There are two effects in play. A wing entering ground effect will see the center of pressure move forward which tends to destabilize the glider. A pilots feels this as increased elevator "twitchiness" when near the ground. This effect is particularly noticeable in G103's.

Any increase in drag will tend to damp pitch oscillations. A free flight demonstration is easy. Just set up a stick-free Phugoid oscillation then open the spoilers and watch it damp out.

The only issue I have with Carswell calling the G103 issue a PIO is that term generally refers to a free flight phenomena not involving ground contact.. Perhaps the G103 specific problem should be called a PIB or Pilot Induced Bounce.


On Thursday, January 31, 2013 9:42:53 PM UTC-7, 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