothing wrong with McCready theory - in theory. The considerations beyond McCready for pilots are, first, as Wil pointed out, the achieved cross-country speed penalty for cruising a bit off McCready optimal STF is not that great and options open up when you fly a bit slower in terms of search radius, ability to sense lift, less G-induced losses in getting slowed down within the diameter the thermals you encounter so you don't miss good lift, etc. There's zero benefit to flying faster than McCready so the bias ought to be slower on average to open up that option value.

Second, McCready applies in a theoretical world of consistency and certainty in terms of lift ahead versus lift behind you as well as search altitude versus inter-thermal distance, lift strength distribution, wind speed and direction versus task leg orientation (which themselves can vary with AATs). Soaring weather is stochastic and the expected probabilities ought to affect how we fly - especially as circumstances like altitude, time of day, the look of conditions ahead all affect that probability distribution.

The fundamental tradeoff in flying fast is the probability of getting stuck or landing out versus the probability of finding a good (or better) thermal ahead before that happens. Again, read the Cochrane article on flying faster that nicely covers many of these topics. There is also the issue of searching for lift (or more importantly the best lift) in areas where you have indications that there ought to be a good thermal, and how much altitude there is to gain if you strike gold with a boomer. No point in searching around close to cloudbase.

Based on the above, there a places and times when I'll S-turn (or even fly a clover-leaf) to find the good lift I expect might be there somewhere, particularly in cases where there's a marker like a cloud, a terrain feature or when I'm coming to the end of a street where prospects ahead are less certain. If lift is widely distributed and variable I more frequently attempt to expand the area searched under a cloud to see what's available. On occasions where I'm flying with another glider I typically find I pay a fraction of a mile for the option to search in an S-turn versus steam ahead - that's when it doesn't pay off. When it does pay off I'll often find that I arrive at the next thermal later but hundreds to more than a thousand feet above the pilot I'm flying with (depending on what he finds up ahead).

That's the great thing about soaring - it pays to think - most of the time.

9B