On Oct 31, 8:36 am, BB wrote:
I think the Duo's airbrakes are better than many people think. The Duo is a
big heavy glider with lots of inertia.
I don't understand the physics here.

Like other duo pilots, I notice that the glide angle in a stabilized,
on-the-triangle glide with spoilers open is decently steep, but that
the duo does not slow down quickly if you use spoilers to stop excess
speed, especially in ground effect.

Here's a theory -- or at least the physical possibility of this
observation. Spoilers, true to their name, do two things -- they add
drag and they spoil lift. Since lift = weight, "spoiling lift" really
that induced drag is raised because of the discontinuous lift
distribution at the spoiler position.

Now, it makes sense that the duo spoiler configuration is more
oriented to the production of induced rather than profile drag. The
spoilers are wide (spanwise) but not very deep (up and down). The span
is bigger than your 15 meter glider, so there is less induced drag to
start with, and increasing it is more effective in reducing glide
angle.

If this were the case, we'd expect the spoilers to be less effective
One thing I haven't seen in this discussion is the fact that the Duo's
spoilers are not located right behind the spar. They are located quite
a bit farther aft and thus produce mostly drag and not as much spoiled
lift. I liken them to the H-301 Libelle spoilers that produced mostly
dray and therefore had to be opened earlier and left out longer to get
the same result as, say the H-201 Libelle which had true spoilers
located at the high point of the airfoil.
JJ


overall at higher speeds, and especially so in ground effect, which
reduces induced drag, but to produce a good glide angle at a
relatively slow approach speed, out of ground effect. This seems to be
about what we observe. This also explains why spoilers on all big
gliders seem less effective at "high parasitic drag" maneuvers.

The bottom line remains, if you fly a duo, accurate speed control in
the pattern is essential. And yes, even if the stabilized glide angle
is the same as other gliders, the spoilers are "less effective"
because bleeding off excess speed is one of the desirable "effects" of
spoilers.

John Cochrane