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Jan Lyczywek (1/21/2011)
My personal speed-to-fly ideas, coming from flying an unballasted Cirrus in the European Alps (though, mainly for the OLC, hardly any central comps):
- Do use the speed-to-fly function of the electronic vario, preferably with sound; it is a great wake-up
call wey every time one is tempted to dillydally about in bad air. Plus the speed-to-fly sound saves a great
deal of workload in terms of thinking about what would be the ideal speed; it just commands it.
- Do not follow the speed-to-fly sound slavishly, but softly in its tendency, and never forget that any
vario information is information from the past, while only the pilot can look ahead and into the future
(admittedly with varying quality of her/his powers of prophecy...).
- As to McCready settings, do not bother with too fine adjustments; one knot steps do fully suffice.
I use a very simple "five-speed gearbox."
(0 kt: never use MC zero, it just makes you far too vulnerable in case of sudden sink, especially with an unballasted club class ship)
- 1 kt: minimum MC setting for all situations of mere survival: setting off in the morning in the first weak thermals; reaching that last chance for a thermal; stretching the glide before an inevitable outlanding; bringing her in on a long, marginal final glide; working your way home in the evening in the last weak thermals
- 2 kt: MC setting in generally weak weather, or below-average weather combined with other difficulties
such as few outlanding fields, unknown landscape, etc. A very very careful setting.
- 3 kt: Seems to be my most commonly used MC setting, typically in predictable, reliable
weather with normal thermals.
- 4 kt: MC setting for good, above-average conditions, reliable lift, high cloudbase,
well marked thermals, an everything's-just-fine MC setting.
- 5 kt: My personal MC setting for throttle fully open.
6 kt: will of course deliver higher average speeds in very good weather in a classic
climb-glide-climb-glide situation, but in the Alps strong weather always brings good lifting
lines and I had the impression that with a MC setting of 6 kts I ran a higher risk of dropping
out of those lifting lines, thus being forced back to a climb-glide-climb-glide strategy which
consumes quite a big deal of the higher speeds.
I know I am not too much in line with classical speed-to-fly theory with this, but from all flying my personal opinion is that the optimum McCready setting for a given situation is not too closely related to the climb rates achieved or expected. Instead, I try to boil my whole assessment of everything that is in any given situation of flight down into something like a "throttle setting". So, climb rates do go in there, but also cloud base altitude, reliability of the clouds so far, any difficulties experienced, my current altitude, my personal level of knowledge of the landscape ahead, and so on, and so on. All this goes into that "throttle setting", i.e. the MC setting, and this in turn ensures that the speeds chosen will fit to the situation (and to the whole situation, not merely mathematically to some climb rate value found previously or expected ahead).
The habit of using this "five-speed gearbox" with its five McCready settings evolved from flight experience only,
so no particular features of my polar have contributed to the choice of these five settings.
The polar is, of course, implemented in my speed-to-fly calculator (my electronic vario, that is),
so that any McCready setting leads to the appropriate speed being commanded by the calculator.
As to the numeric values of the MC settings in use, mine will probably do as a starting point.
Depending on where you fly, they will probably need to be adapted to typical weather conditions
in your area. For example, in an area with typically high cloud base and strong lift, but long
glides in between, the "throttle fully open" mark will probably need to be at a higher McCready
value, say at 7kt (3.5m/s) or even 8kt (4m/s). Also, some adaptation to pilot preferences might
be required. The type of glider however will not make much difference, simply because this is
cared for by their different polars; same with ballast.
I think using five settings suits human perception and psychology well, because it gives you
one "normal" value in the middle, with one "very bad" and one "rather bad" on the one side and
a "rather good" and a "very good" on the other side. It seems to be quite simple to categorize
even complex soaring situations into one of these five settings. Using more settings makes this
assessment more difficult, and classic speed-to-fly theory shows that mathematically five
settings are fully sufficient, errors caused by the steps in between are negligible.
Also, I think that the rule of never using a McCready value of zero is applicable to all gliders.
It is of course theoretically true that a zero setting maximizes gliding distance from a given
height, but this is static theory, i.e. while it does take constant air mass movements into
account (and holds true for constantly rising air or constantly sinking air), it does not depict
*changes* in the vertical air mass movement, particularly not the dynamics of a glider entering
into sudden sink. Flying at MC = 0 leads to very slow airspeeds (on an unballasted Std. Cirrus
theoretically only about 50kts) and consequently high vulnerability to such variations in air
mass movement. A slightly higer McCready setting allows a significantly higher airspeeds
(Std. Cirrus 60kts @ MC=1kt) at the price of a marginally reduced glide ratio. This pays off
as soon as you hit sink: part of the increase in speed now required is already done, and the
remaining speed needed is picked up much quicker.
Following is a closer look at some of my flights.
The flights analysed were thermal flights between 500 and 800 km, with no influence of wind, neither
slope soaring nor wave. All were flown unballasted at 33 kg/m2. I divided the flights into 60 minute
intervals and analysed each hour of flight separately. Every red dot in the diagrams is one full hour of
flight. The cross country speed achieved during each hour is equal to the straight line distance covered
in this 60 minute interval. Altitude differences between beginning and end of each hour of flight were corrected for.
The first diagram shows cross country speed vs. average climb rate.
Obviously, the better the climb rate during the 60 minute intervals, the better the cross country speed that
can be achieved in that hour. Covering more than 80 km in one hour requires at least 1.5 m/s (3 kt) average
climb rate; more than 90 km have almost never been achieved with an average climb rate of less than 2 m/s (4 kt),
and doing more than 110 km an hour calls for climbing at 2.5 m/s (5 kt) or more on average.
The blue line is the cross country speed that according to theory is achieved at any given climb rate,
assuming that the air in between the thermals is not moving. Some of the red dots are below this line;
this means some speed was lost to either gliding in sinking air in between the thermals, or to great
detours or other mischief. Some of the red dots however are quite a bit above the blue line.
This means the cross country speed actually achieved was higher than theory predicted for the given climb rate.
There is only one explanation for that: the air between the thermals was not, as theory expected it, at rest,
but was on average rising. At 2 m/s (4 kt) climb rate, the best cross country speed was 100 km/h, vs. 80 km
that would have been possible when gliding in still air. At 2.5 m/s (5 kt), cross country speeds actually
achieved are around 110 km/h, vs. 90 km/h predicted without lifting lines. So, if the pilot carefully
follows lifting lines, the cross country speed can be significantly higher than expected.
It is typical for mountain gliding, at least here in the eastern part of the European Alps, that good
days do not only bring good climb rates but also well established lifting lines. Vice versa, exploiting
lifting lines almost automatically leads to higher average climb rates, because one will glide further
and thus has more choice which thermal to take.
The second diagram shows glide speeds between thermals vs. average climb rate. The blue line is the glide speed
required by speed to fly theory at the given climb rate when gliding in still air. In practice, the speeds
flown are mainly below this line, especially at good climb rates. This is once again due to gliding in rising air,
which allows moderate glide speeds despite high climb rates and despite high McCready settings. Most of the time I
seem to fly my Cirrus between 65 and 75 knots.
The third diagram shows cross country speeds achieved vs. the percentage of time spent circling.
To me this seems to be the key to high cross country speeds: obviously, for achieving more than 100 km in one hour,
less than 30% of the time should be spent circling, for achieving more than 110 km, it should be less than 20 %.
This emphasizes that high cross country speeds are not achieved by racing like mad, but much more effectively by
minimizing the time spent circling. Good climb rates are essential for that purpose, yet not sufficient: it is even
more important to carefully choose the flight path in order to glide in good, rising air.
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