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Old October 26th 14, 12:54 PM posted to rec.aviation.soaring
John Cochrane[_3_]
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Default alternative McCready theory and variometers

Frankly, people who think that "MacCready theory" is wrong and need an "alternative theory" don't understand MacCready theory.

That doesn't mean pushing and pulling and following the vario around. Vario lags make that strategy unproductive. MacCready theory with lags in the vario would say so. However, if you see a beautiful long cloud up ahead with all the signs of strong lift in it that will last several minutes, by all means slow up! If you're stuck in a long lasting river of sink, speed up!

The speed to fly mode remains, in my view, the most useful way to set up a vario for cruising flight. Set it on speed to fly mode with a narrow deadband and a pretty short time constant. So, if you're in zero air flying 80 knots at MacCready 2, it will be silent. If you're flying 60 it will give you a gentle boop boop to remind you that you're cruising too slowly, which your netto would not do. If you had set it on regular TE vario it would be screaming down. What's the point in that?

Then, the speed to fly vairo naturally tells you what the air around you is doing. If it starts rising, you're in better air. That's interesting information. You don't immediately pull back on the stick, as you're flying a basically constant speed. But if the trend is good, the air is smooth, and you see some clouds, then you do gently slow down. And vice versa.

The confusion is in the name. It says "speed to fly" mode, which gives off the air of listening to it and doing huge pulls and pushes. Maybe we should rename it "listen to the air in cruise mode" which is how it works.

This setup works best if the vario units remain in vertical knots, i.e. that the sensitivity of the vario is the same in cruise and climb mode. If you hit 2 knots of lift a previously silent vario should always make the same "2 knots" sound. Of my varios, the 302 and clearnav do that. The LX, while otherwise a very nice vario, is much more sensitive to ups and downs in cruise mode than in climb mode, rendering it pretty useless for this sort of flying.

John Cochrane