In the mid '60s
Witold Kasper began investigating the stability of a small flying wing,
the "BKB-1". Previous flight tests conducted by his partners,
Stefan
Brochocki (the
designer)
and Fred Bodek, in Canada had shown a tendency to mush at low
speed.
Kasper's flights confirmed that, with full up elevon, the rate of
sink was 600 feet per minute at an airspeed of 40 mph, with good
control.
Kasper wanted to explore the plane's stall characteristics so, before a
later flight, he adjusted the elevon linkages for more upward travel
and
attached tufts to the trailing edge so he could see when the air flow
separated.
As he raised the nose beyond the earlier test the plane continued to
slow
until it reached a minimum indicated airspeed of 20 mph, still
with
full control. This post stall behavior is commonly called the
"mush
mode" and Kasper spent several years studying it. He clamed that
the rate of descent during this mush was only 100 feet per minute but
that
hasn't been confirmed by other investigators. Since the only
BKB-1
was destroyed in a crash and a replica has not been built the validity
of his observations can't be confirmed at this time.
Kasper theorized that a large
vortex system, like that depicted in figure 6, was formed when the
angle
of attack was greater than 30 degrees. Actually he originally
thought
there was only one vortex, shown rotating clockwise, but wind tunnel
tests
showed a secondary counter-rotating vortex as well. He apparently
thought the secondary vortex was nothing but bad news because much of
his
effort was directed toward diminishing it's size.
Figure 6
As it turns out, effort to
suppress
the secondary vortex may have been misspent. The most obvious
reason
being that, in nature, vortices usually occur in counter rotating
pairs.
It is also interesting that what Kasper called the "wake bubble"
mirror's
the bound/starting
pair *
**
but raised entirely above the wing. Remember, in order for a
vortex
to be stationary one of two conditions must be satisfyed:
- either energy (in the form of new material) must go
through it or
- it must be closed at bouth ends. Either by walls or by
being
bent
around into a closed loop
Since new fluid enters at the periphery and exits from the
core. In the system shown above, the vortices are
getting
energy from air flowing around the wing . As long as the two
vortices
are close together the system may not consume very much energy and it
dose
produce some extra lift, maybe as much as 100% over the airfoil
2D
CLmax (Kasper clamed he experienced CL
greater
than 3)
See this link for a NASA
deep stall experiment using a modified sailplane.
