"Bob Kuykendall" wrote in message
...
Earlier, Mat Redsell wrote:

the air going over the top is separating
due to the high angle of attack...

Sounds like a stalled condition to me.

Bob K.

Again, not quite. Mat is describing an incipient stall, not a full stall.

The PII is under-elevatored with respect to stalling the aircraft but not
with respect to achieving CLmax. The Marske PII elevator authority
limitation is directly related to the incipient stalling of the wing. At
airspeeds above stall, the elevator authority is as large or larger than
conventional gliders. But as the incipient stall begins, the elevator
authority is sharply limited. (Another way to say this is that there is a
large non-linearity in elevator authority that sharply reduces elevator
effectiveness as the incipient stall begins.) This is not a linear function
of AOA or airspeed. As far as I know, this behavior is unique to plank-type
flying wings - particularly those with the elevators on the inboard trailing
edge of the wing such as the Marske planform.

On the other hand, with a conventional tailed aircraft, there is no link
between the elevator authority and the stalling behavior of the wing. In
the case of a conventional glider with a horizontal tail located some
distance aft of the wing, the wing may be fully stalled or fully flying, or
anywhere in between, with no effect on the elevator authority. Depending on
the elevator area, up angle limit, airspeed and CG location, the elevator
may be able to bring the wing to stall or beyond without the wing stall
imposing any limits on the up elevator authority at all. The designer may
choose to impose limits on up elevator authority to achieve benign stalling
behavior - this is not the case with the Marske planform.

As to Mat's report of near zero airspeeds, there are certainly large ASI
errors involved and the true airspeed is almost certainly not zero. To get
a true reading, Mat would need a nose boom with a gimballing pitot/static
probe and a sensitive calibrated ASI.

However, there are many reports of special conditions where a spanwise
vortex forms above the wing at high AOA preserving a large amount of lift.
One prototype of the Velocity canard pusher encountered this phenomenon with
the forward wing which resulted in the aircraft settling in a flat attitude
at low sink rates. The aircraft was not recoverable from this condition and
the test pilot elected to ride it into Tampa Bay at 200FPM sink and lived
to tell the tale. The Kasper Wing apparently can do this as well but is
recoverable.

It would be interesting to see a video of yarn tufts on the upper wing
surface to help visualize the flow with near-zero indicated airspeed.

Bill Daniels