On Thu, May 13, 2010 at 5:24 AM, Frans Veldman <frans@privatepilots.nl>wrote:
"As my previous attempts to explain how I think the anti servo tab works
failed, I thought that a "look and feel" experience would make clear
what I mean."
The confusion around this topic (thus your "failure" ;-) may be (*at least
it was in my case* :-P ) that one has to accept the fact that the design's
torque tube placement results in a (symmetrical airfoil) tailplane is
*aerodynamically balanced* for Frans' functional description to make
sense. ie- no matter what the steady-state angle of incidence is of the
tailplane, there are no resultant stick forces which need to be zero'd out
by a portruding trim-tab ( which is the anti-servo tab in this case).
This was a very hard sell for myself. I had always assumed that that the
tailplane's torque-tube location was *slightly forward* of the aerodynamic
balance point, to make sure that the tailplane would always aerodynamically
"trail" and thus be inherently stable (since it is mass-balanced). My
rational was that if the design was perfectly *aerodynamically* balanced
about the torquetube, and a builder had somehow managed to move the fulcrum
slightly *rearward* of the aerodynamic balance point (build tolerances?),
then the tailplane would be inherently unstable and that would be a disaster
waiting to happen (in that case, once the tailplane is moved slightly
producing an AOA ><0, positive feedback would quickly take hold and slam it
to the stop).
So, if one were to go down the same **incorrect** thought path as myself
(the trailing tailplane idea), then indeed the anti-servo tab would need to
remain deflected out in the airflow to keep any AOA other than zero (and
also performing the function of a flap? .. this visualization still makes my
head hurt).
So, since we are dealing with an "approaching perfect" design (an
aerodynamically balanced tailplane), Frans' assertion of the anti-servo tab
always nulling itself out is correct (boundary layer effects being
ignored/taken care of by the flettner strips), regardless of airspeed, COG,
airplane AOA etc.
I'm still curious how close to perfectly aerodynamically balanced the
tailplane design is. Don Dykins sure was a clever fellow.
Cheers,
Pete
A239
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