This page addresses the techniques used for searching for, entering, and centering thermals. Discussion of variometer use assumes a netto variometer is **not** used. ## Searching ### On Tow Ideally, a glider pilot can save the time and expense of a higher tow and search for lift by releasing directly into a thermal. To do this, the glider pilot should be familiar with the glider and towplane's performance in neutral air (no lift or sink), and have a minimum safe release altitude in mind. The glider pilot should note: - Rate of climb (variometer indication) on tow in neutral air - Rate of descent (variometer indication) off tow in neutral air, at minimum sink speed Upon releasing from tow, the glider pilot can anticipate the drop in variometer reading to be approximately the difference between these two values. With this in mind, the glider pilot can cross-check the variometer in areas of suspected lift to ensure that a strong enough thermal is encountered to make releasing worthwhile. As an example, let's say a particular glider and towplane average +5kts climb on tow, and the glider sinks at -2kts in neutral air on a given day. The glider pilot would expect the variometer to drop by 7kts upon releasing from tow. If the glider releases in an air mass with the variometer reading +7kts on tow, the glider would be in an area of "zero-sink", where the air mass lift equals the glider's sink rate. Releasing in anything above +7kts, the glider pilot should expect to be able to gain altitude. In practice, this means that the variometer should be cross-checked against physical sensations of lift (vertical acceleration, or "surges") on tow, to determine if the cause is a thermal (organized, sustained lift), or erratic turbulence (fluctuating, inconsistent lift). Adhere to an appropriate minimum release altitude, sample a few thermals on the way up, and release when an average or better thermal is encountered. It is important to note that the rate of climb on tow is *highly* variable, based on a number of factors related to [[Performance During Takeoff and Tow]]. ### Location If unable to release into a thermal, make note of where thermals are encountered during the tow. During local thermalling flights, ensure that the search **always** remains safely within glide distance to the airport; in strong wind conditions, this may limit the search to upwind of the airport. Identifying markers of thermal lift, and leveraging knowledge of the current conditions and local area can make [[Locating Thermals]] significantly easier. ### Speed-to-Fly During the search for thermal lift, different speeds can be selected to optimize for different tasks: - To cover a maximum search area *through the air mass*, best glide speed should be used - To optimize distance covered over the ground, best glide **corrected for wind** should be used - To optimize average speed and minimize flight time, MacCready speed should be used ## Entering Ironically, the first indication that the glider is approaching a thermal is often increased sink, and turbulence associated with the shear from neutral, to sinking, to rising air. If the pilot intends to work this thermal, they should begin slowing to the appropriate minimum sink speed. ![[thermal horizontal structure.png]] When the glider contacts the rising air of the thermal, significant vertical acceleration, or a "surge" is felt; to the pilot, this physical sensation is similar to an elevator beginning to rise. In addition, the transition from sinking to rising air is often audible, with a "whoosh" heard as the relative flow of air about the glider changes. Finally, the variometer will begin to trend upward; note that the amount of lag present in the variometer will vary considerably with the type and condition of the instrument/system. ### Speed-to-Fly and Bank Angle The glider's net rate of climb in a thermal is determined by subtracting the glider's sink rate from the rate of air mass ascent. For example, if the air mass is rising at +10kts, and the glider sinks at -2kts, the net rate of climb (and variometer indication) will be +8kts. The [[Anatomy of a Thermal#Horizontal Structure]] of a thermal usually does not provide uniformly strong lift; the strongest lift is typically found near the center, or **core**, with lift diminishing towards the edges. Selecting the appropriate speed-to-fly and bank angle, then, is a compromise between minimizing the glider's sink rate, and maneuvering to stay in the area of strongest lift. [[Correcting Performance Speeds]], applied to the glider's minimum sink speed and selected bank angle, will result in the minimum sink rate and maximum net climb rate. In some conditions though, it may be desirable to deviate from this speed; in especially tight thermals, flying a slightly slower speed to decrease [[Radius of Turn]] may offset the increased sink rate by keeping the glider in stronger lift. Conversely, in strong turbulence, a slightly higher speed may be preferable to retain adequate control authority and margin from a stall. Sink rate increases **exponentially** as bank angle increases; shallow turns are rarely practical, however, as the strongest lift in the core of a thermal may only be a few hundred feet in diameter. Therefore, the pilot should select the **minimum bank required to stay