On Thursday, January 31, 2013 11:42:53 PM UTC-5, Larry Suter wrote:
In his article on avoiding PIO in Grob 103's, http://www.soaringsafety.org/pilots/ic8.htm Dean Carswell writes, "the more the airbrakes are closed, the less pitch stability the Grob will have, making a PIO more likely if otherwise mishandled" Is there a simple explanation why opening the spoilers increases the pitch stability? Does it somehow move the center of lift further aft? I believe increasing the separation between the cg and center of lift is the classic way to increase pitch stability. And if that's how it works, why does the center of lift move aft? I would guess spoilers destroy the lift downwind of their location, causing the center of lift to move forward..... Thanks, Larry

The event described is indicative of a failure to recognize that the attitude of the glider is the most important element of not knocking the wheels off the end of the glider.
When watching one of these events, it is common to see pitch changes and spoiler changes all mixed into a confused over correction event.
I would expect that application of spoilers may change the trimmed condition that the glider will tend to seek on it's own, but I doubt there is a significant change in the stability of the glider.
I teach the principle that once the glider is in the round out and pre touchdown
position, the spoilers/brakes change from rate of descent control to rate of deceleration control.Given that, the message is leave the left hand fixed unless the glider is going to land short.
FWIW
UH