Kiteboard Theory - Thread 8: Telltales

This is a short one.

Generally, to achieve the maximum possible lift, your kite should be trimmed with the highest AOA (angle of attack) possible without causing the airflow above the kite to become separated from the kite surface, and turbulent. Airflow separation starts near the trailing edge, and with progressively higher AOA, caused by tighter bar trim, the area of airflow separation increases, working forward toward the leading edge. Total separation is called "(aerodynamic) stall", and partial separation can be considered as "partial stall", which reduces lift and decreases the L/D (lift/drag) ratio.

Most reasonably experienced kiteboarders can tell when a kite is suffering total aerodynamic stall because it stops flying effectively and begins to fall backward, as though it were a parachute or giant Glad garbage bag. But it is virtually impossible to detect a subtle, "partial stall" by feel alone.

"Telltales" are simple but effective devices for detecting any degree of aerodynamic stall. They are typically strips of ripstop sailcloth, yarn or cassette tape that are adhesively fastened to the upper surface of the kite near the trailing edge. When they stream smoothly backward, they indicate smooth airflow, but when they flutter erratically, they indicate airflow separation and turbulence. I recommend attaching about four telltales across the top of the kite, about six inches forward of the trailing edge. You can add more if you want to see what the airflow is doing all over your kite, but it's not necessary.

For optimal lift (for maximum power, speed, upwind performance, jumps or whatever), the bar should be trimmed, or "sheeted in" until the telltales just begin to act erratically, and then the bar should be immediately eased just enough to make the telltales stream smoothly. If you keep repeating this procedure, you'll always know that your kite is perfectly trimmed.

Sailmakers and marine suppliers like "The Dinghy Shop" sell very tidy and effective telltale kits, but alternatively you can improvise your own. And just because most kiteboarders don't use telltales doesn't mean for a second that they shouldn't. For the same reason, there is barely a competitive racing sailboat in the word that doesn't have telltales on its sails.

©2007