We can beat up theory all day, but what about practice?

I don't give a sh!t what I do with the stick when I open the brakes, nor
what is the arc hyperbolic cosine of the angle of the dangle, nor what
the glider does in response. When I deploy the boards, I also operate
the flight controls to make the flight path do what I want.

Or you can be debating theory as you proceed merrily towards the ground.

On 6/6/2015 7:32 AM, wrote:
On Saturday, June 6, 2015 at 7:59:13 AM UTC-5, Martin Gregorie wrote:

No. The main effect of well-designed airbrakes is to reduce effective
lifting surface, but, as a side effect is to add drag, most gliders will
need the stick to be eased forward a little to keep the airspeed
constant. However, the wind gradient and turbulence will probably be more
significant: you'll think you're correcting for these factors rather than
for the extra drag from the brakes. But, as I said above, it does depend
on the glider. For the G103 to behave as it does, its drag must reduce
along with the lift as you open the brakes. Conversely, the Puchacz has
huge speed-limiting upper and lower surface brakes: so much so that it
would be surprising if shoving those out in the breeze didn't show it
down.

One could steer this conversation in the direction of transient versus steady-state effects. For example, if we start with a typical sailplane L/D ratio (say 30:1), and then we deploy some device that doubles the drag coefficient and halves the lift coefficient, we'll experience a temporary deceleration due to the sudden drag, but as the flight path curves downward, it must be the case that we'll finally come to equilibrium at a much higher airspeed than we started with.

Because "scaling up" the L and D vectors, by increasing the airspeed, is the only way to create a closed vector triangle of L D and W.

Assuming that angle-of-attack is held constant throughout.

When we open airbrakes without moving the stick, there's no reason to assume that angle-of-attack stays constant. We're killing the lift over one part of a wing which has twist (washout), so we're making a change in the average incidence of the "working" part of the wing. And we may be changing the airflow over the tail as well.

So a question of interest remains-- let's forget about airspeed entirely-- in wings-level flight or at some shallow bank angle, does the stick need to be further aft (closer to the aft stop) to induce a stall with airbrakes open than with airbrakes closed?

S

--
Dan Marotta