Flying the Standard Cirrus Cross Country

John H. Cochrane (5/4/1999)

MacCready Theory with Uncertain Lift and Limited Altitude (PDF)

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.

Scott Alexander (1/24/2011)

Everybody has suggestions for how to fly faster. I have really enjoyed this topic so far. What I would really like to know is what kind of speed you achieved, with the days average vario from SeeYou and what inter thermal speed you were using.

For instance on 2.0-3.0 days I get between 40-45 mph, flying no faster than 70 kias between thermals.

On 3.0-4.0 days I get between 45-50mph, flying no faster than 75 kias between thermals.

On one specific day last year, it was an average of 4.5, I did 13% circling, I never did faster than 70 kts between thermals and achieved a 52 mph speed. It was all thermal.

The times I have gone with a 80 knot or greater speed between thermals I have routinely fallen below the lift band and wasted alot of time digging myself out. Also, I think thermal density plays a big part in choosing what speed to fly. Overall I agree with the gut feeling to guide you best, which comes from experience.

Jari Hyvärinen (1/25/2011)

There is also a mental aspect of flying to slow between thermals. If you spend a long time flying between thermals even without loosing a lot of altitude there is a tendency to be inpaitiant and start to climb way to early. At least it's been like this for me.

I have statistics on competition flight over the past 10-years: 75% of all I fly are in competition. I have gradually, as confidence and experience has increased, increased my c/s-speed. The result has been that I climb fewer times during a flight and my average speed has increased (with this I have also climbed on the score boards).