On Mon, 29 Sep 2003 21:18:08 GMT, "Michael Stringfellow"
wrote:

A few points that might be worth adding:

1) The application of MacCready theory optimizes your speed over a
cross-country course if you fly at the speed indicated for the expected rate
of climb

"expected" rate of climb says a lot. It isn't necessarily the rate of
climb of the whole next thermal.

2) MacCready flight optimizes time at the expense of height. If you are low
or trying to clear a mountain ridge, it isn't appropriate to fly as fast as
indicated. If you slow down, you will lose less height (often even when in
sink). John Cochrane has given a good description of the trade-off betweeen
speed and height.


Speed to fly theory assumes you *will* get the next thermal before you
hit the ground.

3) It probably doesn't pay to follow the guidance of an audio speed-to-fly
vario too aggressively - flying at a more constant speed may often be more
efficient.

Which is why the speed to fly command should be damped considerably
compared to the vario. This is in line with now 20 year old German
research.

It has also been shown that it isn't necessary to fly at the exact
speed to get most of the benefit.

Severe dolphining is detrimental and likely to make you sick.

and, most important......

4) My experience has shown that people grossly overestimate the achieved
climb rate on a cross-country flight. Here in Arizona, days with memorable
8 to 10 knot thermals rarely produce more than a 5-knot flight average and
2.5 to 4 knot days are more common. If you climb in a rare 10-knot
thermal on a day with a 4-knot average and set your MacCready setting to 10,
you'll be in trouble in a hurry! A good flight computer that will give you
the climb average for a flight will keep you honest!

Mike the Strike


Anyone who has been around soaring for a while will know that when the
pilot says" it was a ten knot thermal" he means "the vario once
indicated 10 knots during the climb".


This is extended to the day: "there were ten knot thermals that day"
means that "in one thermal that day the vario momentarily read 10
knots"

Maybe we aren't so different from sport fishermen.

The things that John Cochrane mentions in his Macready paper were all
discussed at greater length 30 or more years ago by Anthony Edwards -
the" Armchair Pilot" in Sailplane and Gliding. We are in danger of
reinventing the wheel.

Mike Borgelt