by wingspan33 » Sat Sep 15, 2012 11:45 am
Thanks for the reply Bill!
I think your analysis is close to right on. What I originally did was simply look to see (on the flight's GPS track log) what I could not remember because of the jarring my head took. I solidly remember planing a different and specific approach for my landing. However, the GPS track log revealed that I altered that approach. I have no memory of why I did so.
I also noted that during my final few seconds in the air, I was losing altitude much faster than should have been expected. The track graphic in Google Earth revealed that clearly. In fact, I lost 85 ft. in about 4 second's time.
But at the same time that I was losing that altitude, my ground speed was indeed quite swift. And conditions weren't such that I had much chance to have encountered a 25 - 30 mph westerly tail wind (which would have resulted in a stall - which is one way to explain the quick 85 ft loss in altitude). That leaves mechanical turbulence and I suspect some form of rotor induced downward air flow. This could have come from a freaky cross and down wind air flow off the hill, or a powerful thermal ripping off just beyond my area of the sky. This last situation could have drawn air downward from around the area surrounding the base of the thermal release "zone" - resulting in the apparent serious sink that I believe I must have been encountering.
As to the 7 foot altitude gain, I hadn't noticed that. But where that was, was between the transition from the ground speed reading of 47 mph and the next reading of 30 mph. Note, that at the same time that I am gaining a small amount of positive altitude, I am also "losing" 17 mph of ground speed. This is because I was involved in my turn back toward the west. With my excess(?) ground speed (52 - 47 mph) just before this point, I would guess that the 7 foot "bounce" could have been connected with a quick transition back into a head wind (a stick thermal without having to move the stick).
But it could also have been associated with a change in the very local air mass at this lower altitude. It may have been denser (cooler?) or the air might have been "bouncing" off the ground after down washing as a sinking air mass. I have very clearly encountered this kind of effect in the past, but it has always been within 10 - 15 feet of the ground. So, I think that it is acceptable to bet on my slight altitude gain as being the result of the transition from going fast down wind to swinging that gathered kinetic energy around back into the wind. An effective "stick thermal".
Also, in doing some subtraction involving my altitude loss between (2 second interval) GPS readings, I get -
(During the final moments of my flight)
- 36 ft
- 49 ft
+ 7 ft
- 43 ft
- 13 ft
In 10 seconds (on a topless wing) I lost 141 feet of altitude.
In another 10 second sample during my (sinking - not soaring) glide about a minute before landing, I lost only 38 feet.
There was something unusual going on during that last section of my flight. Some kind of seriously sinking air, is probably a good guess.
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