Early cross-country flights, including small practice triangles within gliding range of the home field, are excellent preparation and training for longer cross-country flights. The FAI Gold Badge requires a 300-kilometer (187 statute miles) cross-country flight, which can be straight out distance or a declared triangle or out-and-return flight. An average cross-country speed of 20 or 30 miles per hour (mph) may have been adequate for a 32-mile flight, but that average speed is too low on most days for longer flights. Flying at higher average cross-country speeds also allows for farther soaring flights.
Improvement of cross-country skills comes primarily from practice, but reviewing theory as experience is gained is also important. A theory or technique that initially made little sense to the beginner has real meaning and significance after several cross-country flights. Postflight self-critique is a useful tool to improve skills.
In the context of cross-country soaring, flying faster means achieving a faster average groundspeed. The secret to faster cross-country flight lies in spending less time climbing and more time gliding. This is achieved by using only the better thermals and spending more time in lifting air and less time in sinking air. Optimum speeds between thermals are given by MacCready ring theory and/or speed-to-fly theory, and can be determined through proper use of the MacCready speed ring or equivalent electronic speed director.
On most soaring days there is an altitude range, called a height band, in which the thermal strength is at a maximum. Height bands can be defined as the optimum altitude range in which to climb and glide on a given day. For instance, a thermal in the 3,000 feet AGL range may have 200 to 300 fpm thermals, increasing to 500 fpm at 5,000 feet AGL range then weaken before topping out at 6,000 feet AGL. In this case, the height band would be 2,000 feet deep between 3,000 feet and 5,000 feet AGL. Staying within the height band gives the best (fastest) climbs. Avoid stopping for weaker thermals while within the height band unless there is a good reason.
On another day, thermals may be strong from 1,000 feet to 6,000 feet AGL before weakening, which would suggest a height band 5,000 feet deep. In this case, however, depending on thermal spacing, terrain, pilot experience level, and other factors, the height band would be 2,000 feet or 3,000 feet up to 6,000 feet AGL. Avoid continuing to the lower bounds of strong thermals (1,000 feet AGL) since failure to find a thermal there gives no extra time before committing to a landing. [Figure 11-12]
NOTE: Automated Flight Service Stations (AFSS) report cloud levels as AGL in METARS, and PIREPS are reported as MSL. Area forecasts gives clouds as MSL if above 1,000′ AGL. Pilots must be careful to determine which value is being presented. This is very important when glider pilots travel to higher elevation airports and must subtract field elevation from MSL reports to ensure cloud clearances.
Determining the top of the height band is a matter of personal preference and experience, but a rule of thumb puts the top at an altitude where thermals drop off to 75 percent of the best achieved climb. If maximum thermal strength in the height band is 400 fpm, leave when thermals decrease to 300 fpm for more than a turn or two. The thermal strength used to determine the height band should be an average achieved climb. Many electronic variometers have an average function that displays average climb over specific time intervals.
Another technique involves simply timing the altitude gained over 30 seconds or 1 minute.
Theoretically, the optimum average speed is attained if the MacCready ring is set for the achieved rate of climb within the height band. To do this, rotate the ring so that the index mark is at the achieved rate of climb (for instance, 400 fpm) rather than at zero (the setting used for maximum distance). A series of climbs and glides gives the optimum balance between spending time climbing and gliding. The logic is that, on stronger days, the extra altitude lost by flying faster between thermals is more than made up in the strong lift during climbs. Flying slower than the MacCready setting does not make the best use of available climbs. Flying faster than the MacCready setting uses too much altitude between thermals; it then takes more than the optimum amount of time to regain the altitude.
Strict use of the MacCready ring assumes that the next thermal is at least as strong as that set on the ring and can be reached with the available altitude. Efforts to fly faster must be tempered with judgment when conditions are not ideal. Factors that may require departure from the MacCready ring theory include terrain (extra height needed ahead to clear a ridge), distance to the next landable spot, or deteriorating soaring conditions ahead. If the next thermal appears to be out of reach before dropping below the height band, either climb higher, glide more slowly, or both.
To illustrate the use of speed-to-fly theory, assume there are four gliders at the same height. Ahead are three weak cumulus clouds, each produced by 200-fpm thermals, then a larger cumulus with 600 fpm thermals under it, as in Figure 11-13.
- Pilot 1 sets the ring to 6 knots for the anticipated strong climb under the large cumulus, but the aggressive approach has the glider on the ground before reaching the cloud.
- Pilot 2 sets the ring for 2 knots and climbs under each cloud until resetting the ring to 6 knots after climbing under the third weak cumulus, in accordance with strict speed-to-fly theory.
- Pilot 3 is conservative and sets the ring to zero for the maximum glide.
- Pilot 4 calculates the altitude needed to glide to the large cumulus using an intermediate setting of 3 knots, and finds the glider can glide to the cloud and still be within the height band.
By the time pilot 4 has climbed under the large cumulus, the pilot is well ahead of the other two pilots and is relaying retrieve instructions for pilot 1. This example illustrates the science and art of faster cross-country soaring. The science is provided by speed-to-fly theory, while the art involves interpreting and modifying the theory for the actual conditions. Knowledge of speed-to-fly theory is important as a foundation. How to apply the art of cross-country soaring stems from practice and experience.
Tips and Techniques
The height band changes during the day. On a typical soaring day, thermal height and strength often increases rapidly during late morning, and then both remain somewhat steady for several hours during the afternoon. The height band rises and broadens with thermal height. Sometimes the top of the height band is limited by the base of cumulus clouds. Cloud base may slowly increase by thousands of feet over several hours, during which the height band also increases. Thermals often “shut off” rapidly late in the day, so a good rule of thumb is to stay higher late in the day. [Figure 11-14]
It is a good idea to stop and thermal when at or near the bottom of the height band. Pushing too hard can lead to an early off-field landing. Pushing too hard leads to loss of time at lower altitudes because the pilot is trying to climb in weak lift conditions.
Another way to increase cross-country speed is to avoid turning at all. A technique known as dolphin flight can be used to cover surprising distances on thermal days with little or no circling. The idea is to speed up in sink and slow down in lift while only stopping to circle in the best thermals. The speed to fly between lift areas is based on the appropriate MacCready setting. This technique is effective when thermals are spaced relatively close together, as occurs along a cloud street.
As an example, assume two gliders are starting at the same point and flying under a cloud street with frequent thermals and only weak sink between thermals. Glider 1 uses the conditions more efficiently by flying faster in the sink and slower in lift. In a short time, glider 1 has gained distance on glider 2. Glider 2 conserves altitude and stays close to cloud base by flying best L/D through weak sink. To stay under the clouds, he is forced to fly faster in areas of lift, exactly opposite of flying fast in sink, slow in lift. At the end of the cloud street, one good climb quickly puts glider 1 near cloud base and well ahead of glider 2. [Figure 11-15] The best speed to fly decreases time in sink and therefore decreases the overall amount of descent but produces the best forward progress. Being slower in sink increases time descending and slows forward progress, while being fast in lift decreases time in lift and altitude gained.
On an actual cross-country flight, a combination of dolphin flight and classic climb and glide is frequently needed. In a previous example, the two pilots who decided not to stop and circle in the weaker thermals would still benefit from dolphin flight techniques in the lift and sink until stopping to climb in the strong lift.