In article ,
says...
Eric Greenwell wrote:


I think this may not be a correct analysis. In my ASW 20, during steady
straight flight, I could open the spoilers while holding the stick
steady. The glider would maintain it's attitude, but begin sinking. The
G force was reduced very momentarily, then returned to 1 G.

Correct. In steady straight flight, as the brakes destroy
lift in the wing central area, the glider begins to
accelerate towards the ground under the influence of
gravity. The change in path causes an increase in AOA
everywhere until lift has increased to equal weight.

However, we aren't talking about "steady straight flight" at
1g but curved flight at 1g and the only question is how
much greater than 1g we're going to impose on the structure.

The wing
tips would bend upwards, indicating they were producing additional lift.

Agreed.


The additional sink rate increased the angle of attack of the wing, and
this caused the additional loading on the wing tips. In other words, the
lift tends to shift to the wing tips without the pilot doing anything
besides opening the spoilers.

Correct - in the case of "steady straight flight" at 1g.
However, in the case of nose down curving high-g pullup,
there is no force corresponding to gravity that will cause
the glider's path to automatically change to increase the
AOA.

I haven't tried it, but I think the glider will change it's path; in
fact, it must, since the lift on the wings has changed. How could it
continue in the same direction with one of the forces on it cut by a
large amount?

I don't think it matters if the glider is in a turn or pulling out
from a dive, what matters is the reduction in lift force. Because of
this reduction, the glider will accelerate "downward" (relative to the
lift on the wing). This change in path will increase the AOA on the
wing, increasing the lift at the tips, and increasing the load on
them.

If this analysis is correct, simply opening the spoilers will result
in shifting some of the lift to the wing tips. It isn't necessary to
change the elevator position to make this happen.

That would only happen if there was a 5 g force
outside the curve of the pullup pulling the glider outward.
There isn't. If the pilot has an AOA set to produce 5 g's
and opens the brakes, he suddenly loses a major part of the
lift and gets 3 g's and a larger radius pullup. There's no
5 g gravitational force outside the pullup path imposing 5
g's on the structure. The glider is happy to fly at the
larger radius reduced g path produced when the wing remains
at the original AOA with the brakes open.

The "trap" for the pilot is that we tend to hold a constant
g-level by modulating back pressure on the stick. As the
brakes are opened, we will automatically pull back to
increase AOA and shift the load to the tips, even if the
glider doesn't do it automatically.

This may also happen, which would make the situation worse, of course.


I haven't tried it, but I assume this would also happen in a 3 G turn.
If true, the only way to avoid exceeding the "open spoiler G limit"
would be reduce the G loading before opening the spoilers.

We're not talking about a "turn," but rather a pullup from
nose down attitude.

The basic forces on a glider in a 2 G turn (for example) are
indistinguishable from a 2 G pullup.

You can't really compare the situation
where gravity imposes 1 g to the case where the g-load is
not externally imposed.

I think you can. Aren't the forces on a glider pulling 1 G at the top
of a loop the same as a glider gliding steadily and upright? Of
course, there are some minor differences because the airflow in
curving flight isn't exactly the same as straight flight, so the
elevator would have a slightly different force on it. The influence of
gravity is to change the path of the glider, but the forces on the
structure depend on the G loading, not the orientation of the glider.

--
!Replace DECIMAL.POINT in my e-mail address with just a . to reply
directly

Eric Greenwell
Richland, WA (USA)