Monday, May 31, 2010

Tehachapi Day 3 - Mystery Partly Solved

Very few people flying today, so hard to tell if anyone stayed up. It was a bit breezier but not bad. Conditions didn't look good for a retry of my long flight for Silver. I dawdled trying to decide whether to try to fly shear line (due to the wind) or wait for it to heat up. Someone asked me, "What's that little box on your hat?" I replied that it is a mini audio variometer which I can use if the one in the ship fails (or in Blaniks that don't have built-in audio varios.) She asked me, "Do they fail often?" I said no, but I had the whole battery fail once (the PW5 has no pneumatic-only vario), and I had definitely used it then. (See my first Dust Devil Dash post.)

One club member stayed up about a half hour, working shear line lift at about 6000' MSL, so I figured I'd at least give it a try. As I was pushing out, he came back, which was not a great sign.

During the tow, I noticed the vario needle swinging wildly, something I had seen yesterday but attributed to the bumps we flew through. I let off at 3000' AGL, well upwind, where I thought I would start hunting for shear line (convergence) lift. But something wasn't right. I was seeing sink on the vario, but it didn't feel like I was sinking nor was the altimeter unwinding. I thought to turn on my clip-on electronic vario and sure enough, it beeped to indicate lift. I also checked the "Vario" number on the display of my PDA/GPS, and it showed "up" as well. (Not sure if it's calculated from GPS info, or fed from the pressure sensor in the Volkslogger.)

So I had conclusive proof that the vario or the pneumatic system that drives it was faulty. Yesterday's terrible flights were not entirely my fault. Thinking I was in sink, I sped up to try to get out of it, which only made things worse, since the PW5's polar curve drops off pretty steeply with speed. I had slowed down to minimum sink speed a couple of times to try to reduce the drag and see if I was fooling myself, but maybe at those times I really was in sink, because it didn't help. I don't know why I didn't think of checking against the other vario and the GPS yesterday - I guess I believed the instrument too much.

I ignored the bad instruments and flew by feel and by my clip-on vario. I found some moderate lift and then a halfway decent thermal. I worked it back up to 7100' MSL, nearly my release altitude. I couldn't seem to get higher than that. I don't think it was shear line lift, because it was wide enough to circle in pretty consistently. I left it in search of other lift, but kept coming back to it to "tank up". Eventually I went downwind to circle under another glider that was higher than me, but that didn't work out and I came back for a landing after 33 minutes. I didn't care that it was fairly short - I had solved yesterday's mystery and successfully worked the lift on a not-so-great day. (And I experimented with another way of working lift in wind, but that's a story for another blog post.)

My landing on runway 27R was terrific - really smooth touchdown, and I worked hard to get a very long straight rollout with a light gusty wind about 40 degrees off my nose. That's tricky in the PW5, with its non-castering wheels and tiny rudder. The runway was completely clear so I was able to roll all the way to the west end, and use the favorable crosswind to roll off gently, wings level, into the dirt right in front of my tie-down spot. Nice!

Back on the ground, another club member and I worked on diagnosing the problem. To make a long story short, we found a kink in the Total Energy probe line, and think it's pinched or caught somewhere under the cockpit floor. Another member who's staying over at Tehachapi is going to pull the floor and look for problems.

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