On Friday, February 12, 2016 at 9:14:46 PM UTC-7, Steve Koerner wrote:
On Friday, February 12, 2016 at 6:55:40 PM UTC-7, Tim Taylor wrote:
Johnathan,

Two things:

I always set MC to true average climb rate that I am willing to stop in.. This is the average for the whole thermal, not just a few seconds.

My best calculation of speed to fly in lift if you do not need to climb is the MC speed for your current MC setting minus the lift strength. So if you are flying a MC of 5 and the thermal is 2 knots slow to the speed for a MC of 3.

Tim

I really like Tim's second paragraph suggestion.

Both paragraphs, though, have a caveat that is left out. That is, when the ground is threatening to interrupt your plans, then the correct action is to dial down the MC setting. Most of the time when I am flying, I do feel that certain threat from the ground and the dial is set lower than optimal to accommodate that threat. The closer I get to the ground, the more the dial is turned back from the free soaring optimum setting. I'm pretty sure that Tim does the same thing.

For myself, when conditions dictate a 5 knot expectation, my setting will likely average around 3 or 3.5. That may sound way conservative, but the problem is that over the period of a 7 day contest (or just any long cross country flight) there will be lots of opportunity for gravity and the ground to conspire against you. That forces an altitude conserving bias. You never want to let the ground win. Flying a bit slower than optimal doesn't hurt overall performance very much (you're operating near the plateau of a curve). Landing out hurts performance a lot.

Tim's MC setting becomes absolutely correct on final glide when the ground no longer threatens. That's why the final glide thermal is such an important one in a contest. At that point you always get to buzz home at an optimum MC speed.

A key skill in efficient soaring is to know when to dial back and by how much. It's hard to quantify and tends to take years to learn. You may be dialing back because the expectation for the next thermal has declined or because you want to be sure not to drive your glider into the ground or some combination of the two.

Besides the ground threat, there is actually more good reason to cruise somewhat slower than true MC. Flying slower allows you to do a better job of finding, sorting and tracking in good air. Flying slower also gives you a better chance of doing an accurate pull up into the next thermal. So biasing to a lower setting has those secondary benefits.

Yet another reason that a lower setting is preferred is that all thermals are not equal. You get a better overall performance when you can select for the best thermals. Cruising slower at a lower than optimal setting, presents more thermal opportunities with every glide. That means you can be more choosey and end up with a better average climb rate over the flight.

MacReady theory should best be thought of as an upper bound. Practical considerations dictate a lower setting.