5Z wrote:
Chris Nicholas wrote:

Now, why does opening the airbrake (usually called spoilers in the USA
for some unknown reason) help on some gliders? And did it ever help on
gliders that really did have spoilers (UK-speak for things that spoil
lift but don't add much drag, unlike airbrakes [in UK-speak], which do
both in spades)?


The best argument FOR this technique was presented in Soaring magazine
quite a few years ago.

At low speed the wing is producing some lift over the entire span.
This provides a damping effect to any rolling tendency. Imagine a
whole bunch of weak springs attached to the spar every few inches and
attached to the ceiling. The plane will resist a wing drop. Now,
remove the springs in the spoiler region and the glider will tip more
easily, as there are fewer springs working to keep it level.

So with the spoilers extended, the wing is easier to pick up as the
aileron has a little bit less force to fight.

The "spring" analogy doesn't sound right to me: it implies the wing is
attempting to hold the glider level, which is desirable. I suspect it is
the opposite: when a wing moves down a bit, it moves into slower moving
air nearer the ground, and loses some lift (and vice versa for the wing
going up a bit). This loss of lift tends to make that wing go down even
faster. Or if the wing is already down, it's definitely in slower moving
air compared to the up wing. The ailerons have to overcome this
disparity in lift and CG shift from the glider being tilted to the side.

So, with the spoilers out, the lift disparity is reduced (both wings
lose some lift), and it's easier for the ailerons to lift the wing off
the ground.

The above argument probably makes more sense when there is a wind. If
it's calm and the air motion is just the glider rolling on the ground
through the air, it might still make sense if the air near the ground is
more disturbed (somehow) the air several feet higher.

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Eric Greenwell
Washington State
USA