in the strongest lift** - the exact bank angle will vary considerably with the glider, and the nature of the thermals on a given day. In practice, a pilot should be familiar with the performance of their glider: how different speeds and bank angles affect turn radius and sink rate; and work to build a mental profile of thermal characteristics (strength, size, lift distribution, etc) for the current conditions. ### Initiating the Turn Ideally, the pilot turns at the correct time, and in the correct direction, to establish a circle in the core of the thermal. **Before turning**, the pilot must visually clear the area of traffic, and continually practice [[Collision Avoidance During Thermal Soaring]]. When to initiate the turn depends on where the pilot estimates the thermal's core to be; while this may require some guesswork, a few general guidelines can be helpful. As the flight progresses and more thermals are encountered, the pilot's mental profile of average thermal size and lift distribution can make estimating the core's location easier. Paying attention to the **trend** of vertical acceleration ("surge") can help to identify the core - sustained vertical acceleration indicates the glider is still flying into stronger lift; as this vertical acceleration decays, the glider is flying into more consistent lift. A very accurate variometer system may assist the pilot with trend information as well (increasing vs consistent climb indication). The size and [[Anatomy of a Thermal]] can vary quite widely. Pilots may need to initiate the turn shortly after encountering lift on a day with tight thermals, while wide thermals may necessitate waiting 10 seconds or more before beginning to circle. The pilot should turn in the direction they estimate the core to be. If the glider banks when the thermal is contacted, stronger lift is present on the side of the rising wing; the turn should be made in this direction. If the glider does not bank, the pilot may use the location of any thermal markers (ie cloud, circling birds, etc) to select the direction to turn. If there is no definitive indication of the correct direction to turn, simply guess - it is better than flying through the thermal while trying to decide, and thermals are rarely centered on the first try anyways. If the wrong direction is selected and the glider encounters sink, a 270° correction can be made. ![[GFH 270 correction.png]] ## Centering A multitude of techniques and philosophies exist on thermalling flight. This section will cover general principles and a basic approach to thermalling. To optimize thermalling flight and center a thermal, the pilot must: - continuously update the mental model of the thermal they are flying in - maneuver to keep the glider circling in the strongest lift The pilot's mental model of the thermal is perhaps the most important element of successfully centering the thermal. This will include: - location of the thermal's core with respect to the current circle being flown - where the lift begins to increase ("surge") and decrease during the circle - size of the thermal - degree of lift gradient; strength at the core vs the edge - the "tilt" of the thermal, or how the thermal is affected by wind - variation in strength with altitude Maintaining orientation with the glider's heading, position, and altitude, is critical to accurately constructing this mental model. To establish a consistent frame of reference for the mental model, a consistent circle must be flown - this can only be achieved with a constant [[Radius of Turn]]. To fly a consistent circle, a constant bank angle **and** constant speed (pitch attitude) must be maintained. After one or more consistent circles, the pilot has enough information to begin constructing the mental model of the thermal, and visualize where the glider is within the thermal. The general principle behind centering a thermal is to shift the glider's circle into the area of the strongest lift. This should be done **gradually**; making only one correction per circle, with a series of small, deliberate corrections rather than gambling on a single large correction. In practice, this usually means shallowing the bank for 2-3 seconds to shift the circle a small amount toward the strongest lift during each correction. ![[GFH centering thermal.png]] Once the pilot has spent several circles in the thermal and refined the mental model, important inferences can be made, such as: optimal bank angle, the effects of wind, any consistent corrections required to stay in the thermal, etc. The pilot may elect to leave the thermal for any number of reasons: maintaining VFR cloud clearance, mitigating passenger discomfort, or reaching the top of the lift, are the most common. [[Thermal Soaring Weather Information]] may indicate what altitude the lift is expected to top out at, but this altitude will certainly not be most efficient, or even achievable for every thermal encountered. In association with the mental model, the pilot should note how the strength of the thermal changes with altitude, and consider if continuing to circle is worthwhile in the context of the goals for the flight. When optimizing flight for time or distance, a "height band", or altitude range in which thermals tend to be strongest, is a key component of the mental model.