Question about spoilers and pitch stability

At 04:42 01 February 2013, 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
st=
ability the Grob will have, making a PIO more likely if otherwise
mishandle=
d"

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
i=
ncreasing the separation between the cg and center of lift is the classic
w=
ay to increase pitch stability.=20

And if that's how it works, why does the center of lift move aft? I would
g=
uess spoilers destroy the lift downwind of their location, causing the
cent=
er of lift to move forward.....

Thanks,
Larry


Hi Larry,

There is indeed a simple explanation as to why opening airbrakes increases
stability in pitch. It is to do with drag.

As I assume you are aware, the reason that the airspeed increases when you
'push the nose down' is because a larger component of the lift force
generated by the wings essentially points in the direction of the gliders
horizontal motion. Obviously the opposite happens when you 'raise the
nose', and again obviously the glider stops accelerating when the sum of
the forces acting on the glider is zero (drag increases as the glider
accelerates).
Clearly the total drag at a given airspeed is greater when the airbrakes
are open than when they are closed. If a glider is experiencing a
particular 'amount' of drag at a given airspeed with the brakes open, then
the same glider with the brakes closed will obviously have to fly at a
greater speed in order to experience the same amount of drag.
All of what I've just written is completely and utterly obvious, and I'm
sorry if I've insulted your intelligence.

However what I'm getting at is that, essentially, if you sit in a glider
and move the stick forward by a certain amount, the glider with the
airbrakes open is in some (rather handwavy) sense already 'closer' to being
in equilibrium (or closer to the constant speed at which it will fly given
the amount of forward stick) than the same glider with closed airbrakes.
In other (probably rather clearer) words the glider with airbrakes open
will not accelerate at the same rate, or for as long, as the glider with
closed airbrakes, and will therefore have a lower speed (remember we are
talking about moving the stick forward by a given amount).
Basically what this results in is, again as you know, the glider feeling
less responsive in pitch if the airbrakes are open - you have to push
forward more with the brakes open in order to accelerate at the same rate
as with the brakes closed, and if you want to fly at a given airspeed the
nose will be lower with airbrakes open than if they were closed. This is
why the glider is more stable in pitch, or if you like less responsive in
pitch, when the airbrakes are open.

I'm not entirely sure any of that made sense, but I hope it helps!

Best,
JK.