Friday, February 1, 2013

Boyang learns a useful lesson on a combined wave and thermal day - the worst of both worlds

Although this past Saturday was not nearly as epic as Sunday, the conditions made for an eventful flight. In the morning, lenticular clouds were hovering along the mountain range, right above the wind mills, indicating a wave day. There was even a rotor cloud over the pumping station. However, by the time FB was assembled and Van passed his checkride, all of the clouds had disappeared. Did the wave disappear as well? Terence and I set to find out.

Without the lenticular clouds marking the location of the wave, the first challenge is to find the wave. On this particular day, two things were in our favor. First, we already had a clue from events earlier in the day. Second, the standing wave nature of this means that areas of lift and sink change much slower than a thermal. Unfortunately, it isn't as simply as flying towards where we last saw the clouds. From the perspective of a person on the ground, a lenticular clouds looks the same whether you are viewing at it straight on or from an angle. This matters because the region of the lift is perpendicular to the direction of the wind. Since there is a 15 knot wind blowing straight down runway 23, I make a request for a high tow towards brushy peak.

Around 1500' on tow, we experienced a prolonged period of 2 knot climb rate. Normally, the climb rate on tow is 6 knots, so we were in some serious sink. On a wave day, this could be a blessing in disguise because that means the front of the wave might be only half a wavelength away. Sure enough, right around 3000 feet, we experienced a very nice lift, got off tow, and started doing figure eights. In a classic wave, there should be consistent lift with minimal turbulence. Flying a line between brushy peak and the reservoir, we went through patches of lift, followed by no lift and sometimes exuberant lift. It wasn't a classic wave, but wasn't a thermal either because flying in circles didn't help much.

Turning back to the airport, we flew over the pumping station, and found signs of a thermal. After a few adjustments, we found the sweet spot and started climbing at up to 6 knots. With some work we soon gained 1000 feet, and reached 3000' a few miles southeast of runway 30. Just when I thought I might be setting a personal record for longest flight from Byron, the vario went from positive 6 knots to negative 6 knots. Pretty soon, we lost all of the altitude we had gained, and set up for a landing. The culprit might have been a hiiden rotor clouds, which we saw right over the pumping station earlier in the day!

In conclusion, I learned a very useful lesson: on a wave and thermal day, you are not going to get the best of both world. In fact, you are probably going to get the worst of both worlds. Waves work best in stable air because stable air is conductive towards laminar flow of each sublayer of the wave. However, on a thermal day, the unstable air disrupts the laminar flow, and reducing the effectiveness of the wave. On the other hand, directly underneath a wave and on the back side of a wave, rotors lurk, which ultimately ended my flight. In hindsight, we should have taking a much higher tow to get above the thermals and go straight for a juicy wave.