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Angle of Attack
11/2010 Sundowner Comment
In response to Marge's request for comment
on a video she posted flapping over the Factory
on Saturday, 11/13/10 [Marge's Article]
Thanks for sharing Teach. The video is entertaining, but it's educational value is missing clarity. It's difficult to track the glider's orientation without a horizon reference, and I suspect the wide angle lens is skewing our perception. The trailing edge isn't visible much of the time so we can't follow control inputs, but if the camera was on a leg looking up, and we can see the hands, then the brakes likely weren't buried.
Please note that I'm not an expert on maneuvers. I've never been to a maneuvers clinic, and spend most of my flight time trying to keep my canopy overhead, so I'm sure others are able to offer more studied advice, but I don't often pass on an opportunity to state my perception when it's solicited, and I have flown in variable air.
When moving up to a higher performance wing, the most notable change is more speed, which results in a greater tendency for the wing to move quicker out front, especially when it's loaded (loading is the driving force). It appeared from the video that the pilot's wing may have been in front and tucked due to a low or negative angle of attack, but the wide angel lens may be distorting my perception.
Higher performance by definition means the wing is operating at lower angles of attack (higher L/D).
As previously published in comments titled "Asymmetric Angle of Attack" (2001 http://paraglide.net/comment/01/asymmetric_aoa.htm ) "Ultimately the angle of attack needs to stay within a range that depends on variables. If the angle of attack gets too high the wing will stall, if it's too low it will tuck. If you're going fast it can front tuck and snap back into a full stall." The wing may tend to surge in front for a number of reasons, including high loading that occurs during recovery from some prior event that built vertical momentum, like recovering from a stall. I personally learned that lesson the hard way on a glider that was beyond my skill level. http://paraglide.net/incident_report/1998_07-01_aspen.htm
When in doubt, a full symmetric stall can be your friend. It's like rebooting your computer. A more serious hazard is getting the wing under you and falling through the lines or into the canopy (getting shrink wrapped).
There is a balance under higher performance wings between letting the glider surge some to let it fly and recover from a stall, but not letting it get too far out front. Braking too hard might result in an asymmetric stall (spin), but letting the canopy de-tension and go negative is arguably a greater hazard in some scenarios. A spin is more likely to occur with higher performance canopies for a number of reasons including more asymmetric span to manage. But if you don't brake hard enough then you may have other issues to deal with. Two common ways to get out of a spin are to let it fly, or reboot using a full symmetric stall. A problem with letting the wing fly is that one side is already flying, surging in front due to increased loading, while the other side may be pinned under by the drag. If the canopy starts to helicopter, then I'd recommend stalling the side that's flying to get back into symmetry. It is obviously wise to practice over water with plenty of altitude because if you fly a high performance glider in the Santa Barbara Mountains on strong thermic days with sharp deferential heating and a little wind mixed in, you will eventually need to deal with gyrations close to terrain.
Most really bad configurations occur during a series of cascading events. One event often swings into the next. When in doubt, if one cascading event is gyrating into another, and you can't visualize the pending sequence, it's nice to get the glider overhead, even if it's stalled. Marge's video does show a few quick views of the full wing chord just prior to the flapping tuck, but not at the moment of tuck. It appears there is minimal deflection along the trailing edge, and the wing likely tucked because the angle of attack went negative. This was a cascading event. The appearance of the wing horizontally in front of the pilot was a result of a prior event that cascaded into a surge. My Monday morning quarterback suspects the pilot could have been more aggressive in arresting the surge and keeping the angle of attack positive by using a lot more brake travel. Sometimes you have to use all the travel your arms can provide. Long lanky people arguably have more control authority. Also note that brake pull force increases with loading, so you might have to pull really hard.
I'm not suggesting Marge needed to full stall the glider. I am addressing the reluctance of budding pilots to utilize more control travel. I've also counseled pilots to avoid excessive braking when their canopy is mushing at a high angle of attack, slow and behind them, but that is not what the photo represents. In turbulence we often fly with the glider slowed down some for a number of reasons, including some upward control travel availability when the canopy starts to rock back. DHV 1 canopies are typically slow with stubby aspect ratios so they tend to self correct with minimal tendency to spin or surge below the pilot. If you wait around for a higher performance canopy to self correct, you might end up packaged.
My subjective comments addressing the photo above are not meant to be an in depth guide to stall recovery. There are many issues that may come into play. For example, many pilots wouldn't let their glider fly out of a full stall if they don't have enough room for the resultant pendulum to swing through. Slamming into the rocks rounding out the bottom of a high speed pendulum is likely the worst of multiple bad options. Give it some thought, discuss it with your instructors and counsel, and try to dial into your canopy pendulum timing under various loading scenarios.
Another minor comment would be about turning aggressively toward strong thermal cores. Sharp thermals want to spit you out, so you need to steer aggressively to get your glider banked into the core. If you are too timid, you will keep getting spit out. It's ok if the outside wing rolls under as the tip goes negative when the pilot muscles through the gradient trying to turn into a sharp core. I've skirted the subject in other comments, but I need to write a real article some day because it requires much more discussion. I will mention that speed control is essential. You can't bank the canopy if you don't have some speed to swing out with, and speed management is essential to controlling your bank angle.
On low pressure smooth days, the air is cleaner in the core and nasty on the edges, so thermaling can be more elegant. On high pressure days, the thermals tend to be more broken and multi-core without a defined center, so you can either bounce around the general area at min sink keeping it flat, or select one of the cores and turn hard into it.
I do have to commend Marge on keeping her emotion in check. A lot happens pretty fast and it's easy to get flustered. A lot of our reaction is intuitive response built from experience, so the more you can mix with the wild air, the better you can hone your intuitive reaction, balance, and timing.
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