I was having a nice glass of red wine when I wrote this late the other evening, and left out some essential details.
DMarley wrote: ↑
Fri Mar 09, 2018 11:37 pm
What Eric didn't account for in his last example is that the air currents are being driven upwards from the cliffs at an angle that is normal to the direction of the cliffs. While flying parallel to the cliffs, the glider's airspeed vector added to the vector of the air currents being thrust upward at an angle, create an apparent 'wind' to the ocean-side (starboard) wing that is at a higher angle of attack compared to the land-side (port) wing, due to the wing's sweep angle. The starboard wing of course experiences more lift, but more importantly, creates more drag than the port wing, therefor the glider yaws to starboard, and only continues past equilibrium of both wings' individual drags until the rotational momentum is damped. The pilot gave zero control input to the glider yawing even though his body was a bit skewed from the glider's initial heading. This happens a lot while slope soaring in a glass-off, does it not?!?!
The only time this situation could occur is if there was an airspeed reduction of the glider, and/or change in direction and/or magnitude of the air currents.
The angle of attack is not really different on both wings, rather it's the flow direction relative to each wing's sweep angle that changes between the two. I.e., if the glider slows it's airspeed down to trim and the air current is flowing from ocean to land at an upward angle, the resultant apparent wind to the wings will appear to clock around to the ocean side by a small amount, something similar to landing in a crosswind. The difference in drag of both wings results from the clocking airflow, causing the apparent flow to encounter a longer effective span on the starboard wing, and a shorter effective span on the port. Thereby the starboard wing generates slightly more lift, but also more drag compared to the port wing.
When the pilot in the vid example releases control of the control bar, he likely is allowing the glider to slow to trim from a slightly higher airspeed. Either that or he is flying into a section of air current that has a slightly higher velocity. Or both.