In checking out pilots in the G-103, I refer to the nose wheel/tail
wheel oscillation as a "tire PIO" to clearly differentiate it from the
more classic PIO associated with the excessive landing speeds and ground
effect destabilization. The correction for the latter is also
classic...FREEZE THE STICK.

The correction for the tire PIO is a bit different and is the more
difficult to perform. This oscillation, as pointed out in another post,
is divergent and VIOLENT! I have seen these result in 3-4 cycles in
about a second. Avoidance is the best cure but if it happens, there is
not enough time left to think of a plan so this is best thought out in
advance.

This is my plan...since it is the ground that is going to smite me, I
get away from it. RE-FLY AND RE-LAND the aircraft. (Here is how I know
this works...once while rolling on the main gear in an ASK-21, carrying
enough energy to taxi to parking, an airplane pulled out in front of me.
I hit the wheel brake hard, the nose pitched down and when the nose
wheel contacted the runway a tire PIO ensued, despite having full
spoilers out. I stowed the spoilers briefly and froze the stick.The
glider resumed flying and I then re-landed). Don't plan on your logical,
rational mind helping much...the motion is too rapid and violent. In my
case, I was acting instinctively. Be spring-loaded to use that excessive
energy that led to the problem in the first place, to get away from the
ground.

Some may disagree with these methods but so far, they have worked for me
and the pilots that I have instructed.

Paul
ZZ


On 02/01/2013 08:18 AM, Bill D wrote:
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